Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 030848 AFDRNK AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BLACKSBURG VA 448 AM EDT THU SEP 3 2015 .SYNOPSIS... HIGH PRESSURE ACROSS THE AREA WILL KEEP A HOT AND HUMID AIRMASS IN PLACE THROUGH THE END OF THE WEEK. A BACKDOOR COLD FRONT WILL CROSS THE REGION LATE FRIDAY NIGHT INTO SATURDAY...BRINGING BETTER CHANCES OF SHOWERS AND STORMS IN ADVANCE OF IT LATE FRIDAY INTO FRIDAY NIGHT. DRIER AND COOLER AIR WILL WORK ITS WAY INTO THE AREA FOR THE LATTER HALF OF THE WEEKEND INTO THE START OF NEXT WEEK. && .NEAR TERM /THROUGH TONIGHT/... AS OF 240 AM EDT THURSDAY... OVERALL PERSISTENCE FORECAST CONTINUES OVER THE NEXT 24 HOURS WITH YET ANOTHER HOT/HUMID AFTERNOON IN STORE TODAY UNDER SURFACE HIGH PRESSURE AND WARM 85H TEMPS. HOWEVER ENOUGH OF A WEAKNESS ALOFT IN THE WAKE OF THE UPPER WAVE FROM WEDNESDAY TO ELIMINATE ANY CAPPING THIS AFTERNOON ESPCLY GIVEN LOWER 5H HEIGHTS AND SLIGHTLY COOLER MID LEVEL TEMPS. THIS AGAIN SUPPORTS THE POTENTIAL FOR AT LEAST SCATTERED CONVECTIVE DEVELOPMENT ESPECIALLY BLUE RIDGE VICINITY AND POINTS WEST INTO EARLY THIS EVENING. LATEST MODEL SOLUTIONS MOSTLY IN LINE WITH THIS SCENARIO BUT DIFFER ON THE DEGREE OF COVERAGE AND WHERE ANY MORE ORGANIZED CLUSTERS MAY TAKE SHAPE. SPC WRF AGAIN SHOWING MORE WIDESPREAD MOUNTAIN TSRA WITH FOCUS ON RESIDUAL WEAK TROFFINESS SLIDING SE FROM THE OHIO VALLEY INTO WEAK LOW LEVEL CONVERGENCE AIDED BY OROGRAPHICS AND HIGH INSTABILITY. OTHER SOLUTIONS HAVE LESS WITH MORE SHOTGUN FLAVOR FARTHER WEST AND A BIT SLOWER TO INIT UNTIL MID OR LATE AFTERNOON. SINCE THE NCAR CAM PROGS SIMILAR TO THE SPC WRF WHICH WAS BEST YESTERDAY WENT WITH A BIT HIGHER POP COVERAGE OUT TO THE FOOTHILLS WITH PERHAPS A SWATH OF BEST COVERAGE NORTHERN BLUE RIDGE BACK TOWARD THE NEW RIVER...JUST SOUTH OF WHERE CONVECTION WAS ON WEDNESDAY. SEVERE THREAT AGAIN LIMITED BY LITTLE SHEAR BUT SLOW MOVEMENT AND UNIFORM BUT WEAK NORTHERLY FLOW ALOFT COULD SUPPORT A FEW STRONGER BANDS OR PULSE CLUSTERS LATE. HIGH TEMPS AGAIN LOOK TO TOP 90 IN THE EAST AND ESPECIALLY WHERE THE GROUND IS DRIER OVER THE SE. OTRW STAYED A BIT ON THE WARM SIDE OF MOS GIVEN HIGHS ABOVE GUIDANCE OF LATE. ANY CONVECTION SHOULD AGAIN FADE THIS EVENING BUT MAY TEND TO LINGER IN SPOTS OVER THE NORTH/EAST GIVEN APPROACH OF WEAK SHORTWAVE ENERGY FROM THE NW LATE. HOWEVER DID GO MOSTLY DRY AFTER MIDNIGHT WITH MORE FOG AROUND ALTHOUGH COVERAGE QUITE DEPENDENT UPON EARLIER RAINFALL. MUGGY LOWS AGAIN MOSTLY 60S TO AROUND 70. && .SHORT TERM /FRIDAY THROUGH SUNDAY NIGHT/...
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AS OF 430 AM EDT THURSDAY... A COMPLEX ARRAY OF WEATHER SYSTEMS ALOFT WILL DICTATE THE WEATHER ACROSS THE REGION THROUGH THE WEEKEND.. HIGH PRESSURE ALOFT WILL BE THE MAIN THEME ACROSS MUCH OF THE EASTERN U.S....WITH AN ANOMALOUS 590DM HIGH ANCHORED ACROSS THE GREAT LAKES. MEANWHILE...A BROAD AREA OF LOW PRESSURE ALOFT...INITIALLY OVER THE REGION TODAY...WILL DRIFT SLOWLY SOUTH THROUGH THE WEEKEND INTO THE SOUTHEAST STATES...WHERE IT IS DESTINED TO LINGER INTO EARLY NEXT WEEK. AS THE UPPER HIGH BUILDS ACROSS THE NORTHEAST U.S...SURFACE HIGH PRESSURE WILL BUILD SOUTHWEST FROM NEW ENGLAND INTO THE MID-ATLANTIC. THIS WILL PUSH A BACKDOOR COLD FRONT THROUGH OUR REGION FRIDAY AFTERNOON/EVENING...THEN INTO SOUTHWEST VA FOR THE WEEKEND...WHERE IT WILL STALL. THE BOUNDARY WILL SERVE AS A FOCUSING MECHANISM FOR CONVECTION FRIDAY AFTERNOON ACROSS MUCH OF THE CWA...BEGINNING IN THE NORTHEAST AT MIDDAY...ADVECTING SOUTHWEST WITH TIME. CONVECTION CHANCES WILL DECREASE ACROSS THE MORE STABLE AREAS BEHIND THE BACKDOOR FRONT SATURDAY...BUT LINGER ACROSS THE SOUTHWEST PART OF THE FRONT SATURDAY AND SUNDAY. THERE WILL BE A DIURNAL SWING TO CONVECTION ON ALL OF THESE DAYS. ALTHOUGH UPPER DYNAMICS ARE VERY WEAK UNDER THE LARGE SCALE UPPER RIDGE...THERMODYNAMICS FRIDAY AFTERNOON ARE QUITE NOTABLE. IN ADDITION...THE TIMING OF THE BACKDOOR FRONT INTO THE REGION DURING PEAK HEATING SHOULD YIELD SOME FAIRLY ROBUST PULSE CONVECTION. CAPES ARE WELL IN EXCESS OF 1000 J/KG ACROSS MUCH OF THE CWA FRI AFTERNOON AND LIS DROP TO AS LOW AS -6 TO -8 IN SOME AREAS...MOST NOTABLY ON THE GFS. PWATS WILL BE CREEPING BACK UP INTO THE 1.5 TO 1.75 RANGE...SO THERE WILL BE LOCALLY HEAVY RAINFALL AS WELL. FORTUNATELY...FROM THAT STANDPOINT...IT HAS BEEN QUITE DRY RECENTLY...SO RAINFALL...OTHER THAN VERY LOCALIZED ISSUES...SHOULD NOT BE A PROBLEM. GREATER CONCERN WOULD BE FOR LOCALIZED DOWNBURSTS AS AFTERNOON TEMPERATURES WILL BE WELL ABOVE NORMAL ONCE AGAIN...WITH 90S COMMON ACROSS THE PIEDMONT AND 80S WEST...EXCEPT MID TO UPPER 70S MOUNTAINS. CONVECTION WILL TEND TO BEGIN IN THE NORTHEAST PART OF THE CWA/I-64 CORRIDOR EARLY IN THE AFTERNOON...WORKING SOUTHWEST TOWARD NW NC AND SW VA BY LATE AFTERNOON. WITH RESPECT TO TEMPERATURES...FRIDAY WILL AGAIN BE A HOT...WELL ABOVE NORMAL DAY AS NOTED ABOVE. MOS GUIDANCE...AS HAS BEEN THE CASE THE PAST SEVERAL DAYS...WAS TOO COOL AGAIN TODAY...AND THUS READINGS WERE RAISED ON AVERAGE 3-4 DEGREES AT MANY LOCATIONS. MIN TEMPS WILL ALSO BE WELL ABOVE NORMAL AGAIN FRIDAY. COOLER/DRIER CONDITIONS WILL FILTER INTO THE AREA FROM THE EAST FOR THE WEEKEND...WITH THE MOST NOTABLE COOLING EAST OF THE BLUE RIDGE...AND TO A LESSER EXTENT IN THE SOUTHWEST PART OF THE CWA NEAR THE STALLED FRONTAL BOUNDARY. OVERALL...GUIDANCE SEEMED TO HAVE A REASONABLE HANDLE ON THE COOLING TREND ACROSS THE EAST.
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&& .LONG TERM /MONDAY THROUGH WEDNESDAY/...
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AS OF 445 AM EDT THURSDAY... NO SIGNIFICANT CHANCES IN THE SYNOPTIC PATTERN EARLY IN THE WEEK WITH A BROAD UPPER LOW/SHEAR/WEAKNESS ZONE LINGERING ACROSS THE SOUTHEAST STATES AND ALONG THE SC/NC COASTLINE. AN UPPER RIDGE PERSISTS FROM THE SOUTH CENTRAL PLAINS INTO THE NORTHEAST U.S...BUT IS BEING DAMPENED BY A STRONG SHORT WAVE MOVING FROM THE PACIFIC NORTHWEST TOWARD THE GREAT LAKES/NORTHEAST EARLY IN THE WEEK. THE ASSOCIATED FRONTAL BOUNDARY WILL BE APPROACHING THE REGION FROM THE NORTHWEST BY THE MID PART OF NEXT WEEK. CONSIDERABLE QUESTION REMAINS AS TO WHETHER OR NOT THE FRONT WILL ACTUALLY MOVE INTO THE CWA...AND ESPECIALLY WHETHER OR NOT IT WILL MOVE SOUTH OF THE CWA...AS THE FLOW IS LARGELY ZONAL AND BLOCKED BY THE UPPER RIDGE. IT MAY TAKE UNTIL LATE IN THE WEEK OR NEXT WEEKEND BEFORE ANOTHER PUSH BY A NORTHWEST U.S. UPPER TROUGH ALLOWS THE FRONT TO PENETRATE FAR ENOUGH SOUTH TO GET THROUGH OUR AREA. CONFIDENCE IN PRECIPITATION CHANCES THROUGH THIS PERIOD IS LOW. FOR THE MOST PART...IT APPEARS TO BE DRIVEN ALMOST ENTIRELY BY DIURNAL AND DIFFERENTIAL HEATING...FOCUSED LARGELY TOWARD THE WESTERN MOUNTAINS. THERE ARE SOME NOTABLE DIFFERENCES IN THE ECMWF/GFS REGARDING POTENTIAL RETURN ATLANTIC MOISTURE FROM OFF THE SC/NC COAST. IT APPEARS THAT THIS MOISTURE COULD ACTUALLY BE ASSOCIATED WITH THE REMNANTS OF FORMER TROPICAL STORM ERIKA. IT IS UNCLEAR WHETHER OR NOT THIS MOISTURE WILL PENETRATE FAR ENOUGH WEST TO REACH OUR CWA. HAVE PREFERRED TO LEAN TOWARD THE CONSENSUS OF SYNOPTIC SCALE MODELS WHICH KEEP THIS DEEP TROPICAL MOISTURE EAST OF OUR CWA...CLOSER TO THE COAST. TEMPERATURES WILL BE WARMING AGAIN THROUGH THE FIRST HALF OF NEXT WEEK...REMAINING WELL ABOVE NORMAL FOR THE MOST PART. TEMPERATURE DEPARTURES WILL BE MOST NOTABLE WITH RESPECT TO MINS...WITH LOWS IN THE LOW TO MID 60S...SOME 7-10 DEGREES ABOVE NORMAL. MAX TEMPS WILL START OUT OVER THE WEEKEND NEAR NORMAL...BUT CREEP BACK UP TO ABOVE NORMAL LEVELS FOR THE FIRST HALF OF NEXT WEEK...WITH MOSTLY 80S WEST TO NEAR 90 EAST...EXCEPT 70S MOUNTAINS.
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&& .AVIATION /09Z THURSDAY THROUGH MONDAY/... AS OF 1258 AM EDT THURSDAY... EARLIER SHOWERS HAVE PUSHED TO THE SOUTH TONIGHT LEAVING MAINLY VFR CONDITIONS UNDER CLEAR SKIES IN PLACE. THIS WILL LEAD TO ANOTHER ROUND OF FOG/STRATUS ESPECIALLY IN THE VALLEYS WITH KLWB LIKELY TO FALL INTO LIFR...WITH A PERIOD OF IFR/MVFR POSSIBLE AT KBCB AND KLYH. ELSEWHERE EXPECTING FOG TO REMAIN SPOTTY WITH ONLY LIMITED MVFR COVERAGE BY DAYBREAK. THURSDAY...VFR CONDITIONS TO RETURN AGAIN BY MID MORNING...FOLLOWED BY INCREASING CU AND CB BUILD UPS ESPECIALLY OVER THE MOUNTAINS. HOWEVER...IN THE AFTERNOON...SHOWERS AND STORMS WILL AGAIN FIRE UP OVER MOUNTAINS AS OLD OUTFLOW AND PERHAPS A WEAK WAVE ALOFT ENHANCE LIFT A BIT. APPEARS KBLF/KLWB MORE IN LINE FOR SHOWERS OR STORMS...BUT STILL ONLY SCATTERED COVERAGE. THUS WILL INCLUDE A VCTS MENTION...AFTER 18Z/2PM WITH VCSH AT KBCB. OTHERWISE CIGS WILL MAINLY BE VFR 4-6KFT. EXTENDED AVIATION DISCUSSION... WILL SEE A WEAK FLOW PATTERN WITH AN UPPER TROUGH EXTENDING FROM EAST-WEST OVER OUR AREA FRIDAY INTO SUNDAY. THIS ALONG WITH HIGH PRESSURE TO THE NORTHEAST WILL CAUSE THE LOW LEVEL FLOW TO TURN MORE EASTERLY IN TIME. AS OF NOW...THE TERMINALS WILL BE VFR DURING THE DAY...WITH POSSIBLE MVFR OR WORSE WITH FOG AT NIGHT INTO FRIDAY NIGHT. THE EASTERLY FLOW EXPECTED BY SATURDAY WILL BRING LOWER CLOUDS IN AND POTENTIAL FOR MVFR CIGS AT LEAST FOR THE MOUNTAINS...AND LOW END VFR ROA/LYH/DAN. WITH THIS UPPER TROUGH THERE WILL BE DAILY THREAT OF SHOWERS...SO SUB VFR IN THESE IS POSSIBLE. BY SUNDAY-MONDAY THE WEAK TROUGH ALOFT MOVES OVER THE SOUTHEAST COAST WHILE UPPER RIDGING ELONGATES OVER THE MID ATLANTIC. WE WILL SEE SOME DRYING TAKE PLACE SO VFR SHOULD OCCUR SUNDAY-MONDAY WITH THE STILL A THREAT OF FOG AT NIGHT AT KBCB/KLWB. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...NONE. NC...NONE. WV...NONE. && $$ SYNOPSIS...JH NEAR TERM...JH SHORT TERM...RAB LONG TERM...AMS/RAB AVIATION...JH/WP

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