SPC AC 221610
   
   DAY 1 CONVECTIVE OUTLOOK  
   NWS STORM PREDICTION CENTER NORMAN OK
   1110 AM CDT WED MAY 22 2013
   
   VALID 221630Z - 231200Z
   
   ...THERE IS A SLGT RISK OF SVR TSTMS THIS AFTN AND EVE ACROSS PARTS
   OF THE OH/TN VALLEYS INTO THE LOWER GREAT LAKES REGION AND NEW
   ENGLAND....
   
   ...SYNOPSIS...
   THE PROMINENT BLOCKING PATTERN NOW PRESENT ACROSS MUCH OF WESTERN
   AND CENTRAL NORTH AMERICA MAY BEGIN TO WEAKEN DURING THIS FORECAST
   PERIOD.  AS A SIGNIFICANT SHORT WAVE TROUGH CONTINUES TO DIG
   SOUTHEAST OF THE HUDSON BAY REGION...THE CLOSED LOW OVER THE UPPER
   MISSISSIPPI VALLEY APPEARS LIKELY TO TRANSITION TO AN OPEN WAVE AND
   SHIFT EASTWARD INTO THE OHIO VALLEY.  A SMALLER SCALE IMPULSE
   CURRENTLY PIVOTING AROUND THE SOUTHERN/EASTERN PERIPHERY OF THIS
   REMNANT CYCLONIC CIRCULATION IS EXPECTED TO SHIFT NORTHEASTWARD
   TOWARD THE UPPER OHIO VALLEY AND LOWER GREAT LAKES REGION THROUGH
   LATE AFTERNOON...BEFORE WEAKENING AND BECOMING INCREASINGLY SHEARED
   WITHIN A CONFLUENT REGIME INTO THE CANADIAN MARITIMES.
   
   AT THE SAME TIME...HOWEVER...LITTLE MOVEMENT IS EXPECTED WITH A
   LARGE AND COLD MID/UPPER LOW NOW CENTERED OVER THE NORTHERN PACIFIC
   COAST STATES.  WHILE SOME DEVIATION OF THE CIRCULATION CENTER IS
   LIKELY...THIS WILL MOSTLY BE IN RESPONSE TO SHORT WAVE IMPULSES
   ROTATING AROUND ITS PERIPHERY.
   
   ...OH/TN VALLEYS INTO THE LOWER GREAT LAKES AND NEW ENGLAND...
   EXTENSIVE CONVECTION AND ASSOCIATED OUTFLOW FROM YESTERDAY HAS
   IMPACTED A SIGNIFICANT PORTION OF THE WARM SECTOR OF THE SURFACE LOW
   NOW CENTERED OVER THE UPPER MIDWEST...FROM THE MISSISSIPPI VALLEY
   EASTWARD TO THE APPALACHIANS.  AND THE FOCUS FOR STORM DEVELOPMENT
   LATER THIS AFTERNOON AND EVENING REMAINS A BIT UNCLEAR. 
   HOWEVER...MODELS CONTINUE TO SUGGEST STRENGTHENING OF LOWER/MID
   TROPOSPHERIC FLOW FIELDS /TO AROUND 40 KTS OR SO/ IN A SOUTHERLY
   BELT ACROSS PARTS OF THE UPPER OHIO VALLEY INTO THE LEE OF THE LOWER
   GREAT LAKES BY LATE THIS AFTERNOON...WHICH SHOULD ENHANCE ANY STORM
   DEVELOPMENT OCCURRING IN THE PEAK HEATING.
   
   SHEAR WILL BE SUFFICIENT FOR ORGANIZED STORM DEVELOPMENT...WHICH MAY
   EVENTUALLY CONSOLIDATE INTO CLUSTERS OR LINES WITH AN ENHANCED RISK
   FOR DAMAGING WIND GUSTS.  BUT DISCRETE STORM DEVELOPMENT ALSO WILL
   BE POSSIBLE...PARTICULARLY NEAR THE CREST OF BROADER SCALE UPPER
   RIDGING ACROSS UPSTATE NEW YORK...WHERE MID/UPPER FLOW WILL BE
   SLOWER TO BACK TO A SOUTHERLY DIRECTION.  MID-LEVEL LAPSE RATES
   APPEAR TO HAVE STABILIZED SOME OVER THE PAST DAY OR SO...BUT SEVERE
   HAIL STILL SEEMS POSSIBLE...ALONG WITH A RISK FOR ONE OR TWO
   TORNADOES...BEFORE STORMS WEAKEN BY LATE EVENING.
   
   ...NORTHERN ROCKIES...
   MOISTURE APPEARS MARGINAL FOR VIGOROUS STORM DEVELOPMENT...BUT
   STRONGER HEATING ALONG AN AXIS FROM NORTHEAST WYOMING THROUGH
   WESTERN MONTANA MAY CONTRIBUTE TO SUFFICIENT DESTABILIZATION FOR
   SCATTERED STORM DEVELOPMENT THIS AFTERNOON.  OROGRAPHIC LIFT WILL BE
   AIDED BY LARGE-SCALE ASCENT BENEATH STRONGLY DIFLUENT AND DIVERGENT
   UPPER FLOW.  GIVEN THE FAVORABLE SHEAR BENEATH STRONG CYCLONIC
   MID/UPPER FLOW...IT DOES NOT APPEAR COMPLETELY OUT OF THE QUESTION
   THAT INSTABILITY ON A VERY LOCALIZED BASIS COULD BECOME ENOUGH FOR A
   SUPERCELL OR TWO WITH THE RISK FOR SEVERE HAIL OR WIND.
   
   ..KERR/DARROW.. 05/22/2013
   
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