January 23 Snow (or Lack Thereof) Update

Before we get to the weather, let me say this: I love The Simpsons.  LOVE.  When I was in college and grad school, I used to tape every episode — I had about 20 full VHS tapes on the shelf by the time DVDs came around.  And then they started releasing whole seasons of TV shows on DVD, and I realized…

So, that’s the reason for the theme to this post: because I wanted to, and because I can usually come up with a Simpsons reference for pretty much any occasion.  Impressive or sad?  You be the judge.

Anyway…the forecast is a little snowier compared to what I discussed in this space 24 hours ago.  Just a tiny bit, but if you’re a snow-lover I’m sure you’re doing this:

And if you’re one those people who could happily live the rest of your days without seeing another snowflake…sorry:

We had some sleet and snow mixed with the cold rain this morning, even some minor accumulations on elevated surfaces to the north and east of Nashville.  But as moist air surges up from the south, it’s dragging along slightly warmer air above ground level — that means we’re in for mostly cold rain showers through this afternoon.  Still, the radar view just before noon looked like this:

Now, most of what you’re seeing in pink (rain/snow mix) and white (snow) isn’t sticking to the ground, just decks and rooftops before it melts.  But it still indicates that the warm air with this system is having a hard time displacing the cold air already in place, and that in turn means the cold air should have an easier time making eastward progress this evening.

Let’s fire up the Futurecast computer…
…and show you the cloud/radar simulation for this afternoon:

Notice the northwestern corner of the Midstate at 5pm.  The cold air looks like it’s going to catch up to the moisture to prompt a changeover from the cold rain and rain/snow mix we’ve seen there for much of the day, to just plain old snow that has a chance to stick.  (IMPORTANT: This is always the most challenging part of any winter weather forecast — if the cold air is slower, or if the showers move faster, they won’t coincide to produce ANY accumulating snow, let alone the small amounts we’re forecasting for this evening and tonight.)

But for right now, here’s how it looks like the rain/snow progression will unfold across the Midstate this evening:

Now, if all of that blue and white on the Futurecast simulation gives you visions of doing this…
…or nightmares of this…
…just slow your roll for a second.

A few factors will limit our snow accumulations, even once the precipitation is all snow:
1) the snow will fall on wet ground, helping to melt the flakes
2) the ground will also be relative warm, helping to melt the flakes
3) air temperatures aren’t likely to drop below freezing until after the snow ends, helping to…well, you get the idea.  Basically, our accumulations will be about half (or less) of what would add up if the snow was to fall on frozen ground.

So, how much will add up?  Here’s what the various models are saying…don’t worry, I won’t just throw computer data at you — my overall forecast is after the model stuff.  But I want to give you an idea of how many different data sources are considered when it comes to putting together these forecasts.  If you want to skip past all the models, just scroll down until you see Comic Book Store Guy (if you don’t know who that is, please re-evaluate your life choices, or at least your television choices).

First up, the “RPM” (the model used in the Futurecast images above):

Sometimes we use the “BAMS” model to feed Futurecast…it’s not impressed with this storm system AT ALL:

The “NAM” model (one of the NOAA computer models) is the snow-friendliest:
Snow lovers, commence your drooling…
…but keep in mind that if the NAM was a Simpsons character, it would be:

The “GFS” model (the other main NOAA model) is a little more reasonable in its depiction…weather nerds recognize how hard it is for me to acknowledge that fact:

The “ECMWF” (the European model you’ve probably heard about in national news stories, because it beats the pants off the American models when it comes to long-range forecasting) ignores the Cumberland Plateau for reasons that mystify me:

The “HRRR” (a very short-range model that sometimes is very accurate, and sometimes not so much) basically says “a little, not a lot”:

Finally, the “SREF” (an ensemble model, or combination of multiple data sources) is almost as unimpressed as the BAMS:

Head spinning?

Here’s what I’m thinking for the “at-most” snowfall potential tonight.  ANOTHER IMPORTANT NOTE: the “at-most” phrasing means that you likely will see less in your back yard.  If you’re in the area that says “Dusting – 0.5″ you’re more likely to see a dusting that you are 0.5″, and you’ll probably get even less than that.  Don’t think of it as “this is what we’ll get,” think of it as “we’ll get less than these amounts unless everything works out juuuuuuuuuuuust right.”  This map is a result of combining the model data, an analysis of how this storm system has been behaving so far (both in terms of temperature and precipitation), and how similar storms in the past have behaved.

Enough snow to shut down society?  No.  Do you need to rush to the store to stock up on perishable items?  NO.  But it is enough that I’ve adjusted the needle on the Panicometer:

LATE-AFTERNOON UPDATE: the National Weather Service revised their predictions (upward), and is now predicting these amounts:

Also, both NWS-Nashville and NWS-Louisville have issued Winter Weather Advisories (purple counties on the next map) for portions of the Midstate — the advisories for the eastern half of southern Kentucky are in effect until 4am Saturday, the advisories for NW Middle TN are in effect until 6am Saturday.

Regardless of how much we get, any snow that does accumulate won’t last long.  The sun will break through quickly on Saturday morning, and temperatures will reach the mid 40s by Saturday afternoon.  Even Hank Scorpio can’t melt snow that fast.
Um…maybe he can.

Yet another chance of mostly-rain-but-a-little-snow is on the way for Sunday and Sunday night.  We’ll keep a close eye on that over the weekend…because there ain’t no party like a weather-nerd party.

Follow us on social media for updates over the next couple of days…links to the relevant accounts are at the bottom of this post. Lisa Spencer and Dan Thomas will have updates this evening, and I’ll be filling in for Nancy Van Camp on the air Saturday morning to put a bow on things as the last of the flurries exit the area.

And if you’re sick of winter…well, before you know it, summer’s heat and humidity will be here.

Social media links

Twitter: @WSMVweather, @PaulHeggenWSMV, @WSMVLisaSpencer, @WSMVDanThomas, @WSMVNancyVanC, @NWSNashville

Facebook: 4WARN Weather, Paul Heggen WSMV, Lisa Spencer, Dan Thomas WSMV, Nancy Van Camp WSMV, NWS Nashville

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January 23 Snow (or lack thereof)

Over the past couple of winters, the same scenario has repeated itself over and over again, to the exasperation of snow enthusiasts and to the delight of snow-haters. The pattern is that either we see precipitation and no cold air, or we see cold air and no precipitation. It looks like a similar pattern is going to unfold over the Midstate over the next 48 hours…but there are two times over the next couple of days when cold air and moisture could coexist over the Midstate long enough to produce a little bit of snow.

The first of those times is early Friday morning. The big question as the storm system approaches is: how early does the moisture move into Middle Tennessee, and how far to the north does it travel? The earlier it arrives, and the farther north it goes, the more favorable conditions will be for a mixture of rain and snow, or possibly even plain old snow showers to start the day Friday morning. It’s something we will have to watch very carefully overnight into early Friday morning…but for right now, the likeliest scenario (shown in these Futurecast snapshots) is that the showers will be confined to southern middle Tennessee, where conditions will be warm enough in the lowest mile of the atmosphere for just cold rain showers. (For you serious weather nerds to who are curious about the model data, this is the RPM model…one of the two in-house models we use here at WSMV.)

As we progress through the morning hours on Friday and into Friday afternoon, the moisture will progress farther and farther to the north. Temperatures will also be warming up — not only near the ground, but further up in the atmosphere. This means that what we are most likely to see through the day on Friday is cold rain. The steadiest and heaviest rain is most likely to fall south of I-40 and east of I-65. During the day Friday, the only locations where I’d be even remotely concerned about a potential winter mix are in far northwestern Middle Tennessee and the western half of southern Kentucky. But even there, accumulations are unlikely.  Again, these Futurecast snapshots give you an idea of the most likely scenario:

Once we get past sunset early Friday evening, things get interesting. (As I’ve said before, you should never ever want a meteorologist to say the word “interesting”.) This is our second opportunity for cold air and moisture to combine for a brief period of snow. The cold air will be moving in from the northwest, catching up to the showers as they move off to the east. If the moisture moves away too fast, we will see no snow. If the cold air comes in too slow, we will see no snow. The most likely scenario is a brief rain/snow mixture and a brief period of snow showers moving quickly from west to east across most of the Midstate…so quickly, in fact, that accumulations are unlikely.  The better chance of snow that lasts a little longer and actually has a chance of accumulating on the ground is in eastern middle Tennessee, especially the higher terrain as you approach the Cumberland Plateau.  This is where my own thinking diverges from the Futurecast model a bit — it’s sticking with plain ol’ rain everywhere except along the Plateau, while I think the last of the showers will be a mix or brief snow. Regardless, it’s got a good handle on the timing, and a nice depiction of the snow bullseye in eastern Middle Tennessee.

Here’s the tricky part: not all of the forecast models are in agreement regarding how this scenario will play out. Some are significantly colder and snowier. Some are significantly drier and thus much less wet and much less snowy. The “boom” forecast for this system in terms of snowfall is for widespread accumulations of around 1 inch (including Metro Nashville) with 2 to 3 inches of accumulation in the higher terrain of eastern middle Tennessee. I do not think this is the most likely scenario — in fact, I’m not even going to create a map for it because it’s so unlikely. The “bust” forecast for this system is for no snow at all…not even a dusting in the high elevations.  The bust maps would just be…well, blank maps.  The “boom” model is the NAM, which looks like this Friday evening and night:

Why the black-lettered caption?  It’s to prevent that sequence making it onto social media…sometimes snow-lovers get a little, um, “enthusiastic”:

The NAM model is also the forecast model that inspired this cartoon, so take that as you will:

The RPM-generated snowfall forecast calls for up to 1″ at most, in eastern Middle Tennessee. That doesn’t mean you won’t see some flakes elsewhere, but it means they won’t stick.

[6pm EDIT: the RPM is now getting friskier with potential snow Friday night. Apologies for the image quality — posting this from my phone.


This would make the snow forecast…complicated. It’s just one run of just one model, so I’m not going to overreact to it — just make you aware that it exists. Next full blog post will be up by midday Friday, and of course you can watch our newscasts for the latest info. END EDIT]

The most likely outcome from the system is what I have outlined in this snowfall forecast. (It results from a combination of analyzing data from various other models that I trust, and looking at how systems like this have behaved in the past.) Notice the phrasing at the top — this is what I anticipate AT MOST out of this storm system…in all likelihood, your backyard amounts will not amount to as much.  (Why phrase it that way?  Just being cautious.)

The National Weather Service is approaching the storm system in a very similar manner. They have not issued any advisories or warnings, but that remains a possibility, especially for eastern middle Tennessee. They also haven’t hazarded a guess as to potential accumulations…at least, not in map form.  For now, they’re saying:


Sorry for the all caps, but that’s how the NWS rolls.

To bundle all of this up, here’s the “Panicometer” for this batch of potential wintry weather.
If you haven’t seen me use the Panicometer on Channel 4 News Today…well, first of all, you should really watch that newscast, because we’re nice people and we try hard…but more importantly, I’ve started using it as a way to light-heartedly summarize what your level of concern should be regarding winter-weather problems. In this case, for most of the Midstate, you don’t need to be worried — you may see Aunt Tabitha post a national-news image on Facebook that has Nashville included in a 2-4″ forecast, or your college roommate tweet a picture of an empty milk fridge at the grocery store, but you can safely ignore all of that. The only places where I’d plan for some late-Friday-night and early-Saturday travel delays is eastern Middle Tennessee. If you live in one of those snow-favored locations, you know who you are…and you’re likely already prepared for it!

As I always say when it comes to winter weather forecasts, stay tuned for further changes. As I’m typing this early Thursday afternoon, we’ve got 12 hours to go before the first minimal snow chance, and about 36 hours until the slightly better chance, so there still time for things to change in one direction or the other. Winter storms are actually pretty easy to forecast on a national or regional scale, but they become very complicated when you try to zoom down to the county-by-county level…which is of course what everyone really wants to know. Follow us on social media for updates over the next couple of days…links to the relevant accounts are at the bottom of this post. Lisa Spencer and Dan Thomas will have updates this evening, and I’ll be on the air tomorrow morning to get you ready for whatever is headed our way!

Finally, since this post has been a little light on memes/gifs/silliness, here’s Don Draper sitting on the Iron Throne.  Just because.

Social media links

Twitter: @WSMVweather, @PaulHeggenWSMV, @WSMVLisaSpencer, @WSMVDanThomas, @WSMVNancyVanC, @NWSNashville

Facebook: 4WARN Weather, Paul Heggen WSMV, Lisa Spencer, Dan Thomas WSMV, Nancy Van Camp WSMV, NWS Nashville

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#StupidCold: The Sequel

The coldest air of the winter so far (and, dare I hope, of the rest of the winter) will be moving into the Midstate tonight and sticking around Wednesday and Thursday.  Unusually cold and windy conditions will settle in by Wednesday morning and stick around through Thursday morning — near-zero and sub-zero wind chills look likely, which leads us to a return of everyone’s favorite* social-media topic, #StupidCold.

*probably not

Most of you have embraced that hashtag on social media, for which I thank you.  For those who asked, there is no formal definition of the term — it is meant as a light-hearted way of having a little bit of fun with what is overall going to be a rather unpleasant weather pattern.  It’s meant to be used as a colloquial synonym for “extremely” — as in, “Wow, these cookies are stupid good!”  However, a few have expressed dismay over my use of the pejorative “stupid,” and to those few I must say:

Anyway.  Let’s talk about the details.

First off, since everyone has been asking about it, let’s talk about the snow.  There isn’t going to be much.  The air mass over the Midstate is quite dry, so even the “squeeze” produced by a big rush of Arctic air isn’t going to be able to produce much more than flurries Wednesday morning.  At most, I expect a dusting (a non-measurable layer of flakes) in the highest elevations of the eastern Highland Rim and the Cumberland Plateau:

But, the cold air is sufficient by itself to prompt us to deem Wednesday and Thursday “4WARN Weather Alert” days.  The coldest air will be moving in after midnight tonight — temperatures will steadily drop until Thursday morning, as shown in this sequence:

Right now I’m forecasting a low of 4 degrees in Nashville on Thursday…which is really cold, but nowhere near the record of -9.  Regardless, the weather model data gives us a good sketch of how things will shape up.

Of course, temperatures are just part of the story.  The winds will be quite unpleasant throughout the day on Wednesday, sustained from the northwest around 15-25 mph, with gusts over 30 mph.  Those winds will produce wind chill values shown in this sequence:

The National Weather Service has issued a Wind Chill Advisory for the entire Midstate from 3pm Wednesday until 9am Thursday:

Sub-zero wind chills are deeply unpleasant, but they’re not flat-out dangerous unless you’re outside for a prolonged period of time and inadequately dressed for the weather.  The coldest wind chill in the forecast is -10 in Crossville at 12am Thursday — even at that point, it takes more than 30 minutes for frostbite to occur on exposed skin!  Here’s the wind chill chart that also shows expected frostbite times:

So why make a big deal of the cold snap?  Refer back to the “deeply unpleasant” phrasing of the last paragraph…it’s really not going to feel great outside.  The cold snap has the strong potential to cause inconveniences, and if you’re not careful there’s an outside chance it will be dangerous.  Here are some mostly common-sense tips for the next few days, along with a few that maybe slipped your mind:

– Dress in layers.  Hat, gloves, scarf, the whole deal.  Minimize the amount of your skin that’s exposed to the cold and wind.

– Take care of outside animals.  Consider letting outdoor pets become indoor pets for a couple of nights.  At minimum, make sure they have shelter and plenty of blankets to nest in.  And refresh their water so it doesn’t freeze over.

– Make sure your furnace is working properly.  Better (and cheaper) to do preventive maintenance than emergency repairs.
The freezing girl about a heater

– If you use space heaters or a fireplace for added warmth, please be careful.  Keep space heaters away from drapes and curtains, and TURN THEM OFF when you leave the room.  Not a great idea to go to bed with a roaring fire in the fireplace/stove, either.

– Add extra blankets to the bed.  (One of the few things about winter that I personally enjoy.)

– Let the car warm up in the morning, even if you’re running late.  Just a few minutes to let the engine heat up will help things run more smoothly.  Make sure your antifreeze is topped off as well.
frozen car

– Check the air pressure in your tires.  Cold air is more dense, which means it takes up less space — low tires in normal weather become flat tires in cold weather.  (If you have this vehicle, don’t worry about it.)

– I don’t think it’s going to be cold enough, long enough for widespread frozen/ruptured water pipes to be a concern around here, but it never hurts to check to be sure anything along an outside wall or in a basement is well-insulated.

Bottom line wisdom:

And as always, follow us on Twitter or Facebook for more weather-y goodness…

Social media links

Twitter: @WSMVweather, @PaulHeggenWSMV, @WSMVLisaSpencer, @WSMVDanThomas, @WSMVNancyVanC, @NWSNashville

Facebook: 4WARN Weather, Paul Heggen WSMV, Lisa Spencer, Dan Thomas WSMV, Nancy Van Camp WSMV, NWS Nashville

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Cold November = Cold Winter???

Hey, remember back in July when we I asked and answered the question: does a cooler-than-normal July correspond to a colder-than-normal winter?  (The answer was: no. No, it does not.)  Well, now that we’re heading to a mid-November cold snap, I thought it might be useful to look at whether colder-than-normal November temperatures are followed by colder-than-normal winter temperatures (“winter” here meaning December, January and February).  And since I’m lazy, I’m using exactly the same “Sherlock” memes and gifs to help explain things along the way.

We’re heading into a prolonged stretch of cold weather.  Temperatures for the next 10-14 days look like they’re going to be below normal…but our on-air forecast only goes out to the next 7 days.  Those 7 days are definitely going to be below-average — at least, in terms of temperatures.  Whether or not they’re below-average in terms of life experience is kind of up to you.

If we take the data from the first 11 days of the month (about -3.5 degrees compared to “normal” so far), AND if we assume that our forecast for the next 7 days is accurate (don’t laugh), AND then if we optimistically hope that the remaining 12 days of November bring us a return to just-average temperatures, this would still add up to the 9th-coldest November on record.  Exciting.

Because no one can ever be satisfied just enduring a cold snap while it’s here, I’ve been getting a LOT of questions about what our current cold weather is a harbinger of what’s to come this winter.  (Yes, I used the word “harbinger.”  Don’t judge.)

My experience as a meteorologist tells me that short-term weather patterns are rarely reliable indicators of long-term climate trends…

…but any halfway-decent scientist wants proof.  So, I dove into the record books to look for answers.  First, here are the numbers I’ll be referring back to throughout the rest of this post.  These are the 30 coldest Novembers on record, along with the average December-January-February temperatures for the following winter (and their overall rank in the 143 years of Nashville records):

A few things of interest within those numbers:
1) The “normal” winter temperature over the last 143 years is 40.4 degrees.  The 20 coolest Novembers on record were followed by 12 colder-than-average winters…leaving 8 as above-average.
2) Of the 20 coldest Novembers in Nashville, 4 were followed by Top-20-coldest winters.
3) Of the 30 coldest Novembers, 9 were followed by Top-30-coldest winters.
3) Looking at the flip side, of the 20 coldest Novembers, only 2 subsequent winters ranked in the Top 20 warmest.
5) The Top 30 coldest Novembers were followed by a Top-30-warmest winter only 5 times.

So based on those facts, we can reasonably expect below-normal temperatures this winter, right?

Those five facts above are fun bits of trivia, but they’re picked from a small sample size…they’re not the entire picture.  If you want to definitively state that there is a direct correlation between November temperatures and following-winter temperatures, you have to look at all the data, and plot every November temperature against every winter temperature.  If there’s a direct relationship, the chart would look like this:

Here’s what the chart really looks like for November vs. Winter temperatures:

Quickly glance at that, and you probably notice the line through the dots — that’s the “best-fit” line that attempts to summarize all the data points in a straight line.  “AHA!” you say, “that line goes UP!  Therefore, a cooler July equals a colder winter, and a hotter July equals a warmer winter!”  And Moriarty says…

There’s a way to measure the accuracy of that “best-fit” line…basically, measuring how close the data “dots” correspond to the line.  It’s called the “Coefficient of Determination,” and it’s abbreviated as R-squared.  (Why is it R-squared instead of CD?  Why isn’t it called something simpler?  These are questions I asked my statistics professor in graduate school, and the answers were…unenlightening.)

Anyway…that “Coefficient of Determination” tells us whether the best-fit line does a good job of summarizing the data, or whether it’s just the software doing it’s best to pound a square peg into a round hole.  An R-squared value of exactly 1 means that it’s a perfect fit, while values closer to zero mean that there’s really no relationship between the two things you’re trying to compare.

For November temperatures vs. Winter temperatures, the R-squared value is 0.0447.  That’s really low, and tells us that with almost 150 years of data, there is virtually no predictive relationship between temperatures in November and temperatures the subsequent winter.


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October 13 Severe Weather Threat

(Posted 4:20 p.m., 10/13/2014)

Tornado warnings have started being issued in Hardin, McNairy and Decatur counties west of the Tennessee River.

Stay with 4WARN Weather for updates.

(Posted 3:25pm 10/13/2014)
Just a quick update to let you know that the Storm Prediction Center really hasn’t changed their thinking regarding our threats this evening and tonight — they’re maintaining the Moderate/Slight risk dividing line along the Tennessee River (upper left).  The Moderate Risk corresponds with the highest tornado threat, also some tornadic potential exists across the entire Midstate (upper right).  Within the Slight Risk area, locations west of I-65 have a higher probability of damaging winds occurring within 25 miles (lower left).  All locations have a relatively low risk of large hail (lower right).

Dan Thomas and Lisa Spencer will be on-air this evening during our newscasts and, if necessary, during prime time if/when warnings are issued.


(Posted 1:15pm 10/13/2014)
A few updates to our severe weather outlook, now that things are beginning to come into better focus regarding our potential for nasty weather later today…

It looks like there will be plenty of instability available in the atmosphere to help drive severe storms — as of 1pm, temperatures are in the upper 70s, so we’re on track to reach our forecast highs in the low 80s.  The warm temperatures combined with unseasonably high humidity will result in a “buoyant” atmosphere supportive of surface-based thunderstorms (more capable of producing tornadoes, compared to elevated-base thunderstorms).  Even as the atmosphere cools after sunset, strong southerly winds will keep plenty of warm and unstable air in place ahead of the storms.

The precise timing of when the storms will arrive remains the least-confident part of the forecast…until the squall line shows more and consistent signs of actually slowing down, I’m maintaining this estimate of severe storm “windows”:

If anything, I’m erring on the early side — partly to account for pre-squall-line storms, and partly so the storms in the main line don’t catch you off-guard.  The Nashville office of the National Weather Service is still forecasting a later arrival time — they’ve adjusted the previous version of this map to reflect an earlier time frame, but they’re obviously banking on the squall line slowing down significantly.  (To be clear: I’m providing their map not as a “hey, look what THESE guys are saying LOL” but as an example of how reasonable minds are interpreting data in a slightly different manner):

One concerning possibility is that of isolated supercell thunderstorms developing ahead of the main squall line.  As I wrote this morning (below), squall lines are typically associated with a straight-line wind threat — but any discrete storms that develop ahead of the line will have a higher tornadic potential.  I’m not changing my thinking regarding our relative threat levels — winds and flooding are still the biggest concerns, but the tornado potential definitely isn’t zero.  The pre-squall-line storms are the ones developing down in Alabama as of just after 1pm (also, this image gives you an idea of what’s headed our way):

The Storm Prediction Center has expanded their “Moderate Risk” outlook a little farther east (not far enough, in my opinion — I still think I-65 should be the Moderate/Slight dividing line).


I’m still concerned that SPC hasn’t adequately adjusted for the faster-than-expected movement of the entire complex of storms.  Their next update is scheduled for 3pm — I’ll post a quick update with some thoughts at that point.  We’ll likely have a tornado watch issued for our western counties before then — but the storms themselves will still be in West Tennessee.

This morning’s post is below…


(Posted 9:25am 10/13/2014)
A potentially dangerous and complicated weather pattern is shaping up over Middle Tennessee and southern Kentucky over the next 24 hours.  So let’s take a look at what we know, what we think we know, and what we’re still figuring out.

1. Strong/severe thunderstorms are moving eastward through Arkansas, Louisiana and Texas this morning.
2. Computer models have been WAY too slow in how they’re depicting the progression of these storms (most are 3+ hours behind).  Check out the 9am radar image (left) next to Futurecast’s version of what that radar should look like:
3. Other than isolated showers/storms, the atmosphere over the Midstate will be warming up and becoming progressively more unstable.  It will also be quite windy — a Wind Advisory is in effect this afternoon through tonight:
4. The ground is already saturated, due to last week’s and this morning’s rain.  Storms moving in later today and tonight will be capable of producing flash flooding in a very short period of time, especially considering that rainfall amounts through early Tuesday are forecast in the 2″-3″ range.  A Flash Flood Watch is in effect until 7pm Tuesday:

1. The environment over Middle Tennessee and southern Kentucky as the storms arrive will be especially favorable for damaging straight-line winds.  There’s a lot of wind energy associated with this system, but a majority of it is associated with “speed shear” (change in wind speed as you ascend in the atmosphere) as opposed to “directional shear” (change in wind direction as you go up).  That means that while tornadoes are still possible, the greater threat is from thunderstorm wind gusts over 60mph.
2. As the atmosphere warms up, there will be enough instability over our heads for severe storms to maintain their strength.  However, it does not look there will be enough instability to support much of a hail threat at this point.
3. The earlier the storms move in, the greater our severe weather potential will be.  Which brings us to…

1. When will the storms get here???  Pretty much the million-dollar question, especially considering that “daylight arrival = nastier storms”.

Factoring in the faster-than-anticipated movement of the storms this morning, here’s my latest estimate of when the strongest storms will arrive in the Midstate:

For posterity’s sake, here the NWS-Nashville’s version of that map:

Obviously some pretty major differences.  I think it’s likely that the storms will decelerate as they cross the Mississippi River, but they’re already 3 hours ahead of this morning’s computer guidance.  Speaking of computer guidance, here’s how Futurecast depicts the radar simulation this evening through tomorrow morning — the hot-off-the-press newest model run is significantly faster that previous versions, and has a more-reasonable estimate of the timing (but I’m thinking it’s still an hour or two too slow):

Despite its timing challenges, I do think Futurecast has a pretty good handle on what the storm structure will look like — a squall line (technically, a quasi-linear convective system) with embedded supercell features.  The individual storms that comprise the squall line will be moving generally south-to-north within the line, while the line itself will slow down and move from west-to-east throughout the evening and into tonight.  Notice that some re-development of storms is predicted for early Tuesday morning along and east of I-65.

Squall lines are primarily associated with straight-line wind events, but little areas of rotation can spin up within the line and produce isolated tornadoes.  The heavy rain and slowing movement of the line could produce flash-flooding throughout the night…especially with the potential re-development of storms Tuesday morning.  Overall, I’d rank the threats in this order: straight-line winds, flash flooding, tornadoes, hail:

The Storm Prediction Center has noted the fast forward movement of the storms so far this morning, but their 8am outlook maintained the position of the severe weather “Moderate Risk” to the west of the Tennessee River, with a “Slight Risk” for Middle Tennessee and southern Kentucky:

My concern with SPC’s logic is that the storm line may outrace the Moderate Risk area — I think a better Moderate/Slight dividing line would be roughly along I-65, since that’s how far east the storms could get before the sun goes down (instability levels decrease after sunset).  After sunset, and as the storms progress eastward into a less-favorable air mass, the severe threat will diminish…but it won’t go away entirely.  SPC’s next update is due to be posted at 11:30am, so we’ll see what they say at that point.

The biggest wild-card in all of this is the possibility that this morning’s squall line will move too far ahead of its upper-level support and fall apart…if that happens, we’d be playing a waiting game as another squall line develops to take its place.  (In which case the forecast becomes so complicated that you’ll be able to watch my hairline recede before your very eyes as I try to figure out the forecast.)  There are other wild-cards that could also reduce or rearrange our severe weather potential — daytime clouds, non-severe storms developing ahead of the main line, a layer of warm air in the atmosphere that could inhibit storm development…you get the idea.

Either way, plan on staying weather-aware this afternoon and this evening.  I’ll be on Twitter regularly (and Facebook irregularly) with updates, and of course we’re on the air with newscasts at noon and beginning at 4:00 this afternoon.  Watches and warnings are automatically posted to my Twitter account — and allow me to remind you once again that Facebook is not a good source of time-sensitive information like severe weather alerts.

Above all, don’t freak out.  (Easy for me to say after I just dumped a thousand words worth of “this could be bad” on you!)  This is NOT a situation where you should plan on sleeping in your safe place with a helmet on and a weather radio clutched in your shaking hands…even in high-risk severe weather scenarios, the odds of something happening in your immediate vicinity are pretty low — but it’s always best to be prepared!

Social media links

Twitter: @WSMVweather, @PaulHeggenWSMV, @WSMVLisaSpencer, @WSMVDanThomas, @WSMVNancyVanC, @NWSNashville

Facebook: 4WARN Weather, Paul Heggen WSMV, Lisa Spencer, Dan Thomas WSMV, Nancy Van Camp WSMV, NWS Nashville

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October 6 Severe Weather Threat

We’re in the middle of a pretty turbulent weather pattern…last Thursday we almost tied a record high temperature (we hit 90), we had severe storms Thursday night, then by Sunday morning our temperatures were down into the upper 30s.  Now the warm air is moving back into the Midstate, and we’re facing another potential severe weather event.

I’ll start with the good news: the weather conditions this afternoon won’t be perfectly aligned for severe weather.  But this is one of those situations where conditions don’t need to be perfect, they just need to be good enough.  Let’s break it down by which ingredients will be favorable, and which ones could help to limit our severe storm potential…

Severe thunderstorms require an environment in which conditions near ground-level are warm and muggy, with cold air in the upper atmosphere — this setup allows storms to explosively develop.  The air over our heads is definitely cold…the factor that’s lacking will be warmth and humidity near the ground.  The temperature in Nashville as of 9am is 59 degrees, and the dew point is 55.  For severe storms to develop, that dew point number needs to be at least 55, and really should be 60+, so we’re kind of borderline there.  Southwesterly winds will be pushing more humidity into the region, but it will be an uphill battle.  Additionally, temperatures today will only reach the low to mid 70s…all in all, the not-quite-warm-enough and not-quite-humid-enough air near the ground is our best hope for dodging severe weather.

Another limiting factor is the lack of a strong front to get the storms started in the first place.  A well-defined cold or warm front is a strong trigger for severe storms, and we’re just not going to see one of those in the area this afternoon.  The trigger for our storms this afternoon will be the approach of a strong upper-level storm system — even though it’s high in the atmosphere, it can still influence the behavior of air near the ground.  The lack of a surface front means that storms that develop this afternoon will be loosely organized (at best) — that means we can’t give you a specific “arrival time” map.  The storms will be clustered in different bunches that may behave erratically.

There will be plenty of wind energy in the atmosphere, so the threat of straight-line winds will be present in any storm this afternoon.  The wind profiles this afternoon aren’t terribly impressive from the perspective of a tornado threat, but at this point we can’t rule out isolated tornadoes.  And those cold temperatures in the upper atmosphere that I mentioned earlier will also help to support the threat of large hail.

One last limiting factor: this morning’s rain, which helps to temporarily stabilize the environment.  The atmosphere looks like it will have time to recover enough energy late this morning and early this afternoon, but that’s largely dependent on how much sun breaks through the lingering clouds.

Add it all up, and we’re talking about an environment in the Midstate this afternoon that will support the intensification of thunderstorms…if those storms get going in the first place.  Yes, it’s complicated.  (Isn’t it always?)  Let’s get to some maps, beginning with how Futurecast depicts the development of storms this afternoon and this evening.

So, does that series of images depict exactly what the radar will look like this afternoon?  No, but it does give us a reasonable approximation of where the greatest severe threat will take shape (along and south of I-40), when the storms will develop (mid- to late-afternoon), and the type of storms we can expect (supercell storms, with gaps in between the individual thunderstorms).

The Storm Prediction Center has included all of Middle Tennessee and southern Kentucky in their “Slight Risk” outlook for today:

Because there are so many “if this, then that” factors that could limit the development of severe storms, the percentages are pretty low in terms of the specific wind, hail and tornado threats.  SPC estimates a 15% chance of large hail (1″+ diameter) within 25 miles of any one point in the Midstate…

…and also a 15% chance of damaging straight-line winds…

…and a 5% chance of a tornado within 25 miles:

One method of evaluating forecasts is to look back at similar historical scenarios…one such “analog” forecast found that atmospheric conditions similar to today’s have produced severe weather about 30% of the time:

The individual severe reports from the 15 most-similar days show why we’re taking this so seriously:

BOTTOM LINE: Don’t freak out.  You don’t need to hunker down in your safe place for the rest of the day, just stay weather-aware this afternoon and this evening.  There’s a chance that we’ll get lucky and dodge the severe weather this time around, but it’s still something that bears careful watching.  (In fact, I’m skeptical enough about our storm chances this afternoon and evening that I wouldn’t cancel soccer practice, barbecue plans, or other outdoor stuff this evening — just be flexible!)  We’ll have on-air updates at noon and during our early-evening newscasts, and we’ll let you know right away if any warnings are issued.  If you’re away from the TV, follow us on social media (links at the bottom of this post).  As always, any warnings will be automatically posted on my Twitter stream, @PaulHeggenWSMV.  (And I’ll refrain from tweeting non-weather thoughts today.  You’re welcome.)

Social media links

Twitter: @WSMVweather, @PaulHeggenWSMV, @WSMVLisaSpencer, @WSMVDanThomas, @WSMVNancyVanC, @NWSNashville

Facebook: 4WARN Weather, Paul Heggen WSMV, Lisa Spencer, Dan Thomas WSMV, Nancy Van Camp WSMV, NWS Nashville

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October 2-3 Severe Weather Threat

It’s been a couple of months since we’ve had enough of a severe weather threat to discuss in this space…but autumn is our secondary severe weather season — think of it as the junior varsity to springtime’s varsity.  Tonight will bring us our first potential severe weather event of the fall — the threat is marginal in our part of the country, but it still bears watching.

Unseasonably warm and breezy conditions will prevail today — the record high temperature for October 2 is 91 degrees, and we’ll be awfully close to that this afternoon.  Strong southerly winds will reinforce the warm air, but also pump in higher levels of humidity…not August-level humidity, but still noticeable.  We could see a few spotty storms in western Middle Tennessee this afternoon, but it’s just a 30% chance and they’ll be non-severe.  Those storms are reflected in both Futurecast simulations for this afternoon (BAMS model on the left and RPM on the right, weather nerds).

Severe thunderstorms will develop far to our west this afternoon, in Arkansas and Missouri.  They’ll quickly coalesce into a squall line of storms that will march from west to east, eventually crossing the Mississippi River…and approaching the Tennessee River by midnight.

The storms will continue their eastward progression throughout the late-night and early-Friday-morning time frame.  Best estimate for their arrival in Metro Nashville is around 3-4am.  Keep in mind that these storms don’t even exist yet!  Trying to pin down their specific arrival times in various parts of the Midstate is very tricky.

Those two Futurecast snapshots represent when the storms will pose the greatest severe threat for our neck of the woods.  Most of the Midstate and southern Kentucky are included in the Storm Prediction Center’s “Slight Risk” area for tonight and early Friday morning (specifically, that means a 15% chance of 60+mph wind gusts within 25 miles of any one location in the risk area).

It’s hardly ever possible to completely rule out the possibility of an isolated tornado or some 1″-diameter hail, but both of those threats are very low tonight (2% tornado chance, 5% hail chance).  And since we’ve been so dry lately, the ground will be able to absorb all but the heaviest downpours, so flooding isn’t a major concern either.

As the storms continue on to the east of I-65, they’ll weaken significantly.  At this point, it looks like most of the thunderstorms will be moving off the Cumberland Plateau shortly after sunrise on Friday.

Smoothing out the differences between the various models and allowing a little “flex time” for Mother Nature’s mood swings, here’s the best estimate of when the strongest thunderstorms will impact various portions of the Midstate.

As I mentioned on TV this morning, this is a very borderline severe weather threat — in general I expect strong-but-not-severe storms, but a few stronger cells could prompt severe thunderstorm warnings.  That means a fair amount of lightning and thunder, some heavy downpours, but not much in the way of damage.  These kinds of marginal straight-line wind threats generally result in some tree damage and some spotty power outages, but not much more than that…unless of course a tree falls on your house!  Still, it’s something you’ll want to stay aware of — we’ll have updates during our regularly scheduled newscasts this evening, and we’ll be here monitoring the storms as they move in overnight.

I automatically tweet all Middle Tennessee and southern Kentucky watches and warnings — follow me at @PaulHeggenWSMV for that.  (I promise, I won’t tweet about baseball or football tonight.  But once the storms have passed through, all bets are off!)  Links to the social media profiles of all of WSMV’s meteorologists are at the bottom of this post.

More storms will pop back up around midday on Friday, but these are expected to be non-severe…they’ll be the result of the atmosphere getting one last squeeze from the approaching cold front.  That front, by the way, will drop temperatures significantly by the weekend — parts of the upper Cumberland Plateau could even see some frost on the rooftops Sunday morning!

Social media links

Twitter: @WSMVweather, @PaulHeggenWSMV, @WSMVLisaSpencer, @WSMVDanThomas, @WSMVNancyVanC, @NWSNashville

Facebook: 4WARN Weather, Paul Heggen WSMV, Lisa Spencer, Dan Thomas WSMV, Nancy Van Camp WSMV, NWS Nashville

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