SWODY2
SPC AC 311719
DAY 2 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1219 PM CDT WED AUG 31 2011
VALID 011200Z - 021200Z
...THERE IS A SLGT RISK OF SVR TSTMS THURSDAY AND THURSDAY NIGHT
ACROSS PARTS OF THE NORTHERN PLAINS AND UPPER MISSISSIPPI VALLEY...
...SYNOPSIS...
MODELS INDICATE THAT THE UPPER TROUGH...NOW EVOLVING ACROSS THE
PACIFIC COAST STATES...AND COMPRISED OF AT LEAST A COUPLE OF
SIGNIFICANT SHORTER WAVE IMPULSES...WILL LIFT EAST NORTHEASTWARD
AHEAD OF A STRONG UPSTREAM TROUGH DIGGING FROM THE NORTHEAST PACIFIC
THROUGH THE CANADIAN ROCKIES DURING THIS PERIOD. THE INITIAL
PERTURBATION EMERGING FROM THE LEAD FEATURE APPEARS LIKELY TO BE
ACCELERATING NORTHEAST OF THE CENTRAL CANADIAN/U.S. BORDER AREA BY
12Z THURSDAY...CONTRIBUTING TO SURFACE CYCLOGENESIS OVER SOUTHERN
MANITOBA. THE DEEPENING SURFACE LOW CENTER IS EXPECTED TO TRACK
NORTHEASTWARD TOWARD HUDSON BAY DURING THE DAY...WITH A TRAILING
COLD FRONT ADVANCING THROUGH THE NORTH CENTRAL U.S...AS THE
LINGERING UPSTREAM SHORT WAVE IMPULSE GRADUALLY ACCELERATES EAST
NORTHEAST OF THE NORTHERN INTERMOUNTAIN REGION. EVENTUALLY THIS IS
EXPECTED TO CONTRIBUTE TO MID-LEVEL HEIGHT FALLS ACROSS MUCH OF THE
NORTHERN PLAINS/UPPER MISSISSIPPI VALLEY...AND SUPPRESSION OF
SUBTROPICAL RIDGING TO THE SOUTH...BUT PROBABLY NOT UNTIL THE LATTER
PORTION OF THE FORECAST PERIOD.
...NORTHERN PLAINS/UPPER MISSISSIPPI VALLEY...
IN THE WAKE OF THE INITIAL PERTURBATION LIFTING ACROSS CANADA...
MODELS INDICATE A SIGNIFICANT COLD FRONTAL SURGE SOUTHWARD THROUGH
THE NORTHERN PLAINS IN ADVANCE OF THE NORTHERN INTERMOUNTAIN
IMPULSE. THIS IS EXPECTED TO OCCUR BENEATH A PLUME OF VERY WARM AND
STRONGLY CAPPING ELEVATED MIXED LAYER AIR...WHICH WILL BE MUCH
SLOWER TO ADVECT EASTWARD AS LARGER SCALE UPPER TROUGHING MAINTAINS
A POSITIVE TILT ORIENTATION THROUGH MUCH OF THE PERIOD. THIS
COMPLICATES THE CONVECTIVE AND SEVERE WEATHER POTENTIAL FOR
THURSDAY...WHICH MAY ACTUALLY REMAIN FAIRLY LOW ACROSS MOST OF THE
SLIGHT RISK AREA THROUGH THE DAYTIME HOURS.
IT IS NOT OUT OF THE QUESTION THAT THE SOUTHERN FLANK OF A
STRONG/SEVERE MESOSCALE CONVECTIVE SYSTEM...OR MORE ISOLATED
STRONG/SEVERE CELLS JUST TO ITS SOUTH...MAY BE IMPACTING PORTIONS OF
NORTHERN MINNESOTA AT THE OUTSET OF THE FORECAST PERIOD. THIS
THREAT PROBABLY WILL DIMINISH BY LATE MORNING. HOWEVER...IT IS
POSSIBLE THAT AN ASSOCIATED AREA OF COOLING ALOFT COULD WEAKEN
INHIBITION SUFFICIENTLY TO ALLOW NEW STORM DEVELOPMENT AS THE
BOUNDARY LAYER DESTABILIZES DURING THE AFTERNOON...MAINLY NORTHWEST
OF THE MINNEAPOLIS/ST. PAUL AREA THROUGH THE MINNESOTA ARROWHEAD
AREA. IF THIS OCCURS...FORECAST SOUNDINGS INDICATE AN ENVIRONMENT
CHARACTERIZED BY STEEP MID-LEVEL LAPSE RATES... SIZABLE CAPE...AND A
MODEST FLOW REGIME WITH STRONG SHEAR FAVORABLE FOR SUPERCELLS AND
ORGANIZED STORM DEVELOPMENT. LARGE HAIL AND DAMAGING WIND GUSTS
WILL BE POSSIBLE...AND PERHAPS AN ISOLATED TORNADO OR TWO.
OTHERWISE...INCREASING CONVECTIVE POTENTIAL LIKELY WILL AWAIT
STRONGER MID-LEVEL FORCING WITH THE APPROACH OF THE UPSTREAM
IMPULSE. THIS MAY NOT OCCUR UNTIL AFTER 02/00Z ACROSS PARTS OF
EASTERN WYOMING INTO WESTERN SOUTH DAKOTA AND NORTHWEST NEBRASKA
...AND AFTER 02/06Z ACROSS CENTRAL/EASTERN SOUTH DAKOTA INTO
SOUTHWESTERN MINNESOTA. MUCH OF THIS ACTIVITY WILL BE BASED IN A
RESIDUAL MOIST LAYER ABOVE THE COLD FRONT...LIKELY IN THE 850-700 MB
LAYER. BUT MID-LEVEL LAPSE RATES ARE EXPECTED TO REMAIN STEEP
ENOUGH TO SUPPORT SUFFICIENT CAPE FOR AT LEAST A SEVERE HAIL
THREAT...PARTICULARLY ACROSS EASTERN SOUTH DAKOTA INTO SOUTHWEST
MINNESOTA.
..KERR.. 08/31/2011
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