April 25: Severe Weather Update

5PM UPDATE: A severe thunderstorm watch has been posted for the remainder of southern Kentucky, in effect until 11pm.  No watches on the Tennessee side of the border.
Watch

1PM UPDATE: A tornado watch has been issued for Calloway, Trigg, Christian and Todd counties in southern Kentucky, in effect until 8:00 this evening.
PAUL WATCH
Developing storms are still around and west of St. Louis, but they’ll track along the warm front I talk about below…we’ll keep a careful eye on those storms as they approach!

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(Originally posted 11:30am Saturday)

Some good news and some bad news for today…first, the good news!  Overall, our severe weather potential has diminished and shifted farther to the north.
colbert-dancing

Now, the bad news: our severe weather threat still isn’t zero, and the farther north you go into Kentucky, the greater the severe potential will be later today (primarily this evening).
colbert-we-are-not-friends

Let’s get to the details!

24 hours ago, I was reasonably confident that at least the northern half of Middle Tennessee and all of southern Kentucky would see a significant severe weather threat this afternoon and evening.  But beginning late yesterday morning, my reaction to every new batch of model data has been this:
colbert-slow-smile

Here’s the deal…all week, in this space and on TV, I’ve been talking about how “conditional” our severe weather threat would be — that is, storms would likely be severe IF they developed in the first place.  The environment conducive for severe thunderstorms really hasn’t changed much…if anything, the severe weather parameters have looked even more impressive over the last 24 hours.  But the likelihood of any “trigger” developing to initiate thunderstorm development has consistently looked a lot less impressive, especially on the Tennessee side of the border.  Basically, the best environment for severe storms and the best environment for any storms to develop are in different places on the map.
Colbert-High-Five

Here’s the regional radar view as of late this morning, with the surface analysis (fronts and low/high pressure areas) drawn as well.
VIDEO VIPIR
The area of low pressure in Missouri is the best “trigger” for thunderstorms, and it will ride along the warm front stretched west-to-east.  The warm front itself will be drifting farther north…which is why the severe weather threat has shifted farther north.  Imagine a train (that’s the low pressure) confined to a path along the tracks (that’s the warm front), but the train tracks themselves aren’t stationary.  We have to predict where the low will be, where the warm front will be, and where any storms triggered by the low will be.  Now you’re starting to get a feeling for why weather forecasting is a wee bit complicated.
colbert-jaw-drop

Even though the outlook has significantly improved for much of Middle Tennessee, it’s not all great news.  Southern Kentucky will still be under the gun for severe storms as that low-pressure center moves in this evening.  The latest outlook from the Storm Prediction Center shows how quickly the severe threat escalates as you go north:
PAUL SEVERE RISK REGION

That’s the regional view…on a local level, I’m estimating our storm chances (this is for any storm — regardless of severity) late this afternoon and this evening to shape up like this:
PAUL RAIN TIMING

As usual, Metro Nashville finds itself right on the dividing line.
colbert-clap

Areas to the south of I-4o aren’t completely in the clear — it’s a slight chance of storms overall, but if a thunderstorm manages to pop up it will be like lighting a match in a gasoline refinery.  Northern Middle Tennessee is in that annoying middle ground — by the time storms make it in this evening, the severe weather ingredients won’t be as favorable, but we’ll still be on the lookout for strong to severe storms…especially if they move in faster.  Southern Kentucky, it’s not a guarantee of severe storms for you either — you’ve got the higher storm chances, but the environment over your heads isn’t quite as favorable for those storms to be severe (it’s still looking like it will be “good enough” though).  Overall, the specific severe weather threats shape up like this:
PAUL SEVERE RISK GRAPH 2
colbert-angry

The timing of the greatest threat has been adjusted somewhat as well.  We’ll be on the lookout for any storms to pop up as the atmosphere warms this afternoon, but overall the odds of that are relatively low.  It’s a good idea for you to stay weather-aware just in case, but there’s no real reason for alarm at this point.

This evening, storms will develop to our northwest and move into southern Kentucky and northern Middle Tennessee.  One of the few computer models to handle this whole scenario in a somewhat-reasonable way shows this pattern this evening (notice the dramatic weakening after 10pm):
NAM 8P SAT

Is that going to be perfect?  NOPE.
colbert-facepalm
The storms could be faster, slower, weaker, stronger, you name it.  But it at least gives us an estimate of how things will play out, and that particular model seems to have its head in the right place.

BOTTOM LINE: I’d love to be able to give you a yes-or-no answer on a county-by-county basis regarding whether it will storm, and whether those storms will be severe.  But we deal in probabilities, not certainties.  I wouldn’t cancel any plans this evening, but I would plan on staying weather-aware as you’re out and about…or sitting on the couch watching the Predators game.  Just remember:

  • storms are still possible this afternoon and this evening
  • storm chances increase the farther north you go
  • ANY storm that develops could become severe, even if you’re in a low-storm-chance part of the Midstate

While things have been consistently trending in an encouraging direction, it’s still a situation that could change very quickly.  I’ll be monitoring things throughout the afternoon — if watches are issued (indicating a higher potential for severe weather), I’ll post that information on social media and at the top of this post.  If warnings are issued (indicating severe weather is actually happening), they’re posted automatically to my Twitter feed and at the bottom of your screen on Channel 4…as always, I encourage you to NOT use Facebook as a severe-weather source.

colbert-byebye

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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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April 24: Severe Weather Saturday, Links

Evening update: Latest model data keeps the best storm chances north of the Midstate tomorrow afternoon/evening. Obviously good news, but high instability means any storm that DOES develop would soon become severe. Bottom line: stay weather-aware, but don’t freak out! I’ll post a blog update before noon tomorrow…

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Your daily dose of nerdy science-themed links is at the bottom of this post…some good stuff today, both weather-related and just general science-y material.  But we’ve got a significant severe weather threat shaping up this weekend, so we’ll start there.

 

WEATHER

The stage is set for severe thunderstorms to occur in the Midstate Saturday afternoon and evening…but as usual, it’s a complicated situation.  I’ll go through this today’s and tomorrow’s weather in chronological order, beginning with…

TONIGHT: Showers and thunderstorms will become more likely, moving in from the west/southwest.  While some rumbles of thunder will probably accompany the rain, severe weather is not anticipated in this time frame.

SATURDAY MORNING: Here’s the first point (of three) at which things get tricky.  The Country Music Marathon starts at 7am, and the computer guidance is split 50/50 regarding our chances of rain in that time frame.  Our in-house Futurecast model thinks the heaviest rain will have moved off to the east, but it will be a close call and there will still be scattered storms (a few of which could be strong) hanging out in the area:
RPM 7A SAT

If you’re heading downtown to either run or spectate, I would plan for the worst and hope for the best — that is, plan for rain and hope that we catch a break in the action.
TheWire_Rhonda_eyebrowraise

MIDDAY SATURDAY: As the early-Saturday rain chances depart, we expect breaks in the clouds to develop and expand.  This will allow the late-April sunshine to rapidly warm things up, with afternoon high temperatures approaching 80 degrees.  The warmer we get, the greater our severe weather potential will be…but this is the second point at which things get tricky.  IF the clouds stick around, our temperatures won’t warm up as much, and thunderstorms will have a harder time initiating in the afternoon.  I haven’t abandoned hope in this scenario — but again, plan for the worst and hope for the best.
TheWire_McNulty_soso

SATURDAY AFTERNOON AND EVENING: Assuming our temperatures have warmed up during our midday break, we’ll see thunderstorms develop and rapidly intensify.  Now we’ve arrived at our third point at which things get tricky.  Something needs to initiate storm development — in all likelihood they won’t just spontaneously pop up on their own unless we warm up all the way into the low to mid 80s.  There will be a cold front approaching from the northwest, but it will probably be too far away to trigger storms in the afternoon.  More likely is that leftover “outflow boundaries” from our overnight and early-morning storms will help focus the development of our expected afternoon thunderstorms.  This is an environment in which storms will very quickly go from “growing cloud” to “damaging supercell thunderstorm,” which is why we’re emphasizing the importance of staying weather-aware throughout the afternoon.  The severe-weather threat will move from west-to-east throughout the afternoon and into early evening.  By about 8:00pm or so, things should be quieting down.
TheWire_Lester_BeingAwesome

One of the problems is that computer models don’t do a great job of simulating supercell thunderstorms more than 24 hours in advance — they’re much better at representing squall lines and just plain old rain.  So when you look at Futurecast for Saturday afternoon…
RPM 3P SAT
…you might think, “hey, we’re mostly in the clear!”  But the fact that Futurecast is showing anything for Saturday afternoon is an ominous sign.  Unfortunately, we’ll have to wait for the very short-range computer models to simulate things Saturday morning to see how things will specifically shape up.
TheWire_Bunk_cmon

Is it possible that things will shake out in such a way that the afternoon storm threat fizzles out?  You bet!  The uncertainty with this scenario isn’t with the severe-weather ingredients…it’s with the existence of storms in the first place.  Put another way, we’re not certain that storms will even develop Saturday afternoon in Middle Tennessee and southern Kentucky — but if they do, they’ll likely become severe.
TheWire_McNulty_deskkick

THREATS: At this point, ALL types of severe weather appear possible…that means tornadoes, large hail, and damaging straight-line winds.  The Storm Prediction Center has included the northern half of the Midstate in an “Enhanced Risk” area for severe thunderstorms.  (“Enhanced” is the middle rung on the severe-risk ladder — “Moderate” and “High” risk areas are worse…and SPC has indicated it may upgrade portions of the outlook with those categories.)  Earlier today, SPC had the entire Midstate in the “Enhanced Risk” area…with their early-afternoon update, they trimmed that back (instead of upgrading the northern half to “Moderate”).  Not sure I agree with that, but it does reflect our general lack of confidence that storms will fire up farther south of I-40.
PAUL SEVERE RISK REGION
oh-thats-terrifying

(Okay, I had to break the streak of gifs from “The Wire” for that one.)  Don’t breathe of sigh of relief if you live south of I-40, though…analog forecasts (comparing tomorrow’s weather pattern to similar patterns in the past) show that all types of severe weather have occurred with this type of system…and in significant numbers:
SVRnam212F036

The analog method also yields a better than 50-50 chance of at least one severe report in the Midstate:
SVRall_N1_110km_nam212F036

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 Saturday afternoon and evening:

Moisture: There should be plenty, as dew point temperatures climb into the 60s…but there are some indications that a wedge of slightly drier air will move in and help to stabilize things a bit.  That would be great, but I won’t count on it.

Unstable air: Big-time.  Temperatures near the ground will be around 80 degrees in the 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).  But again, if the air is drier it will help to limit our instability…or if we don’t clear out around midday, we won’t be warm enough for things to really get cooking.  That’s why this is one of the biggest wild cards in tomorrow’s setup.  Two measures of instability are Convective Available Potential Energy and the Lifted Index — CAPE is forecast to be in the 2500-3000 range (which is high), and the LI is forecast to be around -10 (which is really high, and is why we’re concerned about large hail).

Lifting mechanism: I already discussed this factor in the “Saturday Afternoon” portion of the forecast above…but it’s the other big wild card in Saturday’s severe weather potential.

Wind energy: The one thing we’ll lack on Saturday is “directional shear” in the lowest levels of the atmosphere — that is, the change in wind direction with height.  There will be plenty of “speed shear” (winds will be much stronger as you ascend in the atmosphere), which, combined with the high instability, might be enough to overcome the lack of directional shear and enhance our tornado threat.

So if storms develop, the environment looks like it will be ready-to-go in terms of producing severe weather.  In yesterday’s post I went through some of the composite indices that we use to evaluate severe potential, and they’re even more favorable around here than they were yesterday…we’re in a region with higher values of the “Significant Severe” Parameter (anything over 20,000 is favorable for all types of severe weather, and our forecast value Saturday afternoon is 50,000):
SREF-CB
TheWire_Stringer-head-shake

We’re also forecast to be in a region higher values of the Supercell Composite Parameter (values over 3.0 will get our attention, and right now the forecast value is over 9.0):
SREF-SCP
TheWire_WeeBey

Finally, we’re in a non-zero area of the “Significant Tornado” Parameter (anything over 1.0 is something we don’t want to see, and we’re at a 3.0 level according to the latest data):
SREF-STP
TheWire_Daniels_eyeswide

OVERALL: This is the most dangerous setup for severe weather that we’ve seen so far this year.  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 Sunday morning.  This would be an overreaction.
DO: Check the battery levels in your weather radio and flashlights, and make sure your cell phone is fully charged.
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: Lock your kids in the house for the weekend, or cancel all of their/your plans.  Just emphasize the importance of keeping an eye on the weather.
MOST IMPORTANTLY, DO: Plan on staying weather-aware the next couple of days, especially Saturday afternoon.  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 — all watches and warnings are automatically posted to my Twitter feed…they are NOT posted to Facebook.  Facebook is less useful in a minute-by-minute severe weather scenario than just about anything imaginable.  Links to our social media 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.  Even in high-risk scenarios, the odds of severe weather happening in your immediate vicinity are fairly low.  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.
TheWire_Omar_winks

 

LINKS

If all of the severe-weather talk has you a little on-edge, let’s decompress with some geekiness.

  • One of the tools most important for meteorologists in terms of forecasting severe weather is pretty low-tech.
  • Are thunderstorms more likely as urbanization increases?
  • Drones aren’t just sky-robot-killing-machines (or GoPro camera platforms)…researchers are using them to monitor Arctic Sea ice conditions.
  • This time-lapse video of the Chilean volcanic eruption is pretty spectacular:

  • There’s lightning in that time-lapse video and in the pictures of the eruption I shared yesterday.  It’s the same basic physical process that causes thunderstorm lightning, but overall volcanic lightning isn’t very well-understood yet.
  • Space stuff!  More images to celebrate the Hubble Space Telescope’s 25th birthday…one of my favorites is of the proto-planetary disks surrounding newborn stars.  (It’s cool to be able to actually watch the process of planets being formed.)
  • Almost 2000 planets have been confirmed outside of our solar system (“exo-planets”)…now we’re able to directly observe visible light bouncing off one of those planets.
  • So far, we only know for-sure of the existence of life in one place in the universe…Earth.  But there are spots even in our own solar system that have conditions that could support simple organisms — Saturn’s moons Titan and Enceladus, and Jupiter’s moon Europa.

 

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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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April 23: Weekend Severe Storms, Links

The format of yesterday’s post seemed to work out pretty well, so I’ll stick with it for a little while…if you’re clicking here for the first time (or for the first time in a while), here’s the deal: I’ll be updating the blog daily — the posts will lead off with a discussion of our local weather, then I’ll share some nature- and science-related links that I find interesting.  Away we go!

WEATHER

Really nice weather today!  A bit on the cool side for this time of year, but a break from the rain chances is a nice change of pace.  We’ll be dry most of Friday as well, but clouds will thicken throughout the day, with showers moving in Friday evening.  The best chance of showers and storms will arrive after midnight Friday night — at this point, it’s still very up-in-the-air as to whether those showers and storms will still be around when the Country Music Marathon starts Saturday morning.  If you’re running or spectating, plan for rain and hope for a break!

Any storms we get Friday night and early Saturday could conceivably become severe, but the chances aren’t very high during that time frame.  The greater severe weather potential will take shape Saturday afternoon and evening, but it’s still highly conditional at this point…meaning there are a lot of “if this, then that” and “but maybe” statements at play.  The biggest of those: IF we see a substantial break from the clouds and rain chances in the late-morning and early-afternoon on Saturday, THEN our severe thunderstorm chances will increase substantially (and vice versa).

The Storm Prediction Center has already included the Midstate in a “Slight Risk” area for severe weather, along with a sizable chunk of the eastern half of the country:
PAUL SEVERE RISK REGION

Specifically, that means they’re estimating a 15% chance of severe weather (60mph winds, 1″+ hail, or a tornado) within 25 miles of any point in the risk area.  The situation is muddled enough at this point that they’re not outlining any “Enhanced Risk” or “Moderate Risk” areas (and the associated higher percentages) yet.  The uncertainty is not related to the wind profile in the atmosphere or the amount of energy available for storms to intensify — both of those factors look very storm-friendly.  The uncertain is related to how Friday night and Saturday morning’s rain chances will influence the weather set-up by Saturday afternoon…unfortunately, that’s a very difficult thing to predict more than 48 hours in advance.

If storms develop, the environment looks like it will be ready-to-go in terms of producing severe weather.  (There’s some serious weather nerdiness about to hit you, so if you want to skip past the maps I won’t take it personally.)  Some of the composite indices that we use to evaluate severe potential are very favorable around here…we’re in a region with higher values of the “Significant Severe” Parameter (anything over 20,000 is favorable for all types of severe weather, and our forecast value Saturday afternoon is 40,000…with even higher values in Mississippi):
SREF-CB

We’re also forecast to be in a region higher values of the Supercell Composite Parameter (values over 3 will get our attention, and right now the forecast value is about 7.5):
SREF-SCP

Finally, we’re in a non-zero area of the “Significant Tornado” Parameter (anything over 1 is something we don’t want to see, and the southern half of the Midstate is at a 2 according to the latest data):
SREF-STP

All of those indicate a situation that requires our attention.  However: THIS DOESN’T MEAN THAT SEVERE WEATHER IS DEFINITELY GOING TO HAPPEN.  Storms have to exist in the first place before they can become severe, and that’s the big wild card.

The computer-model radar simulations for Saturday are still all over the place, so I’m not even going to bother showing those here.  The thing to remember at this point is that any storms that fire up Saturday afternoon and evening will develop in an environment that will be conducive to severe weather…but it’s far from certain that those storms will even fire up to begin with.  As always, watch it, but don’t worry about it.  We’ll keep you posted.

 

LINKS

All sorts of science-y and dorky goodness today…

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April 22: Showers, Storms, Links!

Happy Earth Day!

I’m still working out the mechanics of transitioning this space from a severe-weather-only update schedule to a daily collection of weather information and other science info.  So I’ll probably be moving things around and experimenting with different formats as I try to find a level of organization that makes the most sense.  For now, the plan is to lead off with some local-weather discussion (whether there’s anything to worry about or not), then finish up with a “Links” section of science nerdiness.

 

WEATHER

Not much to worry about today — some scattered showers north of I-40 this morning, generally along and south of I-40 this afternoon.  No severe concerns whatsoever with this light rain chance.

The next time frame of concern arrives Friday night and Saturday.  Showers and non-severe storms look increasingly likely late Friday evening and overnight, lingering into early Saturday morning.  At the moment, I’m hopeful that a break from the rain will develop around the time of the Country Music Marathon — it’s just a little too far in the future for specific timing estimates, unfortunately.

More thunderstorms could develop Saturday afternoon — if we warm up enough during our break from the rain, some of those afternoon storms could become severe.  Right now we’re NOT included in any long-range outlooks from the Storm Prediction Center…
SPC-Day4

…but some of the “composite” severe weather indices from the Short-Range Ensemble Forecast model indicate the potential for severe storms.  Among those, the “Supercell Composite Parameter,” which combines various forecast elements to indicate whether supercell thunderstorms (more likely to produce severe weather) are likely, is elevated over the Midstate late Saturday afternoon:
SPC_supercell

The map shows the highest supercell probabilities to our west and south, but around here it’s high enough that we’ll definitely need to keep an eye on it.  Also, the analog forecasts (comparing this Saturday’s weather pattern to similar patterns in the past) yields a history of severe weather reports…
SVRgfs212F096

…and a better-than-average chance of at least some severe weather in the Midstate:
SVRall_N1_110km_gfs212F096

But it’s still more than 72 hours away!  Too early at this point for any heightened worry levels — we’ll keep you posted as the weekend gets closer.

 

LINKS

Like in yesterday’s post (and hopefully in future posts), these are links to science-related articles that I found interesting.  Some are weather-related, some aren’t…but I thought they were worth sharing.

  • It looks like the skies over most of Middle Tennessee and southern Kentucky should clear in time for you to check out tonight’s Lyrid meteor shower.
  • I spend a lot of time de-bunking these when I do school visits and other public appearances: Tornado Myths.
  • Global warming isn’t occurring as fast as the worst-case computer simulations…but it is happening.
  • Time itself moves more slowly at your feet than it does at your head.  Really.  We can measure it.
  • So far, almost two thousand exo-planets (planets other than the eight in our own solar system, orbiting other stars) have been discovered.  Including one that’s really REALLY far away.
  • Have a tissue handy when you’re watching this video about a soldier who adopted his soldier-dog after she was injured.  NO, YOU’RE CRYING AT 9:00 IN THE MORNING.
  • A story that merges history, science, and alcohol.  It’s like they wrote it just for me.
  • Finally, a story that won’t be for everyone…but I found it fascinating: a physicist-turned-priest has spent the last two decades exploring the overlap between religion and quantum physics.
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Links Of Interest…Maybe?

Seems like I’m only updating this blog when there’s some kind of inconvenient or flat-out dangerous weather on the way…I’ll be trying over the next few months to post more material.  Some of it will be weather-related, some of it won’t.  We’ll see how much traction this stuff gets, and that will determine whether it becomes a regular occurrence.  Like everything else on the internet: share it if you like it, ignore it if you don’t!  Constructive feedback is always welcome, of course…

There’s no severe weather on the horizon for the next several days, so this seemed like a good opportunity to share some links to interesting science news…or at least, science news that I found interesting!  It’s mostly space-stuff — which shouldn’t be surprising, since meteorologists spend most of their time looking up!

  • I’m optimistic that skies will clear out over our neck of the woods Wednesday night, which would allow for good viewing of the Lyrid meteor shower.
  • Only 4% of the universe is visible matter (the stuff we can see and observe directly).  The rest is dark matter and dark energy…ominous names, but more reflective of the fact that scientists just don’t know that much about them.  That might be starting to change.
  • Speaking of dark…astronomers are getting a better look at the inner workings of supermassive black holes.
  • The time-scales and distances involved in astronomy are astounding, as this article about the “fizzling” of giant galaxies demonstrates.
  • If that article didn’t melt your brain, try this one: scientists are trying to identify what might be the largest known structure in the universe.
  • A little closer to home (actually, a LOT), NASA’s Mercury MESSENGER probe is about to complete its mission in spectacular fashion — by crashing into the little planet.
  • And waaaaaaay too close to home, an asteroid the size of a house zipped by Earth just after 3:00 this morning.
  • Finally, if you’re looking for some reading material, I started reading this book over the weekend.  Don’t be scared by the title — it’s very accessible and easy-to-understand, even if you are intellectually allergic to math and physics.
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April 19 Severe Weather Potential

Just a quick post to update you on our severe weather potential today and tonight.  The entire Midstate is included in the Storm Prediction Center’s “Slight Risk” outlook for today and tonight:
1630z_SPC_Cat

The first round of potentially strong storms will take shape late this afternoon and early this evening.  This is a highly “conditional” event — whether those storms develop and how strong they become will depend on how much we warm up in between early-afternoon clouds and spotty showers.  Our Futurecast model thinks some feisty storms will start showing up between 4-5pm:
2130z_RPM_Apr19

Once the sun goes down, those storms will fizzle.  But another round of storms will move in from the west late tonight, with at least some potential for severe weather as well.  Futurecast anticipates a line of storms to approach the Tennessee River by midnight…
0500z_RPM_Apr20

…race across I-65 between 1:00am and 2:00am…
0700z_RPM_Apr20

…and weaken as it moves off the Cumberland Plateau by 4:00am:
0900z_RPM_Apr20

Again, that’s one run of one computer model…models are useful guidance, but they’re not perfect.  In this case, the other computer models we look at are in generally good agreement regarding the timing and placement of the storms today and tonight.

The main threat anticipated from both this afternoon’s storms and tonight’s storms is from damaging straight-line winds (thunderstorm wind gusts in excess of 6omph).  SPC estimates a 15% chance of those wind gusts within 25 miles of any given point in the Midstate:
1630z_SPC_Wind

Some hail threat exists with the afternoon storms, especially in the warmer and more unstable air mass to the south of I-40:
1630z_SPC_Hail

Overall, our tornado threat isn’t zero, but I would categorize our tornadic potential around here as “not impossible, but unlikely”:
1630z_SPC_Tor

So what are our chances of actually getting severe storms?  Like I said up top, this afternoon’s chance will be determined by how much we warm up (which determines how much instability develops overhead)…and tonight’s storms will be weakening as they move in from the west.  Using an analog forecasting method (comparing past weather events to the current situation) yields an estimate of a 40-60% chance of at least one severe storm impacting the Midstate, with the higher risk farther to our south:
SVRall_N1_110km_nam212F048

I said in yesterday’s post that you should “watch it, but don’t worry about it” — stay weather-aware throughout the afternoon and early-evening, and make sure the weather radio has batteries before you go to bed.  Better safe than sorry, basically.

And now my usual social media plug: all severe thunderstorm and tornado watches and warnings are automatically posted to my Twitter feed and Facebook profile (links below) unless Tumblr is being cranky…Twitter is BY FAR the better resource, since Facebook’s filters won’t display a lot of updates in your news feed.

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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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Weekend Storms, April 18-19

First off, if the formatting of this looks weird, I apologize…I’m writing this on my phone while visiting my in-laws-to-be, and the cell phone service in this part of the world is, shall we say, SPARSE. (Yes, that’s pretty much the textbook definition of a first-world problem.)

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Several rounds of thunderstorm chances will affect the Midstate through the rest of the weekend. The first chance will take shape this afternoon as the humidity slowly increases and temperatures warm up to near 80°. Our Futurecast computer model (the RPM model, for those who care) puts the best chance of scattered storms in southern Middle Tennessee:

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More widespread showers and non-severe thunderstorms will spread up from the south late tonight (likely after midnight) and will probably linger into Sunday morning. Futurecast at sunrise Sunday shows a lot of disorganized activity:

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We’ll catch a break from the highest storm chances midday Sunday, but spotty hit-and-miss storms will be possible again Sunday afternoon and evening. I wouldn’t necessarily cancel outdoor plans, but keep an eye to the sky or occasionally check the radar on the 4WARN mobile app. Futurecast for mid-afternoon Sunday shows the scattered storms, some of which could be strong to marginally severe:

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We’re in the time of year when ANY thunderstorms that develop will have the potential to become briefly severe, but our severe weather threat Sunday afternoon and evening depends on how much we warm up after the morning rain fizzles out.

Another threat of severe weather arrives late Sunday night. The Storm Prediction Center has included the Midstate in a “Slight Risk” of severe thunderstorms for both Sunday afternoon and Sunday night.

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They estimate a 15% chance of seeing severe weather within 25 miles of any point in the risk area. The main concern will be damaging straight-line winds over 60mph. Any tornado threat should remain to our west, in the “Enhanced Risk” (30%) area during the afternoon and early evening on Sunday.

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Futurecast shows hefty storms approaching the Tennessee River around midnight…

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…I-65 and Metro Nashville around 2am…

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…and the storms in a weakened state moving into eastern Middle Tennessee by 4-5am Monday.

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Keep in mind, ALL of the Futurecast images above represent one run of one model. We use those models for guidance, not as a guarantee. There are other models, like the NAM 4km model pictured here, that keep our storm threat very low Sunday night.

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I’m increasingly leaning in the direction of the lower threat Sunday night, but still: stay tuned for updates…we’re always evaluating new information, and we’ll pass that along as the situation unfolds. Like our last severe weather threat, I’d put this squarely in the “watch it, but don’t worry about it” category. I’ll post a few updates via social media throughout the weekend (cell service permitting), and Nancy Van Camp will have updates during our weekend newscasts. And one last social media plug: all severe thunderstorm and tornado watches and warnings are automatically posted to my Twitter feed and Facebook profile (links below)…Twitter is BY FAR the better resource, since Facebook’s filters won’t display a lot of updates in your news feed.

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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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