SWODY1
SPC AC 261240
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0740 AM CDT FRI AUG 26 2011
VALID 261300Z - 271200Z
...THERE IS A SLGT RISK OF SVR TSTMS ACROSS THE NC OUTER BANKS...
...SYNOPSIS...
PRIMARY WEATHER FEATURE OF INTEREST WILL BE HURRICANE IRENE. THIS
SYSTEM IS CURRENTLY MOVING NWD THROUGH THE WRN ATLANTIC...EAST OF
FL/GA...AND IS FORECAST TO BE LOCATED JUST SW OF FAR ERN NC BY
SUNRISE SATURDAY MORNING. ELSEWHERE...UPPER RIDGE WILL REMAIN
ANCHORED OVER THE SOUTH/CENTRAL ROCKIES REGION...WITH WLYS MAINLY
CONFINED ALONG AND NORTH OF THE CANADIAN/U.S. BORDER. WEAK COLD
FRONT IN THE NRN PLAINS WILL SPREAD SWD INTO THE CENTRAL PLAINS AND
EWD INTO THE WRN GREAT LAKES TODAY...AS A MID/UPPER LEVEL SHORTWAVE
TROUGH AND SPEED MAX SHIFT EWD FROM SRN ONTARIO INTO QUEBEC.
...ERN NC REGION...
AS IRENE MOVES NWD DURING THE DAY...OUTER BANDS OF CONVECTION ARE
EXPECTED TO SHIFT NWD AND WWD THROUGH THE DAY AND MOVE ONTO THE NC
COAST LATER TODAY. THE ACTIVITY IS LIKELY TO BECOME MORE INTENSE
OVERNIGHT AS THE WIND FIELDS STRENGTHEN...WITH 0-3 KM SRH EXPECTED
TO RANGE FROM 300-600 M2/S2 TONIGHT. WHILE SHEAR WILL BE EXTREMELY
FAVORABLE FOR ROTATING STORMS AND TORNADOES...INSTABILITY WILL BE
EXTREMELY WEAK. FORECAST SOUNDINGS AND MODEL DATA SUGGEST MUCAPES
FROM 300-500 J/KG ARE POSSIBLE ACROSS THE NC OUTER BANKS...AND THIS
MAY BE ENOUGH FOR A TORNADO OR TWO AND A SLIGHT RISK HAS BEEN ADDED
FOR THIS AREA. HOWEVER...CURRENT FORECAST TRACK OF IRENE AND
SLIGHTLY HIGHER INSTABILITY EAST OF THE COAST INDICATES THE GREATER
POTENTIAL FOR TORNADOES SHOULD BE OFFSHORE.
...UPPER MS VALLEY...
A FEW ELEVATED STORMS WERE MOVING EWD ACROSS THE REGION WITHIN ZONE
OF WARM ADVECTION. THESE STORMS SHOULD CONTINUE TO SPREAD EWD AND
WEAKEN WITH TIME. HOWEVER...CONVERGENCE ALONG FRONT MAY AID IN
RENEWED CONVECTION DEVELOPING ALONG THE FRONT FROM WI SWWD TO NEAR
THE MN/IA BORDER. ASSUMING TEMPERATURES REACH WELL INTO THE
80S...STEEP MID LEVEL LAPSE RATES WOULD YIELD MLCAPES FROM 1000-1500
J/KG. THIS INSTABILITY COMBINED WITH 30-40 KT DEEP LAYER SHEAR WOULD
BE FAVORABLE FOR A FEW SEVERE MULTICELL STORMS.
HOWEVER...SINCE THE DYNAMICAL FORCING WILL BE NORTH OF THE BORDER
AND MOST OPERATIONAL AND STORM SCALE MODELS DO NOT GENERATE MUCH
CONVECTION...ONLY LOW SEVERE PROBABILITIES ARE FORECAST.
...PACIFIC NW EWD INTO WRN MT...
CONVECTION DEVELOPED EARLIER THIS MORNING IN WRN ORE UNDER UPPER
CIRCULATION. AS THIS IMPULSE SHIFTS EWD DURING THE MORNING...THE
CONVECTION SHOULD WEAKEN AS THE FORCING SPREADS INTO THE DRIER AIR
MASS EAST OF THE CASCADES. THE UPPER WAVE WILL WEAKEN AS IT SHIFTS
EWD INTO THE NRN FRINGE OF THE UPPER RIDGE...BUT ENOUGH FORCING IS
EXPECTED TO SCATTERED THUNDERSTORMS TO DEVELOP OVER NRN ID/WRN MT BY
LATE AFTERNOON. WHILE THE DEEP LAYER SHEAR WOULD BE SUPPORTIVE OF
SEVERE STORMS AND EVEN A SUPERCELL OR TWO... MAINTAINING LOW SEVERE
PROBABILITIES DUE TO PRESENCE OF UPPER RIDGING AND FORECAST OF A
RELATIVELY DRY BOUNDARY LAYER/MARGINAL UNSTABLE BY MOST
MODELS...WHICH IS SUPPORTED BY GTF 12Z SOUNDING.
...CENTRAL HIGH PLAINS...
WV IMAGERY SHOWS EMBEDDED UPPER IMPULSES...WITHIN MID/HIGH LEVEL
TROPICAL MOISTURE...PROGRESSING EWD AROUND THE NRN PERIPHERY OF THE
UPPER RIDGE. THESE IMPULSES COMBINED WITH ELY BOUNDARY LAYER WINDS
MAY RESULT IN SUFFICIENT INSTABILITY AND SHEAR FOR A FEW STORMS WITH
LOCALLY STRONG WINDS.
..IMY.. 08/26/2011
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