Friday, April 23, 2010

Moderate Risk in Alabama Saturday




DAY 2 CONVECTIVE OUTLOOK    
NWS STORM PREDICTION CENTER NORMAN OK
1235 AM CDT FRI APR 23 2010

VALID 241200Z - 251200Z

...THERE IS A MDT RISK OF SVR TSTMS OVER PARTS OF SWRN AND S-CNTRL
KY...WRN AND MIDDLE TN...ERN AR...NWRN LA...MUCH OF MS...WRN AND NRN
AL AND FAR NWRN GA...

...THERE IS A SLGT RISK OF SVR TSTMS FROM LOWER ELEVATIONS OF THE
CNTRL AND SRN PLAINS EWD TO THE CNTRL/SRN APPALACHIANS AND ERN GULF
STATES...

..SYNOPSIS


A DYNAMIC LARGE-SCALE PATTERN WILL CONTINUE THROUGH THE DAY TWO
PERIOD FEATURING A DEEP-LAYER CYCLONE WHICH WILL TRACK FROM THE
CNTRL PLAINS ENEWD THROUGH THE MID MS VALLEY BEFORE RE-DEVELOPING
INTO IL/IND BY SUNDAY MORNING. OF POTENTIALLY MORE SIGNIFICANCE IS
A VIGOROUS VORTICITY MAXIMUM AND ATTENDANT 80+ KT MIDLEVEL JET
STREAK WHICH ARE FORECAST TO ROUND THE LARGE-SCALE TROUGH BASE OVER
TX PRIOR TO ACCELERATING NEWD --AS A NEGATIVELY-TILTED WAVE--
THROUGH THE ARKLATEX/LOWER MS VALLEY AND EVENTUALLY INTO THE OH
VALLEY.

AT THE SURFACE...A CYCLONE ASSOCIATED WITH THE PRIMARY UPPER-LEVEL
LOW WILL DEVELOP SLOWLY EWD FROM THE MID MO VALLEY PRIOR TO
WEAKENING OVER IA. AN ASSOCIATED COLD FRONT WILL PUSH SEWD THROUGH
THE MID MO VALLEY AND CNTRL PLAINS...EVENTUALLY INTO THE MID MS
VALLEY. MEANWHILE...A SECONDARY CYCLOGENETIC LOW WILL DEVELOP
RAPIDLY NEWD FROM THE VICINITY OF THE ARKLATEX TO NEAR PAH BY MID
AFTERNOON AND INTO NRN IND BY THE END OF THE PERIOD. A COLD OR
PACIFIC FRONT ASSOCIATED WITH THIS LOW WILL SWEEP EWD THROUGH THE
MID/LOWER MS VALLEYS AND MUCH OF THE OH VALLEY SATURDAY INTO
SATURDAY NIGHT.

--TORNADO OUTBREAK POSSIBLE OVER PARTS OF THE LOWER MS AND TN
VALLEYS INTO CNTRL GULF STATES SATURDAY INTO SATURDAY NIGHT--

...THE LOWER OH/LOWER MS VALLEYS EWD INTO THE CNTRL/SRN APPALACHIANS
INTO THE ERN GULF STATES...

THE PRESENCE OF A BROAD...MOIST WARM SECTOR /I.E. BOUNDARY LAYER DEW
POINTS RANGING FROM AROUND 60 F NEAR THE OH RIVER...TO UPPER 60S TO
AROUND 70 F OVER THE LOWER MS VALLEY AND CNTRL GULF STATES/ WILL SET
THE STAGE FOR A SIGNIFICANT...AND WIDESPREAD OUTBREAK OF SEVERE
THUNDERSTORMS THROUGH MUCH OF THE DAY TWO PERIOD. THIS MOISTURE IN
CONJUNCTION WITH RELATIVELY STEEP MIDLEVEL LAPSE RATES AND DAYTIME
HEATING WILL CONTRIBUTE TO A MODERATELY UNSTABLE AIR MASS BY
AFTERNOON OVER THE LOWER MS VALLEY WHERE MLCAPE WILL APPROACH
1500-2000 J/KG. WHILE CONVECTIVE INSTABILITY WILL LIKELY DECREASE
WITH NWD EXTENT OWING TO MORE WIDESPREAD CLOUDS AND PERSISTENT
CONVECTIVE PRECIPITATION...IT WILL STILL BE SUFFICIENT TO SUPPORT
SEVERE STORMS.

CLUSTERS OF SEVERE TSTMS ARE FORECAST TO BE ONGOING SATURDAY MORNING
FROM THE VICINITY OF THE COLD FRONT OVER ERN TX EWD INTO THE LOWER
MS VALLEY. ACTIVITY WILL SPREAD NWD/NEWD THROUGH THE PERIOD IN
ADVANCE OF THE NEGATIVELY-TILTED MIDLEVEL TROUGH RAPIDLY LIFTING
NEWD THROUGH THE LOWER/MID MS VALLEYS. THE COMBINATION OF A 60-90
KT MIDLEVEL WIND FIELD AND A DEVELOPING 50-60 KT LLJ WILL RESULT IN
STRONG VERTICAL SHEAR ACROSS THE WARM SECTOR. THIS SUGGESTS A HIGH
LIKELIHOOD OF MULTIPLE BANDS/CLUSTERS OF SUPERCELLS CAPABLE OF
DAMAGING WINDS...LARGE HAIL AND TORNADOES PERSISTING THROUGH THE
PERIOD.

THE GREATEST THREAT FOR CYCLIC SUPERCELLS CAPABLE OF SIGNIFICANT
TORNADOES WILL EXIST FROM THE VICINITY OF THE SECONDARY LOW TRACK
SEWD INTO THE WARM SECTOR /I.E. LARGELY DELINEATED BY MDT RISK
AREA/. HERE...ENVIRONMENT WILL BE CHARACTERIZED BY DEEP EFFECTIVE
INFLOW LAYERS YIELDING EFFECTIVE SRH IN EXCESS OF 200-300+
M2/S2...MODERATE INSTABILITY AND 50-70 KT OF DEEP-LAYER SHEAR.
SEVERE STORMS CAPABLE OF MAINLY DAMAGING WINDS AND TORNADOES WILL
CONTINUE THROUGH THE NIGHT WITH THE THREAT EXTENDING NWD THROUGH THE
OH RIVER AND AS FAR E AS THE CNTRL/SRN APPALACHIANS AND ERN GULF
STATES.

..MID/LOWER MO VALLEYS INTO THE MID MS VALLEY


A SECONDARY CONCENTRATION OF STRONG TO SEVERE STORMS IS EXPECTED TO
DEVELOP DURING THE AFTERNOON ALONG AND AHEAD OF THE SECONDARY COLD
FRONT AND INVOF THE DEEP-LAYER CYCLONE SLOWLY SHIFTING ENEWD. HERE
RESIDUAL BOUNDARY LAYER DEW POINTS IN THE MID 40S TO MID 50S COUPLED
WITH DAYTIME HEATING AND COLD MIDLEVEL TEMPERATURES /I.E. -20 C AT
500 MB/ WILL YIELD POCKETS OF MODERATE INSTABILITY WHERE MLCAPE WILL
APPROACH 1000-1500 J/KG.

REGION WILL RESIDE ON THE INNER ENVELOPE OF STRONGER DEEP-LAYER
SHEAR ASSOCIATED WITH THE SYNOPTIC SYSTEM. AS SUCH...SETUP WILL BE
FAVORABLE FOR ORGANIZED MULTICELL OR SUPERCELL STRUCTURES CAPABLE OF
MAINLY SEVERE HAIL. ISOLATED TORNADOES WILL ALSO BE POSSIBLE NEAR
THE SURFACE LOW WHERE AMBIENT...VERTICAL VORTICITY WILL BE
MAXIMIZED.

..MEAD.. 04/23/2010

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