March 24: Severe Storms Possible Today

11:45AM UPDATE: We’re not out of the woods just yet, but things are definitely heading in the right direction.  This morning’s few thunderstorms have remained well below severe thresholds, and the widespread rain and cloud cover has helped keep the atmosphere relatively stable.  There’s still a ton of wind energy overhead, but I’m increasingly hopeful that we won’t see any storms that are capable of tapping into that wind energy.

The Storm Prediction Center dropped the northern half of the Midstate from the Slight Risk area as of their 11:30am update, so they’re obviously thinking along the same lines:
PAUL SEVERE RISK REGION

We’ll still have to wait and see how the line of showers and storms right along the cold front behaves late this afternoon.  That’s the last wild-card left in the deck for this particular storm system.  But all in all, things have shaped up in a way that makes the “best-case scenario” (outlined below in this morning’s original post) much more likely!

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Posted 9:15am

WEATHER

A complicated forecast today, with at least some threat of severe thunderstorms.  The easy part of the forecast is this: it’s going to rain.  The tricky part is pinning down when the different waves of storms will move through, and how much energy will be available in the atmosphere for each wave to work with.

A diminishing batch of showers and thunderstorms will move in from the west this morning, reaching I-65 by about midday (earlier west, later east).  These storms aren’t going to bring much of a severe weather threat.  Futurecast (using the RPM and HRRR models side-by-side) shows the progression, and also shows how well the two models agree with each other:
RPM 11A THU RPM 12P THU RPM 1P THU

The second, and potentially more powerful, batch of storms will develop closer to the cold front that’s driving this whole system.  Those storms will also move west-to-east, reaching Nashville around late afternoon (once again, earlier west of I-65 and later to the east):
RPM 2P THU RPM 3P THU RPM 4P THU RPM 5P THU RPM 6P THU RPM 7P THU

I’m moderately confident in that progression, but of course Mother Nature loves throwing us curveballs, so we’ll be ready to re-evaluate things on an hour-by-hour basis.  The big questions are:
1) How fast will the atmosphere destabilize this morning?  (i.e. Will it be warm and muggy enough for the first batch of storms to be severe?)  Cloud cover and early-morning showers are definitely helping to limit the warming process, which is a good sign.
2) Will there be enough time and clearing after the first round for the atmosphere to recover and become unstable enough for the second round of storms to be severe?

Those two questions are virtually impossible to answer with any level of certainty, which is why I’m not getting carried away with the phrasing I’m using here and on-air.  Throughout the morning newscast, I used words like “potential” and “possibility” instead of words like “outbreak” — I also used the word “if” a LOT.  It’s a borderline threat, which is why the Storm Prediction Center has categorized it as a Slight Risk (level 2 of 5) of severe thunderstorms, rather than one of the higher categories:
PAUL SEVERE RISK REGION

The SPC’s SREF (short-range ensemble forecast) model shows a 70+% chance of “organized” storms this afternoon:
SREF_prob_combined_supercell__f009 SREF_prob_combined_supercell__f012

How organized?  The Supercell Composite Parameter measures that — SCP values hover around 3 most of today, which really isn’t that bad.  The higher values stay down to our south:
SREF_SCCP_MEDIAN_MXMN__f009 SREF_SCCP_MEDIAN_MXMN__f012

The Significant Tornado Parameter isn’t zero, which makes sense considering the amount of wind energy in the atmosphere today (more details on that down the line).  But STP values around 1 also aren’t that bad, and again the higher values are off to our south:
SREF_SIGTOR_MEDIAN_MXMN__f009 SREF_SIGTOR_MEDIAN_MXMN__f012

This morning’s weather balloon data shows some good news (I’ll explain below the image):
BNA
The rightward squiggle in the red line (circled) is an inversion — a layer of warm air a couple of miles off the ground that will act like a cap, to suppress thunderstorm development.  That cap isn’t a permanent feature, though — it could gradually be eroded throughout the day, particularly as the first showers and storms move through.  But the fact that it’s there in the first place is a good sign.  Another good sign is the presence of relatively dry air near the ground — the dew point at the time of the balloon launch was only 56 (the little green number).  The humidity will really have to increase before a significant severe weather threat could shape up.

A not-so-good sign presented by the balloon information is the wind data.  Winds are strong near ground level, and they get stronger and change direction with height…what we call wind shear.  The wind energy in the atmosphere overhead is more than sufficient to organize and rotate thunderstorms, IF (there’s that word again) the storms can find enough thermodynamic energy to tap into the wind field.  Confused yet?  This is why meteorology is a science of probabilities, not certainties.

With that, let’s talk more about the probabilities.  Every storm system like this brings a range of possibilities with it, from “no big deal” to “EVERYONE PANIC.”
Best-case scenario: clouds and showers this morning prevent the atmosphere from destabilizing, the midday rain further stabilizes things, and the storms along the cold front late this afternoon don’t have enough energy to do anything remarkable.  We don’t even have any severe thunderstorm warnings.
Worst-case scenario: temperatures warm up to the 70s thanks to a few breaks in the clouds, dew points make it up to the 60s.  The first round of storms produces a few severe wind reports, AND it moves through fast enough that the atmosphere recovers in time for the second batch of storms.  The second batch produces widespread wind damage, some large hail, and even a couple of isolated tornadoes.  Likely?  NO!  Possible?  Yeah, barely.  The HRRR model shown above leans a little closer to this possibility, but it’s been heading in a calmer direction.
Most-likely scenario: the first round of storms brings mostly plain ol’ rain instead of actual storms, and it also muddies the waters sufficiently to sap the strength of the later storms.  Some severe thunderstorm warnings are issued with the “main line,” there are a few straight-line wind damage reports, but we mostly get garden-variety springtime thunderstorms.  The RPM model shown above represents this quite well.  Packaging everything up:
PAUL SEVERE RISK GRAPH

We’ll be here all day keeping an eye on things.  All watches and warnings (severe thunderstorm or tornado) are automatically posted to Twitter (links at the bottom of this post).  Severe thunderstorm warnings will show up in the “ticker” at the bottom of your TV screen on Channel 4, and we’ll break into programming if any tornado warnings are issued.

All in all, prepare — don’t panic.  I’m quite optimistic that this will come and go without any significant severe weather around here, but I’m certainly not going to let my guard down.  I’ll share some thoughts on social media throughout the day, and if anything “big” changes I’ll post updates at the top of this post.

Temperatures will be cooler as skies clear out tomorrow, then Saturday is looking simply wonderful.  I wish I could say the same about Easter Sunday:
WSMV 7 Day AM
I’m still optimistic that Easter Sunday morning will be dry, but thunderstorms look likely again late in the day and Sunday night.  A few of those storms could be strong as well, and the Storm Prediction Center has already outlined us for another heightened risk of severe weather:
day4prob
It’s that time of year.

 

LINKS

Like I said above, there’s no need to panic about today’s severe weather threat.  Just pass the time by checking out today’s nerdiness…

  • The same storm system bringing us the severe weather threat today brought over a foot of snow to Denver, shutting down Denver International Airport.
  • We talked about this a little bit on the newscast this morning — a company in Texas tore down the wrong tornado-damaged house.
  • Measuring global temperatures is a complicated process, which means figuring out how much we’ve warmed up is also a complicated process.
  • I found this fascinating: how scientists figured out where the dinosaur-killing asteroid/comet impact crater was located.
  • You know the “man in the moon”?  Turns out that feature didn’t always face Earth.
  • How can astronomers estimate the mass of a planet?
  • What would it mean if physicists discover a new subatomic particle?
  • …but the discovery of such a new particle (“tetraquark”) hasn’t been confirmed by the Large Hadron Collider.  Yet.
  • Now that the existence of gravitational waves has been confirmed, it’s time to put that knowledge to use.
  • Why does it always seem like the technology of sustainable nuclear fusion is 30 years away?
  • Sleeping through the night might be a relatively recent concept.
  • Hidden text in a very old Bible reveals the history of the Protestant Reformation.
  • A leftover link from National Puppy Day yesterday: 7 surprising benefits to dog ownership.

 

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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

Instagram: PaulHeggenWSMV

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About paulheggen33

Morning meteorologist for WNCN-TV in Raleigh.
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