Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 240441 AFDRNK AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BLACKSBURG VA 1141 PM EST TUE DEC 23 2014 .SYNOPSIS... ABUNDANT MOISTURE WILL RUN OVER THE TOP OF A SHALLOW DOME OF COLDER AIR BANKED UP AGAINST THE EAST SIDE OF THE APPALACHIANS OVERNIGHT... BRINGING PERIODS OF RAIN TO THE REGION THROUGH TOMORROW. A STRONG COLD FRONT WILL THEN MOVE THROUGH LATE ON CHRISTMAS EVE AND BRING MORE RAIN AND SOME GUSTY WINDS AS IT PASSES. HIGH PRESSURE WILL THEN BUILD OVER THE EASTERN UNITED STATES INTO THE WEEKEND WITH GENERALLY FAIR WEATHER EXPECTED INTO SATURDAY. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... AS OF 825 PM EST TUESDAY... RAIN OVERSPREADING THE REGION THIS EVENING ASSOCIATED WITH WEAK ISENTROPIC LIFT OVERTOP THE STRONG BUT SHALLOW WEDGE IN PLACE. THIS SEEN ON EVENING SOUNDINGS PER THE BOUNDARY LAYER INVERSION WITH SOME WEAK INSTABILITY SEEN ABOVE THE CAD...AND SUPPORTED BY HEAVIER ELEVATED SHRA THAT RECENTLY TRACKED NEAR DANVILLE. ALSO AS A WEAK EMBEDDED WAVE ALOFT NEAR THE COASTAL FRONT LIFTS NORTH...WOULD EXPECT THE MORE SUBSTANTIAL RAINFALL TO AFFECT FAR EASTERN SECTIONS OVERNIGHT. ELSW APPEARS LIGHTER AMOUNTS ON TAP WHERE WILL BE IN BETWEEN THE INITIAL PRECIP BAND TO THE SE...AND THE MAIN AXIS ALONG THE FRONT/TRIPLE POINT BACK TO THE SW. MAY ALSO SEE SOMEWHAT OF A CUTOFF OF DEEPER MOISTURE LATE IF THE CONVECTIVE LINE OVER AL/GA HEADS EAST SO SOMETHING TO WATCH. OTRW UPDATING TO INCLUDE MORE FOG AND TO LOWER WESTERN QPF...AS WELL AS LOWERING CURRENT TEMPS IN SPOTS...WHILE RAISING SOME RIDGES WHERE IN THE 50S...AND LIKELY NOT TO FALL MUCH GIVEN MIXING ABOVE THE WEDGE. OTRW KEEPING TEMPS ABOUT STEADY UNTIL AROUND MIDNIGHT FOLLOWED BY SOME RISES THROUGH DAYBREAK. WINDS SHOULD BE GUSTY ABOVE 3K FT BUT THINKING THE STRONGER JET SHOULD STAY JUST ABOVE THE RIDGETOPS OVERNIGHT. THUS BUMPED UP WINDS AS WELL WEST BUT WITHOUT HEADLINES FOR NOW. PREVIOUS VALID DISCUSSION AS OF 330 PM EST TUESDAY... WATER VAPOR IMAGERY SHOWS THAT THE UPPER LONG WAVE TROF APPROACHING FROM THE WEST HAS ESTABLISHED A DEEP SWLY FLOW OF MOISTURE INTO THE EASTERN UNITED STATES. THIS MOISTURE...ALONG WITH SOME ISENTROPIC LIFT AND SHORT WAVE ENERGY...WILL RUN OVER THE TOP OF OUR SHALLOW SURFACE HIGH PRESSURE WEDGED UP AGAINST THE APPALACHIANS AND KICK OFF A GOOD AMOUNT OF RAIN ACROSS THE REGION STARTING TONIGHT. ANTICIPATING QPF THROUGH TONIGHT IN THE ONE HALF TO THREE QUARTERS OF AN INCH RANGE AND GIVEN DRY ANTECEDENT CONDITIONS THIS WILL NOT CAUSE ANY HYDRO ISSUES THROUGH DAYBREAK. RAIN WILL LIKELY COME DOWN AT A GOOD CLIP OVERNIGHT BUT MID/UPPER LAPSE RATES SHOWN IN MODEL SOUNDINGS DO NOT APPEAR TO SUPPORT THUNDER SO WILL NOT MENTION FOR THE OVERNIGHT PERIOD. LOW LEVEL JET WILL BE RAMPING UP FROM THE SOUTHEAST OVERNIGHT AND SINCE THE COLD AIR WEDGE IS SO SHALLOW...RIDGE TOPS FROM THE BLUE RIDGE WEST WILL START TO SEE WINDS ON THE INCREASE AS WE GET CLOSER TO DAYBREAK. THIS WILL ALSO MAKE FOR AN INTERESTING TEMPERATURE FORECAST SINCE READINGS IN THE LOWER 50S WILL BE FOUND ALONG THE RIDGES THANKS TO WARMER AIR MOVING IN WITH THE STRENGTHENING LOW LEVEL JET...BUT UNDER THE SHALLOW DOME OF COLD AIR IN THE WEDGE EXPECTING LOWER 40S. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH FRIDAY NIGHT/... AS OF 405 PM EST TUESDAY... MAIN STORY WIDESPREAD RAINFALL TO START THE DAY WITH ABUNDANT GULF MOISTURE OVERRUNNING ERODING WEDGE AS DEEPENING SFC LOW TRACKS NORTHWARD INTO THE OHIO VALLEY REGION AND EVENTUALLY OVER GREAT LAKES. MODERATE MORE STEADY RAIN WILL EVENTUALLY SHIFT EAST BY AFTERNOON...WITH LIGHTER SHOWERS IN THE WEST TOWARD END OF THE DAY AND MOST HIGH RES MODELS SHOWING SIGNIFICANT DRY SLOW MOVING IN BEFORE ANOTHER NARROW LINE OF SHALLOW CONVECTION MOVES IN FROM WEST ALONG SFC FRONT BY EVENING. TRICKIER FCST CHALLENGES ARE HOW THE WEDGE WILL ERODE...IMPACTING HOURLY TEMPERATURES AND ALSO RESULTING IN SOME VERY WEAK INSTABILITY BOTH ALONG SW EDGE OF FCST AREA AS WELL AS FAR SOUTHEAST AS THE BOUNDARY SQUEEZES IN FROM BOTH SIDES. KEPT THE HEART OF THE CWA COOLER AGAIN UNTIL EARLY AFTERNOON...BUT MODELS SEEM MORE AGGRESSIVE AT ERODING WEDGE A LITTLE QUICKER IN THE FACE OF A VERY STRONG SSW 850 JET. DECIDED TO ADD KEEP SLIGHT CHC THUNDER ACROSS PIEDMONT ALONG WEDGE FRONT GIVEN SFC BASED CAPES OF 100-200 J/KG...AND KEPT THE MENTION OF GUSTY WINDS TOO. SOME MINOR CONCERN THAT SOME CELLS COULD BE MORE ISOLATED IN THE PIEDMONT AND THIS DEVELOP SOME ROTATION GIVEN INCREDIBLE HODOGRAPH...BUT INSTABILITY IS CERTAINLY LIMITED FACTOR. BEST CHANCES FOR ANY VERY SHALLOW ROTATING STORMS WILL LIKELY BE SOUTH AND EAST OF OUR FCST AREA. ALSO ADDED MENTION OF SLGT CHC THUNDER IN FAR WEST BY 00Z AS FINE LINE ALONG COLD FRONT IS INDICATED IN HIGH RES MODELS AND MAY ALSO HAVE GUSTY WINDS ASSOCIATED WITH IT. WHETHER OR NOT THIS CAN TRACK ALL THE WAY ACROSS THE FCST AREA OR DIE IN THE MTNS REMAINS TO BE SEEN. CERTAINLY COULD REDEVELOP LATE EVENING IN THE PIEDMONT...SO THREAT OF SOME GUSTY WINDS MAY CONTINUE THERE UP UNTIL ALMOST MIDNIGHT. SOME MAJOR DIFFERENCES IN TIMING OF THIS FRONT AND LAST LINE OF CONVECTION HOWEVER...WITH 12Z 12KM NAM THE SLOWEST...AND THE HIGH RES ARW WRF THE FASTEST...AND WITH NO REAL INDICATION OF SHALLOW CONVECTION ALONG FRONT MAKING IT INTO PIEDMONT. LEANING TOWARD THE FASTER SOLUTION...BUT SOMEWHERE IN BETWEEN...AND DID NOT GET TOO DETAILED WITH TIMING THE POPS EXACTLY GIVEN THE UNCERTAINTY. OVERALL RAINFALL SHOULD NOT BE ENOUGH TO CAUSE ANY SIGNIFICANT FLOODING CONCERNS...BUT WILL HAVE TO WATCH NEAR BLUE RIDGE IF THE UPSLOPE CAN ENHANCE RAINFALL THERE...BUT WEDGE MAY RESULT IN MORE OF A GRADUAL LIFT VS. SHARP UPSLOPE. IF THE CONVECTIVE LINE CAN HOLD TOGETHER ACROSS THE BLUE RIDGE INTO WED EVENING...THE RATES MAY BE HIGH ENOUGH FOR SOME LOCAL FLASH FLOOD CONCERNS..BUT CONFIDENCE VERY LOW AND WOULD NOT BE WIDESPREAD ENOUGH FOR WATCH AT THIS POINT. JUST AHEAD OF FRONT WINDS MAY BE QUITE GUSTY ALONG HIGHEST RIDGES GIVEN STRENGTH OF LOW LEVEL JET...BUT OTHERWISE HARD TO KNOW JUST HOW EFFECTIVELY ANY WILL MIX DOWN TO VALLEYS AND WOULD BE ISOLATED. WILL HAVE TO HANDLE THAT WITH A SHORTER TERM ADVISORY OR SPECIAL WEATHER STATEMENTS...POSSIBLE SVR WARNINGS...DEPENDING ON HOW THIS CONVECTIVE LINE PLAYS OUT. BEHIND FROM INTO EARLY THURSDAY...WESTERLY GRADIENT INCREASES IN COLD ADVECTION...AND MORE LOCATIONS WILL EXPERIENCE GUSTY WINDS WITH BETTER MIXING. LOOKING AT FCST WIND VERTICAL PROFILES...AM NOT SEEING MUCH THREAT OF ADVISORY LEVEL CRITERIA...EXCEPT PERHAPS ON HIGHEST RIDGES. WILL HOLD OFF ON HEADLINE SINCE PROBABILITY SEEMS LOW OF MEETING THESE CRITERIA. BY EARLY THURSDAY MORNING LOW LEVEL THERMAL PROFILES FINALLY GET COLD ENOUGH FOR UPSLOPE SHOWERS BEHIND FRONT TO CHANGE TO SNOW AT HIGHEST ELEVATIONS...AND WHEN UPPER TROUGH AXIS COMES ACROSS AROUND 12Z OR SO THURSDAY MORNING...THIS WILL BE BEST CHANCE FOR ANY ACCUMULATING SNOW IN FAVORED AREAS...BUT THIS IS EXPECTED TO BE LESS THAN AN INCH GIVEN MARGINALLY COLD ENOUGH AIR AND SHORT- LIVED UPSLOPE...WITH UPPER FLOW TURNING ZONAL BY MIDDAY. HAVE BARELY TRACE SNOW ACCUM IN NW NC MTNS TO JUST UNDER AN INCH IN WRN GREENBRIER. SKIES BEGIN TO CLEAR EAST OF THE WESTERN SLOPES THROUGHOUT CHRISTMAS DAY...AND DESPITE COLD ADVECTION TEMPS SHOULD BE FAIRLY MILD FOR THIS TIME OF YEAR WITH SUNSHINE AND WEAKENING COLD ADVECTION. 40S MTNS TO 50S PIEDMONT. SFC HIGH MOVES IN OVERHEAD FOR EARLY FRI MORNING RESULTING IN COLD LOWS...WENT A COUPLE DEG BELOW COLDEST GUIDANCE. HIGH DOMINATES FRIDAY FOR A VERY MILD AND MOSTLY SUNNY DAY. TEMPS MAY APPROACH 60 IN PIEDMONT. && .LONG TERM /SATURDAY THROUGH TUESDAY/... AS OF 400 PM EST TUESDAY... FOR SATURDAY...SHORT WAVE ENERGY IS PROGGED TO BE LIFTING NORTHEAST THROUGH THE GREAT LAKES AND OHIO VALLEY WITH A SURFACE LOW OVER THE GREAT LAKES AND A FRONT TRAILING SOUTH THROUGH THE OH/TN VALLEYS INTO THE WESTERN GULF. MODELS ARE IN AGREEMENT THAT UPPER LEVEL SUPPORT WILL PASS WELL NORTH OF THE FORECAST AREA YIELDING LITTLE SUPPORT FOR PRECIP ALONG THE FRONTAL BOUNDARY AS IT APPROACHES THE FORECAST AREA LATE SATURDAY. BIGGEST BY PRODUCT WILL BE INCREASING MID/HIGH LEVEL CLOUDINESS...BUT LITTLE OR NO PRECIP THREAT. THE FRONT IS PROGGED TO GO THROUGH THE AREA DRY...OR MAY EVEN STALL UPSTREAM OF THE FORECAST AREA PENDING THE MODEL SOLUTION YOU BELIEVE FOR THE WEEKEND. CONTINUE TO ENTERTAIN A MODEL BLENDED FORECAST... CONSENSUS/COMPROMISE/MEAN...WHATEVER YOU WANT TO CALL IT FOR THE WEEKEND AND FIRST PART OF NEXT WEEK. THERE ARE BIG DIFFERENCES IN THE MODELS DUE TO UPPER LEVEL TIMING/PHASING OF SHORT WAVE ENERGY WITHIN THE MID CONUS TROUGH WHICH WILL BE WORKING ITS WAY EASTWARD FROM THE ROCKIES DURING THE PERIOD. THE ECMWF HAS THE GREATEST AMPLIFICATION WITH SHORTWAVE ENERGY PASSING THROUGH THE TROF RESULTING IN A SLOWER BUT STRONGER PROGRESSION OF SURFACE FEATURES LATER IN THE WEEKEND...WHEREAS THE GFS ENSEMBLES ARE FASTER AND LESS AMPLIFIED. THE GFS PUSHES THE SURFACE FRONT THROUGH THE FORECAST AREA LATE SATURDAY FOLLOWED BY A WEAK WAVE RIPPLING ALONG THE FRONT LATE SUNDAY. A SOLUTION THAT WOULD LEAVE THE FORECAST AREA DRY UNTIL THE ARRIVAL OF PRECIP LATE IN THE WEEKEND...POSSIBLY MIXED WITH SNOW SINCE WE WOULD BE ON THE COLD SIDE OF THE BAROCLINIC ZONE. THE ECMWF ON THE OTHER HAND...STALLS THE FRONT TO OUR WEST ON SATURDAY WITH LOW PRESSURE DEVELOPING TO OUR WEST AND MOVING NORTHEAST ALONG THE WESTERN SLOPES OF THE BLUE RIDGE SUNDAY INTO MONDAY. THIS SOLUTION IS WARMER AND MUCH WETTER. DIFFERENCES REALLY SHOW UP IN THE SOUNDING PROFILES BY 00Z MONDAY (7PM SUNDAY) WITH THE ECMWF INDICATING 85H TEMPS OF +10 DEG C VS 0 DEG C ON THE GFS. ALL THINGS CONSIDERED WOULD TEND TO WANT TO BELIEVE THE FLATTER MORE 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 A COMPROMISE WITH INITIAL WEAK FRONTAL PASSAGE BY SAT NIGHT AND SOME INCREASE IN POPS BY SUNDAY IF AT LEAST A WEAK WAVE DEVELOPS. WITH A BLENDED SOLUTION...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 /04Z WEDNESDAY THROUGH SUNDAY/... AS OF 1120 PM EST TUESDAY... WEDGE SCENARIO CONTINUES TONIGHT WITH IFR/LIFR CIGS ALONG AND EAST OF THE BLUE RIDGE WITH MAINLY MVFR/IFR ACROSS THE WEST ON THE PERIPHERY OF THE COOL POOL. VSBYS ALSO MVFR/IFR IN FOG/-RA/-DZ AS WELL ESPCLY FROM KBCB EAST WITH POOR FLYING CONDITIONS LIKELY TO PERSIST OVERNIGHT UNDER PATCHY RAIN/DRIZZLE DURING THE NEXT FEW HOURS...AND LIKELY SOME HEAVIER RAIN BY DAYBREAK WEDNESDAY PER LATEST RADAR LOOPS. CIGS AND VSBYS MAY VARY OVER THE NEXT COUPLE OF HOURS GIVEN CURRENT COVERAGE OF RAINFALL AND MORE IN THE WAY OF FOG/DRIZZLE UNDER STRATUS. HOWEVER ONCE THE NEXT ROUND OF MORE STEADIER RAINFALL SETS IN LATE...AND LOWER CIGS BECOME MORE UNIFORM...WOULD EXPECT MORE OF A STEADY STATE WITH IFR/LIFR CLOUD BASES AND MVFR TO IFR VSBYS IN PERIODS OF RAIN AND PERSISTENT DENSE FOG THAT MAY LOWER VSBYS TO LIFR AT TIMES ESPCLY WEST WHERE MOUNTAIN OBSCURATIONS WILL PREVAIL OVERNIGHT. THIS SITUATION LIKELY TO CONTINUE THROUGH MUCH OF WEDNESDAY WITH SOME EROSION OF THE WEDGE POSSIBLE OVER THE SE AND FAR WEST WHERE COULD BREAK INTO MVFR OR BETTER DURING THE AFTERNOON. HOWEVER THIS LOOKS UNCERTAIN GIVEN SO MUCH RAIN AROUND...WITH ANY IMPROVEMENT ALOFT ABOVE THE COOL POOL PER ARRIVAL OF A LATE DAY DRY SLOT FROM THE SW. IF FASTER MODELS DO WORK OUT THEN SPOTS SUCH AS KBLF/KLWB AS WELL AS KBCB AND KDAN COULD BREAK OUT SOONER. OTHER CONCERN WITH SURFACE WINDS ACROSS THE RIDGES AFFECTING LOCATIONS SUCH AS KBLF WHERE WILL LIKELY OSCILLATE IN AND OUT OF THE WEDGE INTO WEDNESDAY...AND THE STRONG JET ALOFT ESPCLY WED MORNING ACROSS THE ENTIRE AREA. WINDS AT MOST TAF SITES SHOULD GENERALLY BE LIGHT E/SELY IN THE COLD AIR WEDGE...THOUGH KBLF WILL SEE SOME GUSTS DEVELOPING AS WE GET INTO LATE TONIGHT AND WED MORNING. HOWEVER...THE LOW LEVEL JET ABOVE THE WEDGE WILL INCREASE OVERNIGHT AND BE IN THE 45-60 KT RANGE FROM WED MORNING INTO THE AFTERNOON. THIS WILL NECESSITATE THE CONTINUED INCLUSION OF OF LLWS IN ALL TAFS OVERNIGHT...AND EVEN MORE SO ON WED WITH THE EXCEPTION OF KBLF WHERE WILL AGAIN MIX DOWN GUSTS THROUGH THE COLUMN MORE READILY...THUS LIMITING THE DEGREE OF SHEAR. COLD FRONT TO THE WEST SHOULD FINALLY ARRIVE FROM WEST TO EAST WED EVENING PRECEDED BY PERHAPS ANOTHER LINE OF SHOWERS THAT WILL SWEEP RAPIDLY EAST BY MIDNIGHT. CONTINUED POTENTIAL OF LLWS JUST AHEAD OF THE FRONT ESPCLY WITHIN THE LINGERING COLD POOL WED AND MORE SURFACE BASED WINDS WITH ANY SHOWERS INTO WED EVENING. EXTENDED DISCUSSION... BEHIND THE COLD FRONT WEDNESDAY NIGHT AND THURSDAY...SURFACE WINDS SHOULD TURN TO THE WEST...AND BECOME GUSTY BY CHRISTMAS DAY. MVFR CEILINGS AND UPSLOPE RAIN/SNOW SHOWERS SHOULD PERSIST IN THE MOUNTAINS...WHILE VFR CONDITIONS WILL RETURN IN THE PIEDMONT. THURSDAY NIGHT AND FRIDAY HIGH PRESSURE SHOULD BUILD OVER THE SOUTHEAST...WHICH WILL PROVIDE VFR WEATHER WITH LIGHTER WINDS. ANOTHER COLD FRONT WITH ASSOCIATED LOW PRESSURE WILL APPROACH THE REGION SATURDAY AND BE OVER OR JUST WEST OF THE AREA SUNDAY. GIVEN THE SLOWER TRENDS...APPEARS MAY STAY VFR FOR SATURDAY WITH MORE IN THE WAY OF SUB-VFR CONDITIONS RETURNING WITH CLOUDS/PRECIP ON SUNDAY FOR NOW. && .HYDROLOGY... AS OF 335 PM EST TUESDAY... A SIGNIFICANT WETTING RAIN IS LIKELY TONIGHT INTO WEDNESDAY EVENING. FORECAST STORM TOTAL RAINFALL IS FROM AROUND 1.00 TO 1.50 INCHES MOST LOCATIONS...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 MODEL ENSEMBLE BASIN-AVERAGE RAINFALL SUGGEST ONLY VERY SMALL CHANCES THAT SOME RIVERS SUCH AS THE DAN FROM DANVILLE TO SOUTH BOSTON MAY EXPERIENCE MINOR FLOODING LATE IN THE WEEK. THERE IS A VERY GOOD CHANCE HOWEVER THAT THE RAINFALL WILL RESULT IN MODEST RISES ON SOME RIVERS...BUT CURRENTLY ONLY SOUTH BOSTON IS FORECAST TO APPROACH ACTION STAGE BY THURSDAY NIGHT. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...NONE. NC...NONE. WV...NONE. && $$ SYNOPSIS...MBS NEAR TERM...JH/MBS SHORT TERM...SK LONG TERM...PM AVIATION...AMS/JH/MBS HYDROLOGY...AMS/PC/SK

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