Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 230453 AFDRNK AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BLACKSBURG VA 1153 PM EST MON DEC 22 2014 .SYNOPSIS... HIGH PRESSURE WILL REMAIN WEDGED EAST OF THE APPALACHIANS THROUGH TUESDAY NIGHT...BRINGING US A GOOD DOSE OF CLOUDS...DRIZZLE...AND FOG. A STRONG COLD FRONT WILL MOVE THROUGH ON WEDNESDAY WITH RAIN AND SOME GUSTY WINDS. HIGH PRESSURE WILL THEN BUILD OVER THE EASTERN UNITED STATES AND BRING US FAIR WEATHER THROUGH THE END OF THE WEEK AND INTO THE WEEKEND. && .NEAR TERM /THROUGH TUESDAY/... AS OF 900 PM EST MONDAY... EVENING RNK SOUNDING INDICATED SUBFREEZING LAYER AROUND 3200 FT ABOVE MEAN SEA LEVEL (MSL)...WITH TEMPERATURES ABOVE FREEZING EITHER SIDE OF THIS LAYER. MATTER OF FACT...IT IS WARMER ABOVE THE LAYER COMPARED TO THE IMMEDIATE SURFACE WITH TEMPERATURES NEAR 40 DEG F BETWEEN 4200-5300 MSL. BASICALLY ITS WARMER ON TOP OF MOUNT ROGERS VA AND BEECH MOUNTAIN NC THAN IT IS ANYWHERE ELSE IN THE CWA. SINCE THE SUBFREEZING LAYER IS SO SHALLOW...WILL CONTINUE TO USE A SPECIAL WEATHER STATEMENT TO ACCOUNT FOR ANY ICING THAT IS TAKING PLACE AT 3200 FT MSL. SALEM VDOT HAS CONFIRMED SOME ICING ON CATAWBA MOUNTAIN...AND WE HAVE ALSO HAD SOME REPORTS OF ICING ON BENT MOUNTAIN...BOTH LOCATIONS NEAR 3200 FT MSL. TEMPERATURES DURING THE OVERNIGHT SHOULD REMAIN FAIRLY STEADY. CURRENT THINKING IS THE AREA OF RAIN THAT IS MOVING NORTH FROM THE SOUTHERN APPALACHIANS WILL REACH OUR FCST AREA AFTER MIDNIGHT. AS THIS STEADIER RAIN FALLS THROUGH THE WARM LAYER BETWEEN 4200-5300 FT MSL...IT SHOULD MODIFY THE SUBFREEZING LAYER BELOW AND RESULT IN TEMPERATURES AT 3200 FEET MSL RISING A DEGREE OR TWO DURING THE OVERNIGHT. THE SAME IS TRUE FOR THE LOWER ELEVATIONS TOO...TEMPERATURES POTENTIALLY RISING A DEGREE. THE ONLY THING THAT WOULD PREVENT THIS FROM HAPPENING WOULD BE REINFORCING COOLER AIR FROM THE NORTHEAST. A NORTHEAST WIND WILL PERSIST THROUGHOUT THE NIGHT...COLD AIR ADVECTION MAINTAINING A STRONG WEDGE ON THE EAST SIDE OF THE APPALACHIANS. THE 1030MB PARENT HIGH WELL TO OUR NORTHEAST HAS SOLIDLY WEDGED DOWN THE EASTERN SEABOARD AND WILL STAY LOCKED IN THROUGH TOMORROW WITH LOW CLOUDS...DRIZZLE...AND FOG WITH NEARLY STEADY TEMPERATURES. SOME POCKETS OF FREEZING RAIN/DRIZZLE WILL REMAIN POSSIBLE FOR ISOLATED PORTIONS OF THE ALLEGHANY HIGHLANDS THROUGH TONIGHT BUT THIS WILL NOT REQUIRE AND ADVISORY AND CAN READILY BE HANDLED IN AN SPS. JUST ABOVE THE SURFACE THINGS ARE WARMING UP CONSIDERABLY IN SWLY FLOW AND 850MB TEMPS SHOULD BE AROUND +10C BY LATE TUESDAY. ISENTROPIC LIFT ASSOCIATED WITH THIS WARMING WILL COMBINE WITH SOME WEAK SHORT WAVE ENERGY ALOFT TO BRING A PERIOD OF MORE COHESIVE PRECIPITATION TO THE REGION OVERNIGHT THOUGH QPF WILL AGAIN BE LIGHT AT JUST A TENTH OR SO. WESTERN PERIPHERY OF THE WEDGE MAY ERODE ENOUGH FOR SOME WARMER TEMPERATURES TO ENCROACH INTO PORTIONS OF THE MOUNTAIN EMPIRE REGION BUT THE REMAINDER OF THE AREA WILL STAY LOCKED UNDER CLOUDS WITH TEMPERATURES RISING ONLY EVER SO SLIGHTLY. && .SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY NIGHT/... AS OF 410 PM EST MONDAY... MODELS IN FAIRLY GOOD AGREEMENT WITH TIME OF ARRIVAL OF MORE SIGNIFICANT RAINFALL TO SPREAD NORTHWARD ACROSS ENTIRE FCST AREA BY EARLY TUESDAY EVENING...IN ASSOCIATION WITH STRONG SOUTHERLY JET AHEAD OF DEEP TROUGH...EFFICIENTLY TAPPING GULF MOISTURE. SHOULD BE GENERALLY BROAD LIFT OVER TOP OF COLD AIR WEDGED IN PLACE...WITH LESS INFLUENCE FROM UPSLOPE THAN THERE WOULD BE OTHERWISE BECAUSE OF WEDGE...ALSO DUE TO FACT THAT LOW LEVEL JET IS MORE FROM SSW COMPARED TO SOUTHEAST. THIS DIRECTION ALONG WITH WEDGE WILL KEEP WINDS FROM BEING STRONG TUES NIGHT WITH EXCEPTION OF THE VERY HIGHEST RIDGES. TEMPS SHOULD REMAIN NEARLY STEADY OR ONLY DROP SLIGHTLY OVERNIGHT DUE TO WEDGE FIRMLY IN PLACE. ON WEDNESDAY MANY MODELS TRY TO QUICKLY ERODE THE WEDGE ACROSS ENTIRE FCST AREA DUE TO SOUTHERLY JET...BUT HIGH RES VERSIONS OF NAM SUGGEST PERHAPS NOT IN THE HEART OF THE AREA FROM NEW RIVER VALLEY UP THROUGH SRN SHENANDOAH VALLEY AND TOWARD LYH...BUT WILL LIKELY ONLY ERODE ALONG SOUTHWEST AND SOUTHEAST FRINGES UNTIL LATE IN THE DAY. HIGHS MAY ACTUALLY OCCUR IN MANY PLACES AT 5 OR 6PM. THUS UNDERCUT ALL MOS GUIDANCE FROM BCB TO ROA TO LYH FOR TEMPS...AND TRIED TO DEPICT NEARLY STEADY TO ONLY SLOWING RISING TEMPS FOR THE AREAS WHERE WEDGE HOLDS MOST OF THE DAY. TEMPS MAY CONTINUE TO WARM FOR WED EVENING...ESPECIALLY IN THE EAST...UNTIL FRONT ARRIVES LATE EVENING TO MIDNIGHT...AND JUST AHEAD OF FRONT COULD BE A LINE OF HEAVIER RAIN WITH SHALLOW CONVECTION. INSTABILITY EXTREMELY LIMITED AND CHOSE TO ONLY INCLUDE MENTION OF THUNDER IN PIEDMONT VERY LATE WED INTO THE EVENING JUST AHEAD OF FRONT. BEST CHANCE FOR GUSTY WINDS WITH ANY SHALLOW CONVECTION IF THIS CAN MIX DOWN STRONG MOMENTUM FROM ABOVE. RAINFALL TOTALS FROM TUES NIGHT THROUGH WED COULD EXCEED AN INCH IN MANY PLACES...AND NEAR BLUE RIDGE AND DEPENDING ON WHERE ANY SHALLOW CONVECTION CAN FORM...PERHAPS LOCALLY UP TO TWO INCHES. WORSE CASE SCENARIO COULD BE SOME LOCALIZED MINOR FLOODING OF LOW LYING AREAS AS WELL AS DITCHES...BUT FOR THE MOST PART THIS WILL NOT BE ENOUGH RAIN TO CAUSE ANY ISSUES. BEHIND FRONT LATE WED NIGHT INTO CHRISTMAS DAY...WINDS TURN WESTERLY WITH COLD ADVECTION...BUT IT NOT ESPECIALLY COLD AND WINDS MORE WESTERLY AS OPPOSED TO NORTHWEST...SO NOT ESPECIALLY FAVORABLE FOR UPSLOPE SNOW ACCUMULATION. WITH WARM AND WET GROUND...AND TEMPS ONLY FALLING TO NEAR OR JUST BELOW FREEZING IN HIGHEST ELEVATIONS...AT THIS POINT EXPECTING LESS THAN AN INCH ACCUMULATION IN FAVORED AREAS. WILL HAVE A HARD TIME STICKING. BY MIDDAY CHRISTMAS...UPPER LEVEL FLOW TURNS ZONAL AGAIN...SKIES SHOULD BEGIN CLEARING ALL BUT FAR WESTERN UPSLOPE AREAS...AND TEMPS WILL BE ABLE TO CLIMB INTO THE 40S MOST LOCATIONS. BREEZY...BUT WINDS AT RIDGE TOP LEVEL REMAIN BELOW ADVISORY THRESHOLDS WITH POSSIBLE EXCEPTION OF NW NC MTNS...SO AT THIS POINT HAVE SOME GUSTS TO NEAR 40 MPH...BUT WINDS GENERALLY IN THE 15 TO 25 MPH...AND MAINLY FIRST PART OF THE DAY AS HIGH PRESSURE QUICKLY MOVES IN FROM WEST BY OVERNIGHT. && .LONG TERM /FRIDAY THROUGH MONDAY/... AS OF 225 PM EST MONDAY... FRIDAY INTO SATURDAY BEGINS WITH MILD HIGH PRESSURE OVER MID ATLANTIC WHICH SLOWLY HEADS OFF THE COAST AS NEXT COLD FRONT APPROACHES FROM THE MIDWEST. MODELS IN GENERAL AGREEMENT THROUGH EARLY SATURDAY...SUGGESTING THIS FRONT WILL BE RELATIVELY DRY AS IT APPROACHES THROUGH SATURDAY GIVEN MOISTURE FROM GULF INITIALLY BLOCKED...BUT DIFFERENCES START TO SHOW UP AFTER THAT. ECMWF AND CANADIAN CONSISTENTLY HOLDING ONTO AN UPPER WAVE BEGINNING TO DIG INTO THE SOUTHERN PLAINS LATER SATURDAY...WHICH IN TURNS SLOWS THE ARRIVAL OF FRONTAL SYSTEM WITH A SFC WAVE DEVELOPING ALONG THE WESTERN GULF COAST. GFS AND MOST OF ITS ENSEMBLE MEMBERS ARE MORE PROGRESSIVE OVERALL...BRINGING A WEAK FRONT THROUGH BY LATE SAT...WITH A WEAK SFC WAVE REMAINING DOWN IN GULF AND CROSSING NRN FLORIDA INTO SUNDAY. THIS SITUATION HAS SOME SIMILARITIES TO THE POTENTIAL EVENT FROM THIS PAST WEEKEND WHICH NEVER AMPLIFIED...AND THUS THOSE EARLY ECWMF FORECASTS ENDED UP BEING OVERDONE. CURRENTLY...THE ECWMF FROM 00Z AND NEW RUN FROM 12Z THIS MORNING WOULD BRING A SFC LOW UP ON THE WEST SIDE OF APPALACHIANS WITH MUCH MORE PRECIP BY SUNDAY...LIKELY MOSTLY RAIN...WITH UPSLOPE MTN SNOW ON BACK SIDE BY MONDAY. GFS IS MAINLY DRY THROUGH THIS PERIOD WITH SFC HIGH PRESSURE DOMINATING AGAIN SUNDAY INTO MONDAY. CONSENSUS AT THIS TIME IS TO COMPROMISE SOMEWHAT WITH A WEAK SFC WAVE PASSING JUST TO SOUTH AND GIVEN THE HIGH DEGREE OF UNCERTAINTY...KEEPING POPS IN A LOW CHC CATEGORY FOR NOW. WOULD TEND TO WANT TO BELIEVE THE FLATTER MOVE PROGRESSIVE GFS BASED ON RECENT HISTORY...BUT WITH BETTER HISTORICAL TRACK RECORD OF ECMWF AND ITS INTERNAL CONSISTENCY IN LAST FEW RUNS...CERTAINLY CAN`T RULE OUT THE IDEA OF STRONGER WAVE. FCST GRIDS REFLECT THIS COMPROMISE WITH INITIAL WEAK FRONTAL PASSAGE BY SAT NIGHT AND SOME INCREASE IN POPS BY SUNDAY IF AT LEAST A WEAK WAVE DEVELOPS. TEMPS GENERALLY TOO WARM FOR ANY SNOW EXCEPT POSSIBLY AT HIGHEST ELEVATIONS...BUT ALL THIS WOULD BE SUBJECT TO THE ULTIMATE TRACK OF ANY DEVELOPING SFC LOW. CERTAINLY WORTH KEEPING A CLOSE EYE ON POTENTIAL DEVELOPMENTS FOR NEXT WKND. && .AVIATION /06Z TUESDAY THROUGH SATURDAY/...
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AS OF 100 AM EST TUESDAY... COLD AIR WEDGE IS IN PLACE ALONG THE EASTERN SEABOARD AND IT WILL REMAIN THE DOMINANT FACTOR CONTROLLING OUR WEATHER THROUGH THE 24 HOUR TAF PERIOD. AS SUCH...ANTICIPATE WIDESPREAD IFR ALONG AND EAST OF THE APPALACHIAN DIVIDE...BASICALLY ENCOMPASSING ALL RNK TAF SITES...WITH PERSISTENT NE WINDS...CIGS BLO 1KFT AND VSBYS AOB 3SM. VSBYS MAY FLUCTUATE FROM TIME TO TIME AS INTERMITTENT DRIZZLE/LIGHT RAIN AFFECTS SITES BUT DO NOT EXPECT THIS TO CHANGE FLIGHT CATEGORIES SINCE CIGS WILL REMAIN LOW...SO WILL KEEP TAFS VERY STRAIGHTFORWARD WITH VERY FEW CHANGE/TEMPO GROUPS. THE EXCEPTION WILL BE KBLF AND KLWB SINCE THEY WILL BE ON THE WRN PERIPHERY OF THE WEDGE. THE WEDGE IS EXPECTED TO PERSIST THROUGH EARLY WEDNESDAY...FINALLY ERODING DUE TO A FRONTAL PASSAGE. UNTIL THEN EXPECT CONDITIONS TO BE PREDOMINATELY IFR WITH THE MOUNTAINS OBSCURED. ANOTHER HAZARD...FOR JUST TONIGHT...TO BE CONCERNED WITH IS A SUB FREEZING LAYER AROUND 3200 FT MSL. THE TEMPERATURE WITHIN THIS LAYER IS AROUND 30 DEG F AND IS RESULTING IN SOME LIGHT ICING ON CATAWBA...POOR...AND BENT MOUNTAINS NEAR ROANOKE. THIS LAYER IS EXPECTED TO WARM ABOVE FREEZING TUESDAY. EXTENDED DISCUSSION... BY WEDNESDAY...A STRONG COLD FRONT WILL SWEEP OVER THE MID ATLANTIC...AND ERODE THE WEDGE. MODEL SOUNDINGS CONTINUE TO SHOW THE POTENTIAL FOR LOW LEVEL WIND SHEAR AHEAD OF THIS FRONT...WITH A POTENT LOW LEVEL JET DEPICTED. RAIN AND MVFR CONDITIONS WILL ACCOMPANY THE FRONTAL PASSAGE. BEHIND THE COLD FRONT...SURFACE WINDS SHOULD TURN FROM THE SOUTHEAST TO THE NORTHWEST...AND BECOME RATHER GUSTY BY CHRISTMAS DAY. MEANWHILE...MVFR CEILINGS AND UPSLOPE RAIN/SNOW SHOWERS SHOULD PERSIST IN THE MOUNTAINS...WHILE VFR CONDITIONS WILL RETURN IN THE PIEDMONT. HIGH PRESSURE SHOULD BUILD OVER THE SOUTHEAST BY THURSDAY NIGHT...WHICH WILL PROVIDE GOOD FLYING WEATHER WITH LIGHTER WINDS ON FRIDAY.
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&& .HYDROLOGY... AS OF 345 PM EST MONDAY... A SIGNIFICANT WETTING RAIN IS LIKELY TUESDAY NIGHT AND WEDNESDAY...FORECAST STORM TOTAL QPF...IS IN THE 1.00 TO 1.50 INCH RANGE ACROSS THE ENTIRE CWA...WITH LOCAL AMOUNTS NEAR THE BLUE RIDGE OF OVER 2.00 INCHES POSSIBLE. THIS WILL ACTUALLY BE WELCOME FROM A HYDROLOGIC STANDPOINT AS MUCH OF THE PIEDMONT HAS BEEN DRIER THAN NORMAL OVER SEVERAL MONTHS AND DECEMBER HAS HAD BELOW NORMAL PRECIPITATION IN MOST OF THE CWA. SHOULD NOT SEE ANY EXCESS RUNOFF ISSUES..I.E. FLOODING...ALTHOUGH SOME DITCHES WILL LIKELY FILL UP AND MAYBE A FEW PONDING ISSUES WHERE HEAVIER RAINFALL RATES. RIVER FORECASTS FROM NORTH AMERICAN AND GEFS ENSEMBLES PORTRAY BASIN- AVERAGE QPF GENERALLY IN THE 0.75 TO 1.25 INCH RANGE WITH SOME OUTLIERS AS USUAL BUT ALL SOLUTIONS AT THIS POINT PRODUCE ONLY MINOR TO MODEST WITHIN-BANK RISES ON MAINSTEM RIVERS BY LATER IN THE WEEK. && .EQUIPMENT... THE KFCX RADAR WILL NEED TO BE TAKEN DOWN FOR A FEW HOURS ON TUESDAY TO REPLACE A PART. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...NONE. NC...NONE. WV...NONE. && $$ SYNOPSIS...MBS NEAR TERM...MBS/PM SHORT TERM...PM/SK LONG TERM...SK AVIATION...PM HYDROLOGY...PC/SK EQUIPMENT...PM

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