SWODY2
SPC AC 210553
DAY 2 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1253 AM CDT THU JUL 21 2011
VALID 221200Z - 231200Z
...THERE IS A SLGT RISK OF SVR TSTMS FROM THE UPR MS VLY NWWD TO THE
NRN HIGH PLAINS...
...SYNOPSIS...
UPR HIGH WILL CONTINUE TO EXPAND IN AN E-W FASHION OVER THE SRN HALF
OF THE COUNTRY ON FRI. THIS WILL ALLOW THE PROGRESSIVE WLYS TO DIP
FARTHER INTO THE NRN STATES. EMBEDDED WITHIN THIS FLOW WILL BE
SEVERAL IMPORTANT MID-LEVEL SHORTWAVE TROUGHS. LEAD
TROUGH...CURRENTLY OVER THE CANADIAN PRAIRIES WILL BECOME ABSORBED
INTO THE POLAR LOW OVER SERN CANADA BY THU NIGHT. THIS FEATURE WILL
THEN EJECT QUICKLY EWD ACROSS NRN NEW ENGLAND FRI AFTN.
MEANWHILE...SECOND IMPULSE...NOW DIGGING ACROSS WRN BRITISH
COLUMBIA...WILL TURN EWD ACROSS THE NRN ROCKIES DURING FRI
AFTN/NIGHT.
AT THE SFC...A COLD FRONT SETTLING INTO THE GREAT LAKES...CORN BELT
AND CNTRL PLAINS WILL DEVELOP NWD AS A WARM FRONT IN ADVANCE OF THE
SECOND SYSTEM. A NEW COLD FRONT WILL BEGIN TO SWEEP SEWD ACROSS THE
NRN PLAINS FRI EVE/NIGHT. BOTH FRONTS WILL BE THE FOCUS FOR PSBL
SVR TSTMS DURING THE DAY 2 PERIOD.
...UPR MS VLY WWD ACROSS THE NRN PLAINS...
LLVL MOISTURE...CHARACTERIZED BY 70S SFC DEW POINTS...WILL BEGIN TO
SURGE NWD FRI AS SLY FLOW ACCELERATES AHEAD OF THE NEXT UPR SYSTEM.
CONCOMITANT INCREASE IN H85-H7 TEMPERATURES WILL RESULT IN A FAIRLY
SUBSTANTIAL CAP...PARTICULARLY ALONG/SW OF THE MO RVR VLY.
HOWEVER...00Z GFS/NAM AND 12Z ECMWF HINT THAT A SUBTLE SUB-TROPICAL
WAVE MAY EJECT AHEAD OF THE PRIMARY TROUGH... REACHING PARTS OF
SD/ND BY MID-AFTN FRI. STRONG DIABATIC HEATING...STEEP MID-LVL
LAPSE RATES...AND WEAK ASCENT/MOISTENING OF THE BASE OF THE EML WILL
PROBABLY YIELD WDLY SCTD SFC-BASED TSTM INITIATION ALONG THE WARM
FRONT/LEE LOW IN SRN ND AND NRN/CNTRL SD. 0-8KM SHEAR WILL BE MORE
THAN SUFFICIENT FOR SUPERCELLS WITH ALL FACETS OF SVR WEATHER
PSBL...INCLUDING TORNADOES. LACK OF SUBSTANTIAL FORCING AND
QUESTION ON NUMBER OF STORMS GIVEN WARM MID-LVL TEMPS PRECLUDES
HIGHER SVR PROBABILITIES ATTM...BUT SIGNIFICANT SVR WEATHER COULD
OCCUR SHOULD STORMS INDEED FORM. WHETHER SFC-BASED STORMS FORM OR
NOT...LLJ INCREASING TO 40 KTS AND INCREASING LARGE-SCALE ASCENT
ASSOCIATED WITH THE PARENT TROUGH WILL CONTRIBUTE TO CONSIDERABLE
ELEVATED TSTMS FRI NIGHT FROM THE ERN DAKOTAS SEWD INTO THE UPR MS
VLY. IT IS CONCEIVABLE THAT SRN FRINGES OF ACTIVITY COULD BE
SUFFICIENTLY ROOTED AT THE SFC FOR DMGG WIND THREATS TO SPREAD EWD
TOWARD THE UPR MS VLY OVERNIGHT...OTHERWISE...ELEVATED STORMS WILL
PRIMARILY BE MRGLLY SVR WITH HAIL/HVY RNFL.
MEANWHILE...COMPARATIVELY STRONGER FORCING FOR ASCENT WILL SPREAD
ACROSS THE NRN ROCKIES INTO THE NRN HIGH PLAINS FRI AFTN/EVE.
HIGH-BASED STORMS DEVELOPING OVER THE HIGHER TERRAIN WILL ENCOUNTER
AN INCREASINGLY UNSTABLE ENVIRONMENT OVER ERN MT/WRN ND FRI EVENING.
STRENGTHENING VERTICAL SHEAR AND ROBUST MID-LVL LAPSE RATES WILL
FOSTER BOTH SUPERCELLS AND FAST MOVING LINE SEGMENTS WITH HIGH
WINDS/HAIL AND PSBLY A COUPLE TORNADOES. THIS ACTIVITY WILL LIKELY
JOIN STORMS FARTHER E OVERNIGHT RESULTING IN A RATHER EXPANSIVE AREA
OF CONVECTION ACROSS THE NRN PLAINS FRI NIGHT.
...NRN MAINE...
ALTHOUGH LLVL MOISTURE WILL LIKELY BE SCOURED OUT BY FROPA THU
NIGHT/EARLY FRI...APPROACH OF THE LEAD SHORTWAVE TROUGH WILL INDUCE
A BURST OF MOISTURE ADVECTION ALONG/E OF THE ST. LAWRENCE
VLY/INTERNATIONAL BORDER FRI AFTN. IF CAPPING CAN BE
OVERCOME...ISOLD TSTMS WILL LIKELY FORM WITHIN THE INSTABILITY AXIS
AS THE TROUGH SWINGS THROUGH THE REGION. DEEP AND FAST WLY FLOW /H5
50+ KTS/ MAY YIELD A COUPLE SUPERCELLS AND/OR BOWS WITH DMGG WINDS
THE PRIMARY THREAT.
..RACY.. 07/21/2011
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