SPC AC 291942
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0242 PM CDT FRI MAY 29 2009
VALID 292000Z - 301200Z
...THERE IS A SLGT RISK OF SVR TSTMS ACROSS PARTS OF THE OH
VALLEY...MID-MO VALLEY AND CNTRL PLAINS...
...THERE IS A SLGT RISK OF SVR TSTMS ACROSS PARTS OF THE NRN PLAINS
AND WRN MN...
...THERE IS A SLGT RISK OF SVR TSTMS ACROSS PARTS OF THE ERN
SEABOARD...
...CNTRL AND NRN PLAINS...
THE MAIN CHANGE FOR THIS OUTLOOK IS TO TAKE THE SLIGHT RISK OUT OF
SERN SD...ERN NEB...SW MN AND SW IA. LEFT A NARROW CORRIDOR IN THE
SLIGHT ACROSS WCNTRL NEB WHERE INSTABILITY AND SHEAR PROFILES APPEAR
SUFFICIENT FOR ISOLATED SUPERCELL DEVELOPMENT. THIS ACTIVITY SHOULD
HAVE A LARGE HAIL AND WIND DAMAGE THREAT. THE SLIGHT ALSO IS PLACED
ACROSS NE KS WHERE SFC TEMPS ARE VERY WARM AND LOW-LEVEL CONVERGENCE
SHOULD INCREASE WITH STORM INITIATION POSSIBLE ALONG A COLD FRONT
LATE THIS AFTERNOON.
ELSEWHERE...THE OUTLOOK REMAINS NEARLY THE SAME WITH NO CHANGES TO
THE REASONING FROM THE 1630Z OUTLOOK.
..BROYLES.. 05/29/2009
.PREV DISCUSSION... /ISSUED 1114 AM CDT FRI MAY 29 2009/
...SYNOPSIS...
STRONG TROUGH SCENTRAL CANADA DIGGING SEWD TOWARD UPPER GREAT LAKES
THIS FORECAST PERIOD. IN ADVANCE OF THIS SYSTEM A WEAKER TROUGH
TN/UPR OH VALLEY MOVES EWD TO OFF MID ATLANTIC COAST TONIGHT. LARGE
RIDGE CONTINUES WRN U.S. HOWEVER WEAK UPPER LEVEL LOW REMAINS
TRAPPED IN THE RIDGE POSITION CENTERED OVER THE SWRN STATES WITH
ASSOCIATED STEEP LAPSE RATES AND SUFFICIENT MOISTURE FOR DIURNALLY
DRIVEN THUNDERSTORMS VICINITY HIGHER TERRAIN.
...MID-ATLANTIC AND SERN STATES...
MORNING SATELLITE INDICATES STRONG HEATING TAKING PLACE FROM
DELMARVA TO ERN GA WHERE A MOIST LOW LEVEL AIR MASS RESIDES. BY MID
AFTERNOON MLCAPES WILL RISE TO AOA 1500 J/KG THIS SAME AREA AHEAD
OF UPPER S/WV TROUGH. SFC-6KM SHEAR OF 25-30KT MARGINAL FOR
ORGANIZED CONVECTION...HOWEVER COMBINATION OF EXPECTED INSTABILITY
AND STEEP LOW/MID LEVEL LAPSES WILL SUPPORT SCATTERED THUNDERSTORMS
MUCH OF THIS AREA BY MID AFTERNOON. PRIMARY THREAT WILL BE
MARGINALLY SEVERE HAIL AND LOCAL DAMAGING WINDS AS STORMS WORK
TOWARD THE COAST LATER THIS EVENING PRIOR TO WEAKENING.
...CENTRAL/NRN PLAINS TO UPPER MS VALLEY....
WHILE STEEP LAPSE RATES COVER MUCH OF THIS AREA AND DEEP LAYER SHEAR
WILL BECOME INCREASINGLY FAVORABLE FOR SCATTERED THUNDERSTORMS LATER
THIS AFTERNOON...THE PRIMARY LIMITATION OF THE SEVERE THREAT IS THE
MARGINAL MOISTURE AND RESULTANT INSTABILITY. HOWEVER WITH LAPSE
RATES GENERALLY 8C/KM OR GREATER AND INCREASING UPPER SHEAR AS
TROUGH DROPS SEWD OUT OF CANADA THERE WILL BE A POTENTIAL FOR SEVERE
THUNDERSTORMS BEGINNING BY MID AFTERNOON PORTIONS OF UPPER MS VALLEY
ALONG AND ON THE CYCLONIC SIDE OF THE STRONG NWLY JET ACROSS ERN ND
INTO MN. HAVE INCREASED THE SEVERE PROBS AND EXTENDED THE SLIGHT
INTO THIS AREA FOR THIS AFTERNOON...WHERE SUFFICIENT CAPE GIVEN THE
STEEP LAPSE RATES AND SHEAR SHOULD LEAD TO A FEW SEVERE STORMS
INCLUDING LARGE HAIL/DAMAGING WINDS.
ADDITIONAL STRONG/SEVERE STORMS WILL ALSO BE POSSIBLE SWD THRU ERN
NEB AND THEN EWD VICINITY OLD FRONTAL ZONE ACROSS SRN IA INTO IL BY
LATE AFTERNOON. ANY SEVERE THREAT SHOULD BE LIMITED TO MARGINAL HAIL
AND WIND GIVEN THE LACK OF ANY SOURCE OF STRONG INSTABILITY. STEEP
LAPSE RATES AND THE INCREASING DEEP LAYER SHEAR WILL BE THE PRIMARY
CONTRIBUTOR TO ANY STRONG CONVECTION.
..CNTRL/NRN CA INTO ORE/ID...
SLIGHTLY STRONGER MID/UPR LVL FLOW IS EXPECTED OVER THIS REGION
TODAY AS GRADIENT STRENGTHENS ON N SIDE OF LWR CO VLY UPR LOW.
SATELLITE AND GPS DATA SHOW PW OVER CA/ORE NOW AVERAGING AROUND .75
INCH. COMBINATION OF INCREASED INSTABILITY WITH MODEST...
UNIDIRECTIONAL...DEEP SSE TO SSW FLOW MAY SUPPORT A FEW
FORWARD-PROPAGATING STORM CLUSTERS. THIS ACTIVITY APPEARS TO BE
MOST LIKELY FROM THE NRN CA SIERRAS TO THE ERN SLOPES OF THE ORE
CASCADES. A SMALL CLUSTER OR TWO MAY ALSO PROPAGATE WNW ACROSS
CNTRL/NRN PARTS OF THE SACRAMENTO VLY. THE STORMS WILL POSE AN
ISOLD RISK FOR SVR HAIL/DMGG WIND THROUGH EARLY TONIGHT.
..SW/S CNTRL TX...
CONVECTION OVER REGION YESTERDAY...AND GENERAL PATTERN OF LOW LVL
RIDGING...WILL FURTHER WEAKEN OLD FRONTAL ZONE OVER S TX TODAY.
NEVERTHELESS...COMBINATION OF WEAKLY CONFLUENT AND PERSISTENT LOW
LVL ELY /UPSLOPE/ FLOW BENEATH ENHANCED HI-LVL WLY FLOW ON NRN
FRINGE OF SUBTROPICAL JET LIKELY WILL SUPPORT DEVELOPMENT OF SCTD
STG AFTN/EVE STORMS OVER SW TX AND THE MOUNTAINS OF N CNTRL MEX W OF
THE RIO GRANDE. WITH SBCAPE OF 1500-2000 J/KG...A FEW MULTICELL
CLUSTERS/BRIEF SUPERCELLS COULD OCCUR POSING A RISK FOR ISOLD LARGE
HAIL/GUSTY WINDS. STORM COVERAGE SHOULD...HOWEVER... BE REDUCED
OVER S TX RELATIVE TO PREVIOUS DAYS.
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