National Weather Service United States Department of Commerce

Unsettled Weather Continues for the Northern U.S., Alaska, and Hawaii

Low pressure will bring strong winds and heavy snow to the northern Plains and upper Great Lakes into Monday. Farther south, strong to severe thunderstorms accompanied by locally heavy rainfall are possible over the ArkLaTex and lower Ohio Valley through tonight. Meanwhile, a major winter storm will affect western and central Alaska. Heavy rainfall and flooding are possible for much of Hawaii. Read More >

Day 1 Convective Outlook issued at 1255 UTC 27 April 2011

Hail probabilityTornado probabilityWind probability

SPC AC 271255
   
DAY 1 CONVECTIVE OUTLOOK  
NWS STORM PREDICTION CENTER NORMAN OK
0755 AM CDT WED APR 27 2011
   
VALID 271300Z - 281200Z
   
...THERE IS A HIGH RISK OF SVR TSTMS TODAY INTO EARLY TONIGHT FOR NE
MS...CENTRAL/NRN AL...NW GA...SE TN...
   
...THERE IS A MDT RISK OF SVR TSTMS SURROUNDING THE HIGH RISK...FROM
NRN/CENTRAL MS TO SRN AL...NW GA...MIDDLE/ERN TN...WRN SC...WRN
NC...WRN VA...AND ERN KY...
   
...THERE IS A SLGT RISK OF SVR TSTMS FROM THE N CENTRAL GULF COAST
TO THE OH VALLEY/APPALACHIANS AND NY...
   
--DANGEROUS TORNADO OUTBREAK IS EXPECTED FROM LATE MORNING INTO LATE
EVENING ACROSS THE TN VALLEY REGION--
   
...SYNOPSIS...
AN INTENSE MIDLEVEL SHORTWAVE TROUGH AND ASSOCIATED 100 KT 500 MB
JET WILL PIVOT EWD FROM TX/OK THIS MORNING TO LOWER MS/TN VALLEY
REGION BY THIS EVENING...AND CONTINUE NEWD TO THE CENTRAL
APPALACHIANS OVERNIGHT.  THE PRIMARY SURFACE CYCLONE WILL DEVELOP
NEWD FROM WRN AR THIS MORNING TO WRN KY/SRN INDIANA BY THIS
EVENING...AND THEN CONTINUE NEWD TO THE LOWER GREAT LAKES OVERNIGHT.
A TRAILING COLD FRONT AND REMNANT DRYLINE FEATURE WILL SWEEP EWD
ACROSS THE ARKLAMISS THIS AFTERNOON...PROVIDING A FOCUS FOR SEVERE
STORM DEVELOPMENT.  GIVEN THE PRESENCE OF VERY STRONG WINDS
THROUGHOUT THE TROPOSPHERE AND AN INFLUX OF RICH MOISTURE FROM THE
GULF OF MEXICO...THE ENVIRONMENT WILL BE FAVORABLE FOR A DANGEROUS
OUTBREAK OF FAST-MOVING TORNADIC SUPERCELLS AND EMBEDDED BOW ECHOES
TODAY INTO EARLY TONIGHT.
   
...MS/AL/TN/KY/GA AREA THROUGH TONIGHT...
THE REMNANTS OF AN INTENSE OVERNIGHT MCS /INCLUDING EMBEDDED
SUPERCELLS AND BOWS/ IS WEAKENING OVER NE AL AND MIDDLE TN AS OF
SUNRISE.  OTHER ELEVATED THUNDERSTORM DEVELOPMENT IS UNDERWAY TO THE
W FROM NE LA INTO SE AR/NRN MS.  SOME FORM OF THIS ELEVATED
CONVECTION WILL LIKELY PERSIST THROUGH THE DAY WHILE SPREADING ENEWD
OVER NRN MS INTO WRN/MIDDLE TN.  S OF THIS ACTIVITY AND IN THE WAKE
OF THE EARLIER MS/AL MCS...IT APPEARS THAT THE WARM SECTOR WILL
RECOVER QUICKLY DURING THE MORNING/EARLY AFTERNOON IN RESPONSE TO
CLOUD BREAKS AND STRONG LOW-LEVEL SLY FLOW.  A RESERVOIR OF 70-72 F
BOUNDARY LAYER DEWPOINTS FROM SRN LA TO SRN AL WILL SPREAD NWD IN
THE WAKE OF THE MORNING STORMS...BENEATH THE REMNANTS OF STEEP
MIDLEVEL LAPSE RATE PLUME SPREADING EWD FROM TX/LA.  SURFACE HEATING
WITHIN THE MOIST WARM SECTOR WILL BOOST MLCAPE VALUES TO 2500-4000
J/KG ALONG AND S OF THE REMNANT OUTFLOW BOUNDARY...AND REDUCE
CONVECTIVE INHIBITION BY ABOUT MIDDAY.  THIS WILL ALLOW THE
DEVELOPMENT OF SCATTERED-NUMEROUS WARM SECTOR SUPERCELLS ALONG
CONFLUENCE BANDS E OF THE COLD FRONT/DRYLINE BY EARLY AFTERNOON.
   
THE VERTICAL SHEAR ENVIRONMENT WILL BECOME VERY FAVORABLE FOR
TORNADIC SUPERCELLS...CHARACTERIZED BY LONG/CURVED HODOGRAPHS WITH
EFFECTIVE BULK SHEAR IN EXCESS OF 70 KT AND EFFECTIVE SRH OF 300-600
M2/S2 IN THE UNSTABLE WARM SECTOR.  GIVEN THE EXPECTATION OF SEVERAL
HOURS OF DISCRETE SUPERCELL DEVELOPMENT IN THE WAKE OF THE MORNING
STORMS...A VERY MOIST BOUNDARY LAYER...THE STRONG INSTABILITY...AND
INTENSE VERTICAL SHEAR...THE SCENARIO APPEARS FAVORABLE FOR MULTIPLE
LONG-TRACK SUPERCELLS WITH THE POTENTIAL TO PRODUCE INTENSE/DAMAGING
TORNADOES IN THE HIGH RISK AREA.
   
...AR NEWD TO THE LOWER OH VALLEY TODAY...
ALONG THE PATH OF THE MIDLEVEL TROUGH AND SURFACE CYCLONE...THERE
WILL BE A FOCUS FOR ADDITIONAL SEVERE THUNDERSTORM DEVELOPMENT LATER
TODAY.  THE DEGREE OF INSTABILITY IS IN QUESTION GIVEN THE IMPACTS
OF OVERNIGHT AND ONGOING CONVECTION FARTHER TO THE SE.  STILL...ANY
CLOUD BREAKS WILL ALLOW DESTABILIZATION BENEATH COOL MIDLEVEL
TEMPERATURES...IN AN ENVIRONMENT OF STRONG DEEP-LAYER SPEED SHEAR. 
DAMAGING WINDS/HAIL AND A COUPLE OF TORNADOES WILL BE POSSIBLE WITH
ANY CLUSTERS IMMEDIATELY IN ADVANCE OF THE SURFACE LOW.
   
...UPPER OH VALLEY TONIGHT...
THE PRIMARY SURFACE CYCLONE AND STRONG KINEMATIC PROFILES ARE
EXPECTED TO REACH THE UPPER OH VALLEY EARLY TONIGHT.  THE MAGNITUDE
OF WARM SECTOR INSTABILITY IS IN QUESTION GIVEN WIDESPREAD
CONVECTION ONGOING AND EXPECTED TO THE SOUTH.  HOWEVER...VERY STRONG
LOW-LEVEL SHEAR IN A MOIST ENVIRONMENT COULD SUPPORT A FEW TORNADOES
WITH EMBEDDED BOWS AND SUPERCELL STRUCTURES.
   
..THOMPSON/COHEN.. 04/27/2011
   
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