SPC AC 271959
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0259 PM CDT MON JUL 27 2009
VALID 272000Z - 281200Z
...THERE IS A SLGT RISK OF SVR TSTMS ACROSS MOST OF THE ERN SEABOARD
FROM ME SWD TO THE CAROLINAS...
...THERE IS A SLGT RISK OF SVR TSTMS ACROSS PORTIONS OF THE UPR MS
VLY WWD INTO THE CNTRL PLAINS AND CNTRL ROCKIES...
FCST CHANGES WERE MINIMAL FOR 20Z UPDATE. SEE PREVIOUS OTLK
DISCUSSION AND ONGOING MESOSCALE DISCUSSIONS FOR SHORT-TERM FCST
DETAILS.
...NEW ENGLAND...
RISK OF SVR EXPANDED EWD INTO CNTRL MA AND INTERIOR CT.
HERE...TSTMS THAT DEVELOPED INVOF LEE TROUGH ARE EXPECTED TO LAST
INTO THE EARLY EVENING BEFORE DIMINISHING.
...SERN STATES...
EXPANDED SLGT RISK SWD TO INCLUDE THE SAVANNAH RVR VLY AND MOST ALL
THE CAROLINAS. COLD POOL ALOFT /H5 TEMPERATURES AOB MINUS 8 DEG
C/...HIGH PWAT VALUES AND THE APPROACH OF A VORT MAX WILL CONTRIBUTE
TO STRONG/SVR STORMS THROUGH EARLY EVENING. A BLEND OF DISCRETE
CELLS /PSBLY DEVELOPING BRIEF SUPERCELL CHARACTERISTICS/ AND SHORT
LINE SEGMENTS WILL GIVE LARGE HAIL AND DMGG WIND GUSTS.
...PLAINS...
COMPLEX SFC PATTERN EXISTS WITH SVRL LLVL CONVERGENCE ZONES/FRONTS
THAT WILL FOCUS CONVECTION THROUGH THE EVENING. SLGT RISK WAS
ADJUSTED SWD TO ALONG THE NEB/KS BORDER AND INTO SRN KS. OUTFLOW
BOUNDARY/FRONT HAS CLEARED MOST OF NEB AND TCU/CBS HAVE BEEN
DEVELOPING ACROSS NRN KS ALONG THIS FEATURE AND AHEAD OF IT IN THE
HOT BOUNDARY LAYER OVER CNTRL KS. DMGG WIND GUSTS/HAIL CAN BE
EXPECTED WITH THE STRONGER STORMS.
FARTHER N...ISOLD CBS HAVE ALSO DEVELOPED ALONG A SECONDARY FRONT
OVER SRN SD...ALONG SRN EDGE OF LARGE SCALE HEIGHT FALLS SPREADING
SWD FROM ND. VERTICAL SHEAR WAS COMPARATIVELY LESS THAN FARTHER TO
THE SOUTH...SO ONLY ISOLD AND BRIEFLY ORGANIZED STORMS WITH
HAIL/GUSTY WINDS CAN BE EXPECTED.
...CNTRL ROCKIES/HIGH PLAINS...
ADJUSTED RISK FOR SVR TSTMS WWD ALONG/AHEAD OF THE DIGGING UPR
FEATURE OVER ERN ID/NWRN WY. DIFFLUENT ENVIRONMENT AHEAD OF THE
FEATURE COMBINED WITH MODEST BUOYANCY AND UPSLOPE FLOW WILL
CONTRIBUTE TO A FEW STRONG/SVR TSTMS CAPABLE OF HAIL/GUSTY WINDS.
..RACY/COOK.. 07/27/2009
.PREV DISCUSSION... /ISSUED 1137 AM CDT MON JUL 27 2009/
...CORRECTED FOR GRAMMAR/STATE ABBREVIATIONS...
...NRN ROCKIES TO ADJACENT HIGH PLAINS...
WV IMAGERY DEPICTS COMPACT SHORT WAVE TROUGH EMBEDDED WITHIN
DEEP-LAYER NWLY FLOW SPINNING SEWD ACROSS ID/MT ATTM. EXPECT ASCENT
ASSOCIATED WITH THIS IMPULSE...AND DOWNSTREAM DIFFERENTIAL HEATING
ACROSS THE MOUNTAINOUS AREAS OF WY/CO...TO RESULT IN AFTERNOON TSTM
DEVELOPMENT. MODEST MID-LEVEL FLOW OF ABOUT 30KT AND THERMODYNAMIC
REGIME CHARACTERIZED BY DEEP AND WELL-MIXED BOUNDARY LAYER SHOULD
RESULT IN CLUSTERS OF STORMS AND PERHAPS A SUPERCELL OR TWO WITH THE
POTENTIAL TO PRODUCE HAIL AND LOCALLY STRONG DOWNBURST WINDS.
FARTHER SOUTH AND EAST...FRONTAL SURGE SWD ALONG THE CO FRONT RANGE
IS INDICTED IN LATEST GUIDANCE. IN COMBINATION WITH INCREASING MID
LEVEL FLOW AND DIURNAL DESTABILIZATION...AND NUMEROUS STORMS
SPREADING SLOWLY EAST OFF THE HIGHER TERRAIN...ADDITIONAL ACTIVITY
APPEARS POSSIBLE IN UPSLOPE FLOW DEVELOPING FROM NERN CO AND ALONG
THE FRONT RANGE. LATEST VISIBLE IMAGERY SHOWS THAT MUCH OF THE AREA
WILL EXPERIENCE STRONG HEATING AND DESTABILIZATION IN ADVANCE OF
THIS DEVELOPMENT. THUS...EXPECT SCATTERED STRONG TO SEVERE STORMS
WITH BOTH SEVERE WIND AND HAIL POTENTIAL. THIS ACTIVITY MAY CONGEAL
INTO SMALL MCS SPREADING SWD ACROSS SERN CO/NERN NM THROUGH LATE
EVENING.
...CNTRL PLAINS TO UPR MS VALLEY...
COMPLEX FORECAST SCENARIO ACROSS THESE AREAS WITH WEAK FRONTAL ZONE
CURRENTLY ANALYZED FROM NEB ENEWD ACROSS NWRN IA TO A WEAK LOW IN
MN. WV IMAGERY AND MODELS DEPICT SHORT WAVE TROUGH DIGGING SEWD
ACROSS ND/MN ATTM AND THIS FEATURE IS ALSO EMBEDDED WITHIN BELT OF
40-50KT WNWLY FLOW. WHILE MODEST HEIGHT FALLS WILL OCCUR ACROSS NRN
AREAS...ONLY WEAK FRONTAL AND DIABATIC FORCING WILL OCCUR ALONG AND
SOUTH OF THE FRONT IN THE PLAINS. EXPECT POCKETS OF STRONG
INSTABILITY TO EVOLVE AHEAD OF THE SURFACE LOW AND FRONT THROUGH THE
AFTERNOON WITH DEEP CONVECTION NEAR THESE FEATURES GRADUALLY
INCREASING IN BOTH COVERAGE AND INTENSITY.
DEEP-LAYER SHEAR ACROSS THE UPR MS VALLEY IS FORECAST TO STRENGTHEN
AS THE UPPER TROUGH APPROACHES. THIS SHOULD SUPPORT AN INCREASING
POTENTIAL FOR A FEW SUPERCELLS...OR LONGER LIVED STORMS. THIS
ACTIVITY MAY CONSOLIDATE NEAR THE LOW TRACKING FROM ERN MN TO CNTRL
WI THROUGH THE EARLY EVENING. IF SUFFICIENT INSTABILITY CAN DEVELOP
COINCIDENT WITH THIS DEVELOPMENT...A CORRIDOR OF SEVERE WIND AND
HAIL MAY DEVELOP...WITH SOME CHANCE FOR A TORNADO TOO.
STORMS MAY BE NUMEROUS BUT PERHAPS LESS ORGANIZED ALONG THE TRAILING
FRONT ACROSS IA/NEB/KS WHERE BOTH LARGE SCALE ASCENT AND SHEAR WILL
BE MORE LIMITED/WEAKER. HOWEVER...CONVECTION ALONG THE BOUNDARY WILL
DRIFT SWD IN AN AXIS OF STRONG INSTABILITY ACROSS THESE AREAS
THROUGH THE EVENING. MARGINALLY ORGANIZED MULTICELL STORMS IN BANDS
OR CLUSTERS COULD ATTAIN SEVERE LEVELS WITH SOME HAIL AND WIND
POSSIBLE.
...NORTHEAST...
A WEAK COLD FRONT WILL CONTINUE TO SPREAD EWD/SEWD INTO WARM AND
UNSTABLE AIRMASS FROM ERN NY/NRN NEW ENGLAND SEWD ACROSS PARTS OF
ERN PA AND NJ. LIFT AND POCKETS OF STRONGER MID LEVEL FLOW NEAR THE
APPROACHING FRONT WILL COINCIDE WITH MAXIMUM IN DIURNAL
HEATING/DESTABILIZATION IN A RELATIVELY NARROW CORRIDOR ACROSS THESE
AREAS FROM THIS AFTERNOON INTO EARLY EVENING. EXPECT A FEW
STORMS/LINE SEGMENTS TO BRIEFLY ATTAIN SOME POTENTIAL FOR DAMAGING
WINDS.
...CNTRL APPALACHIANS/MID ATLANTIC...
SHORT WAVE TROUGH IN THE BASE OF THE LARGER AND MORE PERSISTENT ERN
TROUGH WILL SPREAD EAST FROM WV WITH ASCENT ACTING ON AN
INCREASINGLY UNSTABLE AIRMASS ACROSS THE DELMARVA AREA. A FEW
STRONGER STORMS APPEAR POSSIBLE INTO THE EARLY EVENING WITH THE
GREATER THREAT BEING DOWNBURST WINDS. SEE LATEST MCD FOR PARTS OF
THE NC/VA AREA.
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