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…


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.



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:

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.

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.

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.

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…
…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.

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.

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.

(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:

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

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):

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):

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):

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.



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.


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

About paulheggen33

Morning meteorologist for WNCN-TV in Raleigh.
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