Storm Prediction Center has highlighted a broad area to our west during their Day 6 outlook:
Notice our area has yet to be included. That’s partly because model consistency takes a huge hit after the Day 6 timeframe. What we do have though, and models HAVE been fairly consistent about this, is a large, dynamic system coming in across the Rockies and High Plains. Typically, this type of system has causes some weather-related trouble across the Deep South and Mid MS/TN Valleys. Is this to say we’re going to have some crazy weather next week? Certainly not, but indications are that this upcoming system will be much more dynamic and the “mandatory” atmospheric parameters we look for to support nefarious weather will be in abundance…more so than yesterday’s system. On the flip side, this whole outlook can get flipped on it’s head if one or two large-scale factors change. We’ll be watching to see if they do.
I think we’re finally honing in on an ETA for the upcoming storms…
Question is, how strong will they be when they get here? Let’s take a look at some data from the SREF – Short Range Ensemble Forecast model:
500 j/kg worth of CAPE is usually enough to support winter-time severe weather events, but given that spring-time atmospheric machinations are more robust, more instability is needed to sustain severity.
Dew points are progged to be between 55-60F. That’s some relatively moist air that these storms are going to run into. Typically, the higher the dew point, the “richer” the storm fuel. Think about it in terms of gasoline octane: 60F dew points are equivalent to 87 octane; 70F dew points are equivalent to 89 octane; and 80F dew points are equivalent to 93 octane. Bottom line, the air should be moist enough to sustain storm severity.
An integral component of any storm system and it’s severity is the low level jet. Seen on the above map, there appears to be an area of 40kt winds streaming into the storm system upon arrival. This, coupled with the upper level jet, should help to maintain storm severity.
This is a rather complicated storm system and trying to determine an accurate forecast has been akin to herding cats…it sounds easy in theory, but is rather difficult to put in practice.
What do I expect? I expect a squall line roll through the area around lunch time, bringing with it heavy rain, thunder, and possibly a few tornado warnings, given the adequate amount of atmospheric shear. Where those tornado warnings show up, though, is a whole ‘nother ball game and I forgot to bring my glove.
The Storm Prediction Center has seen fit to include our area in both their Day 2 and Day 3 SLIGHT risk areas. This isn’t because we expect two days of severe weather, but because the impending arrival of our storms will straddle two days (Wednesday-Thursday). Here are the graphics:
Day 2 graphic:
Day 3 graphic:
We’re still expecting a QLCS – weather speak for squall line with damaging winds and some hail. Tornadoes are a possibility, as well, due to the amount of shear in the atmosphere.
Timing issues continue to confound the weather gurus behind the curtain at the NWS. One model. the Global Forecast System (GFS) paints an afternoon arrival, the North American Mesoscale (NAM) shows a mid-morning arrival, and the European model (Euro or ECMWF) shows an afternoon arrival. See the images below:
GFS – 1:00 pm
NAM – 10:00 am
Euro – 4pm
System ETA is all over the board and the severity of the system is highly dependent upon the atmospheric parameters it runs into. As with virtually any severe weather episode we see, small-scale features that the models can’t pick up on yet will ultimately make or break it. If for any reason the weather models are underestimating instability values, wind shear values, atmospheric moisture content, etc., the impact of the event could be enhanced. We should know what kind of atmosphere we’re going to have to work with after the NWS weather balloon launches tomorrow, and this should help to nail down an arrival time.
The Storm Prediction Center has highlighted portions of Mid TN in a SLIGHT risk for late Wednesday (Day 3):
The risk area stops just west of I-65, but we fully expect to see it shift eastward as we get closer to midweek. Here’s an excerpt from the SPC’s Day 4-8 Convective Outlook discussion:
SLIGHT-RISK EQUIVALENT SEVERE POTENTIAL SHOULD PERSIST
INTO D4 AND PERHAPS EARLY D5 MAINLY ACROSS PARTS
OF THE SOUTHEAST.
HOWEVER...CONTINUED EVOLUTION DIFFERENCES WITH THE
SYNOPTIC PATTERN ALONG WITH LESS CONFIDENCE IN THE
DEGREE OF INSTABILITY IN THE WARM SECTOR AIR
MASS AHEAD OF AN EWD-PROGRESSING COLD FRONT
RENDER SEVERE PROBABILITIES BELOW 30 PERCENT ATTM.
There remains some weather model discrepancies concerning the arrival of the weather system. Some data suggests a Wednesday afternoon arrival, while some suggest a Thursday afternoon arrival. The NWS suggests a possible ETA of Wednesday night into Thursday morning. This nocturnal arrival helps to further lessen the significant tornado threat, but does little to lessen the OVERALL tornado threat. As with any severe weather event, there’s an inherent chance of a tornado, and this is just the case. There appears to be enough “spin” in the atmosphere that any mature storm cell could begin to rotate. Take the time now to replace your weather radio batteries, ensure you have multiple modes of media at your disposal that can retrieve pertinent weather information, and ensure a safe and workable disaster plan.
After taking the weekend off to enjoy a bit of family time, enjoy the springtime weather, and let the models catch up to this system, it’s time to jump back in it. Data is still spread concerning the more finer-scale issues (instability, wind shear, moisture return), but most of the more “popular” model suites have all hit upon the same basic evolution of the upper level trough. There still could be some shifts and dips, but wholesale changes, IMO, are pretty much done.
Virtually all model suites – GFS, NAM, European, SREF – paint a powerful QLCS through the area with an estimated ETA overnight into the morning on Thursday for the Nashville area. Given the overnight arrival for the area, the overall significant tornado threat will be minimized.
However, that’s not to say there WON’T be a tornado threat. Instability values look to wane as the night progresses (starting out with ~1500j/kg, but dips to 500-750j/kg across West TN and about the same for us), but a 50kt low-level jet ramps up during frontal passage. Combine that with ~50kt worth of 0-6km bulk shear (a measure of atmospheric “turning”, any convective complex that can become sufficiently rooted near the surface could rotate.
I know there was earlier discussions about a warm sector tornado outbreak, but I just don’t see the evidence for that right now. If temperatures and dew point values verify, the LCLs (cloud bases) will be a tad bit higher than I’d like to see them and capping issues will undoubtedly be an issue given the lack of lower level forcing needed to effectively erode the cap. I think this will still be an issue even if a secondary surface low forms given the rather close proximity of it’s progged location. Shear and surface convergence will surely increase, but there just won’t be enough time between cyclogenesis and it’s passing to have much impact.
Bottom line, I expect a loud and robust QLCS – squall line – (there could even be some embedded supercellular-like structures) with an attendant tornado chance. Wind fields don’t look too impressive, so the damaging wind threat looks to be mitigated, except for thunderstorm wind gusts. Don’t think hail is going to be an issue given the meager instability values. We’ll see how the models play this all out over the next 48 hours, and I reserve the right to change my thoughts given any changes.
As previously mentioned, the mid MS Valley is under the gun for a potential severe weather outbreak by the middle of next week. Several different players are involved this potential event and they’re all suited up…we just don’t know exactly where they’re going to play. Confounding matters is the fact that one of the two more prominent model suites we use is in it’s “dead zone”, in other words, it’s having a hard time determining where – or even how – each respective player will line up. I fully believe, however, that this resolution problem will be resolved by the weekend.
While it is too early to determine storm magnitude, I will say that there could be a sizable tornado threat with this event, but where that threat area ends up being is determinate upon the respective players and how they end up lining up. In my own personal opinion, anyone from Birmingham, AL northward to Ohio will be under the gun. Please take the time NOW to review your disaster plan, change your batteries in your weather radios, and ensure you have multiple forms of media at your disposal to receive weather information.