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April 18-20th Severe Weather


Chicago Storm

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Elevated complex that developed in MO late Monday night/Tuesday morning.

Still capped where it counts.

Ok, but look at the sounding I just posted.

Even if the mode trends more linear with time, I am concerned about a tornado threat lingering well into the night.

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Ok, but look at the sounding I just posted.

Even if the mode trends more linear with time, I am concerned about a tornado threat lingering well into the night.

I was only disputing the cap issue.

The overall threat is legit and will last well after dark...otherwise I wouldn't be chasing.

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Still like the area between St.Louis, Springfield IL, and Hannibal Missouri. The new NAM dropped things a bit further south compared to previous runs, but hopefully the trend's about over. The SREF looks a bit too far east as usual. I'm not too worried about the cap with the incoming shortwave and associated height falls.

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DAY 2 CONVECTIVE OUTLOOK

NWS STORM PREDICTION CENTER NORMAN OK

1259 AM CDT MON APR 18 2011

VALID 191200Z - 201200Z

...THERE IS A MDT RISK OF SVR TSTMS ACROSS PORTIONS OF

AR/MO/IL/INDIANA/KY AND FAR WESTERN TN...

...THERE IS A SLGT RISK OF SVR TSTMS FROM THE SOUTHERN PLAINS TO THE

MIDWEST/OH RIVER VALLEY...

..MIDWEST/OZARKS AND OH/MIDDLE MS RIVER VALLEYS

A VERY ACTIVE SEVERE WEATHER DAY IS EXPECTED ACROSS THE REGION ON

TUESDAY/TUESDAY NIGHT...INCLUDING THE POSSIBILITY OF SOME STRONG

TORNADOES IN ADDITION TO LARGE HAIL AND WIDESPREAD DAMAGING WINDS

TUESDAY AFTERNOON/NIGHT.

INITIALLY...EARLY DAY ELEVATED TSTMS /SOME CAPABLE OF SEVERE HAIL/

WILL BE ONGOING ACROSS THE MIDWEST/OH RIVER VALLEY...ON THE NOSE OF

A STRONG SOUTHWESTERLY LOW LEVEL JET AND ON THE EDGE OF A

NORTHEASTWARD BUILDING ELEVATED MIXED LAYER /EML/. SUCH A HAIL

THREAT COULD INCLUDE DOWNSTATE PORTIONS OF IL/INDIANA INTO

KY/SOUTHERN OH THROUGH THE MORNING.

OTHERWISE...BENEATH AN EML-DRIVEN CAP...LOW LEVEL MOISTURE WILL

CONTINUE TO AGGRESSIVELY RETURN NORTH-NORTHEASTWARD AS STRONG

SOUTHWESTERLY LOW LEVEL WINDS ARE MAINTAINED THROUGH THE DAY. WITH

RELATIVELY RICH MOISTURE /AROUND 70 F SURFACE DEWPOINTS AND

REFERENCE THE 00Z OBSERVED CORPUS CHRISTI RAOB/ ALREADY RETURNING

INLAND ACROSS SOUTHEAST TX EARLY TODAY...IT IS PLAUSIBLE THAT A

BROAD CORRIDOR OF LOWER TO MIDDLE 60S F SURFACE DEWPOINTS WILL

BECOME INCREASINGLY ESTABLISHED TO THE EAST OF A LOWER MO VALLEY

SURFACE LOW AND ALONG/SOUTH OF A NORTHWARD MOVING WARM FRONT ACROSS

THE OH RIVER VALLEY/MIDWEST. BENEATH THE STRONG EML/PLUME OF STEEP

MID LEVEL LAPSE RATES...MODERATELY TO STRONG INSTABILITY WILL LIKELY

EXIST ACROSS THE PRE-COLD FRONTAL WARM SECTOR BY AFTERNOON...WITH

UPWARDS OF 2000-4000 J/KG OF MLCAPE POSSIBLE WITHIN A CORRIDOR FROM

EASTERN OK AND AR INTO THE SOUTHEAST HALF OF MO/DOWNSTATE IL AND

ADJACENT PORTIONS OF THE MIDDLE MS VALLEY.

A SOUTH-CENTRAL ROCKIES/CENTRAL PLAINS SHORTWAVE TROUGH EARLY

TUESDAY IS EXPECTED TO GRADUALLY AMPLIFY AND ADVANCE

NORTHEASTWARD/TAKE ON A MORE NEGATIVE TILT ESPECIALLY TUESDAY NIGHT

TOWARD THE UPPER MS RIVER VALLEY/UPPER GREAT LAKES REGION. ATTENDANT

LARGE SCALE FORCING FOR ASCENT/HEIGHT FALLS AND THE ASSOCIATED COLD

FRONT SHOULD ERODE AN OTHERWISE STRONG CAP...WITH SURFACE BASED

TSTMS LIKELY TO ERUPT BY MID/LATE AFTERNOON INTO EARLY EVENING AHEAD

OF THE DEEPENING LOWER MO VALLEY/MIDDLE MS VALLEY SURFACE LOW AND

NORTHEAST-SOUTHWEST ORIENTED COLD FRONT. SUCH VIGOROUS DEEP

CONVECTIVE DEVELOPMENT CURRENTLY APPEARS MOST LIKELY TO INITIALLY

OCCUR ACROSS CENTRAL/SOUTHWEST MO AS WELL AS WESTERN IL AND

NORTHWEST AR/NORTHEAST OK.

AMPLE LOW LEVEL MOISTURE/VERY STEEP LAPSE RATES AND DEEP LAYER

SHEAR OF 40-50 KT COINCIDENT WITH THE COLD FRONT/ADJACENT WARM

SECTOR WILL BE FURTHER ACCENTUATED BY LONG/CURVING LOW LEVEL

HODOGRAPHS AIDED BY 40-55 KT WINDS WITHIN THE LOWEST 1-2 KM. WHILE

VERY LARGE HAIL AND TORNADOES /PERHAPS A FEW STRONG/ WILL BE

POSSIBLE WITH INITIAL SUPERCELLULAR DEVELOPMENT...SEVERAL FACTORS

IMPLY A QUASI-LINEAR EVOLUTION IS PROBABLE SEVERAL HOURS AFTER DEEP

CONVECTIVE INITIATION /ESPECIALLY MID EVENING AND BEYOND/ ACROSS THE

MIDWEST/OH VALLEY. THIS INCLUDES THE LIKELIHOOD/DOMINANCE OF COLD

FRONTAL-TYPE INITIATION /GIVEN A STOUT EML OTHERWISE ACROSS THE WARM

SECTOR/...AS WELL AS RELATIVELY MODEST HIGH LEVEL FLOW/SHEAR AS

COMPARED TO THE LOWER-MIDDLE TROPOSPHERE. UPSCALE GROWTH INTO AN

INCREASINGLY WELL-ORGANIZED/ACCELERATING SQUALL LINE...INCLUDING

LEWP/BOW EVOLUTIONS....IS PROBABLE TUESDAY NIGHT ACROSS THE OH

VALLEY. AS SUCH...A WIDESPREAD DAMAGING WIND AND ISOLATED TORNADO

THREAT SHOULD EVOLVE BY MID/LATE EVENING...AND CONTINUE INTO THE

OVERNIGHT. A RELATIVELY BROAD WARM/MOIST SECTOR ASIDE...THIS WILL BE

SUPPORTED ON THE LARGE SCALE BY THE ANTICIPATED DEEPENING PHASE OF

THE SURFACE CYCLONE TOWARD THE UPPER GREAT LAKES AND A

REINVIGORATION OF A NOCTURNALLY ENHANCED LOW LEVEL WIND FIELD /50-60

KT SOUTHWESTERLY WINDS AROUND 1-2 KM/.

..SOUTHERN PLAINS/ARKLATEX

PARTIALLY ATTRIBUTABLE TO MORE MODEST LARGE SCALE FORCING FOR ASCENT

AT THESE LATITUDES...A RELATIVELY MORE ISOLATED SEVERE THREAT WILL

BE POSSIBLE IN VICINITY OF THE NORTH-SOUTH DRYLINE ACROSS EASTERN

OK/NORTHEAST TX INTO WESTERN AR/FAR NORTHWEST LA. SUPERCELLS CAPABLE

OF LARGE HAIL AND PERHAPS A TORNADO WILL BE POSSIBLE TUESDAY LATE

AFTERNOON/EVENING. THE COLD FRONT WILL GRADUALLY OVERTAKE THE

DRYLINE FROM NORTHWEST TO SOUTHEAST ACROSS EASTERN OK INTO AR AND

EVENTUALLY NORTH TX/FAR NORTHWEST LA TUESDAY NIGHT. FARTHER

WEST...OTHER MORE ISOLATED/MARGINAL SEVERE TSTMS WITH A HAIL/WIND

RISK COULD DEVELOP ALONG THE WESTWARD EXTENSION OF THE

SOUTH-SOUTHEASTWARD MOVING COLD FRONT ACROSS OK AND THE TX PANHANDLE

TUESDAY LATE AFTERNOON/EARLY EVENING.

..GUYER.. 04/18/2011

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Honestly, I'm leaning towards this being more of a widespread damaging wind event than a massive tornado outbreak like we've seen. The reasons are very well outlined in the 06z Day 2 outlook. This is not to say that the tornado threat is minimal at all, though, as I expect quite a few to form Tuesday regardless, with a few strong.

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Honestly, I'm leaning towards this being more of a widespread damaging wind event than a massive tornado outbreak like we've seen. The reasons are very well outlined in the 06z Day 2 outlook. This is not to say that the tornado threat is minimal at all, though, as I expect quite a few to form Tuesday regardless, with a few strong.

Yeah I would have to agree here, the highest threat will be widespread wind damage with tornadoes being the second significant threat.

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Another point SPC brought up that I noticed as well is the placement of upper forcing relative to the instability. Below is the 00z GFS run 0-4km thetae and jet winds. Notice that the core of the strongest winds remains on the cool side of the surface boundary (hence initiation being tied to that boundary).

Compare those images to the day of the eastern Oklahoma outbreak, which had the core of strongest winds aloft punching directly over the warm sector.

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I'm gonna sound like a total noob here but oh well, so what does that mean for Tues? ^^^^^^

Pretty much confirming some of the analysis already in this thread. The warm sector will remain capped until surface boundary impinges on the instability and initiates convection along it. Initially supercells will be supported, but owing to the fact that the forcing will be along a cold front with upper level flow parallel to the boundary, upscale growth into linear segments is favored.

An isolated tornadic threat will continue with the line after that due the large low level shear, and near the triple point/warm front any semi-discrete structure will have a chance to produce as well.

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Pretty much confirming some of the analysis already in this thread. The warm sector will remain capped until surface boundary impinges on the instability and initiates convection along it. Initially supercells will be supported, but owing to the fact that the forcing will be along a cold front with upper level flow parallel to the boundary, upscale growth into linear segments is favored.

An isolated tornadic threat will continue with the line after that due the large low level shear, and near the triple point/warm front any semi-discrete structure will have a chance to produce as well.

Winds are actually maxed closer to 700mb with weaker winds aloft at 500 and 300. There's a pretty pronounced and well defined jet streak at the 700mb and 850 levels, producing a very non-classical look to the wind profiles. Since we're dealing with what looks to be a wind max near or just below the LND, I'd have to think that we'd have to look near the nose of that streak to find where the cap breach is most likely at first.

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12z NAM is impressive with lapse rates and parameters tomorrow over southern MO into IN. If supercells or a supercell can form in eastern MO into Illinois then the potential is certainly there for a strong tornado. Soundings show winds backing tomorrow evening over the KPAH region. Low is a bit further south and east than some other runs.

Moderate risk seems well placed - question will be storm mode. Squall line is a given - question remains on discreet cells. The CAP is very impressive tomorrow over the region. Weakens late in the day and evening.

Though SPC might put a slight over us for some hailers tonight.

post-783-0-10809000-1303137271.gif

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