Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 120557 AFDRNK AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BLACKSBURG VA 157 AM EDT SAT JUL 12 2014 .SYNOPSIS... WEAK HIGH PRESSURE WILL CONTINUE TO INFLUENCE THE AREA THROUGH SATURDAY...BEFORE DEEPER MOISTURE BEGINS TO RETURN SUNDAY AND INTO EARLY NEXT WEEK AHEAD OF A STRONG COLD FRONT THAT WILL BE PASSING THROUGH THE REGION TUESDAY OR TUESDAY NIGHT. HIGH PRESSURE FOLLOWING THE FRONT SHOULD BRING MUCH DRIER AND COOLER AIR FOR THE MIDDLE AND END OF NEXT WEEK. && .NEAR TERM /THROUGH TODAY/... AS OF 1125 PM EDT FRIDAY... GOING TO SEE MAINLY CLEAR SKIES OVERNIGHT WITH SOME SCT SC/AC IN THE MTNS. OVERALL LOOKING DRY TONIGHT WITH PATCHY FOG...ESPECIALLY VALLEYS. NO OTHER MAJOR CHANGES TO THE FORECAST AT THIS UPDATE. PREVIOUS VALID DISCUSSION... RELATIVELY DRY HIGH PRESSURE IN CONTROL THROUGH SATURDAY. SOME CONCERN ABOUT SOME STRATUS DEVELOPING ALONG BLUE RIDGE WITH WEAK UPSLOPE AND CONVERGENCE BUT THINK MODELS HAVE BACKED OFF A LITTLE FROM THIS AND FLOW IS VERY WEAK. SATURDAY ONLY A VERY SLIGHT INCREASE IN MOISTURE AS VERY WEAK SOUTHERLY TO SW FLOW DEVELOPS LATE IN THE DAY...BUT THIS IS VERY SUBTLE IN THE FCST SOUNDINGS AND MID LEVEL DRY AIR STILL SHOULD DOMINATE. OVERALL THOUGH EXPECT SOME ISOLATED COVERAGE IN MTNS...PROBABLY A LITTLE MORE THAN WHAT WE ARE SEEING TODAY...BUT AGAIN MOST LOCATIONS DRY. TEMPS A COUPLE DEGREES WARMER THAN TODAY...GOING CLOSE TO GUIDANCE AND PREVIOUS FCST WITH LOWER 90S PIEDMONT AND MID 80S IN THE MOUNTAINS. && .SHORT TERM /TONIGHT THROUGH MONDAY NIGHT/... AS OF 230 PM EDT FRIDAY... EXPECT MAINLY DRY CONDITIONS TO PREVAIL ACROSS THE BLACKSBURG FORECAST AREA SATURDAY NIGHT THROUGH SUNDAY NIGHT...WITH PERHAPS EXCEPTION OF SOME DIURNALLY DRIVEN WIDELY SCATTERED THUNDERSTORMS IN THE MOUNTAINS INTO THE FOOTHILLS AS RETURN UPSLOPE FLOW ON THE BACKSIDE OF DEPARTING SURFACE RIDGE IMPACTS THE EASTERN FLANKS OF THE APPALACHIANS...WHILE SOUTHWESTERLY WINDS IMPINGE UPON THE WESTERN FLANKS OF THE APPALACHIANS SETTING UP A BROAD ZONE OF LOW LEVEL CONFLUENCE. WEAK FORCED ASCENT COMBINED WITH SLOWLY INCREASING MOISTURE AND DIFFERENTIAL HEATING MAY BE SUFFICIENT TO OVERCOME WEAKENING CAPPING INVERSION...ESPECIALLY BY SUNDAY AFTERNOON/EVENING AS WARM MOIST ADVECTION CONTINUES TO SLOWLY INCREASE OVER THE AREA. APPROACH OF A VERY STRONG AND DEEPENING LONG WAVE TROF DIGGING SOUTH INTO THE UPPER GREAT LAKES WILL BEGIN TO IMPACT THE BLACKSBURG FORECAST AREA MONDAY INTO MONDAY NIGHT. SUBTLE BACKING OF LOW LEVEL WINDS IN ADVANCE OF THIS SYSTEM AND CONTINUED THETA-E ADVECTION SHOULD BE MORE THAN SUFFICIENT TO ERODE ANY LINGERING CAP...EVEN ACROSS THE PIEDMONT...AND WITH MORE OF A SOUTHWESTERLY LOW LEVEL FLOW...ANY UPSTREAM CONVECTION DEVELOPING OVER/NEAR THE WESTERN FLANKS OF THE APPALACHIANS WILL BE MORE LIKELY TO CROSS EAST OF THE BLUE RIDGE BY MID-LATE AFTERNOON INTO THE EVENING. INCREASING THERMODYNAMICS AND SHEAR WARRANTS AN INCREASING CONCERN FOR PRE-FRONTAL SEVERE WEATHER ON MONDAY AFTERNOON/EVENING...BUT AREAL EXTENT AND OVERALL THREAT WILL BE LARGELY INFLUENCED BY HOW MUCH BACKING OF LOW LEVEL WIND IN ADVANCE OF THE TROF WILL OCCUR (A MORE WESTERLY WIND WILL LIMIT PRIMARY THREAT TO THE WESTERN FLANKS OF THE APPALACHIANS AS OPPOSED TO EXPANDING THE THREAT FURTHER EAST). REGARDLESS...SUFFICIENT CONCERN EXISTS THAT THE STORM PREDICTION CENTER HAS HIGHLIGHTED PORTIONS OF THE BLACKSBURG FORECAST AREA IN A POTENTIAL SEVERE WEATHER THREAT ON DAY 4 (MONDAY)...AND THIS SEEMS PRUDENT AT THE PRESENT TIME AND HAS ALREADY BEEN REFLECTED IN THE BLACKSBURG HAZARDOUS WEATHER OUTLOOK. LOSS OF DAYTIME HEATING SHOULD BE SUFFICIENT TO ALLOW CONVECTIVE ACTIVITY TO WANE BY MID-LATE MONDAY EVENING...PERHAPS ENDING ALTOGETHER EAST OF THE BLUE RIDGE. FURTHER WEST...APPROACH OF COLD FRONT AND ASSOCIATED CONVERGENCE AND BETTER JET DYNAMICS SHOULD MAINTAIN CHANCE OF ONGOING OR REDEVELOPING NEW CONVECTION THROUGHOUT THE NIGHT WEST OF THE BLUE RIDGE. THIS SCENARIO ESPECIALLY TRUE IF MUCH MORE PROGRESSIVE GFS MODEL SOLUTION VERIFIES...WHICH IS A GOOD 12 HOURS FASTER IN FRONTAL MOVEMENT/POSITIONING THAN ECMWF MODEL AT THE PRESENT TIME. NEAR SEASONAL TEMPERATURES EXPECTED THROUGHOUT THE SHORT-TERM PERIOD...MAINLY 60S AT NIGHT...WITH MID 70S TO MID 80S MOUNTAINS TO UPPER 80S TO LOWER 90S PIEDMONT. && .LONG TERM /TUESDAY THROUGH FRIDAY/... AS OF 1230 PM EDT FRIDAY... TUESDAY A SURFACE LOW WILL RESIDE OVER THE EASTERN GREAT LAKES WITH A 500MB LOW CENTERED OVER LAKE MICHIGAN. THE COLD FRONT AT 12Z WILL BE TO THE WEST OF CHARLESTON WV. A SURFACE HIGH SOUTH OF THE FL PANHANDLE WILL STREAM PLENTY OF MOISTURE INTO THE REGION AHEAD OF THE COLD FRONT. 1-2K J/KG MOST UNSTABLE CAPE VALUES...30KT MEAN LAYER FLOW...AND MODEST LAPSE RATES WILL ALLOW SHOWERS AND THUNDERSTORMS TO DEVELOP AHEAD OF THE FRONT. THE KEY ON TUESDAY WILL BE THE TIMING OF THE FRONTAL PASSAGE. THERE IS STILL SIGNIFICANT DIFFERENCES BETWEEN THE 12Z GFS AND THE LAST RUN OF THE ECMWF. THE GFS WANTS TO RACE THE FRONT THROUGH THE REGION BETWEEN 12-18Z ON TUESDAY...WHILE THE ECMWF HANGS THE FRONT UP LONGER WITH PASSAGE BETWEEN TUESDAY NIGHT AND WEDNESDAY MORNING WHICH WOULD ALLOW FOR MORE DESTABILIZATION AND THE POTENTIAL FOR STRONG STORMS DURING THE AFTERNOON AND EVENING ON TUESDAY. THE GFS SEEMS TOO ROBUST WITH THE FRONTAL PASSAGE SO THE FORECAST HAS TRENDED MORE TOWARDS THE ECMWF. WHAT IS CERTAIN IS THAT AFTER THE COLD FRONT PASSES THROUGH THE AREA A 1020MB SURFACE HIGH BUILDS IN ALONG WITH MUCH COOLER TEMPERATURES. A NOTICEABLE DROP IN HUMIDITY WILL BE FELT AS DEWPTS CRASH INTO THE 40S AND 50S. HIGH TEMPS WILL BE SEVERAL DEGREES BELOW NORMAL WEDNESDAY AND THURSDAY WITH LOW TO MID 70S ACROSS THE MOUNTAINS AND NEAR 80 EAST OF THE BLUE RIDGE. NIGHT TIME LOWS WILL ALSO BE BELOW NORMAL...WILL NOT BE SURPRISED TO SEE A FEW MOUNTAIN LOCATIONS DROP INTO THE MID TO UPPER 40S ON THURSDAY MORNING...DEPENDING ON CLOUD COVER. && .AVIATION /06Z SATURDAY THROUGH WEDNESDAY/... AS OF 745 PM EDT FRIDAY... SATELLITE PICTURES SHOWED SCATTERED MID AND HIGH CLOUDS AT 05Z/1AM. STILL ENOUGH CLEARING THAT TEMPERATURES HAVE LOWERED CLOSE TO THE DEW POINT AT KLYH/KDAN/KLWB AND KBCB. MODERATE TO HIGH CONFIDENCE OF MVFR FOG FORMATION AT THESE LOCATIONS BY 09Z/5AM. LOWER CONFIDENCE THAT LIFR FOG WILL DEVELOP. MOST LIKELY LOCATIONS IF THAT DOES OCCUR WOULD BE AT KBCB AND KLWB. PLENTY OF DRY AIR ABOVE THE SURFACE BASED INVERSION THAT FOG WILL MIX OUT QUICKLY AFTER SUNRISE. SCATTERED TO BROKEN VFR STRATOCUMULUS...AROUND 4000 FT WILL DEVELOP DURING THE LATE MORNING. ISOLATED THUNDERSTORMS ARE EXPECTED ALONG THE BLUE RIDGE IN THE AFTERNOON. PROBABILITY OF OCCURRENCE IS TOO LOW AT ANY ONE AIRPORT TO INCLUDE IN THE TAF. EXTENDED AVIATION DISCUSSION... MOISTURE AND MORE WIDESPREAD SHRA/TSRA LIKELY TO RETURN FOR SUNDAY AND EARLY NEXT WEEK AS A BROAD UPPER TROUGH DIGS INTO THE GREAT LAKES REGION AND RESULTANT CYCLONIC FLOW PROVIDES INCREASINGLY UNSTABLE CONDITIONS ACROSS THE REGION. BEST CHANCE OF WIDESPREAD MVFR CIGS/VSBYS WILL BE LATE MONDAY AND INTO TUESDAY WITH THE PASSAGE OF A STRONG COLD FRONT WITH OCNL IFR CIGS/VSBYS IN WITH THE AFTERNOON/EVENING TSRA. PATCHY LATE NIGHT/EARLY MORNING FOG WILL RESULT IN LOCAL MVFR TO IFR CONDITIONS DURING THE LATE NIGHTS AND EARLY MORNINGS...ESPECIALLY IN VALLEY LOCATIONS AND WHERE EARLIER AFTERNOON RAINFALL OCCURRED. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...NONE. NC...NONE. WV...NONE. && $$ SYNOPSIS...JH/SK NEAR TERM...SK/WP SHORT TERM...WERT LONG TERM...CF AVIATION...AMS/RAB

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