Mar 18-24 Plains/MW/OV/GL Severe Weather, SPC Risk Level: Day 1 Slight Risk - Forecasts & OBS Options

[MDH]

Strong trough moves through beginning on the 19th. It appears early on as though this may be a major outbreak if pre-frontal shear can remain strong enough.

Two of the long term models tend to disagree on the exact trajectory of the front or its speed but both seem to agree in intensity and the level of moisture return over most of the eastern United States.

This post has been edited by jdrenken: Mar 14 2012, 12:42 PM

[WeatherMonger]

DAY 4-8 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0325 AM CDT TUE MAR 13 2012

VALID 161200Z - 211200Z

...DISCUSSION...

LATEST GFS AND ECMWF BOTH AGREE THAT A WRN U.S. TROUGH WILL EVOLVE
THIS WEEKEND IN RESPONSE TO A FAIRLY STRONG BLOCKING HIGH THAT WILL
DEVELOP OVER THE ERN U.S. HOWEVER...THERE IS CONSIDERABLE
DISAGREEMENT REGARDING THE EWD MOVEMENT OF STRONGER MID-HIGH LEVEL
FLOW INTO THE PLAINS WHERE STRONGER SHEAR WOULD NOT ONLY SUPPORT BUT
ENHANCE THE PROSPECT FOR SEVERE STORMS GIVEN THE EXPECTED
MOISTURE/INSTABILITY. THE ECMWF IS CONSIDERABLY SLOWER WITH HEIGHT
FALLS ACROSS THE ROCKIES AND THUS STRONGER FLOW IS EXPECTED TO
ADVANCE NO FARTHER THAN WY/CO/NM BY THE END OF THE PERIOD. FOR THIS
REASON WILL NOT INTRODUCE A SEVERE RISK UNTIL TIMING IS RESOLVED
WITHIN THE MODEL GUIDANCE.

..DARROW.. 03/13/2012

[Illinois blizzar...]

[Illinois blizzar...]

Henry..
Late in the weekend and early next week, severe weather in the Plains will become the focus.


This post has been edited by Illinois blizzard: Mar 13 2012, 10:17 AM

[Superstorm93]

BIG potential for this one.

Flashbacks of May 2007

[Illinois blizzar...]

Reed Timmer: Meteorologist and Extreme Storm Chaser
Looks like March 18-20 could be significant dry line severe weather outbreaks each day for the same areas in southwestern Great Plains. Looks like west Texas will get active again. It's been awhile.

[The Snowman]

For anyone interested...
A little write-up I made on my blog.

I am watching the threat for a potential severe weather event next week, mainly from the weekend into about midweek. It will be such a long timeframe because it will take a while for the system to dig south enough to eject into the Southern Plains.

This storm has many characteristics that put me on edge. First, it will be a negatively tilted storm. This means that cold air will be overrunning the top of the system way up in the atmosphere. As we know, instability is created when warm air rises into air cooler than the warm air at the surface is. If the cold air overruns the system, it adds more cool air to the mix, thus increasing instability. This is why negatively tilted storms usually produce the worst severe storms ("usually" meaning they typically have the most potential to).

Secondly, we will have a strong ridge in the East US. This means warm temperatures for the entire area east of the Rockies. However, when one gets to a ridge of this magnitude, it means that there are strong mechanisms both keeping it in place and supporting that strength. This is when theta-e comes into play. In simplest terms, higher theta-e increases instability in the atmosphere. Seeing the strong ridge makes me worry that there will be a strong theta-e, adding to the instability.

Next, we focus on a more important piece- wind shearing. The system in question is forecasted to be digging (strengthening) strongly as it progresses eastward. This is reflected on the image above, with the lines on the images being dragged south and around the mass of colors, which indicates the presence of the storm system. Notice how these lines then shoot vertically north into the high Plains. This is because of the ridge. However, because there is such a tight gradient (difference) between the two items in play (storm system and the ridge) shown by the lines, there will be some shearing.
To me, this is a very good set-up for wind shear. A strong ridge is rubbing against a strong storm system, and this friction is one of the classic textbook scenarios for quality wind shear. Now, I take a look at the GFS forecast for the same timeframe as the image at the top of this post (hour 180), and my fears are confirmed- upwards of 70 knots of deep layer shear (DLS) is possible in the area. DLS is absolutely crucial for tornadoes, and when you get into this high of shearing wind speeds, serious things may happen.
Another way you can tell that there will be high winds are the big wind barbs that have a bunch of lines sticking out of them in the area circled in red. The more lines (or opaque triangles) on a wind barb indicates a higher wind speed. Wind barbs with one small line mean a very low wind speed.

The area I have encircled in red identifies the area I believe is at risk based on the current forecast. Of course things will change (possibly dramatically), but for now, the area circled appears to be the epicenter for the highest instability and wind shearing- the two ingredients necessary for severe thunderstorms and tornadoes.

I have a bad feeling about this after seeing all of these clues put out by the GFS. If it does verify, I am definitely concerned for those that may be affected.

[Illinois blizzar...]

For anyone interested...
A little write-up I made on my blog.

Great write up, Snowman

[Illinois blizzar...]

Reed Timmer

Reed Timmer: Meteorologist and Extreme Storm Chaser

Latest NAM showing tornado potential in the Texas Panhandle afternoon-evening Friday, March 16. See the composite precip forecast for 00z. Main trough still back in the Pacific. That hurricane looking deal is going to cause major severe weather outbreaks March 18-20

[Illinois blizzar...]

00z NAM has a 983mb low off the NW coast late Friday. This will cause the outbreaks.


This post has been edited by Illinois blizzard: Mar 13 2012, 11:24 PM

[Illinois blizzar...]

New Day 5

DAY 4-8 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0341 AM CDT WED MAR 14 2012

VALID 171200Z - 221200Z

...DISCUSSION...

LATEST MEDIUM RANGE GUIDANCE IS IN REASONABLE AGREEMENT REGARDING
THE EVOLUTION OF THE WRN U.S. TROUGH THIS WEEKEND. BOTH THE GFS AND
ECMWF ALLOW STRONGER FLOW ALOFT TO SPREAD ACROSS THE ROCKIES INTO
THE HIGH PLAINS BY DAY5. GIVEN THE EVOLUTION OF THE LONG WAVE
TROUGH IT APPEARS THAT A BROAD WARM SECTOR WILL HAVE SPREAD ACROSS
MOST OF THE PLAINS INTO THE SRN PROVINCES OF CANADA BY
120HR...SUNDAY. LATEST THINKING IS CONVECTION WILL LIKELY
CONCENTRATE ALONG THE DRY LINE/LEE TROUGH WHERE BOUNDARY LAYER
HEATING WILL BE MAXIMIZED AND THIS WILL COINCIDE WITH STRONGER FLOW
ALOFT THAT WILL SUPPORT ORGANIZED UPDRAFTS. SEVERE WILL LIKELY
ADVANCE DOWNSTREAM DURING THE DAY6 PERIOD BUT THE INCREASINGLY
MERIDIONAL NATURE OF THIS UPPER SYSTEM WILL LEAD TO WEAKENING LAPSE
RATES WITH TIME. WILL ADDRESS THE SEVERE THREAT BEYOND THE DAY5
PERIOD WHEN CONFIDENCE INCREASES REGARDING THE CONVECTIVE
OVERTURNING ACROSS THE HIGH PLAINS AND THE TIMING OF THE UPPER
TROUGH.

This post has been edited by Illinois blizzard: Mar 14 2012, 04:03 AM

[andyhb]

We need to watch here for the flow becoming too meridional, leading to issues like subsidence and veer-back-veer wind profiles, which would be unfavorable for tornadic supercells. With that said, most trends seem to indicate the majority of the meridional flow holding off until at least Monday, so it looks like we may see at least one significant event out of this. Combining a late winter/early spring setup in terms of winds aloft and resultant surface cyclogenesis with a mid/late spring-esque warm sector in terms of temperatures, instability and moisture (with cool mid level temperatures to both enhance lapse rates and help make the cap breakable, like you usually won't see in a late spring/summer setup) usually is not a good combination. There are several analogs that these setups aloft and at the surface remind me of, three of the most obvious would have to be May 4th/5th, 2007; May 22nd/23rd, 2008; and March 28th, 2007. Others would include March 13th, 1990 and April 26th, 1991.

The three former ones would probably be the closest analogs I'm seeing, and they all were major outbreaks...time will tell if something like that comes into fruition, but this definitely has significant potential and will need to be closely monitored over the next several days.

This post has been edited by andyhb: Mar 14 2012, 04:25 AM

[Illinois blizzar...]

Reed Timmer: Meteorologist and Extreme Storm Chaser
Here is the NAM forecast 0-3 km EHI (left) and convective inhibition (right) for 4 pm CDT. Storms fire in the TX Panhandle right around this time. Low-level shear will only increase through evening. http://TVNweather.com/ definitely chasing this!

[Illinois blizzar...]

Reed Timmer: Meteorologist and Extreme Storm Chaser
Sunday looks BIG somewhere from southern NE south through western OK into Northwest TX. Substantial convergence with southeasterly sec winds along dry line and strong southerly low-level jet collocated with instability axis.

[andyhb]

DAY 4-8 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0334 AM CDT THU MAR 15 2012

VALID 181200Z - 231200Z

...DISCUSSION...

ECMWF/GFS CONTINUE TO SUGGEST A PROGRESSIVE...ALBEIT
SLOW-MOVING...UPPER TROUGH WILL TRANSLATE ACROSS THE WRN U.S. INTO
THE PLAINS DURING THE MEDIUM RANGE PERIOD. LEADING EDGE OF STRONGER
FLOW ALOFT SHOULD SPREAD ACROSS THE LEE TROUGH/DRY LINE FROM WRN NEB
INTO WEST TX BY SUNDAY EVENING. ANY STORMS THAT DEVELOP ALONG THIS
BOUNDARY COULD POTENTIALLY ORGANIZE AS 500MB FLOW INCREASES IN
EXCESS OF 50-70KT. BY DAY5 STRONGER FORCING WILL SPREAD DOWNSTREAM
AND THE GREATEST SEVERE THREAT IS EXPECTED TO EXTEND FROM THE KS/OK
BORDER...SWWD INTO THE HILL COUNTRY OF TX. BEYOND DAY5 THERE IS
CONSIDERABLE UNCERTAINTY REGARDING THE SPEED/MOVEMENT OF THE UPPER
TROUGH...AS WELL AS CONVECTIVE MODE.

..DARROW.. 03/15/2012

[Illinois blizzar...]

Reed Timmer: Meteorologist and Extreme Storm Chaser
Sunday the 500 mb "ski jump" ejects. Potential exists for a widespread tornado outbreak across much of the Central/Southern Plains based on this model run. Stay tuned.

[Illinois blizzar...]

And..

Reed Timmer: Meteorologist and Extreme Storm Chaser
Saturday now is shaping up to have greater tornado potential up and down the dry line from western KS through the TX Panhandle. Low- and deep-layer shear is much stronger than the day before. Birthday intercept with D1 imminent.

[andyhb]

Reed needs to look at the LCL heights...

[jdrenken]

DAY 3 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0230 AM CDT FRI MAR 16 2012

VALID 181200Z - 191200Z

...THERE IS A SLGT RISK OF SVR TSTMS ACROSS A LARGE PORTION OF THE
PLAINS STATES...

...SYNOPSIS...
WITH LARGE-SCALE RIDGING TO FURTHER INCREASE ACROSS THE ERN HALF OF
THE U.S. THIS PERIOD...PRIMARY FOCUS IN TERMS OF SEVERE WEATHER
POTENTIAL WILL SHIFT WWD TO THE PLAINS REGION.

THIS SEVERE POTENTIAL WILL BE ASSOCIATED WITH THE ADVANCE OF A
LARGE/SHARP TROUGH COVERING MUCH OF WRN NOAM. AS THIS TROUGH
ADVANCES SLOWLY BUT STEADILY EWD...A SURFACE COLD FRONT WILL EXIT
THE ROCKIES AND EMERGE INTO THE HIGH PLAINS DURING THE SECOND HALF
OF THE PERIOD...PRECEDED BY A DRYLINE. THE DRYLINE -- AND LATER THE
COLD FRONT -- SHOULD FOCUS A SLOWLY-MOVING AXIS OF STRONG/SEVERE
CONVECTION FROM EVENING INTO THE OVERNIGHT HOURS.

...THE PLAINS...
AS THE UPPER TROUGH ADVANCES AND A COLD FRONT BEGINS SHIFTING INTO
THE HIGH PLAINS...LOW-LEVEL MOISTURE WILL CONTINUE TO ADVECT NWD
WITHIN THE PRE-FRONTAL/PRE-DRYLINE BOUNDARY LAYER. THIS COMBINED
WITH STEEP LAPSE RATES AND DIURNAL HEATING WILL RESULT IN MODERATE
DESTABILIZATION...THOUGH CAPPING SHOULD HINDER CONVECTIVE
DEVELOPMENT INTO THE AFTERNOON.

EVENTUALLY HOWEVER...EXPECT STORM DEVELOPMENT TO OCCUR -- AIDED BY
THE WEAKENING CAP AS HEIGHT FALLS ALOFT OVERSPREAD THE REGION
ENHANCING LOW-LEVEL CONVERGENCE.

AS STORMS DEVELOP...STRENGTHENING FLOW FIELD ALOFT WILL RESULT IN
FAVORABLE SHEAR FOR SUPERCELLS...AND THUS STORMS SHOULD QUICKLY
ORGANIZE/BECOME SEVERE. ATTM...MODEST MOISTURE /DEWPOINTS ONLY IN
THE UPPER 50S TO NEAR 60 IN MOST AREAS/ AND ASSOCIATED/FAIRLY DEEP
BOUNDARY LAYER SUGGESTS PRIMARY SEVERE POTENTIAL WILL BE VERY LARGE
HAIL AND LOCALLY DAMAGING WINDS. WHILE OVERALL STORM COVERAGE MAY
REMAIN FAIRLY ISOLATED THROUGH THE AFTERNOON...AN INCREASE IN
CONVECTION INTO THE EVENING COMBINED WITH SLOW EWD MOVEMENT OF THE
FRONT/DRYLINE SUGGEST EVENTUAL COVERAGE OF SEVERE EVENTS SHOULD
WARRANT 30% SEVERE PROBABILITY LINES.

..GOSS.. 03/16/2012

[jdrenken]

DAY 4-8 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0347 AM CDT FRI MAR 16 2012

VALID 191200Z - 241200Z

...DISCUSSION...
MEDIUM-RANGE MODELS IN FAIRLY GOOD GENERAL AGREEMENT THROUGH DAY 4
/MON 3-19/...BUT BEGIN TO DIVERGE DAY 5 /TUE 3-20/ AS THE GFS CLOSES
OFF A LOW IN THE BASE OF THE SLOWLY ADVANCING LARGE SCALE
TROUGH...WHILE THE ECMWF KEEPS THE SYSTEM OPEN.

THE ECMWF EVENTUALLY DEVELOPS A LOW DAY 6...BUT IS MUCH MORE
PROGRESSIVE WITH THE LOW AS COMPARED TO THE GFS -- WHICH KEEPS THIS
FEATURE INVOF OK FOR ROUGHLY 48 HOURS. WHILE A VARYING DEGREE OF
SEVERE POTENTIAL WOULD EXIST DAY 5 -- AND PERHAPS BEYOND -- IN
EITHER MODEL SOLUTION...DUE TO THESE DIFFERENCES WILL REFRAIN FROM
HIGHLIGHTING ANY THREAT AREAS BEYOND DAY 4.

THE PRIMARY SEVERE THREAT DAY 4 IS EXPECTED TO RESIDE OVER ROUGHLY
THE ERN HALF OF THE SRN PLAINS...AS THE INITIAL COLD FRONT ADVANCES
WITH TIME. WHILE SOME THREAT WILL LIKELY EXTEND MUCH FARTHER NWD
AHEAD OF THE FRONT...PERSISTENT CONVECTION MAKING VERY SLOW EWD
PROGRESS DUE TO VERY STRONG MERIDIONAL FLOW ALOFT SHOULD LIMIT
INSTABILITY WITH NWD EXTENT.

FARTHER S HOWEVER...A MORE SWLY BELT OF FLOW ALOFT -- AND THUS MORE
FAVORABLE DIRECTIONAL SHEAR WITH HEIGHT -- IS PROGGED. AS THE FRONT
ADVANCES ACROSS TX/OK INTO A DESTABILIZING AFTERNOON
AIRMASS...SEVERE STORM DEVELOPMENT APPEARS LIKELY. WITH THREATS FOR
HAIL...DAMAGING WINDS...AND POSSIBLY TORNADOES APPARENT ATTM...WILL
MAINTAIN THE ONGOING SEVERE THREAT AREA ACROSS PARTS OF THE SRN
PLAINS REGION -- WHERE THE MOST WIDESPREAD/SUBSTANTIAL THREAT
APPEARS TO EXIST.

..GOSS.. 03/16/2012