December 22: Severe Storms Wednesday

Lots to talk about weather-wise today: record high temperatures today through the weekend, numerous rain chances (including Christmas Day), but most importantly…we’re looking at an increasing likelihood of severe thunderstorms in the Midstate late Wednesday afternoon into Wednesday night.  Let’s dive into the details.

This morning’s showers will diminish to just an isolated shower chance this afternoon and this evening.  A few breaks in the clouds, combined with southerly winds, will help push our temperatures up to record territory:
Today’s record is 67, set back in 1933.  We’re likely to set (or at least threaten) more record high temperatures over the next several days as well:

Rain will spread back up from the south tonight, mainly soaking areas along and east of I-65, as Futurecast’s radar simulation shows:
This activity will NOT be severe, and it will be moving off to our northeast by sunrise Wednesday.

Wednesday is the tricky day in the forecast, because of the increasing severe weather potential.  I want to be clear about this right away: severe thunderstorms are not guaranteed tomorrow, but they are looking more likely. I’m going to get into some serious weather-geek stuff for the next several paragraphs, so if you want to skip to the “bottom line” section at the bottom of the post, I won’t be offended.  I just want to show the logic that’s going into the severe-weather forecast…even if you don’t care about the details, it’s helpful to establish the trail of breadcrumbs that will lead us to the Big Bad Wolf.  (I never paid attention to fairy tales, obviously.)

I’ve made the “cookie analogy” before, comparing severe weather factors to recipe ingredients…so let’s run down the ingredient list for tomorrow and see how things stack up.

1. Unstable air.  I’m cheating a little bit by calling this one ingredient, but it’s actually a combination of sub-ingredients — warm air near the ground, cold air in the upper atmosphere, and lots of low-level moisture.  All of those factors will be in place tomorrow.  We measure instability with a statistic called “CAPE” (Convective Available Potential Energy) — springtime CAPE values typically run in the 2000-3000 range, but you can easily get severe weather this time of year with CAPE in the 500-1000 range.  And look where the CAPE values will be tomorrow:
Good enough.

2. Favorable wind shear.  High wind shear (meaning the wind both increases in speed and changes direction with height) helps organize severe thunderstorms by either tilting or rotating the updrafts that fuel those severe storms.  This is the most severe-weather-friendly ingredient we’ll have in place tomorrow, which is distressing.  We measure shear with a statistic called Storm-Relative Helicity, which lets us quantify the combination of speed and direction changes (thanks, calculus!).  SRH values over 250 are favorable for tornadoes, and tomorrow’s forecast helicity numbers are in the 300-500 range, depending on which forecast model you believe:
This model, the 4km-resolution North American model, forecasts helicity values over 600 in a few spots!

3. Trigger mechanism.  You need something to get the storms started.  In this case, our trigger will be a weak cold front out to the west.  It’s certainly not the strongest front in the world (as evidenced by the fact that our temperatures won’t change much after this storm system moves through), but it looks like it will be sufficient to spark storm development in West Tennessee and Mississippi tomorrow afternoon.  Once those storms develop, they’ll race southwest-to-northeast into and across the Midstate, traveling 50-60 mph.  The good news is, we’ll be able to see them coming…the bad news is, they’ll be hauling.  Our in-house RPM model depicts a complicated pattern, with multiple severe-storm clusters developing from 5pm-10pm:
It’s not very often that you see a computer model’s radar simulation depicting discrete supercells over 24 hours in advance!  After 10pm, the intensity of the storms decreases quite a bit, but they’ll still bear watching:
Please keep in mind, this is ONE version of ONE model.  I have a queasy feeling that it might be too slow, which means the storms would be moving across the Midstate a couple of hours earlier, when the atmosphere is even more unstable.

The Storm Prediction Center has included the most of the Midstate in an “Enhanced Risk” (level 3 of 5) area in their severe thunderstorm outlook for tomorrow:
Within that Enhanced Risk area, I think locations along and south of I-40 and west of I-65 are at greatest risk, because the environment there looks like it will be even more favorable for severe thunderstorms.  It’s possible that the SPC will even upgrade parts of the Enhanced Risk to a “Moderate Risk” (level 4 of 5).  Their next update drops at 11:30am today, so I’ll update this part of the post if there are any changes.  (EDIT: The 11:30am update contained very few changes — I’ve updated the appropriate images, but no additional analysis is necessary at this point.  The next SPC update is after midnight tonight.)

To put some numbers on things, the SPC estimates a 30% chance of severe weather (tornado, 60+mph winds, 1″+ diameter hail) within 25 miles of any one point within the Enhanced Risk area.  They also have outlined an outlook for “significant” severe weather in the hatched area on this map — a 10% chance (again, within 25 miles) of EF-2+ tornadoes, 75+mph winds, or 2″+ diameter hail:

How realistic are those threats?  Again, all we have to go on is forecast model data — but the SPC’s short-range ensemble forecast (SREF) model is a good one, since it combines the output of a couple dozen individual models.  This map is the SREF’s estimate of the Supercell Composite Parameter late tomorrow afternoon/early tomorrow evening:
The Supercell Composite Parameter simply measures how favorable the atmosphere will be aligned for the production and maintenance of supercell thunderstorms.  Anything over 6 puts a twist in meteorological knickers…forecast values for the Midstate tomorrow are twice that.  But you can have the best ingredients in the world for storms to become severe, and it won’t matter if storms don’t develop in the first place!  Unfortunately, the SREF model estimates the odds of that in the 50-70% range late tomorrow afternoon and tomorrow evening:
SREF_prob_combined_supercell__f036 SREF_prob_combined_supercell__f039
All of that means that damaging straight-line winds and large hail are a distinct possibility late Wednesday.

The million-dollar question: what about tornadoes?  I wish I had better news.  The SREF model simulates a statistic called the Significant Tornado Parameter (the name should be pretty self-explanatory there).  Anything over 2 or 3 is concerning, and right now the forecast is for STP values to be over 4 in the western half of the Midstate between 3pm and 6pm:

One more bit of weather-nerdery to look at, and that’s the analog forecast, using similar historical weather patterns to forecast the current pattern.  The analog forecast points to a 30-35% chance of 5 or more severe weather reports tomorrow, especially from Nashville to the southwest:
Interestingly, the weather pattern with the highest analog score (meaning it’s most-similar to tomorrow’s pattern) is December 21, 2013, when a significant straight-line wind event raced through the Midstate.

If you’re looking for some positive news, I do have some sorta-kinda encouraging things to say:
– The instability values are borderline, and there are a few ways that the atmosphere could trend in a more stable direction: abundant cloud cover, storms along the Gulf Coast to block the flow of humid air, or a later storm-arrival time (at night, when the atmosphere is calmer).  We don’t need all of those, just one, to help us out!
– It doesn’t look like there will be a strong area of low pressure nearby to help enhance the tornado threat.  (Unfortunately, even a weak one would suffice given the other ingredients present.)
– This isn’t a favorable time of year for severe storms to develop.  Short daylight hours and a very low sun angle severely reduce any contribution the sun could make to how unstable our atmosphere becomes.  (But again, keep in mind that significant straight-line wind event almost exactly two years ago, on December 21, 2013.)
– Something unexpected could happen!  This is all more than 24 hours away, and Mother Nature loves throwing us curveballs.  I’d be absolutely thrilled to be swinging at a pitch in the dirt on this one (to stretch a metaphor to its breaking point).


Your handy bullet-point list of things to take from this discussion…

  • Thunderstorms with heavy rain are likely late Wednesday and Wednesday night.  The odds that at least some of those storms will become severe have increased quite a bit.
  • Damaging straight-line winds and tornadoes are the primary threat, but large hail and localized flooding are also a concern.
  • Strong straight-line winds can do just as much damage as a tornado.  Pay attention to both severe thunderstorm warnings AND tornado warnings, if they’re issued.
  • DON’T FREAK OUT.  The possibility of severe weather (or even likelihood) is not the same as a guarantee of severe weather.
  • Plan on staying weather-aware Wednesday, particularly during the afternoon and evening.  We’ll keep you updated during our newscasts, I’ll update the blog a couple of times tomorrow, and any watches/warnings are automatically posted to my Twitter feed (social media links at the very bottom of this post).
  • Staying weather-aware means NOT cutting yourself off from the flow of weather information…which means, get your last-minute shopping done early!  You don’t want to get caught out and about if things turn nasty.  Be done with that stuff by 3pm…or noon, if you really want to play it safe!

Overall, my personal level of concern is at “uneasy.”  Not yet “anxious” or flat-out “worried.”  My gut feeling is that areas along/south of I-40 and west of I-65 could be in for a rough several hours late Wednesday afternoon and Wednesday evening.

I’m very much hoping that the data we get the rest of today and tomorrow points in a less-ominous direction.  We’ll keep you posted.  In the meantime, let this dog’s ears chill you out:

Just a quick look ahead at the rest of the Christmas weekend forecast…more showers and NON-severe thunderstorms are likely Friday (Christmas Day) and again Sunday night/Monday.  No end to the warm spell in the 7-day forecast!
7 Day PM

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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