SWODY1
SPC AC 190543
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1243 AM CDT THU AUG 19 2010
VALID 191200Z - 201200Z
...THERE IS A SLGT RISK OF SVR TSTMS LATE THIS AFTN INTO TONIGHT
ACROSS PARTS OF THE NRN PLAINS AND UPPER MS VALLEY...
...SYNOPSIS...
A HIGH LATITUDE BLOCKING PATTERN APPEARS LIKELY TO PERSIST THROUGH
THIS FORECAST PERIOD...WITH A COMPLEX CYCLONIC VORTEX ELONGATING TO
ENCOMPASS MUCH OF CANADA AND THE CANADIAN/U.S. BORDER AREA. TO THE
SOUTH OF THIS FEATURE...A LINGERING BELT OF WESTERLIES IS PROGGED TO
BECOME MORE ZONAL...WITH EMBEDDED SHORT WAVE IMPULSES CONTRIBUTING
TO THE FLATTENING OF THE SUBTROPICAL RIDGE...WHICH IS LIKELY TO
BECOME INCREASINGLY CONFINED TO THE SOUTHERN TIER STATES.
A FRONTAL ZONE ASSOCIATED WITH THE STRONGER WESTERLIES IS EXPECTED
TO BECOME THE FOCUS FOR SCATTERED THUNDERSTORM ACTIVITY TODAY AND
TONIGHT ACROSS PARTS OF NEW ENGLAND AND THE LOWER GREAT LAKES
REGION...THROUGH THE UPPER MISSISSIPPI VALLEY...THE NORTHERN
PLAINS...INTO THE ROCKIES. ADDITIONAL THUNDERSTORMS ARE LIKELY
ACROSS THE EASTERN GREAT BASIN AND SOUTHERN PLATEAU INTO THE CENTRAL
AND SOUTHERN ROCKIES...WHERE MONSOONAL MOISTURE RETURN CONTINUES.
MEANWHILE...WIDESPREAD THUNDERSTORM ACTIVITY IS EXPECTED AGAIN
ACROSS THE GULF AND SOUTH ATLANTIC COAST STATES. THIS IS WHERE
MOISTURE CONTENT WILL REMAIN HIGH TO THE SOUTH AND EAST OF A
LINGERING MID-LEVEL CYCLONIC SHEAR AXIS...TRAILING FROM A TROUGH
WITHIN THE WEAKER SOUTHERN BRANCH OF THE WESTERLIES...WHICH IS
PROGGED TO ADVANCE OFF THE MID ATLANTIC COAST.
...NORTHERN PLAINS INTO THE UPPER MISSISSIPPI VALLEY...
VERY STEEP MID-LEVEL LAPSE RATES...ASSOCIATED WITH WARM ELEVATED
MIXED LAYER AIR ADVECTING EAST OF THE ROCKIES...WILL CONTRIBUTE TO
INSTABILITY NEAR THE FRONTAL ZONE...STALLED IN A GENERALLY EAST-WEST
ORIENTATION ACROSS THE REGION. EAST OF A THERMAL LOW OVER SOUTH
DAKOTA...SURFACE DEW POINTS AROUND 70F WILL CONTINUE TO SUPPORT
LARGE CAPE NORTH/NORTHEAST OF THE MID MISSOURI VALLEY...FROM PARTS
OF EASTERN SOUTH DAKOTA INTO PARTS OF NORTHERN IOWA AND SOUTHERN
MINNESOTA. HOWEVER...MOISTURE ADVECTION ON EASTERLY FLOW ALONG AND
NORTH OF THE FRONT IS ALSO EXPECTED TO CONTRIBUTE TO SIZABLE CAPE
ACROSS THE NORTHERN HIGH PLAINS.
WHILE THE NOSE OF THE PLAINS SOUTHERLY LOW-LEVEL JET REMAINS FOCUSED
NORTHEAST OF THE MID MISSOURI VALLEY...THE ELEVATED MIXED LAYER MAY
REMAIN INHIBITIVE TO VIGOROUS CONVECTIVE DEVELOPMENT THROUGH MUCH OF
THE DAY. HOWEVER...MODELS HAVE BEEN CONSISTENT IN SUGGESTING THAT
AN AREA OF STRONGER MID-LEVEL FORCING...EMERGING FROM THE
INTERMOUNTAIN WEST...WILL OVERSPREAD SOUTHEAST MONTANA AND PARTS OF
NORTHWESTERN SOUTH DAKOTA INTO WESTERN NORTH DAKOTA BY EARLY THIS
EVENING. THIS SEEMS LIKELY TO BE SUFFICIENT TO SUPPORT THE
INITIATION OF STORMS...IN THE PRESENCE OF SUFFICIENT SHEAR AND
INSTABILITY FOR AN EVOLVING STORM CLUSTER. IF THIS
OCCURS...THERMODYNAMIC PROFILES SHOULD BE FAVORABLE FOR DOWNBURSTS
AND SURFACE COLD POOL DEVELOPMENT...WITH POTENTIAL FOR DAMAGING
SURFACE WINDS.
THE TENDENCY SHOULD BE FOR PROPAGATION EASTWARD ACROSS THE DAKOTAS
DURING THE EVENING HOURS. AT THE SAME TIME...WEAKENING MID-LEVEL
INHIBITION AND STRENGTHENING LOW-LEVEL WARM ADVECTION MAY BECOME
SUFFICIENT TO SUPPORT NEW DEVELOPMENT FARTHER TO THE EAST...ACROSS
PARTS OF THE EASTERN DAKOTAS INTO SOUTHERN MINNESOTA. THIS IS WHERE
BOUNDARY LAYER INSTABILITY WILL REMAIN STRONG WITH THERMODYNAMIC
PROFILES ALSO FAVORABLE FOR THE EVOLUTION OF A MESOSCALE CONVECTIVE
SYSTEM CAPABLE OF PRODUCING DAMAGING WINDS AND HAIL.
...LOWER GREAT LAKES INTO NEW ENGLAND...
WITH MID-LEVEL LAPSE RATES NOT LIKELY TO STEEPEN APPRECIABLY
TODAY...PRE-FRONTAL MOISTENING IS NOT EXPECTED TO BECOME SUFFICIENT
TO SUPPORT ANYTHING MORE THAN GENERALLY WEAK INSTABILITY. THIS IS
EXPECTED TO LIMIT SEVERE POTENTIAL WITH STORMS DEVELOPING ALONG AND
AHEAD OF THE FRONT ADVANCING SOUTHEASTWARD INTO THE REGION LATE THIS
AFTERNOON AND EVENING.
...CAROLINAS...
IT IS NOT OUT OF THE QUESTION THAT A MESOSCALE CONVECTIVE VORTEX NOW
MIGRATING INTO THE SOUTHERN APPALACHIANS REGION COULD SUPPORT
ANOTHER STORM CLUSTER FARTHER EAST...ACROSS THE CAROLINAS...LATER
TODAY. MOISTURE CONTENT WILL REMAIN HIGH...BUT IS NOT YET CLEAR
THAT SUFFICIENT SURFACE HEATING WILL OCCUR TO CONTRIBUTE TO THE
DEVELOPMENT OF A FAVORABLE ENVIRONMENT FOR A SURFACE COLD POOL
CAPABLE OF PRODUCING DAMAGING WIND GUSTS.
..KERR/ROGERS.. 08/19/2010
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