DAY1SVR: Day 1 Convective Outlook

ACUS01 KWNS 211958
SWODY1
SPC AC 211956

DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0256 PM CDT FRI SEP 21 2012

VALID 212000Z - 221200Z

...THERE IS A SLGT RISK OF SVR TSTMS THIS AFTERNOON AND EVENING
ACROSS PARTS OF IL AND IND...

...IL/IND THIS AFTERNOON AND EVENING...
TRENDS IN THE MOST RECENT RUNS OF THE HRRR AGREE WITH THE 12Z WRF 4
KM NMM AND 4 KM NSSL SUGGESTING ADDITIONAL TSTM DEVELOPMENT SHOULD
ALSO OCCUR S OF THE ONGOING ACTIVITY...WHICH EXTENDED ESEWD FROM
WEST CENTRAL TO CENTRAL IL PER LIGHTNING DATA/REGIONAL RADAR
IMAGERY. THIS LATTER ACTIVITY WAS GENERALLY LOCATED E OF A
DEEPENING AREA OF SURFACE LOW PRESSURE...ANALYZED OVER NERN MO/FAR
WEST CENTRAL IL AT 18Z...AND ALONG A ZONE OF STRONGER PRESSURE FALLS
AND THE NWD RETURN OF GREATER LOW LEVEL MOISTURE. THESE
OBSERVATIONS ARE IN AGREEMENT WITH THE CONVECTIVE/SEVERE WEATHER
THREAT SCENARIO PER THE 1630Z DAY 1 OUTLOOK. GIVEN THE
AFOREMENTIONED TRENDS IN THE HRRR AND THE POTENTIAL FOR FURTHER
DESTABILIZATION DOWNSTREAM INTO MORE OF SWRN-CENTRAL IND...SEVERE
PROBABILITIES AND SLIGHT RISK AREA HAVE BEEN EXPANDED A FEW COUNTIES
TO THE SE AND E.

...CENTRAL NY TO CENTRAL PA...
REGIONAL RADAR IMAGERY AND LIGHTNING DATA SHOWED TWO AREAS OF
CONVECTION WITH EMBEDDED...THOUGH GENERALLY ISOLATED...TSTMS ACROSS
1/ PARTS OF CENTRAL PA TO ADJACENT SRN NY...WITH THIS ACTIVITY
LOCATED ALONG A PRE-FRONTAL TROUGH...AND 2/ INVOF KROC EXTENDING
NNEWD ACROSS LAKE ONTARIO...WITH THIS ACTIVITY ALONG A COLD FRONT.
AS INDICATED BY THE EARLIER FORECAST...THIS CONVECTION IS LOCATED
WITHIN A STRONGLY SHEARED ENVIRONMENT...BUT INSTABILITY REMAINS
QUITE WEAK...AND THUS SEVERE WEATHER PROBABILITIES CONTINUE TO NOT
BE WARRANTED.

..PETERS.. 09/21/2012

.PREV DISCUSSION... /ISSUED 1112 AM CDT FRI SEP 21 2012/

...SYNOPSIS...

AN AMPLIFIED...FULL-LATITUDE TROUGH WILL BE MAINTAINED E OF THE
ROCKIES THROUGH THE D1 PERIOD. WITHIN THIS BROAD-SCALE REGIME...MID
AND UPPER-LEVEL JET STREAKS FROM THE CANADIAN PRAIRIE PROVINCES INTO
THE NRN PLAINS WILL PROPAGATE SEWD INTO THE OH VALLEY IN CONJUNCTION
WITH A SERIES OF VORTICITY MAXIMA. IN THE LOW LEVELS...A SURFACE
CYCLONE OVER WRN IA WILL DEEPEN WHILE DEVELOPING SEWD INTO CNTRL IL
BY 22/00Z...BEFORE WEAKENING AND EVENTUALLY REFORMING OVER SWRN
ONTARIO AT 22/12Z. AN ASSOCIATED WARM FRONT CURRENTLY ANALYZED
ALONG THE OH RIVER WILL ADVANCE NWD AHEAD OF THE MIGRATORY LOW WHILE
A TRAILING COLD FRONT SETTLES SWD/SEWD THROUGH THE OZARK PLATEAU AND
MID SOUTH.

...IL/IND THIS AFTERNOON AND EVENING...

THE 12Z SGF SOUNDING AND SATELLITE-DERIVED PW IMAGERY INDICATE A
CORRIDOR OF HIGHER MOISTURE CONTENT /I.E. PW VALUES AROUND AN INCH/
OVER THE OZARK PLATEAU WHICH WILL BE ADVECTED INTO THE REGION TODAY
ALONG A STRENGTHENING SWLY LLJ. THIS MOISTURE WILL ALIGN WITH
500-MB TEMPERATURES OF -16 TO -18C AND DAYTIME HEATING TO YIELD
AFTERNOON MLCAPE VALUES APPROACHING 1000 J/KG WITHIN SURFACE WARM
SECTOR.

AN ENHANCED ZONE OF LOW-LEVEL CONVERGENCE INVOF THE SURFACE LOW WILL
COMBINE WITH A COUPLED UPPER-LEVEL JET STRUCTURE TO FOSTER WIDELY
SCATTERED TSTM DEVELOPMENT BY MID-LATE AFTERNOON OVER CNTRL/ERN IL.
THE COLLOCATION OF 50-60 KT WLY MID-LEVEL FLOW AND A 30-40 KT LLJ
WILL RESULT IN A FAVORABLE WIND PROFILE FOR SUPERCELLS CAPABLE OF
HAIL...DAMAGING WINDS...AND PERHAPS A COUPLE TORNADOES. SEVERE
WEATHER POTENTIAL SHOULD BEGIN TO DIMINISH BY LATE EVENING AS THE
BOUNDARY LAYER COOLS AND STABILIZES.

...CNTRL NY THIS AFTERNOON...

CONVERGENCE ALONG A WEAK SURFACE FRONT/TROUGH WILL BE AUGMENTED BY
DCVA AHEAD OF A SHEARED VORTICITY MAXIMUM MOVING ACROSS THE REGION
TO SUPPORT WIDELY SCATTERED TSTMS THIS AFTERNOON INTO EVENING.
MODIFICATION OF THE 12Z BUF SOUNDING FOR ANTICIPATED AFTERNOON
CONDITIONS IN THE BOUNDARY LAYER YIELDS A WEAK UNSTABLE AIR MASS IN
THE PRESENCE OF RATHER STRONG DEEP-LAYER SHEAR. CURRENT THINKING IS
THAT STORM VIGOR AND RESULTANT SEVERE WEATHER POTENTIAL WILL BE
LIMITED BY THE MARGINAL INSTABILITY.

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