Potential Severe Wx Outbreak Fri/Sat in the Mid-South

[jkt21787]

NAM and GFS both indicate a classic pattern for severe weather including tornadoes Friday into Saturday from the Ozarks all the way to the Appalachins, extending northward into the Ohio Valley south to perhaps the Gulf Coast. Both show deepening lows (~995 mb) as it moves east into the Ohio/Tn/Lower MS Valley. Strong instability develops ahead of the associated cold front with capes progged off the NAM at 2500+, Lis -5 and below, SWEAT 500+, in addition to steepening lapse rates. Powerful jet dynamics are also expeceted. Wind shear is also strongly evident, with helicity values over 200 with some areas in the western OH Valley peaking at 500+ at 0z Sat, and EHI 3+. Moisture return is indicated to be quite sufficient with tds 60+ and some areas approaching 70. This was recently overestimated with the most recent event, but in any event this would not be a problem. The Euro shows a slightly different setup, keeping some of the main energy further west, and limiting the severe weather threat, but for now am going with NAM/GFS.

This all points to the potential for big time severe thunderstorms, including supercells and tornadoes. The best threat, including maybe some long tracked and damaging tornadoes would appear to be Eastern AR and Southeast MO Friday Evening, however a tornado threat could exist everywhere. This should spread east on Saturday into the Southeast. One thing of concern would be wind profiles along the front, and whether they go unidirectional. as well as the amount of forcing. This may lead to more linear organization, but its too soon to say for sure.

Storm motions also appear to be very fast, so chasers in this area (there are actually some in these parts) could have a hard time with this one. But, if the models are right, chasers are probably going to go out anyway and not miss this opportunity.

Of course 4 days out plenty of things can change, but I feel the threat appears to be quite significant, as many others are saying. I am not a met, just an enthusiast, but know severe weather patterns, and this one is. Pro chasers and mets are also commenting, and are saying much the same, even making comparisons to some of the big outbreaks from the springs of 1995 and 1998.

Should be very interesting to watch this as it unfolds. Those in the area keep in touch with later forecasts as the timing and threat is fine tuned.

[ALhurricane]

IF the moisture return is sufficient enough, then look out! This would most likely be a big outbreak fro the middle and lower MS valleys.

However, I am going to hold back from barking too loud right now. The reason being is that the models this spring have been TERRIBLE on forecasting dewpoints. Time and time again they have been overforecasting dewpoints. Having said that, if the return flow sets up in earnest by 12z Thursday, then I think there will be ample time for the dewpoints to recover.

Just taking a first look at things, it does look like a significant event may be on the way.

[azsnowman]

Yup....wait until our next system exits the Rockies and heads east, "Look OUT!" I've already posted the AFD from my area in another thread, this looks to be a HUGE SYSTEM!

Dennis

[Brent]

This is DAY THREE!!!



DAY 3 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0231 AM CDT WED APR 27 2005

VALID 291200Z - 301200Z

...THERE IS A SLGT RISK OF SVR TSTMS LOWER/MIDDLE OH VALLEY TO
ARKLATEX REGION AND CENTRAL GULF COAST...

...SYNOPSIS...
PERSISTENT CYCLONE ALOFT -- NOW OVER NWRN ONT -- IS EXPECTED
RETROGRADE WWD ACROSS MB BEFORE PERIOD...THEN BECOME QUASISTATIONARY
AS SERIES OF NRN STREAM SHORTWAVES MOVE THROUGH CYCLONIC FLOW THAT
WILL COVER MOST OF CENTRAL CONUS. PRONOUNCED MID/UPPER LEVEL LOW --
NOW EVIDENT IN MOISTURE CHANNEL IMAGERY OF PACIFIC COAST...IS
EXPECTED TO DEGENERATE INTO OPEN WAVE TROUGH AS IT MOVES INLAND
EARLY DAY-2...THEN MOVE RAPIDLY ACROSS 4 CORNERS AND SRN ROCKIES
REGIONS. POSITIVELY TILTED TROUGH SHOULD CONTINUE EWD ACROSS SRN
PLAINS TOWARD OZARKS BY 30/00Z...THEN ACCELERATE NEWD AND ASSUME
NEUTRAL TO SLIGHTLY NEGATIVE TILT ACROSS LOWER-MIDDLE OH VALLEY
OVERNIGHT.

ASSOCIATED SFC FRONTAL-WAVE LOW SHOULD DEEPEN THROUGHOUT MOST OF
PERIOD AS IT LIFTS NEWD FROM SRN OK ACROSS AR TO VICINITY WRN OH/ERN
INDIANA. MEANWHILE SFC COLD FRONT SHOULD MOVE SWD ACROSS OK AND N
TX AND EWD ACROSS ERN OK/WRN AR BY 30/00Z...WITH DRYLINE SWWD ACROSS
ERN/SRN TX. WARM FRONT SHOULD LIFT NWD ACROSS LOWER OH VALLEY
REGION.

...ARKLATEX TO OH VALLEY...
POTENTIALLY SIGNIFICANT SEVERE WEATHER EVENT POSSIBLE...ESPECIALLY E
OF SFC LOW DURING DAYLIGHT HOURS...WHERE VERTICAL SHEAR/BUOYANCY
COMBINATION SHOULD BE MAXIMIZED. PRELIMINARY GUIDANCE INDICATES
SOME THREAT FOR TORNADOES...POTENTIALLY LONG-TRACK GIVEN STRONG
DEEP-LAYER FLOWS AND ANTICIPATED FAST CELL MOTIONS...WITH DAMAGING
WIND ALSO A SIGNIFICANT CONCERN. RELATIVE WEIGHT OF WIND/TORNADO
THREATS WILL BE DEPENDENT STRONGLY ON DOMINANT CONVECTIVE MODE --
LINEAR VERSUS SUPERCELLULAR -- WHICH IS TOO STRONGLY TIED TO
MESOSCALE PROCESSES TO CALL THIS FAR OUT IN TIME. HOWEVER...SOME
PART OF THIS OUTLOOK AREA MAY REQUIRE CATEGORICAL UPGRADE IN LATER
UPDATES ONCE SOME UNCERTAINTIES ARE LOWERED REGARDING BOTH
CONVECTIVE CHARACTER AND PLACEMENT/TIMING OF STRONGEST AFTERNOON
DESTABILIZATION.

PRECURSORY/DAY-2 SFC LOW -- PROGGED TO DISSIPATE OVER OH VALLEY BY
29/12Z -- SHOULD INITIATE MOIST RETURN FLOW PROCESS THAT WILL
CONTINUE THROUGH THIS PERIOD S AND SE OF DAY-3 LOW. THIS SHOULD
CULMINATE DAY-3 IN FAVORABLE LOW LEVEL MOISTURE BEING OVERSPREAD BY
STRENGTHENING VERTICAL SHEAR. 60S F SFC DEW POINTS SHOULD BE COMMON
OVER WARM/MOIST SECTOR WITHIN ABOUT 150-200 NM E OF COLD
FRONT...SWWD TO JUST AHEAD OF DRYLINE. RELATIVELY VEERED SFC FLOW
IN MUCH OF MOIST SECTOR MAY REDUCE DRYLINE CONVERGENCE AND
CONCENTRATE CONVECTIVE LIFT ALONG/AHEAD OF COLD FRONT...AND INVOF
SFC LOW. HOWEVER STRONGEST PRESSURE FALLS AND LARGEST HODOGRAPHS
WILL BE NEAR AND E OF SFC LOW...AND ALONG/S OF WARM FRONT...AS
CYCLONE TRACKS NEWD ACROSS WRN/NRN PORTIONS OF OUTLOOK AREA.

AS MOISTURE CONTINUES TO INCREASE OVERNIGHT AND COLD FRONT SWEEPS
EWD ACROSS LOWER MS/TN VALLEYS...SEVERE CONVECTION MAY BUILD SWD
TOWARD GULF COAST.

[ChiTownMC]

From my NWS discussion:

Friday and Friday evening continues to appear as a potentially sig severe
event well south of the County Warning Area across portions of the lower Ohio
River valley and arklatex.

The mets in Chicago are seeing this pattern.

[ALhurricane]

Well the 12z April 27 runs of the GFS and ETA are very similar. There is a little disagreement with the track of the low, but this far out it is not that huge of a discrepancy.

If these model runs are correct (and they have been saying the same thing for days now) then this is what Midsouth tornadic outbreaks are made of. If the dewpoints can reach 65 or so (both models progging that) then long track tornadoes are certainly possible. There will also be squall line likely after that with damaging winds as a big threat.

The big thing we will have to watch is the strength of the return flow. Keep in mind that on the morning of May 4th, 2003, Jackson, TN was sitting with dewpoints in the 50s. By that night they were in the 60s and had an F4 tornado. Now I am not forecasting anything like that. All I am saying is that there is definitely a big potential here. SPC and the local WFOs are talking about this very well and definitely have a good handle on it.

[krysof]

when will it get severe in the northeast?

[wx247]

This is looking to be a big deal just east of here, but I am watching this closely as well.

[PurdueWx80]

Keepin' a close eye on this for my parents in Louisville - severe weather is the last thing they need after insurance nearly dropped them last summer (tornado and lightning damage within a month of the other).

The wind field is about as good as it gets for supercellular development, and the strength of the low/mid and upper level jets suggests extremely large hail (if instability gets up there) and some strong tornadoes are likely. I honestly don't think moisture and the resulting instability will be a problem...the progged surface trajectories seem to point to air coming straight up from the southern Gulf, where DP's are in the 70's now. Model QPF in the 2-3+" range will likely pan out as well, and I'm sure a few places could see 4 or 5 inches where training prevails.

[jkt21787]

My thoughts continue exactly as they were yesterday, a signficant severe weather outbreak including tornadoes, some strong and long tracked.

Conerns remain about moisture return (td is 33 at KMEM this afternoon), but things should change quickly by tomorrow morning. Also watching the wind profiles along the front, as they suggest quick transition to squall line. Even so, I believe at this point there should be several hours of supercell storms before a big squall line.

The potential is there for an event that rivals some of the Mid-South's bigger events to ever happen in recent years. Again, its potential at this point, but with each model run confidence continues to increase.

[Windy]

I'm somewhat concerned for Memphis at this point. If this verifies, this is an "Armageddeon" class tubefest with a bullseye right on their general area.

[wlfpack81]

Same here Windy. My 2 sisters and 2 nieces live there and it looks like I'll be calling them tomorrow to give them the heads up for Friday. I'll be doing a lot of radar watching etc. come Friday.

[PurdueWx80]

I think we may see a lot of models shift over the next 48 hours, because the upper level system responsible for the future development of the surface low is still out over the poorly sampled ocean west of California. Many of the non-US models show solutions that differ quite a bit from the NAM and GFS - so even though they have been VERY consistent over the past couple of days, I'm not entirely sure how accurate their solutions are.

Another thing I'm interested in following is the VERY THICK smoke now south of the front over Mexico and the Southern GOM - there are rampant wildfires over much of central and southern Mexico right now. The front has pushed the smoke to the south, but as soon as the return flow starts, this will head north straight towards the United States. I would imagine skies could become relatively hazy because of this. I'm still trying to figure out if the smoke particles will act as condensation nuclei (more clouds and heavy precipitation) or if they will hamper the solar insolation, acting to reduce instability on Friday. Skies are currently quite hazy south of the front, so it will be interesting to see tomorrow what happens to all that smoke. Stay tuned!

[jeff]

I think we may see a lot of models shift over the next 48 hours, because the upper level system responsible for the future development of the surface low is still out over the poorly sampled ocean west of California. Many of the non-US models show solutions that differ quite a bit from the NAM and GFS - so even though they have been VERY consistent over the past couple of days, I'm not entirely sure how accurate their solutions are.

Another thing I'm interested in following is the VERY THICK smoke now south of the front over Mexico and the Southern GOM - there are rampant wildfires over much of central and southern Mexico right now. The front has pushed the smoke to the south, but as soon as the return flow starts, this will head north straight towards the United States. I would imagine skies could become relatively hazy because of this. I'm still trying to figure out if the smoke particles will act as condensation nuclei (more clouds and heavy precipitation) or if they will hamper the solar insolation, acting to reduce instability on Friday. Skies are currently quite hazy south of the front, so it will be interesting to see tomorrow what happens to all that smoke. Stay tuned!

Ah yes, the smoke. We dealt with it last Thursday and Friday here in SE TX. It did hamper daytime heating last Friday with forecasted highs in the mid to upper 80's and actual highs in the low 80's. A very hazy and extremely filtered sun.

[Windy]

The 0Z NAM makes things look pretty nasty for AR/AL/TN intersection for 0Z Saturday.

Classic triple point:





Td's in the 60s:



Truely rippin' LLJ:



Strong veering winds in the lower atmosphere:



Rip-roarin' 500mb winds:



AND a jet maxima with divergence right over the region:



Makes we wish I could be there. Good model run consistancy on this one, and GFS and NAM agree. I would be shocked as a cow on an electric fence if SPC didn't got to MDT with tonight's Day 2 and go to HIGH with a PSWO Day 1 tomorrow night.

[PurdueWx80]

Actually, the best divergence, in the LFQ of the jet, is closer to the Paducah, KY area (a bit north of Memphis) although there is decent diffluence further south from a quick look of the 250 mb prog.



In fact, much of the Ohio Valley would be in a favorable double jet structure (which will aid the rapid intensificaiton of the surface low, and will add to the flooding potential). Storms will still form in the front right quad of the upper jet, but the best support for vertical motion is to the left of the jet axis. What that could do is allow for more discrete storms in AR/TN (as opposed to storms that would rapidly congeal into a derecho).

[wlfpack81]

Man this is looking worse and worse by the moment for the Memphis area. I have a feeling too windy that a MDT risk for Friday will eventually be put out by the SPC.

[PurdueWx80]

Yeah, this outbreak has high risk written all over it (and it should be justified this time as long as very little changes from tonight's NAM run). What's interesting is that the 13-km RUC (which is about to become operational) shows the surface low much further north (near St. Louis) by Friday afternoon. This model is by far the best I've seen out to 24 hours, but is a bit lacking further out (only goes to 48)...probably because the mid-level system is still outside the model domain. If that's the case, the mid-MS valley would get in on more action (the upper jet strongly favors subsidence over the lower- MS valley in that case). I guess we'll see on Friday what pans out.

If anyone understands skew-T's and hodographs, check out http://www.wxcaster.com/model_skewts.htm. Enter in the 4 letter station identifier for whatever city you want, and you can get both of these charts from the NAM, GFS or MM5 out to 48 hours. Try the 42, 45, and 48hr progs for ksdf, kbwg, kbna, kpah, klit, and kmem (Louisville, Bowling Green (KY), Nashville, Paducah, Little Rock, and Memphis respectively). The hodographs (and resulting SRH) near the warm front are incredible, but the NAM shows very little CAPE that far north.

KMEM has decent helicity values at 45 and 48 hours (TONS of CAPE) but the SRH values are generally high due to the speed of the low/mid level winds (mostly unidirectional flow = very strong winds at the surface, high CAPE and low LI = giant hail).

[Windy]

Here are some more eyepopping SVR index parameters. All of these are taken from the 0Z NAM and are for 0Z Saturday:

0-1KM EHI



SIGSVR Index:



SIGTOR (pegging >7 in TN!!):



Supercell Composite:



CAPE (solid colors), SWEAT index (red lines), Helicity (blue lines):



Total Totals bullseye:



One thing I noticed is just how freakin' fast these storms will be moving. Maybe I'm glad I'm NOT chasing, as it would be very hard to keep up with a torndao clocking 60mph:

[wlfpack81]

PurdueWx80 thanks for that link. Other than using Bufkit I didn't know of any other ways to get model soundings.