AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE PITTSBURGH PA 525 AM EDT FRI AUG 2 2013 .SYNOPSIS... A CROSSING COLD FRONT WILL INCREASE RAIN CHANCES FOR TONIGHT AND SATURDAY. && .NEAR TERM /THROUGH TONIGHT/... NO CHANGES NEEDED FOR THE NR-DAWN UPDATE. ALTHOUGH A WEAK SHORTWAVE AND SURFACE TROUGH WILL CONTINUE TO SUPPORT ISOLATED SHOWERS TO THE SOUTH OF I 70 THIS MORNING...DRY WEATHER WILL PREVAIL OVER THE REMAINDER OF THE REGION UNTIL AN APPROACHING SHORTWAVE /JET STREAK INCREASES PRECIPITATION CHANCE LATER THIS AFTN AND TNGT...AND PULL A COLDFRONT OVER THE AREA. RUN-TO-RUN CONSISTENCY ISSUES CONT...AS THIS MORNING NAM SOLUTION AGAIN DIVERGES FROM PREVIOUS RUNS AND PROGS A FASTER FRONTAL PASSAGE. AS THE LATEST GFS IS CLOSE TO THIS SCENARIO...HAVE ADJUSTED TIMING AND INCREASED POPS TO CUMULATE IN LIKELY NUMBERS FOR ALL BUT THE EXTREME SRN ZONES TNGT. HAVE ALSO ADDED A THUNDER MENTION...AS INSTABILITY PROGNOSIS MAY JUST BE SUFFICIENT TO SUPPORT AN EMBEDDED POTENTIAL UNDER THE CROSSING JET STREAK. OTHERWISE...CHANGES TO THE TEMP FORECAST ARE NOT SIGNIFICANT...AND WERE AGAIN BASED ON MOSGUIDE BC. && .SHORT TERM /SATURDAY THROUGH MONDAY/... LATEST DETERMINISTIC SOLUTIONS ARE PROJECTING A MORE DECISIVE SWD FRONTAL PENETRATION ON SATURDAY...WHICH WOULD SUPPORT A SHIFT OF HIGHER POPS MORE RAPIDLY TOWARD THE I 70 CORRIDOR. HOWEVER...AS MENTIONED ABOVE...CONFIDENCE IN THE SCENARIO IS SHAKEY AND HAVE DECIDED TO HEDGE WITH LIKELY POPS FOR ALL ZONES THROUGH MIDDAY SATURDAY...WITH A DOWNWARD TREND THEREAFTER. THINK INSTABILITY FIELDS WILL REMAIN SUFFICIENTLY DAMPENED TO PRECLUDE A SEVERE ISSUE...WHICH THE DEEP LAYER SHEAR FORECAST WOULD SEEMINGLY SUPPORT. BUILDING SURFACE HIGH UNDR TROFFG...TO NW FLOW ALOFT WILL SUPPRESS PCPN POTENTIAL FOR SUNDAY AND MONDAY. HOWEVER...LOW-LEVEL LAPSE RATES SHOULD STEEPEN SUFFICIENTLY AMID COLD ADVECTION TO SUPPORT A HEALTHY AFTN STRATOCU FIELD TO ACCOMPANY THE COOLER TEMPS...WHICH WERE FORECAST 5 TO 10 DEGREES UNDER THE AVERAGES USING MOSGUIDE BC. && .LONG TERM /MONDAY NIGHT THROUGH THURSDAY/... GFS AND ECMWF ARE IN GENERAL AGREEMENT IN SCENARIO OF UPPER LOW RETROGRADING OVER MANITOBA BY MIDWEEK...WITH TEMPS MODERATING TOWAD THE SEASONAL AVERAGES OVER THE UPPER OH REGION AS THE FLOW ALOFT FLATTENS. SHRTWV CROSSING N OF THE GREAT LAKES IS FORECAST TO SEND A FRONT TWD THE AREA AND INCRS RAIN CHANCES FOR LATER WED AND THU...ALTHOUGH DEEP MSTR SURGE IS NOT ANTICIPATED GIVEN THE PROGGED POSITIONING OF BROAD RIDGE OVR THE GULF COAST STATES. IN GENL...WPC GUIDANCE REPRESENTED THE SITUATION AND ONLY ADJUSTMENTS COMPLETED WERE TO HIGH TEMPS AND PCPN TIMING.
Posted on: Fri, 02 Aug 2013 11:59:30 +0000
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