Thanksgiving Week Severe Weather

[Hoosier]

Despite some model differences and the possibility (maybe likelihood) of marginal instability, I am sold on an eventual upgrade to slight risk. Both the 00z NAM and GFS have 900 mb winds cranking up to 50 kts here late Monday night with basically no inversion. Even fairly shallow convection should be able to mix down some strong to severe gusts.

[baroclinic_instability]

Despite some model differences and the possibility (maybe likelihood) of marginal instability, I am sold on an eventual upgrade to slight risk. Both the 00z NAM and GFS have 900 mb winds cranking up to 50 kts here late Monday night with basically no inversion. Even fairly shallow convection should be able to mix down some strong to severe gusts.

Yeah pretty much my thinking too. Differences now really don't matter much, I think a wind event is a pretty good bet and a slight would more than cover that.

[Stebo]

Despite some model differences and the possibility (maybe likelihood) of marginal instability, I am sold on an eventual upgrade to slight risk. Both the 00z NAM and GFS have 900 mb winds cranking up to 50 kts here late Monday night with basically no inversion. Even fairly shallow convection should be able to mix down some strong to severe gusts.

Yeah I could see a 15% for the area that was in the 5% today, and a 15% down to probably Little Rock along the front.

[Hoosier]

Just glancing at the 00z Euro and it has a 1000 mb surface low near Green Bay at 00z Tue. Frontal timing looks to be closer to the GFS, maybe a hair slower.

[Stebo]

Still only see text

SPC AC 210659

DAY 2 CONVECTIVE OUTLOOK

NWS STORM PREDICTION CENTER NORMAN OK

1259 AM CST SUN NOV 21 2010

VALID 221200Z - 231200Z

...NO SVR TSTM AREAS FORECAST...

...SYNOPSIS...

STRONG/BROAD CYCLONIC FLOW WILL BE PREVALENT OVER THE MAJORITY OF

THE CONUS ON MONDAY VIA A GRADUALLY EASTWARD SHIFTING LONGWAVE

TROUGH. WITHIN THIS REGIME...A SHORTWAVE TROUGH IS EXPECTED TO RACE

NORTHEASTWARD FROM THE CENTRAL ROCKIES/HIGH PLAINS TO THE UPPER

MIDWEST/GREAT LAKES VICINITY BY EARLY TUESDAY...WITH A SECONDARY

IMPULSE CROSSING THE CENTRAL PLAINS/MO VALLEY MONDAY NIGHT...ALL

AHEAD OF A MORE SUBSTANTIAL UPPER TROUGH DIGGING SOUTHWARD OVER

BRITISH COLUMBIA/PACIFIC NORTHWEST. THE PRIMARY CONVECTIVE POTENTIAL

FOR THE PERIOD WILL BE TIED TO A DEEPENING SURFACE LOW/EASTWARD

ADVANCING COLD FRONT ACROSS THE MIDWEST/OZARKS/MID-SOUTH.

...MIDWEST/WESTERN GREAT LAKES TO OZARKS/MID-SOUTH...

A STRONG EARLY DAY SOUTHWESTERLY LOW LEVEL JET AND ASSOCIATED WARM

ADVECTION/MOISTURE TRANSPORT REGIME ABOVE THE SURFACE WILL CONTINUE

A NORTHWARD FLUX/PARTIAL RETURN OF A MODIFIED MARITIME AIRMASS INTO

THE MIDWEST...AHEAD OF A DEVELOPING SURFACE LOW/EASTWARD ADVANCING

COLD FRONT. BY AFTERNOON...50S F SURFACE DEWPOINTS WILL BE PREVALENT

ACROSS MUCH OF THE PRE-COLD FRONTAL WARM SECTOR...WITH LOWER 60S F

SURFACE DEWPOINTS ACROSS THE ARKLATEX/OZARKS.

WHILE ELEVATED TSTMS WILL BE POSSIBLE ACROSS THE LAKE MI GENERAL

VICINITY DURING THE MORNING/EARLY AFTERNOON...CURRENT THINKING IS

THAT THE PRIMARY TIMING OF NEAR-COLD FRONTAL TSTMS AND ANY

ASSOCIATED SEVERE THREAT MAY NOT OCCUR UNTIL THE EVENING/PERHAPS

LATE NIGHT HOURS. THIS WOULD BE ROUGHLY COINCIDENT WITH THE EXPECTED

ARRIVAL OF A CENTRAL PLAINS/LOWER MO VALLEY SECONDARY SHORTWAVE

TROUGH/JET STREAK. MARGINAL MOISTURE/WEAK BUOYANCY...AND A NOCTURNAL

POST-SUNSET TENDENCY FOR MUCH OF THE CONVECTION...ARE ALL LIMITING

FACTORS. NONETHELESS...PROVIDED WEAK BUOYANCY OF SEVERAL HUNDRED

J/KG CAPE MONDAY AFTERNOON/NIGHT...A RATHER STRONG LOW/MID

TROPOSPHERIC WIND FIELD AND APPRECIABLY INCREASING DEEP FORCING FOR

ASCENT...ESPECIALLY LOWER MI TO THE CONFLUENCE OF THE OH/MS

RIVERS...WILL BE CONDUCIVE FOR RELATIVELY LOW-TOPPED/FAST MOVING

STRONG/SEVERE TSTMS CAPABLE OF WIND DAMAGE/PERHAPS A BRIEF

TORNADO...AT LEAST ON AN ISOLATED BASIS.

..GUYER.. 11/21/2010

[Hoosier]

Still only see text

No real surprises there. I think there will probably be an upgrade at some point but they may wait until day 1. Guess we'll see in about 12 hours.

[Stebo]

No real surprises there. I think there will probably be an upgrade at some point but they may wait until day 1. Guess we'll see in about 12 hours.

Don't you love how they are always cute with the lines in Eastern Michigan, to them we don't exist.

[janetjanet998]

DAY 4-8 CONVECTIVE OUTLOOK

NWS STORM PREDICTION CENTER NORMAN OK

0400 AM CST SUN NOV 21 2010

VALID 241200Z - 291200Z

..DISCUSSION

00Z ECMWF/GEFS GUIDANCE ARE IN RELATIVELY GOOD AGREEMENT WITH THE

CONTINUED EASTWARD TRANSITION OF A BROAD UPPER TROUGH ACROSS THE

CENTRAL/EASTERN CONUS THROUGH THE MIDDLE TO LATTER PART OF THE

WEEK...WITH A TREND TOWARD A MORE POSITIVELY TILTED UPPER TROUGH

ACROSS THE EAST-CENTRAL CONUS BY AROUND DAY 5/THURSDAY.

THE MAIN FOCUS FOR SEVERE POTENTIAL IS ON THE INITIAL DAY

4/WEDNESDAY PERIOD...AS A RELATIVELY MOIST AIRMASS WILL QUICKLY

RETURN/BE IN PLACE AHEAD OF A CENTRAL STATES UPPER TROUGH/COLD

FRONT. IT DOES SEEM PROBABLE THAT AT LEAST SOME SEVERE WEATHER

THREAT WILL EXIST ON DAY 4/WEDNESDAY INTO DAY 5/THANKSGIVING DAY.

SUCH A SEVERE THREAT COULD INCLUDE PORTIONS OF THE MID-SOUTH/OZARKS

TO THE LOWER OH VALLEY VICINITY. UNCERTAINTY REGARDING THE DEGREE OF

PRE-COLD FRONTAL DESTABILIZATION AT THE DAY 4 TIME FRAME...AND

PERHAPS A LESS THAN OPTIMAL COLLOCATION OF THE GREATEST FORCING VS.

WARM SECTOR PER LATEST GUIDANCE...PRECLUDES CONSIDERATION OF A 30

PERCENT EQUIVALENT RISK AREA AT THIS TIME.

[Indystorm]

I don't know if there have been any formal studies of correlation, but I have noticed over time that when svr warnings are issued in California there seems to be a significant chance of a fairly strong system that will impact the Midwest/Ms valley with significant svr weather. Of course things can change but I tend to think of this as a canary in the mine sort of heads up.

[Stebo]

I don't know if there have been any formal studies of correlation, but I have noticed over time that when svr warnings are issued in California there seems to be a significant chance of a fairly strong system that will impact the Midwest/Ms valley with significant svr weather.

Just as something a lot of us have watched over the years, it tends to be fairly correlated. Normally due to the fact that you need a fairly dynamic system to cause severe weather in California.

[David Reimer]

I'm beginning to pay more attention to Wednesday Evening in the Northeast Texas area. With dewpoints in the mid to upper 60's It won't take much surface heating to get some instability to build. The 12Z NAM has a 45 knot low level jet established over the region, so it's definitely something I'm going to watch.

[patrick7032]

Despite some model differences and the possibility (maybe likelihood) of marginal instability, I am sold on an eventual upgrade to slight risk. Both the 00z NAM and GFS have 900 mb winds cranking up to 50 kts here late Monday night with basically no inversion. Even fairly shallow convection should be able to mix down some strong to severe gusts.

I agree and that was the case with the tors on the superstorm....which that in itself was amazing....how often do you see tor warns in the middle of the line? Answer: about as often as the LIONS making 4 wins a season.

[patrick7032]

No real surprises there. I think there will probably be an upgrade at some point but they may wait until day 1. Guess we'll see in about 12 hours.

Wouldn't surprise me if they waited until the 13z Day 1 to get a grip on the mesoanalysis.

[patrick7032]

Don't you love how they are always cute with the lines in Eastern Michigan, to them we don't exist.

I told ya, it's the "NOOOOO....NOT DTX....WE'RE ALL GONNA DIE....AAAARRRRGGHHH" attitude. If and when they do, it's usually by mistake as they take it back w the next run.

[Indystorm]

I know these maps have to be taken with a grain of salt...nevertheless....hmm...as we now get into RUC range for Monday.

http://wxcaster4.com/ruc/CENTRAL_RUC236_ATMOS_STP_18HR.gif

[janetjanet998]

per 00z NAM for tommorrow

i would go with a slight risk with 5% tornado probs form Chicago to Memphis....S MI may also get into the action

[tornadotony]

I think we're game-on for potentially a few tornadoes across nrn IL and srn WI tomorrow afternoon. NAM and GFS now in fairly good agreement with forming a pool of high low-level CAPE (0-3km CAPE of over 200J/kg on the NAM by 21z tomorrow) in the presence of fairly high helicity (0-1km SRH at or above 180J/kg). Not saying this is going to be a major event, but a supercell or two and a few tornadoes would not at all surprise me tomorrow afternoon. The NAM low-level instability is particularly ominous, though...

[Fred Gossage]

I know these maps have to be taken with a grain of salt...nevertheless....hmm...as we now get into RUC range for Monday.

http://wxcaster4.com...OS_STP_18HR.gif

Just for the record because I see this mentioned a lot by several folks in severe weather threads..... these actual products themselves are useful. It's not that things like the SCP and STP and such are unreliable or need to be taken with a grain of salt. The problem is that a LOT of people will want to look at them for an instant answer to the forecast problem, without looking deeper into the setup to see why the values are what they are... low or high... and what part of the story they may not be telling. Parameters like this are VERY useful, even further out beyond the nowcasting timeframe, but you need to use them a little more like dummy lights than anything else. You need to use them as another source of confirmation of what you already know by studying the surface/upper air map progs and forecast soundings/hodographs. You need to have a good handle on the synoptic and mesoscale pattern at work before looking at the indicies such as this.

[tornadotony]

Looking at the charts from the 00z NAM, most of the CAPE tomorrow will be situated in the lowest 3km on the atmosphere, and the axis of this high low-level CAPE isn't exactly what I'd consider to be skinny. Look at the following maps for 0-3km CAPE at 21z and 00z.

When you look at those maps and then consider that total CAPE is going to probably be in the neighborhood of 500-750J/kg max, then you really get a feel for how volatile the low-levels could be. Any overnight/morning elevated convection that occurs will likely only to enhance any potential threat tomorrow by leaving boundaries to focus moisture/instability pooling and to enhance low-level shear in the warm sector.

[tornadotony]

And the 00z GFS brings 60 Tds to the IL/WI border by 21z....fantastic...

[Thundersnow12]

Fred and I are talking about this as well right now...we agree that if that amount of 0-3 CAPE is close to verifying then there could be some problems. If the updrafts can sustain themselves then things are going to start spinning pretty fast. I also can see temps tomorrow busting on the high side depending on the amount of clouds.

[Hoosier]

Looking at the charts from the 00z NAM, most of the CAPE tomorrow will be situated in the lowest 3km on the atmosphere, and the axis of this high low-level CAPE isn't exactly what I'd consider to be skinny. Look at the following maps for 0-3km CAPE at 21z and 00z.

When you look at those maps and then consider that total CAPE is going to probably be in the neighborhood of 500-750J/kg max, then you really get a feel for how volatile the low-levels could be. Any overnight/morning elevated convection that occurs will likely only to enhance any potential threat tomorrow by leaving boundaries to focus moisture/instability pooling and to enhance low-level shear in the warm sector.

Seems like a lot of the SRH is concentrated in the 0-1 km layer too, at least farther north where I've been more focused. I could see a 5% tornado day...then again, I wouldn't be shocked if they had it at 2% in the 6z outlook.

[Thundersnow12]

[Fred Gossage]

Like Thundersnow12 mentioned, and what we all three have been discussing a little.... that low-level CAPE is disturbing... even if it busts a little too high on the model progs there. The main question would be whether the updrafts can stand up. To put it bluntly... they need to take their Viagra tomorrow. This is going to be a different situation than late last month, where we had excruciatingly strong forcing plowing into marginal instability... with insane vertical shear. While shear and forcing is strong tomorrow, it may not necessarily be too overpowering, especially if mid-level lapse rates are decent... and we have dry air in the mid-levels. Those are usually the two red flag ingredients that make or break a cool season tornado day. Total column CAPE starts to take a back seat as we head deeper into fall... in two ways. One, if it's low... it may be concentrated in the lower parts of the atmosphere, so that there is strong potential energy near the surface, co-located with strong low-level shear. Or secondly, there can be situations where there's 2000 j/kg of total atmospheric CAPE... but it's "skinny" because of marginal lapse rates... and you end up with the same kind of vertical accelerations as you would in a low instability environment.

The low-level shear is there, the low-level instability is there, the low-level moisture is there. We just need adequate mid-level lapse rates (and 6.0 c/km is plenty high enough this time of year) and some dry air in the mid-levels.....

[Moneyman]

I'm probably gonna be a bit too far north I think (about 50 miles north of Milwaukee) should be an interesting event to follow.

[Moneyman]

Yeah, I just checked the 00z WRF and it shows convection here between hours 18 and 24. So who knows at this point.

[Stebo]

From what I have seen on tonight's runs and the fact that both models are busting hard on the current temperatures, with DTW sitting at a balmy 61 at midnight, I would expect a slight risk to be coming out at the top of the hour.

[baroclinic_instability]

I just don't see much in the way of tornadoes. 2% maybe as Hoosier said. I don't know exactly what SPC is thinking, but any counter-rotating supercells that do manage to get going will be undercut by the front for the left-movers and the right-movers won't be supported with much if any effective inflow. Mass vertical shear will likely keep them short lived. Granted tornadoes can form very fast, but it seems momentum transport in wind will be a more effective severe wx producer.

[Hoosier]

Only a see text on the day 1 outlook.

[Moneyman]

WHILE OVERALL SEVERE POTENTIAL SHOULD REMAIN LOW...AFOREMENTIONED

VERY STRONG FLOW FIELD SUPPORTS LOW-PROBABILITY POTENTIAL FOR WIND

AND/OR AN ISOLATED TORNADO OR TWO. IF A FEW STRONGER/ORGANIZED

CONVECTIVE FEATURES CAN OCCUR WITHIN THE LARGER-SCALE FRONTAL

CONVECTIVE BAND...SEVERE THREAT WOULD BECOME ENHANCED LOCALLY.

ATTM...DELINEATING ANY MORE LIKELY AREAS FOR SUCH LOCAL CONVECTIVE

ENHANCEMENT WOULD PROVE DIFFICULT...AND THUS WILL NOT INTRODUCE ANY

SLIGHT RISK AREAS. HOWEVER...A HIGHER-END SEVERE WEATHER REPORT OR

TWO IS POSSIBLE -- DESPITE THE LOW OVERALL SEVERE PROBABILITY...AS A

RESULT OF THE HIGHLY-FAVORABLE KINEMATIC CONDITIONS EXPECTED.

No slight risk yet.