November 29: Another Severe Storm Threat

WEATHER

Last night’s rain has moved off to the east, but we have another chance of very-welcome rainfall on the way late this evening and overnight…the problem is, the rain will be associated with a greater threat of severe weather (compared to yesterday).

Before those storms arrive, we’ll see near record high temperatures in the low 70s:
tuesday-highs
Today’s record high is 75, set in 2006.

The storms will move in from the southwest after sunset — the RPM model has been consistent in its depiction of “elevated” storms (not as likely to be severe) initially this evening, followed by stronger storms south of I-40 in the late evening and overnight, some of which show supercell characteristics:
rpm-6p-tue rpm-8p-tue rpm-10p-tue rpm-12a-wed rpm-2a-wed rpm-4a-wed
“Supercell” is always a word that should grab your attention when it’s used in a forecast — supercell storms are characterized by stronger rotation and a greater potential to become tornadic.  Last night’s storms were not supercells…in fact, they were barely storms!  There wasn’t even enough instability (“storm fuel”) to produce lightning and thunder — but today’s near-record temperatures will help juice the atmosphere for more-unstable conditions (whether the humidity will rebound is a separate matter).  The HRRR model shows a messier set-up, which would be fine — the less-organized the storms, the less likely it will be for them to become severe:
hrrr_2016112912_ref_nashville

Here’s where it gets complicated — we don’t know exactly where the center of this storm system is going to go.  Some forecast models, like the HRRR, keep it on a more southerly path (good news!), others push it farther north (bad news)…so, we’re dealing with probabilities rather than certainties.  To evaluate the odds of a significant severe weather threat, we need to figure out the probability that 1) storms will occur in the first place, 2) that they’ll occur in an unstable environment, AND 3) that there will be enough wind energy (“shear”) to rotate and organize the storms.  The Storm Prediction Center’s short-range ensemble forecast model (SREF) merges the output from over 25 different models, so we can map the probability that all three of those factors will occur this evening:
sref_prob_combined_0-01_eshr30_500__f024-2
That map shows a 70+% chance of supercell ingredients along and south of I-40 this evening.  Notice how quickly the chances decrease as you go farther north of I-40 — that’s why the path of the storm system matters so much.  A 10 to 15-mile “wobble” in either direction would make a big difference!  To (maybe) simplify that a bit, look at the SREF’s estimate of the “significant tornado parameter” (a statistic that should be pretty self-explanatory):
sref_sigtor_median_mxmn__f024
The farther south you go, the greater the odds are that you’ll have to worry about damaging weather this evening and tonight.  None of the severe weather ingredients are forecast to be off-the-charts, but “good enough” is all that the atmosphere requires.

The SPC’s official convective outlook outlines an Enhanced Risk (level 3 of 5) of severe thunderstorms for southwestern Middle Tennessee, with a Slight Risk (level 2 of 5) for most of the rest of the Midstate:
paul-severe-risk-region
I wouldn’t be surprised if the Enhanced Risk area is expanded to include more of southern Middle Tennessee…but I think the I-40 corridor will remain outlined in the Slight Risk region.  (I’ve been mentally looking at I-40 as the dividing line between “probably not” and “better keep an eye on this.”)  The SPC posts updates at 10:30am and 2:00pm, and I’ll share those on Twitter and Facebook when they come out (social media links are at the bottom of this post).

Let’s not lose fact of the good news in all of this — we’re going to get more rain!  The RPM model’s rainfall estimate shows widespread 1″-2″ accumulations, with the spots in red picking up closer to 3″ of desperately-needed rainfall:
paul-rpm-12km-precip-accum

A few points to sum things up for this evening and tonight:
– It’s likely to rain, and we’re much more likely to see/hear genuine thunderstorms.
– The storms that occur before midnight will have the highest tornado potential, with the straight-line wind threat taking over later in the night.
– The greatest severe weather threat will occur farther south of I-40, but even there it’s not guaranteed to happen.
– The lowest severe threat will occur in southern Kentucky, but even there you’re not completely in the clear.
– Plan on staying weather-aware this evening and tonight.  A conditional severe weather threat like this (i.e. a lot of “ifs” and “howevers” involved) shouldn’t cause you to panic, but it should at least have your attention.

The storms will be gone by early Wednesday morning, leaving us with calmer and cooler weather for the rest of the work week:
wsmv-7-day-am
Another chance of rain heads our way for the weekend — I’ve definitely tilted my forecast toward the wetter European forecast model, which shows widespread rain Sunday and Sunday night:
ecmwf_apcp_f138_us
The American GFS model shows some rain, but not nearly as much:
gfs_op_apcp_f132_us
Not only is the European model more reliable in general, it’s also been more consistent in terms of how it’s been depicting this particular scenario.  But, as always, time will tell!

The nerd-links will return tomorrow…I think the severe weather breakdown is enough geekiness for everyone today.

——————–
Social media links

Twitter: @WSMVweather, @PaulHeggenWSMV, @WSMVLisaSpencer, @WSMVDanThomas, @daphne_deloren, @NWSNashville

Facebook: 4WARN Weather, Paul Heggen WSMV, Lisa Spencer, Dan Thomas WSMV, Daphne DeLoren, NWS Nashville

Instagram: PaulHeggenWSMV

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About paulheggen33

Morning meteorologist for WNCN-TV in Raleigh.
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