Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 250346 AFDRNK AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BLACKSBURG VA 1146 PM EDT THU JUL 24 2014 .SYNOPSIS... A COLD FRONT WILL DROP SOUTHEAST OF THE AREA OVERNIGHT WITH HIGH PRESSURE BUILDING IN ON FRIDAY. A DEVELOPING TROUGH OVER THE MIDWEST WILL ALLOW SEVERAL UPPER DISTURBANCES TO POSSIBLY AFFECT THE AREA OVER THE WEEKEND FOLLOWED BY A STRONG COLD FRONT LATER MONDAY. && .NEAR TERM /THROUGH FRIDAY/... AS OF 1130 PM EDT THURSDAY... CONVECTION HAS ENDED...BUT NORTHEAST SFC WINDS BEHIND FRONT PROLONGING LOW CLOUDS AND EVEN SOME PATCHY -RA EAST OF THE BLUE RIDGE. SOMEWHAT OF AN ATYPICAL SITUATION BEHIND AN EASTWARD MOVING COLD FRONT. HOWEVER...INCREASING CONCERN THAT LOW CLOUDS WILL LIKELY PERSIST ALL NIGHT...ESPECIALLY EAST OF THE BLUE RIDGE AND AREAS WHERE LOW CLOUDS DO NOT PERSIST WILL LIKELY SEE FOG DEVELOPMENT AS DRIER AIR WILL LIKELY NOT ADVECT SIGNIFICANTLY INTO THE CWA UNTIL AFTER DAYBREAK FRI. HAVE ADDED SOME PATCHY DENSE FG AND -DZ TO AREAS IMMEDIATELY EAST OF THE BLUE RIDGE...HENCE BEST UPSLOPE AREAS. TEMPERATURES GENERALLY APPEAR ON TRACK. AS OF 830 PM EDT THURSDAY... CONVECTIVE THREAT FOR THE CWA HAS ENDED AS DEWPOINT FRONT APPEARS TO BE ALMOST ENTIRELY SOUTH OF THE CWA. ALL SHRA/TSRA HAVE PUSHED SOUTH OF THE CWA...EXPECT FOR SOME SPOTTY -SHRA ALONG THE WIND SHIFT FROM LEXINGTON-ROANOKE-FLOYD. HAVE ADJUSTED POPS DOWNWARD SLIGHTLY MORE QUICKLY THAN EARLIER ADVERTISED. FOR 10 PM UPDATE...WILL BE REMOVING ALL MENTION OF TSRA COMPLETELY AND JUST HAVE SMALL/SLIGHT CHC POPS EAST OF THE BLUE RIDGE WHERE WEAK WIND CONVERGENCE STILL IN PLACE. TEMPERATURES RUNNING A LITTLE WARMER THAN FORECAST...SO SLOWED COOLING TREND SLIGHTLY WITH EARLIER UPDATE. AS OF 330 PM EDT THURSDAY... SCATTERED TO NUMEROUS STORMS STILL ONGOING LATE THIS AFTERNOON FROM THE WV BORDER WELL OUT INTO THE PIEDMONT. ACTIVITY IS IN ASSOCIATION WITH A SLOW-MOVING COLD FRONT ANALYZED ACROSS THE NORTHERN SECTIONS OF THE CWA AND A SHORT-WAVE CROSSING THE MOUNTAINS. THE FRONT AND SHORT-WAVE ARE FORECAST TO MOVE EAST TONIGHT TAKING THE PRECIPITATION WITH IT. IN THE MEANTIME...A MODEST SEVERE WEATHER THREAT WILL CONTINUE FOR SEVERAL MORE HOURS ESPECIALLY EAST OF THE BLUE RIDGE WHERE WARMER AND MORE UNSTABLE AIR IS LOCATED ALONG AND AHEAD OF THE BOUNDARY. NICKEL SIZE HAIL REPORTS HAVE COME IN FROM SEVERAL OF THE STRONGER CELLS THIS AFTERNOON. IN ADDITION...PWATS 1.5 TO 1.8 INCHES IN THAT SAME AREA WILL ALLOW FOR POCKETS OF HEAVY RAINFALL WITH SOME LOCAL RUNOFF ISSUES POSSIBLE. SEVERAL CELLS HAVE ALREADY DROPPED 2 INCHES OF RAIN IN ABOUT ONE HOUR BUT MOST OF THE ACTIVITY CONTINUES TO TRACK TOWARD THE EAST AT 15 TO 20 MPH WHICH SHOULD PREVENT MORE SERIOUS WATER ISSUES. SKIES WILL BE SLOW TO CLEAR TONIGHT BUT DEPENDING ON HOW FAST THEY DO COULD SEE AREAS OF DENSE FOG TOWARD MORNING...ADDED PATCHY DENSE FOG TO GRIDS AFTER 06Z. FRIDAY LOOKS LIKE A DRY DAY WITH LOWER HUMIDITY AS DEWPOINTS DROP BACK INTO THE UPPER 50S NW AND L/M 60S FAR SE. HIGH TEMPS WILL BE FAIRLY CLOSE TO MID-JULY NORMALS..RANGING FROM UPPER 70S NW TO M/U 80S SE. && .SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY NIGHT/... AS OF 330 PM EDT THURSDAY... MAINLY DRY AND SEASONAL WEATHER EXPECTED FRIDAY NIGHT INTO AT LEAST SATURDAY MORNING AS WEAK SURFACE HIGH PRESSURE LINGERS OVER AREA...THEN DRIFTS EAST AND OFFSHORE BY SATURDAY AFTERNOON. WEAK DISTURBANCE AND ASSOCIATED WARM MOIST ADVECTION IN NORTHERN STREAM FORECAST TO MOVE EAST THROUGH THE UPPER GREAT LAKES INTO NEW ENGLAND SATURDAY AFTERNOON INTO SATURDAY EVENING MAY BE CLOSE ENOUGH TO BLACKSBURG FORECAST AREA TO SUPPORT DEVELOPMENT OF WIDELY SCATTERED SHOWERS/THUNDERSTORMS MAINLY NORTH OF I64. HOWEVER... BETTER THREAT FOR EXPANDING AREA OF PRECIPITATION WILL LIKELY BEGIN TOWARD DAYBREAK SUNDAY CONTINUING INTO SUNDAY NIGHT AS MUCH MORE FORMIDABLE SHORT WAVE TROF IN ITS WAKE HELPS TO AMPLIFY WHAT WILL BECOME AN UNUSUALLY STRONG LONG WAVE TROF OVER THE UPPER GREAT LAKES BY/DURING THE LONG TERM PERIOD. STRONG WARM MOIST ADVECTION AND INCREASINGLY FAVORABLE JET DYNAMICS ASSOCIATED WITH THIS SECOND SHORT WAVE SHOULD SUPPORT THE DEVELOPMENT OF A MORE ORGANIZED CONVECTIVE EVENT...ESPECIALLY DURING THE TIME OF PEAK HEATING ON SUNDAY AFTERNOON INTO SUNDAY NIGHT. SOUTHWEST TO WESTERLY WINDS WOULD IMPLY MOST FAVORABLE AREA FOR SHOWERS/STORMS WOULD BE ACROSS THE WESTERN FLANKS OF THE APPALACHIANS...WITH A LOWER THREAT EAST OF THE BLUE RIDGE...MINIMIZED ACROSS SOUTHSIDE VA INTO NORTH CENTRAL NC DUE TO NEGATIVE INFLUENCE OF DOWNSLOPING...DISPLACEMENT FROM BETTER DYNAMICS...AND PREFRONTAL STORMS THAT MAY ARRIVE WELL AFTER TIME OF BEST HEATING ON SUNDAY. REGARDLESS...SOUTHWESTERLY WINDS IN ADVANCE OF THIS SYSTEM SHOULD HELP TO ADVECT SOME VERY WARM AIR BACK INTO THE REGION...WITH HIGHS OVER THE WEEKEND RANGING FROM THE 70S ACROSS THE HIGHEST ELEVATIONS...TO 90F OR A BIT ABOVE ACROSS MUCH OF THE PIEDMONT. && .LONG TERM /MONDAY THROUGH THURSDAY/... AS OF 330 PM EDT THURSDAY... LONG TERM PERIOD WILL BE CHARACTERIZED BY UNUSUALLY DEEP LONG WAVE TROF THAT WILL REMAIN CENTERED OVER THE UPPER GREAT LAKES THROUGHOUT MUCH OF NEXT WEEK AS SERIES OF SHORT WAVE TROFS HELP TO AMPLIFY THIS FEATURE. COLD FRONT EXPECTED TO MOVE INTO AND THROUGH THE MOUNTAINS MONDAY MORNING...LIKELY PRECEDED AND ACCOMPANIED BY SCATTERED SHOWERS/THUNDERSTORMS...POSSIBLY LINGERING INTO THE EVENING AS COOLER AIR ALOFT AND UPSLOPE INCREASES ACROSS THE WESTERN FLANKS OF THE APPALACHIANS. WITH EARLIER ARRIVAL OF FRONT...AND ASSOCIATED CLOUDS/THREAT OF PRECIPITATION...DAYTIME HIGHS MAY BE HARD PRESSED TO REACH THE 70S ACROSS SOME OF THE HIGHER ELEVATIONS...ESPECIALLY IN WESTERN GREENBRIER COUNTY WV. EAST OF THE BLUE RIDGE...FRONT NOT LIKELY TO CLEAR THE PIEDMONT COUNTIES UNTIL MID TO LATE AFTERNOON ON MONDAY. INCREASING SOUTHWEST TO WESTERLY WINDS AND ASSOCIATED COMPRESSIONAL HEATING...AND BETTER PREFRONTAL INSOLATION MAY HELP DRIVE TEMPERATURES WELL UP INTO THE 80S BEFORE LEVELING OFF WITH ARRIVAL OF SCATTERED FRONTAL CLOUDS/SHOWERS. MUCH OF THE REST OF THE LONG TERM FORECAST APPEARS TO WARRANT MOSTLY DRY AND COOL CONDITIONS AS SURFACE RIDGE OF HIGH PRESSURE NOSES INTO AND THEN OVER BLACKSBURG FORECAST AREA. MOST AREAS WILL REMAIN 5-10F BELOW NORMAL THROUGH THE PERIOD. MANY MOUNTAIN VALLEYS...ALONG WITH THE HIGHEST RIDGES WILL LIKELY EXPERIENCE AT LEAST A COUPLE OF NIGHTS WHERE TEMPERATURES FALL INTO THE 40S. APPROACH OF ANOTHER SHORT WAVE TROF RIDING AROUND THE BASE OF THE UPPER TROF MAY HELP BACK LOW LEVEL FLOW SUFFICIENTLY ON THURSDAY TO WARRANT A LOW CHANCE THREAT FOR SOME SHOWERS ACROSS THE SOUTHWESTERN MOUNTAIN ON THURSDAY...BUT PRECIPITATION AMOUNTS FROM THIS DISTURBANCE...IF ANY...APPEAR TO BE SCANT AT BEST. && .AVIATION /04Z FRIDAY THROUGH TUESDAY/... AS OF 800 PM EDT THURSDAY... COLD FRONT SINKING SOUTHWARD THROUGH THE CWA AIDED BY LINE OF CONVECTION. FEEL THAT THUNDERSTORM THREAT HAS ENDED FOR THE TAF SITES...ALTHOUGH A FEW -SHRA OR -RA WILL BE NEAR/VICINITY OF DAN/LYH FOR A COUPLE MORE HOURS. BIGGEST CONCERN THROUGH THE TAF VALID PERIOD IS CLOUD COVER AND VISIBILITY. ORIENTATION OF FRONT/FOLLOWING HIGH PRESSURE RESULTING IN NE SFC WINDS BEHIND THE FRONT EAST OF THE ALLEGHANY FRONT...WITH LIGHT NW WINDS TO THE WEST. THIS IS RESULTING IN MORE POST FRONTAL CLOUD COVER EAST OF THE ALLEGHANY FRONT THAN NORMALLY WOULD BE SEEN. EXPECT THIS TO CONTINUE THROUGH MUCH OF THE TAF VALID PERIOD...DIMINISHING AFT 18Z. FEEL THAT CIGS OVERNIGHT WILL LARGELY BE IN THE LOW END VFR OR HIGH END MVFR RANGE EAST OF THE ALLEGHANY FRONT...TO INCLUDE BCB/ROA/LYH/DAN. PERIODS OF MVFR BR ALSO POSSIBLE...ESPECIALLY IN AREAS WHICH SAW HEAVIER RAINFALL...NAMELY LYH/DAN...BUT IFR/LIFR BR EAST OF THE ALLEGHANY FRONT NOT EXPECTED. TO THE WEST...INCLUDING BLF/LWB...A SOMEWHAT DIFFERENT STORY. WINDS NW FOR A FEW MORE HOURS WILL PROMOTE TYPICAL UPSLOPE CLOUD COVER. BLF ALREADY SEEING THIS WITH BKN005 OVC012. LWB WILL LIKELY SEE RADIATIONAL FOG LATER AS HIGHER LEVEL CLOUDS DISSIPATE. THUS...EXPECT CONDITIONS IN THESE AREAS TO LARGELY BE IFR OVERNIGHT WITH MVFR-IFR CIGS...POTENTIALLY LIFR BLF...AND MVFR-IFR VSBYS AS THE NIGHT PROGRESSES. MVFR CIGS WILL LIKELY LINGER INTO MID-DAY WEST OF THE ALLEGHANY FRONT. AS NOTED ABOVE...UNUSUAL ATYPICAL POST FRONTAL WIND DIRECTION...GENERALLY NE 5-7KTS THROUGH THE TAF VALID PERIOD...EXPECT BLF-LWB...NW 5KTS...BECOMING VRB-NE AFT 13Z FRI. LOW TO MEDIUM CONFIDENCE IN CIGS THROUGH THE TAF VALID PERIOD. MEDIUM CONFIDENCE IN VSBYS THROUGH THE TAF VALID PERIOD. MEDIUM TO HIGH CONFIDENCE IN WIND DIR/SPD THROUGH THE TAF VALID PERIOD. EXTENDED AVIATION DISCUSSION... AIRMASS WILL DRY OUT BEHIND THE FRONT GOING INTO FRIDAY- SATURDAY. VFR FRI- SAT OUTSIDE OF ANY VALLEY FOG AT KLWB/KBCB EACH MORNING. ANOTHER FRONT AND ASSOCIATED UPPER IMPULSES COULD BRING A GOOD THREAT OF STORMS IN THE SUN-MON TIME FRAME. OTHERWISE...EXPECT MOSTLY VFR CONDITIONS FRI-SAT...EXCEPT FOR LATE NIGHT/EARLY MORNING FOG. && .HYDROLOGY... U.S. DROUGHT MONITOR ISSUED THIS MORNING AND VALID THROUGH 12Z TUESDAY JULY 22 SHOWED EXPANSION OF THE EXISTING ABNORMALLY DRY CONDITIONS INTO MOST OF OUR SE WV COUNTIES AND FAR WESTERN VA...MAINLY THE UPPER JAMES RIVER BASIN. 30-DAY RAINFALL DEFICITS IN MUCH OF THIS AREA ARE RUNNING 2 TO 3 INCHES OR ONLY 25 TO 50 PERCENT OF NORMAL AND 60-DAY RAINFALL NUMBERS NOT MUCH BETTER. JULY IS CLIMATOLOGICALLY THE WETTEST MONTH IN OUR CWA WITH AN AVERAGE RANGING FROM 4 TO 5 INCHES. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...NONE. NC...NONE. WV...NONE. && $$ SYNOPSIS...PC NEAR TERM...PC/RAB SHORT TERM...WERT LONG TERM...WERT AVIATION...PC/RAB/WP HYDROLOGY...PC

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