SPC AC 210557

DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1257 AM CDT SUN AUG 21 2011

VALID 211200Z - 221200Z

...THERE IS A SLGT RISK OF SVR TSTMS FROM THE UPPER OH VALLEY AND
MID ATLANTIC STATES INTO NEW ENGLAND...

...SYNOPSIS...
AN UPPER TROUGH CURRENTLY EXTENDING SWD THROUGH THE GREAT LAKES AND
ANCHORED BY A VORTEX OVER HUDSON BAY WILL UNDERGO FURTHER
AMPLIFICATION AS A SHORTWAVE TROUGH TRACKS ESEWD FROM THE UPPER
GREAT LAKES TO NY/PA THIS FORECAST PERIOD. AT 12Z SUNDAY...A COLD
FRONT ATTENDANT TO THE PARENT LONGWAVE TROUGH SHOULD EXTEND SWWD
FROM SRN ONTARIO THROUGH THE LOWER OH VALLEY TO CENTRAL AR AND WWD
INTO OK. THIS BOUNDARY WILL PROGRESS E/SEWD MOVING THROUGH MUCH OF
THE NERN AND MID-ATLANTIC STATES...AND EXTEND WSWWD INTO THE CENTRAL
GULF COAST STATES BY 12Z MONDAY.

FARTHER W...AN IMPULSE EMANATING FROM AN UPPER LOW ALONG THE NRN CA
COAST IS EXPECTED TO MOVE INTO WRN/NRN MT THIS AFTERNOON AIDING IN
SOME TSTM DEVELOPMENT OVER THE HIGHER TERRAIN. MEANWHILE...HEIGHT
RISES ARE EXPECTED ACROSS THE NRN HIGH PLAINS...TENDING TO LIMIT
TSTM DEVELOPMENT.

...MID-ATLANTIC STATES TO NEW ENGLAND...
00Z NAM/GFS AND 12Z ECMWF AGREE WITH THE TIMING OF THE COLD FRONT
TODAY...REACHING NRN NY TO WRN PA/UPPER OH VALLEY DURING THE
EARLY-MID AFTERNOON. GIVEN THE TIMING OF THE FRONT ALLOWING FOR
SURFACE HEATING/DESTABILIZATION AND THE ARRIVAL OF FORCING ALOFT
DURING THE AFTERNOON/EVENING...THE FORECAST CONFIDENCE REMAINS GOOD
FOR THE DEVELOPMENT OF SEVERE STORMS TODAY INTO TONIGHT ACROSS THE
SLIGHT RISK AREA. SWLY LOW LEVEL WINDS ACROSS THE OH VALLEY TO WRN
PA AND SSWLY LOW LEVEL WINDS IN LEE OF THE CENTRAL APPALACHIANS TO
THE HUDSON VALLEY/NEW ENGLAND WILL EACH SUPPORT MOISTURE ADVECTION
INTO THESE REGIONS. ALTHOUGH MIDLEVEL LAPSE RATES ARE FORECAST TO
BE MODEST /AOB 6.5 C PER KM/ ACROSS THE SLIGHT RISK AREA...SURFACE
HEATING COMBINED WITH THE MOISTURE RETURN WILL RESULT IN MODERATE
INSTABILITY FROM MD/D.C./DELMARVA TO SERN/ERN NY AND SWRN NEW
ENGLAND THIS AFTERNOON. MLCAPE ACROSS THIS CORRIDOR WILL BE UP TO
1500-2000 J/KG WHILE FURTHER W OVER THE UPPER OH VALLEY VALUES WILL
BE UP TO 1000 J/KG.

DIFFLUENT FLOW ALOFT AND HEIGHT FALLS WITH THE APPROACH OF THE UPPER
GREAT LAKES SHORTWAVE TROUGH...AND CONVERGENCE/ASCENT ALONG THE COLD
FRONT AND PRE-FRONTAL/LEE TROUGH EXTENDING FROM VA TO ERN NY WILL
SUPPORT TSTM DEVELOPMENT BY EARLY-MID AFTERNOON IN VICINITY AND
AHEAD OF THESE SURFACE BOUNDARIES. STRENGTHENING WIND FIELDS AND
EFFECTIVE BULK SHEAR OF 40-50 KT COMBINED WITH THE AVAILABLE
INSTABILITY WILL SUPPORT ORGANIZED STORMS. SHEAR VECTORS MORE
NORMAL TO THE LEE TROUGH SUGGEST A GREATER POTENTIAL FOR DISCRETE
STORMS...INCLUDING SUPERCELLS. VERTICALLY VEERING WINDS AND HIGHER
THETAE EXTENDING FROM MD/ERN PA TO SERN NY/SWRN NEW ENGLAND SUPPORT
AN ISOLATED TORNADO THREAT ALONG WITH DAMAGING WINDS AND HAIL.
FARTHER W INTO THE UPPER OH VALLEY...ORGANIZED STRONG TO SEVERE
STORMS WILL BE POSSIBLE BY EARLY-MID AFTERNOON WITH STRONG WIND
GUSTS AND HAIL THE PRIMARY THREATS.

ALTHOUGH INSTABILITY SHOULD BE WEAKENING THIS EVENING...A FURTHER
INCREASE IN DEEP LAYER WIND FIELDS INTO THE NERN STATES AND 60-90
METER HEIGHT FALLS ACROSS THIS REGION SHOULD COMPENSATE THE WEAKER
THERMODYNAMICS FOR AT LEAST A CONTINUED THREAT FOR DAMAGING WINDS
INTO PARTS OF NEW ENGLAND. FORECAST SOUNDINGS SUGGEST A TORNADO
CANNOT BE RULED OUT ACROSS SRN NEW ENGLAND AS STRENGTHENING SLY LLJ
SUPPORTS AN INCREASE IN LOW LEVEL HODOGRAPH CURVATURE.

...WV/VA/CAROLINAS SWWD TO TN/CENTRAL GULF COAST STATES...
A MIDLEVEL SHORTWAVE TROUGH...CURRENTLY MOVING ACROSS IL/IND TO THE
MID SOUTH PER WATER VAPOR IMAGERY...IS EXPECTED TO WEAKEN TODAY AND
MOVE TOWARD THE SRN HALF OF THE APPALACHIANS /WRN VA TO NRN GA/NERN
AL/. ASCENT WITH THIS FEATURE COMBINED WITH MODERATE INSTABILITY
SHOULD SUPPORT TSTM DEVELOPMENT FROM THE TN VALLEY/NRN EXTENT OF THE
CENTRAL GULF COAST STATES INTO THE WRN CAROLINAS AND VA. THE FOCUS
FOR GREATER STORM COVERAGE IS UNCERTAIN AT THIS TIME..AND THUS WILL
MAINTAIN LOW SEVERE PROBABILITIES.

...NRN HIGH PLAINS INTO SRN SD/NRN NEB...
A SURFACE FRONT WILL MOVE SWD TODAY REACHING THE VICINITY OF THE
SD/NEB BORDER. SLY LOW LEVEL WINDS S OF THIS BOUNDARY WILL RETURN
MOISTURE NWD BENEATH A PLUME OF STEEP LEVEL LAPSE RATES/EML
SPREADING EWD INTO SD/NEB. ALTHOUGH MODELS ARE INDICATING MODERATE
INSTABILITY FROM NEB/WRN SD INTO PARTS OF THE ADJACENT NRN HIGH
PLAINS ALONG WITH STRONG EFFECTIVE BULK SHEAR...HEIGHT RISES ACROSS
THIS REGION AND LACK OF SUFFICIENT FORCING ALOFT SUGGEST TSTM
DEVELOPMENT SHOULD BE MINIMAL. THIS OUTLOOK WILL MAINTAIN A LOW
PROBABILITY FOR SEVERE STORMS...GIVEN THE FAVORABLE
INSTABILITY/SHEAR...BUT CONFIDENCE IN GREATER STORM/SEVERE COVERAGE
REMAINS LOW.

..PETERS/SMITH.. 08/21/2011

CLICK TO GET WUUS01 PTSDY1 PRODUCT

NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1300Z
CURRENT UTC TIME: 1212Z (8:12AM), RELOAD THIS PAGE TO UPDATE THE TIME

.SHORT TERM /NOON TODAY THROUGH 6 AM MONDAY/...
UPR LOW SPINNING OVER HUDSON BAY WITH THE JET APPROACHING THE NRN
GREAT LAKES PER WATER VAPOR. H25 U COMPONENT WINDS AND H5 HEIGHT
ANOMALIES WITH THIS SYSTEM ABOUT 2-3 STD/S. 00Z SUITE OF MODELS
CONTINUE TO PROG SCT TSTMS DEVELOPING THIS AFTN ACROSS THE
W...EVOLVING INTO WIDESPREAD TSTMS THIS EVE AND THRU THE OVERNIGHT
AS THE COLD FRONT AND INCREASING DYNAMICS CATCH UP. THIS SOLN HAS
BEEN ACCEPTED FOR THE FCST.

PARAMETERS REMAIN IN PLACE FOR A SEVERE OUTBREAK. 00Z NAM CAME IN
A BIT HIGHER WITH THE BRN...RESULTING IN A TENDENCY TOWARDS
MULTICELLULAR. SYNOPTIC LLVL DIRECTIONAL SHEAR IS
LIMITED...HOWEVER AS STORMS DEVELOP ALLOWING FOR LLVL
INFLOW...EXPECT THIS SHEAR TO BE ENHANCED RESULTING IN AN
ENVIRONMENT MORE SUPPORTIVE OF SUPERCELLS.

ALTHOUGH THERE ISN/T A WARM FRONT INVOF THE CWA WITH THIS
EVENT...EHI STILL IN THE 1-2 RANGE SUPPORTING A TOR POTENTIAL.
INFLOW MAY ENHANCE THESE NUMBERS AS WELL BUT COMPUTATION IS BASED
ON SRH SO CONFIDENCE NOT AS HIGH AS IN THE BRN. ANY OUTFLOW
BOUNDARIES WILL ELEVATE THE SPIN UP RISK...AND AS THE CONVECTION
CONSOLIDATES TONIGHT THE RISK COULD SHIFT TO LI WITH COASTAL
BOUNDARY DEVELOPMENT.

PW/S STILL ON TRACK TO REACH JUST UNDER 2 INCHES SO HVY RAIN HAS
BEEN INCLUDED IN THE FCST. SEE HYDRO SECTION FOR ADDITIONAL INFO.

MODERATE RIP RISK TODAY. THREAT HIGHEST IN THE AFTN AND EVE.

NW WINDS BEHIND THE FRONT TONIGHT. BOUNDARY LOOKS TO CLEAR THE CWA
BY 12Z MON.