Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 031837 AFDRNK AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BLACKSBURG VA 137 PM EST WED FEB 3 2016 .SYNOPSIS... A STRONG FRONTAL SYSTEM WILL SHIFT EAST ACROSS OUR REGION TODAY INTO THIS EVENING. THIS FRONT SHOULD BE EAST OF THE PIEDMONT BY EARLY THURSDAY MORNING. COOL HIGH PRESSURE WORKS IN FROM THE WEST THURSDAY AFTERNOON...AND SHOULD BE OVERHEAD FRIDAY. && .NEAR TERM /THROUGH TONIGHT/... AS OF 1000 AM EST WEDNESDAY... WIND ADVISORY IN THE WEST HAS BEEN CANCELLED. MAIN CONCERN NOW IS THE POTENTIAL FOR FLOODING. MORNING RNK SOUNDING WAS SATURATED AND VERY TROPICAL LOOKING. OBSERVED PWAT WAS 1.24. FOR FEBRUARY 3RD THIS IS OFF THE CHART. PREVIOUS PWAT RECORD (1995 TO PRESENT) FOR THE DATE WAS 0.80 WHICH PLACES THIS AT THE 100 PERCENTILE AS FAR AS EXTREME MOISTURE FOR THIS TIME OF YEAR. PER THE EXCESSIVE RAINFALL DISCUSSION FROM THE WEATHER PREDICTION CENTER...THE PLUME OF MOISTURE THAT IS FLOWING NORTH INTO OUR AREAS IS 4 TO 5 STANDARD DEVIATIONS ABOVE NORMAL IN THE 850/700 MB LAYER. IF YOU LOOK AT WATER VAPOR IMAGERY...THIS MOISTURE PLUME TRAILS ALL THE WAY DOWN INTO THE EQUATORIAL PACIFIC...PRETTY IMPRESSIVE. THAT BEING SAID...RAIN RATES TODAY WILL BE TROPICAL. RADAR IS GROSSLY UNDERESTIMATING RAINFALL...ESPECIALLY OVER THE NC BLUE RIDGE WHERE RAIN EFFICIENCY HAS BEEN EXCEPTIONALLY HIGH AND RADAR IS OVERSHOOTING THE SHALLOW CLOUD TOPS. RAIN RATES IN THAT AREA ARE ABOUT AN INCH AN HOUR. SURFACE ANALYSIS SHOWS AN INCREDIBLE AMOUNT OF LOW LEVEL MOISTURE BEING TRANSPORTED INTO THAT AREA. OBSERVATIONS ACROSS WESTERN NC INDICATED DEWPOINTS IN THE LOWER 60S (STATESVILLE IS 63 AND HICKORY IS OBSERVING 61)...ALL OF WHICH IS ADVECTING NORTH INTO THE BLUE RIDGE ESCARPMENT. WOULD NOT BE SURPRISED TO SEE RAIN TOTALS OF 3 TO 5 INCHES ALONG THE BLUE RIDGE ESCARPMENT FROM BLOWING ROCK NC TO FANCY GAP VA. THIS IS QUITE SERIOUS...SINCE MOST OF THIS WATER IS GOING TO RUN-OFF DUE TO ALREADY SATURATED GROUND FROM RECENT SNOWFALL AND LACK OF VEGETATION TO INTERCEPT. OUR ADVICE TO SCHOOLS IN THAT AREA IS TO SHELTER IN PLACE AND REEVALUATE ROAD CONDITIONS ONCE THE RAIN TAPERS LATER THIS AFTERNOON. HIGHLY RECOMMEND FOLKS TAKE HEAD OF THIS SORT OF SITUATION AS STREAMS WILL COME UP QUICKLY AND COMPROMISE ROAD INFRASTRUCTURE. AS WE PROGRESS THROUGH THE AFTERNOON...THE FRONT WILL MOVE EAST...BUT NOT VERY FAST. MODELS HAVE BEEN TRENDING SLOWER WHICH SUGGESTS HIGHER QPF. WILL CONTINUE TO MONITOR THIS AND MAY EXPAND THE FLASH FLOOD WATCH FARTHER EAST IF NEEDED. LOOKING AT THE BIG PICTURE...A 995 MB SURFACE LOW WAS LOCATED OVER NORTHEASTERN MICHIGAN MOVING NORTHEAST INTO ONTARIO. A STRONG COLD FRONT TRAILED SOUTH FROM THE LOW THROUGH THE EASTERN OH VALLEY ALONG THE WESTERN SLOPES OF THE APPALACHIANS AND INTO THE CENTRAL GULF OF MEXICO. AHEAD OF THE FRONT A VERY STRONG 8H LOW LVL JET....60KTS+...WAS TRANSPORTING MOISTURE NORTH. A SHALLOW WEDGE OF COOLER AIR PERSISTED ALONG THE BLUE RIDGE...REMNANTS FROM YESTERDAYS HIGH PRESSURE. THIS SMALL DOME OF COOL AIR HAS BEEN BLOCKING THE STRONGER WINDS FROM MIXING TO THE SURFACE...BUT AS THIS FEATURE ERODES THIS AFTERNOON LOOK FOR SOUTHERLY WINDS TO INCREASE...ESPECIALLY OVER THE PIEDMONT WHERE DEEPER MIXING OF THE BOUNDARY LAYER WILL OCCUR. CONCERN FOR HEAVY RAIN WILL LINGER OVER THE MOUNTAINS THROUGH EARLY AFTERNOON BEFORE THE FOCUS SHIFTS TO THE PIEDMONT MID-LATE AFTERNOON. PWATS WILL BE IN THE 1.25 TO 1.5 INCH RANGE RUNNING 3-4 SD ABOVE NORMAL. THE HIGHER RESOLUTION NEAR TERM MODELS TAKE THE LINE OF DEEPER CONVECTION WHICH IS CURRENTLY ALIGNED ALONG THE BLUE RIDGE AND MOVE IT EAST ACROSS THE PIEDMONT DURING THE 2 TO 5PM TIME FRAME. TENDENCY OF THE MODELS HAVE BEEN TO SLOW THE FORWARD PROGRESS...SUGGESTING IT MAY TAKE AS LONG AS 8PM BEFORE THE RAIN EXITS THE EASTERN FRINGES OF THE CWA...I.E. CLARKSVILLE/FARMVILLE VA. WITH THE GROUND REMAINING SATURATED FROM SNOWMELT AND 6 HR FFG RUNNING UNDER 1.5 INCHES IN SOME LOCATIONS...WE WILL SEE SOME FLOODING. THE GREATEST RISK WILL BE ALONG THE BLUE RIDGE WHERE THE SOUTH WINDS GET LIFTED UPSLOPE PROMOTING LOCALIZED STRONGER UPWARD VERTICAL MOTION WHICH WILL YIELD HIGHER RAIN RATES/EFFICIENCY. HIGH CONFIDENCE THAT RAINFALL OF 2+ INCHES WILL OCCUR ALONG THE BLUE RIDGE...ESP SW OF ROANOKE INTO NC. ADDITIONAL CONCERN THIS AFTERNOON WILL BE THE POTENTIAL FOR SEVERE THUNDERSTORMS...ALBEIT CONFINED TO JUST A SMALL PART OF OUR CWA. SHEAR IS HIGH...RAIN EFFICIENCY IS HIGH...BUT INSTABILITY IS WEAK DUE TO THE LINGERING STABLE WEDGE. THAT STABLE WEDGE IS ERODING QUICKLY ACROSS THE NC PIEDMONT AND INTO SOUTHSIDE VA...VCNTY OF DANVILLE AND SOUTH BOSTON. THIS WILL BE THE AREA TO WATCH TO SEE IF ENOUGH INSTABILITY CAN INITIATE THUNDERSTORMS. IF THEY DEVELOP...MAIN THREAT WILL BE DAMAGING WIND GUSTS. TEMPERATURES WILL BE ALL OVER THE PLACE THIS AFTERNOON...COOLER WHERE THE WEDGE LINGERS AND MUCH WARMER...CLIMBING INTO THE 60S...WHERE THE WEDGE COMPLETELY ERODES. TONIGHT...THE FRONT SLOWS UP AS IT ELONGATED PARALLEL TO THE UPPER FLOW...BUT SHOULD BE EAST OF US BY MIDNIGHT. HUNG ONTO HIGHER POPS THIS EVENING FROM LYNCHBURG AND DANVILLE EAST WHILE DRYING THINGS OUT WEST. SHOULD SEE DRIER PUNCHING INTO THE PIEDMONT BEFORE DAWN WHILE UPSLOPE STARTS TO GENERATE LIGHT SNOW OR RAIN SHOWERS WEST. COLD AIR MAY LAG GIVEN SLOWNESS OF UPPER TROUGH OVER THE MISSISSIPPI VALLEY. LOWS WILL RANGE FROM THE MID 40S EAST TO LOWER 30S WEST. && .SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/... AS OF 237 AM EST WEDNESDAY... THE COLD FRONT THAT WILL CROSS OVER THE REGION TODAY WILL STALL ALONG THE EAST COAST THURSDAY. AN UPPER LEVEL TROUGH WILL MOVE OVER THE OHIO-TENN VALLEYS BY THURSDAY EVENING...GENERATING A SURFACE REFLECTION OFF THE SOUTHEAST COAST. THIS REFLECTION MAY BUCKLE MOISTURE BACK ACROSS THE COASTAL PIEDMONT THURSDAY EVENING. THE 00Z NAM MOVES AN AREA OF RAIN OVER THE VA/NC PIEDMONT WHILE OTHER MODELS REMAIN DRY. WE WILL RUN WITH THE DRIER SOLUTION AS WESTERLY DOWNSLOPE FLOW PROMOTES DRYING. THE UPPER LEVEL TROUGH SWINGS OVER THE MID ATLANTIC STATES FRIDAY MORNING...PUSHING THE SURFACE REFLECTION AND MOISTURE OUT TO SEA. A CHILLY SURFACE HIGH WILL THEN BUILD OVER THE REGION FRIDAY AFTERNOON AND NIGHT. THERE IS A CHANCE FOR LIGHT SNOW/FLURRIES ACROSS THE MOUNTAINS THURSDAY WITH LIGHT ACCUMULATIONS ALONG WESTERN SLOPES OF SOUTHEAST WEST VIRGINIA. THE 00Z GFS IS THE ONLY MODEL DISPLAYING UPSLOPE SNOW SHOWERS INTO THURSDAY NIGHT. HOWEVER...WITH A MAINLY WEST WIND AND MOISTURE BECOMING VERY SHALLOW THROUGH THE DAY...WILL ONLY CARRY CHANCE POPS FOR ACCUMULATING SNOW UP TO AN INCH IN WESTERN GREENBRIER. THURSDAY AND THURSDAY NIGHT WILL BE BREEZY WHILE TEMPERATURES RUN NEAR NORMAL. AS THE SURFACE HIGH BUILDS OVER THE REGION FRIDAY...TEMPERATURES WILL BE A FEW DEGREES COOLER THAN NORMAL. GOOD RADIATIONAL COOLING CONDITIONS FRIDAY NIGHT WILL SEND TEMPERATURES && .LONG TERM /SATURDAY THROUGH TUESDAY/... AS OF 400 PM EST TUESDAY... HIGH PRESSURE SHOULD DRIFT OFFSHORE ON SATURDAY...WHICH WILL START A PERIOD OF WARM AIR ADVECTION OVER THE MID ATLANTIC. TEMPERATURES SHOULD MODERATE SOMEWHAT ON SATURDAY...BUT THERE WILL BE AN INCREASE IN CLOUDINESS THAT WILL WEAKEN THE WARMING TREND. BY SATURDAY NIGHT...TWO UPPER LEVEL SHORT WAVE TROUGHS APPROACH THE EASTERN UNITED STATES. THE NORTHERN ONE WILL CROSS OVER QUEBEC AND THE NEW ENGLAND STATES...WHILE A SOUTHERN ONE SHOULD MOVE ALONG THE GULF COAST. THE LATEST GFS AND ECMWF MODELS KEEP THIS SPLIT FLOW PATTERN IN PLACE. THE SOUTHERN TROUGH WILL GENERATE A LOW PRESSURE SYSTEM OFF THE CAROLINA COAST...AND TRACK IT TO THE NORTHEAST AS THE SYSTEM INTENSIFIES. THE LOW PRESSURE SYSTEM APPEARS TO STAY JUST OFFSHORE AND NOT IMPACT THE CWA. BY SUNDAY...A BROAD UPPER LEVEL TROUGH WILL EMERGE WITHIN THE NORTHERN STREAM OVER THE DAKOTAS...AND DIG SOUTHWARD TOWARD THE MID ATLANTIC DURING MONDAY AND TUESDAY. A WAVE OF LOW PRESSURE SHOULD ACCOMPANY THE LEADING EDGE OF THIS TROUGH...THAT WILL SPARK AN ALBERTA CLIPPER SYSTEM. TEMPERATURES SHOULD REMAIN ABOVE FREEZING ON MONDAY...TO KEEP THE APPROACHING MOISTURE FROM THE CLIPPER SYSTEM AS RAIN...EXCEPT FOR THE COLDER WESTERN SLOPES WHERE IT MAY START AS SNOW. ONCE A SHOT OF ARCTIC AIR ARRIVES FROM THE UPPER LEVEL TROUGH ON MONDAY NIGHT...THE PRECIPITATION WOULD TURN OVER TO ALL SNOW WEST OF THE BLUE RIDGE. SINCE THERE IS STILL CONSIDERABLE UNCERTAINTY DUE TO THE MODELS DIVERGING ON THE TRACK OF THIS CLIPPER SYSTEM...JUST CHANCE POPS WERE UTILIZED. && .AVIATION /18Z WEDNESDAY THROUGH MONDAY/...
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AS OF 130 PM EST WEDNESDAY... LOWER THAN NORMAL CONFIDENCE IN CIG/VIS CONDITIONS AT TAF SITES EARLY IN THE PERIOD...THEN CONFIDENCE INCREASING WITH TIME. PRECIPITATION AHEAD OF A COLD FRONT MOVING INTO THE REGION FROM THE WEST IS CURRENTLY BRINGING PERIODS OF MDT/HVY RAIN SHOWERS TO THE REGION...BEFORE THE SHOWERS TAPER OFF DURING THE EVENING. CIG/VIS CONDITIONS WERE INTO THE IFR CATEGORY AHEAD OF THE PRECIPITATION...THEN TRENDED UPWARD AS PRECIPITATION HELPED MIX THE BOUNDARY LAYER. HOWEVER CONDITIONS ARE NOW RAPIDLY VARYING BETWEEN VFR AND IFR AS THE LINGERING WEDGE AND COLD GROUND WITH POSSIBLY SOME LINGERING SNOW COVER IS COOLING THE AIRMASS FROM BELOW. EXPECT A HIGH DEGREE OF VARIABILITY IN CONDITIONS THROUGH EARLY TONIGHT BEFORE THE COLD FRONT CAN MAKE IT THROUGH THE REGION AND WINDS SHIFT AROUND TO THE NORTHWEST. THE NORTHWEST FLOW WILL ESTABLISH UPSLOPE CONDITIONS FROM THE BLUE RIDGE WEST WITH MVFR/IFR CONDITIONS EXPECTED THROUGH DAYBREAK. IT IS NOT CERTAIN WHETHER KLYH AND KDAN IN THE PIEDMONT WILL BE ABLE TO MIX OUT THE LINGERING WEDGE OVERNIGHT AND THEY MAY STAY LOCKED INTO IFR CONDITIONS THROUGH TONIGHT. WILL GO WITH A LOW CONFIDENCE OPTIMISTIC FORECAST AND INDICATE IMPROVING CONDITIONS OVERNIGHT FOR KLYH/KDAN. EXPECT ALL SITES TO BE VFR FROM THURSDAY MORNING THROUGH THE END OF THE TAF PERIOD. EXPECT WINDS TO BECOME A BIT GUSTY AS THEY SHIFT TO THE NORTHWEST ESPECIALLY AT KBLF...KROA...AND KBCB. EXTENDED DISCUSSION... BY LATE THURSDAY...HIGH PRESSURE WILL BUILD OVERHEAD TO PROVIDE COLDER AND DRIER CONDITIONS...AND WILL KEEP VFR CONDITIONS AT ALL TAF SITES THROUGH SATURDAY. ANOTHER SYSTEM MOVING ACROSS THE SOUTHEAST APPEARS TO STAY JUST SOUTH OF US SUNDAY WITH POSSIBLE LIGHT PRECIP AND MVFR CIGS EDGING TOWARD THE VA/NC BORDER SUNDAY AFTERNOON.
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&& .HYDROLOGY... AS OF 10 AM EST WEDNESDAY... UNSEASONABLY HIGH PWATS...STRONG LOW LVL JET AND SATURATED GROUNDS COMBINED WITH SLOWER MOVING LINE OF HEAVIER SHOWERS COULD LEAD TO FLASH FLOODING ACROSS THE BLUE RIDGE AND ADJACENT FOOTHILLS TODAY. FLASH FLOOD WATCH HAS BEEN ISSUED WHICH IS IN EFFECT UNTIL 6 PM TODAY. FLASH FLOOD WARNINGS ARE BEING ISSUED AS NEEDED BASED ON RADAR TRENDS. IN ADDITION TO THE SMALL STREAM FLASH FLOOD THREAT...CONCERN IS INCREASING THAT WE MAY NEED TO ENTERTAIN FLOOD WARNINGS FOR SOME OF THE RIVERS WITHIN THE HYDROLOGIC SERVICE AREA...ESPECIALLY THE JAMES/ROANOKE AND DAN RIVER BASINS. EXCESSIVE RAINFALL OVER THESE BASINS TODAY WILL LIKELY RESULT IN SIGNIFICANT RISES. IF BASIN AVERAGES EXCEED 1.5 INCHES...THEN WE SURMISE THAT AT LEAST MINOR FLOODING WILL OCCUR...LAG TIME SUGGESTING THURSDAY AND FRIDAY WILL BE THE PERIOD OF TO WATCH. SOME OF THE MORE FLASHY HEADWATER RIVERS LIKE THE WATAUGA RIVER MAY FLOOD TODAY. AS FOR THE GREENBRIER RIVER...A LOT DEPENDS OF HOW MUCH SNOW MELTS IN THE HEADWATERS. ACTUAL RAINFALL WITHIN THAT BASIN HAS NOT BEEN AS EXCESSIVE...NEVER THE LESS...WILL CONTINUE TO MONITOR THIS RIVER BASIN CLOSELY. THE SAME IS TRUE FOR SOME OF THE HEADWATER STREAMS IN FAR SOUTHWESTERN VA...SUCH AS THE CLINCH AND HOLSTON. A LOT WILL DEPEND ON HOW MUCH SNOW WAS LEFT TO MELT ON THOSE NORTHERN SLOPES. AS FOR THE NEW RIVER...THE HEAVY RAIN WHICH IS FALLING IN THE HEADWATERS IN NC WILL CERTAINLY BE MOVING DOWN STREAM. AT THE VERY LEAST ANTICIPATE A HEALTHY RISE ON THE NEW. && .EQUIPMENT... AS OF 400 AM EST WEDNESDAY... MAINTENANCE WAS BEEN CONDUCTED ON THE KFCX RADAR TUESDAY. HOWEVER...THERE CONTINUES TO BE SOME TRANSMITTER AND RECEIVER ALARMS...SO THE POSSIBILITY EXISTS THE RADAR COULD GO DOWN. TECHNICIANS WILL LOOK AT IT AGAIN TODAY. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...FLASH FLOOD WATCH UNTIL 6 PM EST THIS EVENING FOR VAZ015>017- 022>024-032>035. NC...FLASH FLOOD WATCH UNTIL 6 PM EST THIS EVENING FOR NCZ001>003- 018-019. WV...NONE. && $$ SYNOPSIS...WP NEAR TERM...PM/WP SHORT TERM...RCS LONG TERM...PW AVIATION...MBS/WP HYDROLOGY...PM/WP EQUIPMENT...PM/WP

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