Cool July…Cold Winter???

We’re in the midst of our third stretch of sustained below-average temperatures this month…in fact, July of 2014 will likely be one of the 20 coolest on record.  (The Nashville record book extends back to 1871, so that’s significant.)  All in all, while there have been some hot and humid days, and some strong thunderstorms here and there, the weather this month has been pretty great.
Sherlock_GoodDeduction
Sherlock_BarelyContainMyself

Because no one can ever be satisfied with enjoying pleasant weather while it’s here, I’ve been getting a LOT of questions about what our current cool weather portends for next winter’s weather.  (Yes, I used the word “portends.”  Don’t judge.)
Sherlock_IQ

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

…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 40 coolest Julys 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):
July_Winter_Comparison

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 Julys on record were followed by 13 colder-than-average winters…leaving 7 as above-average.
2) Of the 20 coolest Julys in Nashville, 6 were followed by Top-20-coldest winters.
3) Expand that a bit, and 10 were followed by Top-40-coldest winters.
4) Looking at the flip side, of the 20 coolest Julys, only 2 subsequent winters ranked in the Top 20 warmest.
5) The Top 20 coolest Julys were followed by a Top-40-warmest winter only 5 times.

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

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 July temperatures and following-winter temperatures, you have to look at all the data, and plot every July temperature against every winter temperature.  If there’s a direct relationship, the chart would look like this:
rsquaredone

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

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…
Sherlock_Ordinary

There’s a way to measure the accuracy of that “best-fit” 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.)
Sherlock_eyeroll

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 July temperatures vs. Winter temperatures, the R-squared value is 0.019.  That’s really really low, and tells us that with almost 150 years of data, there is virtually no predictive relationship between temperatures in July and temperatures the following winter.

Sherlock_Laters

Posted in Uncategorized | Tagged , , , | 13 Comments

July 14 Severe Threat

JUST-AFTER-NOON UPDATE: It’s hot, it’s muggy, and so far we’ve seen limited shower and thunderstorm activity.  Combine those factors with the gradual approach of a cold front, and the pump is still primed for severe weather later today.  But WHEN today?  That part of the equation has, if anything, become even more muddled.  Two short-term models (HRRR and RPM) say we’ll get strong afternoon storms (blowing up by 3-4pm) followed by not-as-strong overnight storms.  One other model (BAMS) says the atmosphere will stay “capped” this afternoon, and the overnight storms will be the main event.  I lean toward a compromise: the strongest storms will develop this afternoon, but they’ll be loosely organized.  The well-organized (and more numerous) storms will arrive tonight with the actual cold front, but the after-sunset arrival would limit their severe potential around here.  But as we’ve seen before (in fact, just six days ago): the atmosphere is gonna do what the atmosphere wants to do.
you-got-me

——————–

Let’s start with the good news: today’s severe thunderstorm potential isn’t an off-the-charts, hide-in-your-safe-place-until-Tuesday type of scenario.
bb-high-five

Hang on there, Walt.  (You too, Jesse.)  Even a marginal risk of severe weather is still a risk of severe weather.  This morning’s weather-balloon data shows plenty of moisture in the lowest levels of the atmosphere, which is the fuel for strong thunderstorms.  Whether or not we get strong/severe storms this afternoon depends on whether we get storms this morning.
what

Exactly.  Stay with me here: think of the atmosphere as an iPhone battery.  The amount of moisture in the atmosphere this morning means that the battery is fully charged, at 100%.  This morning’s storms are like playing Candy Crush non-stop for an hour or so — not enough to drain the battery completely, but enough to drop the charge to 50%.  Severe thunderstorms are like using Google maps to navigate a turn-by-turn route for a long drive while downloading a podcast on a roaming network…you need a full 100% charge to make that happen.  So, in areas that are getting non-severe storms this morning, you’re likely not going to have enough “charge” left in the atmosphere for severe weather to be as much of a threat this afternoon.  If you stay dry this morning, the atmosphere is going to be ready to go later today.

(In related news, I may be slightly frustrated with my phone’s battery life.)
shaking_head_breaking_bad

The problem from a forecasting perspective is that even the short-term, high-resolution weather models don’t do a very good job of handling early-morning storms…which means their opinion regarding storm development later in the day are highly suspect.  That said, this series of images (from the HRRR model, for you weather nerds) does a pretty good job of simulating both the timing and placement of this morning’s non-severe storms, which lends it a little more credence regarding the evolution of stronger storms this afternoon:
HRRR 10AM
HRRR 1PM
HRRR 3PM
HRRR 6PM
HRRR 8PM
yeah-science

Keep in mind: these models simulate what the atmosphere is going to do, they’re not a guarantee of specific future timing, placement or strength.  The overall pattern is what we’re looking at, and in this case the overall pattern is:
1) Morning storms in northern Middle Tennessee diminish
2) Strong (possibly severe) storms develop along and north of I-40 this afternoon, and they’ll gradually move farther south
3) More storms will develop in West Tennessee this evening and move into southern Middle Tennessee, increasing flood concerns in that part of the Midstate.

We are included in the Storm Prediction Center’s “Slight Risk” outlook for severe thunderstorms today…here’s that outlook, along with the specific threats for us today:
PAUL SEVERE RISK REGION
PAUL SEVERE RISK GRAPH 2

The problem with this type of weather pattern is that one tiny change in this morning’s weather will have a huge impact on this afternoon’s and evening’s weather.  Returning to the iPhone analogy…if you leave about 20 apps running in the background, there are 20 different ways your battery can be run down without you being aware of it.  Now try to predict exactly which app will cause the battery to crash, and at exactly what time…that’s the “fun” part of trying to predict summertime severe weather in this part of the country.

Overall, I’ll return to where I started: this is a pretty low-end risk of severe weather.  Odds are, in your neighborhood you’ll probably just get some garden-variety storms that produce some heavy rain and lots of lightning and thunder.  (Some of us may avoid even that!)  But the severe risk isn’t zero, and it’s high enough to warrant staying “weather-aware” today, especially in the afternoon and evening.  The best-case scenario is that the morning storms produce enough lingering cloud cover to limit this afternoon’s severe potential…the worst-case scenario is lots of storms producing widespread damaging straight-line winds.  (As usual, the safest bet is “somewhere in between.”)

We’ll keep you updated during our regular newscasts at noon, and beginning again at 4pm.  We’ll also keep you “4WARNed” (branding!) on social media (links below).

——————–
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

Posted in Uncategorized | Tagged , , , , | 2 Comments

July 8 Severe Threat

5PM UPDATE:Obviously we haven’t seen storms firing up in southern Kentucky yet.  That’s good news for folks with outdoor plans this evening, but it doesn’t mean our severe-storm chances are over — it just means those chances will arrive later and in a more “muddled” state.  Which, as a forecaster, is…frustrating.

A couple of reason why storms haven’t developed yet: first, a layer of warm air has moved over the Midstate, several thousand feet off the ground.  This acts like a lid, and prevents thunderstorms from developing as quickly.  Storms can still develop, they just need a stronger push — I still expect that push to arrive in the form of a weak cold front off to our northwest.  The position of that weak cold front is the second reason why we haven’t seen storms yet…it’s moving verrrrrrry sloooooowly in our direction.  As of 5pm, it’s still on the other side of the Ohio River — until it gets closer, our storm chances will remain low.

So, why do I still think we’ll get storms later tonight?  Step outside — feel how muggy it is?  The dew point in Nashville as of 5pm is 70 degrees…that’s pretty tropical, and indicates an environment that’s “ready to go” for thunderstorm development, as soon as that cold front meanders its way down here.  The likely-after-dark timing of the storms means that our severe potential will be slightly lower, but we’ll still be monitoring storms for strong straight-line winds and hail.  Flash-flooding will be possible since the storms will probably be as slow-moving as the front that sparks them.  And we’ll still get quite a lightning show, especially with that after-dark arrival time.

The Nashville NWS just updated their arrival-time map, reflecting similar thinking to what I’ve outlined above:
image7

Lisa Spencer and Dan Thomas are on TV talking about this evening’s weather during our newscasts until 7pm…then Dan will be on again at 10pm.  If (BIG IF) we’re still storm-free at that point, then our severe potential might decrease significantly.  I wouldn’t wager in that direction, though.  All in all, this is a good reminder that while there’s a ton of math and physics directed at trying to accurately simulate how the weather will behave, the atmosphere is gonna do what the atmosphere wants to do.

I’ll leave this morning’s blog post below the break…

—————–

Not much I can say here that I didn’t already say on TV this morning…but in case you missed that, here’s a recap of today’s severe weather potential.

THIS MORNING:  Non-severe showers and thunderstorms have been spreading in from the west.  While these are sufficient to temporarily stabilize the atmosphere, we’re still under the gun for stronger storms this afternoon and evening.

MIDDAY:  I still expect some spotty showers and non-severe storms to be ongoing by lunchtime…once again, not enough to “de-charge” the environment and reduce our severe potential.  In between the spotty showers and storms, it will be hot and muggy — highs will top out around 90 degrees, with the heat index reaching the mid 90s.

AFTERNOON AND EVENING: Thunderstorms will develop by mid- to late-afternoon, most likely in southern Kentucky.  Once they develop, they’ll rapidly intensify and merge into a messy-looking line of storms that will march from NW to SE across the Midstate throughout the evening.  This time series of Futurecast images shows how one weather model thinks things will evolve (this is the BAMS model, for those of you who are serious about your weather geekery):
BAMS 4P TUE
BAMS 6P TUE
BAMS 8P TUE
BAMS 10P TUE
BAMS 12A WED

Smoothing out the differences between this model and a few others (not shown because they haven’t been as consistent), and allowing some “buffer time” on either side, here are the time windows for the greatest severe potential:
PAUL STORM TIMING

As the storms move across the area, the greatest severe threat will be in the form of damaging straight-line winds (thunderstorm wind gusts in excess of 60 mph).  We’ll also see the potential for large hail (1″ diameter or larger) and enough heavy rain for localized flash flooding.  I also think these storms are going to produce tremendous amounts of cloud-to-ground lightning — keep in mind that lightning doesn’t make a thunderstorm severe, so even if you’re not under a warning you’re going to want to stay indoors.  At this point, we’re not very concerned about tornadic potential — it’s never impossible, but tornadoes appear very unlikely considering how little spin is present in the atmosphere:
PAUL SEVERE RISK GRAPH 2

The Storm Prediction Center has included us in their “Slight Risk” area for the next 24 hours, indicating an elevated threat for severe weather.  SPC is mainly concerned about damaging straight-line winds, less so about hail or a tornado:
PAUL SEVERE RISK REGION
July8_wind
July8_hail
July8_tor

The National Weather Service is thinking along similar lines…some subtle differences regarding the timing and specific threats, but we’re more or less on the same page.  (Which is to be expected, considering we’re looking at pretty much the same data!)  Here are their relevant graphics for this evening:
July8_1
July8_2

BOTTOM LINE:Outdoor activities are not a good idea this evening.  The greatest severe threat will shape up along and north of I-40, with thunderstorm strength decreasing after sunset as the storms move into southern Middle Tennessee.  There will likely be a Severe Thunderstorm Watch posted by early afternoon (maybe even a Tornado Watch, if the Storm Prediction Center goes into “Chicken Little” mode), and likely some Severe Thunderstorm Warnings this evening.  Plan on staying weather-aware from late afternoon through this evening — we’ll have our regularly scheduled newscasts on from 4:00-7:00pm, and Dan and Lisa will keep you updated thereafter as well.  You can also follow us on Twitter and Facebook for updates — Twitter is BY FAR the better resource during severe weather events.  (In fact, ALL watches and warnings issued for Middle Tennessee and southern Kentucky are automatically posted on my Twitter feed.)

——————–
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

Posted in Uncategorized | Tagged , , , , , | Leave a comment

June 4-5-6 Severe Threat

CMA Fest kicks off in Nashville tomorrow, so as downtown swells with the arrival of thousands of visitors, what better way to welcome those visitors than with a few consecutive days of severe weather chances?

The first and greatest severe weather threat will arrive this evening and tonight.  In the meantime, the daylight hours will be hot and MUGGY — the heat index will likely climb to the mid 90s this afternoon, so stay hydrated and take it easy.
CMA_heat

Thunderstorms are expected to develop by late this afternoon along the Ohio River, and they’ll head generally north-to-south, which will bring them into the Midstate this evening and tonight.  Trying to pin down the exact arrival time is challenging, to say the least — we can’t know exactly when and exactly where until the storms actually develop (and then we can track them).

That said, we can at least give you a hopefully-good estimate of when storms are mostly likely.  This series of Futurecast images represents one computer’s estimate of where the storms will be throughout this evening (this particular computer model seems to have the best handle on how things will evolve):
RPM 8P WED
RPM 10P WED
RPM 12A THU
RPM 2A THU
RPM 4A THU

So that’s what one computer thinks…factoring some other model information, here are the best-estimate storm arrival time “windows”:
PAUL STORM TIMING 3

Those four-hour windows are longer than what we’d like to be able to give you, but the reality is that the specific times will be dependent not only on when the storms develop, but also on how fast they move once they’re off and running.  I’m not terribly concerned about the storm threat in Nashville as the pre-CMT Award festivities are happening, but it’s certainly possible that there will be strong/severe storms as the show is letting out.

I’ve talked (well, “typed” I guess) in this space before about the “recipe” for severe thunderstorms.  But it’s been a while, so let’s revisit it.

The ingredients for severe thunderstorms are always the same: moisture, unstable air, a lifting mechanism to get things started, and wind energy in the environment.  The “how bad” is determined by the ratio of those ingredients.  If all of the ingredients are ideal, then significant severe weather is possible…but more often, one or more of those ingredients are barely present, while others are there in abundance.  (Think of trying to make chocolate chip cookies with five pounds of flour and only a tablespoon of sugar.)  Here’s how the ingredients will break down this evening and tonight:

Moisture: Walk outside.  Notice how gross it feels.  It’s humid.  Yup, we’ve got enough moisture.

Unstable air: Big-time.  Temperatures near the ground will be around 90 degrees this afternoon, while temperatures farther up in the atmosphere will be significantly colder — a high difference between those readings means that explosive thunderstorm growth will be possible (and large hail becomes more likely).

Lifting mechanism: Thunderstorms are fueled by rising air, but something needs to start that rising motion…you can have all the warm, unstable air in the world in place and still not get storms, if there’s nothing around to get the party started.  This is the big wild-card with this evening’s forecast — storms this morning in Missouri, Illinois and Indiana left behind an “outflow boundary” (kind of a mini-cold front) that will spark the development of storms we expect to occur late this afternoon in the Ohio River valley.  Once those storms develop, they can pretty much create their own lift as they move to the south/southeast.

Wind energy: This tends to be the limiting factor regarding our severe weather potential this time of year.  In the summer months, upper-level winds are usually pretty weak — thunderstorms that develop in that type of environment can still be strong, but are short-lived and don’t rotate much.  But tonight, while the wind-energy levels aren’t going to be as high as they can be in spring, they’ll still be sufficient for strong long-lived storms, and even the possibility of a couple of isolated tornadoes.
what1

Too much detail?  Here’s the bottom line: the tornado potential isn’t our main cause for concern — straight-line winds (gusts over 60 mph) and large hail (1″ diameter or larger) will be the main threats:
PAUL SEVERE RISK GRAPH 2

The Storm Prediction Center and Nashville’s National Weather Service office are thinking along the same lines…the SPC has included most of the Midstate in a “Slight Risk” area, indicating an elevated threat of severe weather:
PAUL SEVERE RISK REGION

Breaking those threats down specifically, the SPC thinks there’s a 2-5% chance of a tornado within 25 miles of any one point in our area…
spc-jun4-tor

…and a 15-30% chance of 60+mph wind gusts…
spc-jun4-wind

…and a 15-30% chance of 1″+ diameter hail.
spc-jun4-hail

Here’s the NWS’ take on tonight’s scenario:
nws-jun4

Once tonight’s storms move to the south and dissipate, we’ll have several hours of calm weather through midday Thursday.  But by Thursday afternoon, it looks like storms will re-fire in southern Middle Tennessee, as shown by this snapshot from Futurecast:
RPM 2P THU

The same scenario will repeat itself on Friday, and parts of the Midstate are included in the SPC’s outlook for both Thursday and Friday:
PAUL SEVERE RISK REGION 2
PAUL SEVERE RISK REGION 3

The severe weather potential for Thursday and Friday isn’t as widespread and isn’t as high, but we’ll still be carefully watching any storms that develop those days for straight-line winds and large hail.

No, you don’t need to head to your “safe place” and hide there until Saturday.  Just plan on staying weather-aware the next few days — we’ll let you know if any watches or warnings are issued, both on Channel 4 and online.

——————–
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

Posted in Uncategorized | Tagged , , , , , , | Leave a comment

May 14 Severe Threat

You didn’t think we’d get through the whole month of May without at least some potential for severe weather, did you?  The good news is that as far as peak-storm-season severe weather threats go, this one is pretty marginal…the bad news is that there is still at least that marginal threat for some strong/severe storms on Wednesday.

WEDNESDAY MORNING:
I expect widespread showers and thunderstorms to be in place across much of the Midstate early Wednesday morning (moving in already after midnight tonight, actually).  A snapshot of Futurecast at 6am Wednesday gives you an idea of what to expect:
BAMS 6A WED

These are not expected to become severe, but they’ll produce some cloud-to-ground lightning and heavy downpours.  Be prepared for a wet commute Wednesday morning.

MIDDAY WEDNESDAY:
Here’s the wild-card…how much of a break do we get after the morning rain, and how long will it last?  The longer the break lasts, the more the atmosphere will be able to re-charge and produce stronger storms later in the day.  At this point, I’m thinking that enough cloud cover will linger overhead to keep temperatures in the upper 70s for Wednesday afternoon highs.  That’s still warm enough for a minimal severe-storm chance in the late afternoon and evening — warmer than that, and our severe weather chances go up…cooler, and they’ll go down significantly.

LATE WEDNESDAY AFTERNOON, WEDNESDAY EVENING:
Storms will re-develop as a slow-moving cold front finally edges into Middle Tennessee.  I estimate the time-frame for the strongest storms to be from 2pm until 10pm…within that range, I’d narrow it down even more to 4pm to 8pm…one of our Futurecast computers has zeroed in on that time frame as well:
BAMS 4P WED
BAMS 6P WED
BAMS 8P WED

(Full disclosure: the other in-house computer model we use produces stronger storms already by 2pm, but I’m just not buying that scenario at this point…that’s why you’re not getting the side-by-side Futurecast images this time around.)

The Storm Prediction Center has placed the majority of Middle Tennessee and southern Kentucky in their “Slight Risk” outlook for tomorrow, indicating an elevated threat for severe weather:
PAUL SEVERE RISK REGION

Specifically, this “Slight Risk” means there’s a 15% chance of severe weather (60+mph wind, 1″-diameter hail, or a tornado) within 25 miles of any one location.  The 15% probability is the lowest that will trigger a threat outlook from the SPC, which means they’re also thinking that this is a marginally favorable setup for severe weather.

So what does “marginally favorable” mean?  It means that SOME of the ingredients for severe weather will be present, but not ALL of them…and that there are significant questions surrounding the degree to which the atmosphere will re-charge for the afternoon/evening storms.  The one ingredient that will almost certainly be missing is wind shear (the change in wind direction and speed as you go up in the atmosphere) — with low wind shear, our tornado threat will be very low (you can never say ZERO around here in the spring).  There will be enough “buoyancy” for storms to rapidly grow, if we get that break around midday — that means our hail and straight-line wind threats will be the primary concern.  In chart form, here are the risk categories for each type of severe weather tomorrow:
PAUL SEVERE RISK GRAPH 2

So what does this “conditional” threat of severe weather mean for you?  As always, it means DON’T PANIC.  There’s a very good chance we’ll get nothing more than “garden-variety” storms throughout the day, with some slightly stronger storms in the late afternoon and evening.  The worst-case scenario would be for numerous storms producing hail more than 1″ in diameter, and damaging straight-line winds — tree branches and power lines would be at greatest risk, along with sheds, barns and other out-buildings.

Just plan on staying weather-aware throughout Wednesday afternoon and evening, and we’ll keep you updated!  We’ll be…um…WATCHING.
HomerSuspicious

——————–
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

Posted in Uncategorized | Tagged , , , , | Leave a comment

April 27-29 Severe Threat — Sunday Update

The severe thunderstorms threat is increasing across Middle Tennessee and southern Kentucky — the greatest threat looks like it will develop Monday afternoon and evening, but other time-frames will bring potentially damaging storms as well.  It’s not often you see phrases like this being used by the Storm Prediction Center:

“Widespread severe storms including strong tornadoes, damaging winds and very large hail appear likely Monday and Monday night across much of Mississippi, northern Alabama, Tennessee, southern Kentucky, and northeastern Louisiana.”

We’ll get to Monday’s threat down the line — but first, let’s set the stage with what’s happening this afternoon…then we’ll progress through the next 3 days in order.

SUNDAY AFTERNOON:Showers and thunderstorms have been moving toward and into Middle Tennessee and southern Kentucky all day.  This activity is not expected to become severe, but it is producing cloud-to-ground lightning, along with heavy rains that will saturate the ground and help to increase our flooding threat over the next few days.  This afternoon’s rain developed along a warm front that’s surging to the north — the air behind that front is warm and humid, and will help to fuel the development of stronger and stronger storms.

SUNDAY NIGHT/EARLY MONDAY: Severe thunderstorms to our west are going to head this way late tonight.  Those storms are responsible for the heightened severe weather threat in Arkansas today, and they’ll march eastward as the night progresses.  At this point, I expect these storms to move into the Tennessee River valley around 3-5am, progressing to I-65 by 5-7am and the Cumberland Plateau by around 7-9am.  Our two Futurecast computer models have some differences regarding the specific timing, but overall depict a similar pattern (a reminder — these are two different computers simulating what the atmosphere will do, which is why the two images don’t always depict exactly the same thing):
RPM 3A MON
RPM 5A MON
RPM 7A MON
RPM 9A MON

As they move east, the storms will be gradually weakening — the greatest severe threat will be along the Tennessee River, which is why you’re included in the Storm Prediction Center’s “Slight Risk” area for late tonight:
PAUL SEVERE RISK REGION SUN

The main severe threat with these storms will be strong straight-line winds and large hail.  The tornado threat isn’t zero, but it should remain limited.

MONDAY AFTERNOON/EVENING: Once the morning storms move east, partial clearing will allow the atmosphere to heat up quickly and reload for the most significant weather threat that we’re facing.  Here’s the wild-card, though: if Monday’s rain moves more slowly, if clouds stick around and block that midday sunshine, it will help to reduce our severe weather potential later in the day.  That’s really our best-case scenario…I don’t think it’s very likely that things will play out that way, but I haven’t abandoned hope just yet.

Unfortunately, the other scenario is more likely: storms will develop in West Tennessee and Mississippi by early to mid-afternoon, then begin trekking our way.  The environment overhead on Monday will comprise an almost-ideal mixture of the ingredients necessary for supercell thunderstorms: lots of moisture to fuel the energy necessary for storms to explosively grow, plenty of deep wind shear to produce rotating storms, and lots of low-level shear for that rotation to reach the ground.

At this point, I expect the first “discrete” (meaning separate from one another) supercells to move into Middle Tennessee by late afternoon.  These storms will move to the east-northeast at a rapid pace, which is an especially dangerous set of conditions.  As the storms move E/NE, the separate cells will likely merge into a squall line.  ALL TYPES OF SEVERE WEATHER WILL BE POSSIBLE WITH THESE STORMS: TORNADOES, LARGE HAIL, DAMAGING STRAIGHT-LINE WINDS, AND FLASH FLOODING.  Here are the Futurecast images for Monday afternoon and evening…again, some differences between the two, but both paint an ominous picture:
RPM 4P MON
RPM 6P MON
RPM 8P MON
RPM 10P MON

I really can’t stress enough how dangerous the weather could become Monday afternoon and evening.  There are still plenty of factors that could develop to help reduce our severe weather potential, but most of the severe-weather parameters I’ve been looking at today (the SPC’s SREF-based Supercell Composite parameter, Significant Tornado parameter, and the Craven-Brooks Significant Severe parameter, for those of you who are serious weather geeks) paint a consistently grim picture, especially for areas south of I-40.

The Storm Prediction Center’s “Day 2″ outlook includes much of Middle Tennessee south of I-40 in their “Moderate Risk” category, with the remainder of the Midstate in the “Slight Risk” area:
PAUL SEVERE RISK REGION MON

The term “Slight Risk” is misleading — it means an elevated threat of severe thunderstorms, up to a 30% chance of severe weather happening within 25 miles of a given point.  I’m honestly surprised that the “Moderate Risk” doesn’t include more of Middle Tennessee, and I’m also surprised that a “High Risk” region hasn’t been outlined at least for northern Mississippi and parts of West Tennessee…but we’ll see how the areas are drawn when tomorrow morning’s outlook is issued (that comes out at 1am, and I’ll have it for you on-air tomorrow morning beginning at 4am).

MONDAY NIGHT:The evening storms should be off the Cumberland Plateau by midnight or shortly thereafter…the rest of the night will give us a bit of a break to get ready for Tuesday’s threat.

TUESDAY: The severe weather threat Tuesday is very dependent on what happens Monday evening.  If Monday’s storms sufficiently “work over” the atmosphere overhead, that will help to limit our severe weather potential.  At this point, the SPC does NOT have us included for anything more than “general thunderstorms” in their Day 3 outlook (except right along the TN/AL border):
PAUL SEVERE RISK REGION TUE

However, as good a sign as that is, I’m still concerned that the atmosphere will be able to recharge once again and give us one more round strong to severe storms late Tuesday.  Here are the Futurecast simulations for Tuesday late afternoon and evening, and they (at this point) support that concern:
RPM 4P TUE
RPM 6P TUE
RPM 8P TUE
RPM 10P TUE

These storms are going to develop along the cold front that will finally sweep this active weather pattern out of here.  While I don’t think the amount of energy (buoyancy, specifically) will be as favorable for severe storms, the lift produced by the front itself might be sufficient to overcome that factor.  The wind field will still be very favorable for severe storms…in fact, maybe even more so than in Monday’s scenario.

The problem here is that before Tuesday’s storms develop we have to make it through today’s rain, tonight’s marginally-severe storms, and then Monday’s severe storms.  Every time one batch of storms moves through, it leaves a changed environment behind…that factor confuses the computer models, so our data becomes a little more suspect the farther into the future we look.  At this point, I’d say that we should worry about Tuesday once we’re done with Monday’s threat…just be aware that Monday isn’t the last of our concerns.

OVERALL: This is the most dangerous setup for severe weather that we’ve seen in a while.  So, let me give you a list of DOs and DON’Ts.

DO: Review your personal severe-weather plan — if your “safe place” doubles as a storage area, make sure it’s cleared out.
DON’T: Go to your safe place now and hide there until Tuesday evening.  This would be an overreaction.
DO: Check the battery levels in your flashlights and weather radio, and make sure your cell phone is fully charged.  Pick up some bottled water, just in case.
DON’T: Clear out the grocery store shelves of non-perishable food items.
DO: Make sure your kids know where to go in the event of a warning.
DON’T: Pull your kids out of school, or drive out in dangerous weather to pick them up from school.  Schools are strongly-built structures (they’re built to withstand children!) and every teacher I’ve ever talked to knows exactly where to take their students in the event of a warning.
MOST IMPORTANTLY, DO: plan on staying weather-aware the next couple of days.  We’ll be on the air with our regularly scheduled newscasts to give you updates on the forecast, and when warnings are issued we’ll be on immediately to let you know.  We’ll be on social media as well — Twitter is better since Facebook will “filter” what appears in your timeline.  (If you’re a Facebook-only person, just have a tab with our 4WARN Weather page open, and refresh it to get updates.)  Links to our Twitter/Facebook pages are at the bottom of this post, along with a link to the NWS-Nashville accounts.

And now, the obligatory disclaimer: undoubtedly, this severe weather event will unfold with at least a few differences compared to what I’ve outlined above.  That’s always the case.  If we’re very lucky, we may even be able to dodge this particular bullet — like I said earlier, I haven’t abandoned hope.  But as of right now, this represents our best interpretation of a complicated and potentially life-threatening situation.

One last important point: don’t freak out.  Yes, I know it can be scary when we’re talking about the potential for tornadoes and damaging storms, and I literally just used the phrase “life-threatening”…but there’s nothing that worrying can do to improve your situation.  Stay calm, and make sure you’ve arranged everything to be able to react quickly if you need to.  The National Weather Service office in Nashville has been putting it this way: “Don’t be scared, be prepared.”

It ain’t Shakespeare, but it’s good advice.

 

——————–
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

Posted in Uncategorized | Tagged , , , , , , | 4 Comments

April 27-29 Severe Threat — Friday Update

As I mentioned in yesterday’s blog post, the severe weather season has been remarkably quiet so far…that trend continued last night, when our “slight risk” of severe weather ended up resulting in absolutely no severe weather in Middle Tennessee and southern Kentucky.  But as I also mentioned yesterday, that was a pretty marginal severe weather scenario — the setup for next week’s severe weather potential looks much more dangerous.

I’m not going to get into too many details yet, because we’re still talking about weather that will occur several days from now…but here’s a day-by-day overview of how it looks like things will unfold over the next several days.

SATURDAY:Storms will develop in “Tornado Alley” — Kansas, Oklahoma and Texas are the states that will get the worst weather.  But every day thereafter, the severe weather threat will creep closer to us…this is going to be a very slow-moving system.

SUNDAY: Humidity levels will be on the rise in the Midstate, and as that atmospheric moisture increases, we could see a few storms popping up around here — a warm and muggy air mass is a fertile environment for strong/severe storms, but they won’t be very numerous and they won’t be very organized.

The greatest severe weather threat on Sunday will develop over eastern Oklahoma and western Arkansas.  That’s where the Storm Prediction Center has already outlined a “Moderate Risk” of severe weather (indicating a significant threat of damaging storms):
SPC-Apr26

Those storms will move to the east overnight, crossing the Mississippi River after midnight.  While the severe threat could persist into West Tennessee, and while weakening storms could move into the Midstate early Monday morning, our severe weather threat for Monday won’t develop until later in the day.

MONDAY:Morning showers and storms could help to stabilize the atmosphere somewhat, but the warm and muggy air mass we’ll be…um, “enjoying”…will allow the atmosphere overhead to rapidly recharge to support much more powerful storms Monday evening and Monday night.  All types of severe weather are possible this time around: tornadoes, large hail, damaging straight-line winds, and localized flooding.  The SPC has included us in their long-range severe weather outlook, indicated in the red-shaded area on this map:

10292528_700508209990936_8523300060275446884_n

TUESDAY:Speaking of that map…see the purple area?  That’s the area the SPC has outlined for a heightened threat of severe weather on Tuesday.  Like I said above, this looks like it’s going to be a slow-moving system.  The air over us will still be warm, it will still be muggy, and once again we could see severe thunderstorms developing, particularly Tuesday afternoon and early evening.  There are still significant differences with how the forecast models are handling Tuesday’s storm chances, but the European weather model (one of the most reliable) indicates that our greatest tornado potential would occur on Tuesday.  Our flash-flooding potential would also be significantly higher, since the ground would be saturated as a result of Monday and Monday night’s thunderstorms.

Okay, so at this point you may feel a little like…
nervs

But let’s take a deep breath.  This is several days down the line.  Severe storms, particularly tornadic storms, require a specific mix of weather conditions to develop.  While it looks like all of those ingredients are going to be in place near Middle Tennessee and southern Kentucky, it still remains to be seen whether the ratio of those ingredients and the timing of their arrival will line up to produce powerful thunderstorms.  Another complicating factor is the multi-day nature of this severe weather outbreak.  This storm system is going to produce waves of thunderstorms on Saturday, again on Sunday, again on Monday, and again on Tuesday.  The exact nature of how each storm outbreak develops will have an effect on how the NEXT storm outbreak will develop.  We’re getting pretty deep into chaos theory here, so I’ll bring it back to a simple point: the weather in the Midstate could get quite dangerous next week, but it’s far from a guarantee that severe storms will impact our specific part of the world.

So, just plan on staying plugged-in to the forecast over the weekend — we’ll have our regularly-scheduled newscasts this evening, Saturday and Sunday, and we’ll have new data every few hours to help us sort out how this is all going to play out.  By midday Sunday, we’ll have some high-resolution computer model data to dive into, which will hopefully allow us to get much more specific regarding the what/where/when/how bad.  I’ll update the blog Sunday afternoon, so be watching for that too!

Posted in Uncategorized | Tagged , , , , , | 1 Comment