Thanksgiving Week Severe Weather

[Srain]

My eye is from NE TX and points NE. The Ozarks being the main target area, IMO.

[csnavywx]

Undoubtedly going to be a pretty unseasonably late severe weather threat, but I don't find the wind fields all that conducive to either tornadoes and or long lived supercells. First the orientation of the front. The GFS keeps trending slightly more amplified with the low amplitude shortwave passing through the plains and into the Great Lakes Monday night. It seems the models are slightly more SW-NE oriented with the cold front in response and it seems most models are initiating convection slightly aloft along the frontal circulation. The problem is the nearly unidirectional SW flow aloft oriented perfectly parallel to the front with any early rotating storms rapidly transitioning into a line of multicell clusters of thunderstorms and heavy QPF. Low level wind fields are not conducive at all to supporting supercells for a very long time. I still think wind is the main threat, and that will be early on before transitioning into a heavy rain event.

It's going to be at night again, so even that is a bit iffy. Still, low level lapse rates aren't terrible, and there's a good deal of low level speed shear, so if those storms can develop ahead of the front and not get undercut right away, then there's a better shot.

[baroclinic_instability]

It's going to be at night again, so even that is a bit iffy. Still, low level lapse rates aren't terrible, and there's a good deal of low level speed shear, so if those storms can develop ahead of the front and not get undercut right away, then there's a better shot.

Yeah, possibly, but I don't see how they will initiate ahead of the front. Models aren't indicating a perfrontal pressure trough, and with the increased amplitude shortwave, it seems the low level front will be positioned well underneath that extreme wind field. I am not denying the potential, I agree with you, there is a chance, but I don't know about "ominous" as I heard earlier. Not yet at least, based off the information. Also don't like the jet stream configuration with this setup.

[baroclinic_instability]

Unidirectional flow? Check.

Flow parallel to the front? Check.

Result:

Now to be fair, a nasty, nasty line (some 70-80MPH wind gusts) did develop and propogate across nrn IL, nrn IN, srn lower MI, and nwrn OH after the supercell that produced two high-EF3 tornadoes moved over the lake...and another nasty line took shape across the Ozarks as the secondary low moved east during the overnight hours...but not after literally dozens of supercells accosted the I-44/I-55 corridors from Tulsa to Chicago...

Interesting maps, thanks for posting. It does seem, under this event, that convection was initiated along a quasi-stationary front with a much stronger south flow in the low levels. The advancing cold front will be acting in a much different way than that event. As CSnavy mentioned, this cold front will be rapidly propagating underneath the deep SW flow. The low level moisture in this case is not inflow into the eastern part of the storms like your event, and that past event would have been far more supportive of the right movers. Also, the jet stream configuration, at least based off the latest guidance, was far more conducive than this event.

[Hoosier]

Yeah, possibly, but I don't see how they will initiate ahead of the front. Models aren't indicating a perfrontal pressure trough, and with the increased amplitude shortwave, it seems the low level front will be positioned well underneath that extreme wind field. I am not denying the potential, I agree with you, there is a chance, but I don't know about "ominous" as I heard earlier. Not yet at least, based off the information. Also don't like the jet stream configuration with this setup.

Maybe a situation where the window for tornadic supercells is brief, like right after initiation?

[baroclinic_instability]

Maybe a situation where the window for tornadic supercells is brief, like right after initiation?

I am not denying the chance, but it seems very low. There is no SRH like the previous event mentioned, these supercells will be short lived and relatively weakly rotating. They have a very small window it seems. And as CSNavy pointed, they will be undercut quickly as per latest guidance. Things can change though, hence why I mention small chance.

[tornadotony]

Interesting maps, thanks for posting. It does seem, under this event, that convection was initiated along a quasi-stationary front with a much stronger south flow in the low levels. The advancing cold front will be acting in a much different way than that event. As CSnavy mentioned, this cold front will be rapidly propagating underneath the deep SW flow. The low level moisture in this case is not inflow into the eastern part of the storms like your event, and that past event would have been far more supportive of the right movers. Also, the jet stream configuration, at least based off the latest guidance, was far more conducive than this event.

The NAM has a much slower frontal progression than the GFS (by 6-12h starting Monday afternoon), so that detail is far from certain.

Also, regarding SRH, I think 200J/kg+ from 0-1km is quite sufficient...

[csnavywx]

Interesting maps, thanks for posting. It does seem, under this event, that convection was initiated along a quasi-stationary front with a much stronger south flow in the low levels. The advancing cold front will be acting in a much different way than that event. Also, the jet stream configuration, at least based off the latest guidance, was far more conducive than this event.

There was a wicked mesoscale jet circulation going on in that case. Here, we're dealing with what is essentially a large shared energy area as the STJ and PJ interact.

[Hoosier]

I am not denying the chance, but it seems very low. There is no SRH like the previous event mentioned, these supercells will be short lived and relatively weakly rotating. They have a very small window it seems. And as CSNavy pointed, they will be undercut quickly as per latest guidance. Things can change though, hence why I mention small chance.

The NAM is progging 0-1 km SRH around 200 in northern IL on Monday evening, so it bears some watching, but I'm certainly cautious like you right now.

[baroclinic_instability]

The NAM is progging 0-1 km SRH around 200 in northern IL on Monday evening, so it bears some watching, but I'm certainly cautious like you right now.

Yeah I saw that too, I did consider that, but mostly didn't because the NAM, like usual, is slower with the low amplitude wave, and its low level mass fields are slightly more S oriented than the GFS. Wind event though, it still seems pretty good given the extreme low level flow as the line of convection propagates E. Both models rapidly strengthen the dynamic/nocturnal LLJ after 0Z. It seems a slight is still reasonable for wind from central IL into Indiana.

[baroclinic_instability]

The NAM has a much slower frontal progression than the GFS (by 6-12h starting Monday afternoon), so that detail is far from certain.

Also, regarding SRH, I think 200J/kg+ from 0-1km is quite sufficient...

Yeah, NAM is phase shifted slow with regards to the GFS in the upper level height field...it has been doing that the last 4 days with each wave. I don't give it much credit right now since it is too slow 95% of the time when phase shifted back of the GFS. Watch as the NAM slowly catches up to the GFS.

[baroclinic_instability]

The classic NAM phase shift. Note the entire height field, from W coast to E coast is phase shifted W wrt the GFS.

Compare them at the same times.

http://www.twisterdata.com/index.php?prog=forecast&model=NAM&grid=221&model_yyyy=2010&model_mm=11&model_dd=20&model_init_hh=12&fhour=66&parameter=RELV&level=500&unit=MB&maximize=n&mode=singlemap&sounding=n&output=image&view=large&archive=false#

http://www.twisterdata.com/index.php?prog=forecast&model=GFS&grid=3&model_yyyy=2010&model_mm=11&model_dd=20&model_init_hh=12&fhour=66&parameter=RELV&level=500&unit=MB&maximize=n&mode=singlemap&sounding=n&output=image&view=large&archive=false

The NAM usually catches up in the first 24 hrs with each run then falls back behind when it gets into this routine.

[northpittweather]

I am really concerned about this severe weather threat for the first part of the thanksgiving holiday week, i am thinking this might be an major event with the threat of strong-long track tornadoes, some one told me many times it doesn't take that much cape to have a major event in fall, I think the European Model is doing really good with the forecasts now, GFS is just doesn't look right, and all the other models are inline with the EURO right now

I think a high end slight risk or moderate risk caliber event is possible, it would be pretty crazy to have a HIGH RISK on thanksgiving

Map below is my area of interest for Tuesday into Thursday Morning or so

[Stebo]

I personally would side with TT on this one trends have been going up, and if you believe the NAM/GFS's temp profile you will end up being wrong, as its showing maxes about 5-10 degrees below what will end up being true.

[tornadotony]

I personally would side with TT on this one trends have been going up, and if you believe the NAM/GFS's temp profile you will end up being wrong, as its showing maxes about 5-10 degrees below what will end up being true.

The other thing of note is by the time the front begins to charge on the northern flank on the GFS, it will be past the peak severe potential time anyway (around/after 00z).

[Stebo]

The other thing of note is by the time the front begins to charge on the northern flank on the GFS, it will be past the peak severe potential time anyway (around/after 00z).

Yeah and in a fast flow pattern I'm not buying a later solution on this one.

[northpittweather]

The other thing of note is by the time the front begins to charge on the northern flank on the GFS, it will be past the peak severe potential time anyway (around/after 00z).

Tornado Tony, i wouldn't really be leaning towards the GFS at this point, for some reason as of late it has been going all over the place during the Medium Range, i might have to lean for the EURO for now

[Hoosier]

The other thing of note is by the time the front begins to charge on the northern flank on the GFS, it will be past the peak severe potential time anyway (around/after 00z).

I was thinking this event might not really get going until around 00z.

[Stebo]

I was thinking this event might not really get going until around 00z.

I'd say Initiation around 21z across IL is my rough initial guess.

[northpittweather]

it is possible that the storms might last all night and storms might be moving over 50mph, my area of concern with the main event will be IL,IN,Southern MI, OH, Western/Central PA, WV, KY

[northpittweather]

This to me is always a sign that severe weather could be widespread when the storm moves into the eastern us

MESOSCALE DISCUSSION 2052

NWS STORM PREDICTION CENTER NORMAN OK

0802 PM CST SAT NOV 20 2010

AREAS AFFECTED...CENTRAL CA COASTAL REGION

CONCERNING...SEVERE POTENTIAL...WATCH UNLIKELY

VALID 210202Z - 210330Z

SOME INCREASE IN CONVECTIVE INTENSITY OBSERVED OVER THE PAST HOUR OR

SO SUGGESTS A SLIGHT ENHANCEMENT OF SEVERE THREAT. THREAT REMAINS

QUITE LIMITED HOWEVER...AND WW IS NOT ANTICIPATED.

LATEST RADAR IMAGERY SHOWS AN INCREASE IN CONVECTION ACROSS CENTRAL

CA AND JUST OFFSHORE...FROM THE SFO BAY REGION SWD. BAND OF

CONVECTION ALONG THE FRONT HAS ORGANIZED SLIGHTLY...WHILE

PRE-FRONTAL CELLULAR CONVECTION ALSO HAS INCREASED IN BOTH COVERAGE

AND INTENSITY...WITH STRONGER CELLS SHOWING OCCASIONAL WEAK

ROTATION. THE CONVECTIVE INCREASE APPEARS TO BE TIED TO A SLIGHT

INCREASE IN INSTABILITY...AS SOME COOLING AT MID LEVELS OVERSPREADS

CENTRAL CA IN CONJUNCTION WITH THE SEWD MOTION OF THE MAIN UPPER

LOW.

WITH BOTH LOW-LEVEL AND DEEP-LAYER SHEAR PROFILES MORE THAN

SUFFICIENT FOR ORGANIZED/SEVERE STORMS...ANY ADDITIONAL INCREASE IN

INSTABILITY AND ASSOCIATED CONVECTIVE UPDRAFT INTENSITY BEARS

WATCHING. ATTM HOWEVER...EXPECT SEVERE THREAT TO REMAIN

INSUFFICIENT TO WARRANT WW ISSUANCE...BUT A LOCALLY-DAMAGING GUST OR

EVEN A VERY BRIEF TORNADO CANNOT BE RULED OUT.

[Hoosier]

I'd say Initiation around 21z across IL is my rough initial guess.

00z NAM looks like it's a couple degrees warmer at the surface, although one of the things I am noticing (even before this run) is that the models are keeping the deeper moisture south/east of northern IL. The 850 mb dewpoints look kinda pathetic.

[Hoosier]

This to me is always a sign that severe weather could be widespread when the storm moves into the eastern us

When a watch isn't likely and the severe threat is expected to be isolated?

[northpittweather]

When a watch isn't likely and the severe threat is expected to be isolated?

i know its isolated, just saying when they mention severe weather threat in California and it's only in the 40s in that area also, just mentioning

[tornadotony]

I was thinking this event might not really get going until around 00z.

I would agree with that for the Ozarks portion of this event...

[baroclinic_instability]

This to me is always a sign that severe weather could be widespread when the storm moves into the eastern us

I am not really understanding what you are trying to relate here

[tornadotony]

Tornado Tony, i wouldn't really be leaning towards the GFS at this point, for some reason as of late it has been going all over the place during the Medium Range, i might have to lean for the EURO for now

Baroclinic_instability makes some valid points, though, about the GFS...if the front ends up moving through that fast up here, that will significantly diminish the severe wx threat. Typically, though, for a front to progress that fast, I would want to see a more amplified trough...but we'll see. We're still 40-48h out from this potential event, so a lot can change...

[tornadotony]

NAM is even slower with the system and now has upper 60 temps almost up to I-80...still 48h out but an interesting trend...the 00z GFS will be very interesting to see...

[northpittweather]

I am not really understanding what you are trying to relate here

With strong storm systems that have produced widespread severe weather and tornadoes have made some type of impact on the west coast in wind damage or brief tornado or two

[baroclinic_instability]

NAM is even slower with the system and now has upper 60 temps almost up to I-80...still 48h out but an interesting trend...the 00z GFS will be very interesting to see...

Yeah I am interested too. I have a feeling they will both keep their own similar solutions and prolly won't converge much until 0z tomorrow. The NAM did correct its phase shift for the first 24 hours then went slower again to the point it is even slower. Sometimes it is hard to read that model. I still think it will end up being more in line with a faster GFS solution, but who fully knows.