Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 190132 AFDRNK AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BLACKSBURG VA 932 PM EDT SAT APR 18 2015 .SYNOPSIS... HIGH PRESSURE WILL DRIFT EAST TONIGHT ALLOWING CLOUDS FROM LOW PRESSURE OVER THE TENNESSEE VALLEY TO SPREAD ACROSS THE REGION OVERNIGHT. RAIN...HEAVY AT TIMES...WILL FALL SUNDAY INTO SUNDAY NIGHT BEFORE A COLD FRONT CROSSES THE REGION WITH A CHANCE OF SHOWERS AND POTENTIALLY STRONG STORMS ON MONDAY. DRY AND COOL CONDITIONS WILL PREVAIL FOR THE MIDDLE PART OF NEXT WEEK. && .NEAR TERM /THROUGH SUNDAY/...
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AS OF 912 PM EDT SATURDAY STILL A FEW DIURNAL SHOWERS ALONG THE BLUE RIDGE BUT THEY ARE FADING FAST. WILL REMOVE POPS FOR THE REST OF THE EVENING...THEN INCREASE POPS TOWARDS SUNRISE AS LOW PRESSURE ENTERS THE TENNESSEE VALLEY. MINOR UPDATES TO OTHER GRIDS. PREVIOUS DISCUSSION... AS OF 300 PM EDT SATURDAY WEAK LOW LEVEL CONVERGENCE JUST SOUTH OF OUR FORECAST AREA HAS IGNITED A FEW SHOWERS AND ISOLATED STORMS. THE HRRR MODEL LIFTS THIS BOUNDARY SLIGHTLY NORTH. THIS COMBINED WITH A WEAK UPSLOPE FLOW MAY FIRE A FEW SHOWERS AND ISOLATED STORMS SOUTH OF HIGHWAY 58 THIS AFTERNOON AND EARLY THIS EVENING...THEREFORE KEPT A SMALL CHANCE FOR POPS IN THE FORECAST. ALL THE MODELS ARE IN GOOD AGREEMENT ON DEVELOPING A STRONG LOW LEVEL SOUTHEAST JET...TRANSPORTING MOISTURE INTO THE AREA OVERNIGHT. A FEW GUSTS OVER 30KTS WILL BE POSSIBLE IN SOUTHEAST WV/CLINCH RIVER VALLEY VA OVERNIGHT AND SUNDAY...BUT VALUES ARE BELOW GUIDANCE CRITERIA. PWATS INCREASE TO 2-3 STANDARD DEVIATIONS ABOVE AVERAGE BY SUNDAY MORNING AND THROUGH MUCH OF THE DAY SUNDAY. MEANWHILE...A LARGE AREA UVV LIFTS NORTH INTO OUR REGION TOWARD DAWN THEN EAST OVER ENTIRE AREA ON SUNDAY. THIS IS IN ADVANCE OF A NEGATIVELY TILTED UPPER TROF LIFTING THROUGH THE OHIO VALLEY SUNDAY WHICH ALSO GENERATES STRONG UPPER DIFFLUENCE. AS A RESULT...EXPECT MODERATE TO HEAVY RAIN TO FALL FOR SEVERAL HOURS...ESPECIALLY ALONG THE EAST SLOPES OF THE BLUE RIDGE. MORE INFORMATION ABOUT THE FLOODING POTENTIAL FROM THIS SYSTEM CAN BE FOUND IN THE HYDROLOGY SECTION. BOTTOM LINE...A FLOOD WATCH WILL BE POSTED FOR THE NW NC MOUNTAINS/FOOTHILLS AND SOUTHERN BLUE RIDGE OF VA. FUTURE SHIFTS MAY NEED TO EXPAND THIS NORTH AND EAST...IF HIGHER QPF TRENDS CONTINUE. CONTINUED TO GO BELOW GUIDANCE FOR HIGHS SUNDAY...WITH WEDGE IN PLACE AND WIDESPREAD CLOUDS AND PRECIPITATION. IN FACT...MANY AREAS IN THE WEDGE LIKELY SEE LITTLE...IF ANY...RECOVERY IN TEMPS.
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&& .SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY NIGHT/... AS OF 335 PM EDT SATURDAY... HEAVIER RAINFALL ASSOCIATED WITH NORTHEASTWARD MOVING STRONGLY NEGATIVELY TILTED UPPER TROUGH AND ASSOCIATED WARM/WEDGE FRONT WILL MOVE INTO THE PIEDMONT THROUGH 06Z MON...THEN EAST OF THE CWA BETWEEN 06Z AND 12Z MON. ADDITIONAL RAINFALL OF 1/2 TO 3/4 INCH DURING THIS TIME FRAME SHOULD BE CONFINED TO THE PIEDMONT. FLOOD WATCH ENDS AT 04Z MON...WHICH SHOULD BE GOOD TIMING. MEANWHILE...A BROAD UPPER TROUGH CONTINUES TO DEEPEN ACROSS THE GREAT LAKES WITH UPPER FLOW REMAINING SOUTHWEST BEHIND THE AFOREMENTIONED UPPER TROUGH. THE NEXT UPPER SHORT WAVE WILL BE APPROACHING THE WESTERN AREAS BY DAYBREAK MONDAY WITH A STRONG COLD FRONT MOVING INTO THE WESTERN TN/OH VALLEY. SCATTERED SHOWERS SHOULD DEVELOP ONCE AGAIN ACROSS THE WESTERN MOUNTAINS IN ADVANCE OF THESE FEATURES. BY AFTERNOON...MUCH OF THE CWA...ESPECIALLY EAST OF THE BLUE RIDGE WILL BE SOLIDLY IN THE WARM SECTOR WITH TEMPERATURES ACROSS THE PIEDMONT NEAR 80 DEGREES AND DEWPOINTS NEAR 60. A 6-HOUR WINDOW OF SUBSTANTIAL CAPE IN EXCESS OF 1000 J/KG...BULK 0-6KM SHEAR OF 20-30KTS...LIS TO -6 ALONG THE VA/NC BORDER...AND SIG SVR PARMS GREATER THAN 20000 COMBINED WITH STRONG PVA AND YET ANOTHER NEGATIVELY TILTED SHORT WAVE STRONGLY SUPPORT THE SLIGHT RISK OUTLOOK FROM SPC EAST OF THE BLUE RIDGE MON AFTERNOON...WITH THE MARGINAL RISK FURTHER WEST INTO THE MOUNTAINS. MAIN CONCERNS WILL BE THUNDERSTORM WIND GUSTS AND HAIL WITH COLD AIR ALOFT ADVECTING INTO THE REGION. A FEW MORE DISCRETE CELLS COULD DEVELOP EAST OF THE FRONT ACROSS THE PIEDMONT IF MAXIMUM INSTABILITY/HEATING IS REALIZED. THE SEVERE THREAT WILL END QUICKLY FROM WEST TO EAST AFT 00Z AS THE THERMODYNAMICS WANE QUICKLY AND THE UPPER SUPPORT SHIFTS NORTHEAST OF THE REGION. RAINFALL WILL NOT BE A CONCERN WITH THIS SECOND EVENT AS ACTIVITY WILL BE BRIEF AND SCATTERED...BUT A QUICK 1/2 INCH OF RAIN MAY OCCUR IN AREAS WITH HEAVIER SHOWERS/THUNDERSTORMS. SCATTERED UPSLOPE RAIN SHOWERS COULD PERSIST ACROSS THE MOUNTAINS INTO TUESDAY...BUT NO SIGNIFICANT DYNAMICS ARE FORESEEN WITHIN THIS TIME FRAME. UNSEASONABLY WARM TEMPERATURES MONDAY WILL GIVE WAY TO MUCH COOLER CONDITIONS ACROSS THE MOUNTAINS TUESDAY WITH VARIABLE CLOUDS AND A FEW SHOWERS...BUT DOWNSLOPE PIEDMONT AREAS SHOULD STILL BE CLOSE TO 70 EVEN TUESDAY. && .LONG TERM /WEDNESDAY THROUGH SATURDAY/... AS OF 350 PM EDT SATURDAY... MODELS IN BETTER AGREEMENT TODAY ON A DEEP AND BROAD UPPER LOW CENTERED ACROSS THE EASTERN GREAT LAKES DOMINATING THE WEATHER THROUGH MOST OF THE REMAINDER OF THE WORK WEEK. THIS WILL LEAVE UNSETTLED CONDITIONS ACROSS THE MOUNTAINS...SOMEWHAT LESS SO ACROSS THE PIEDMONT. THE ECMWF HAS COME INTO BETTER AGREEMENT WITH THE GFS NOW ON A DISTURBANCE ROTATING THROUGH THE BOTTOM OF THE UPPER TROUGH WEDNESDAY...BRINGING A BETTER THREAT OF SHOWERS TO THE REGION. THUS...HAVE INCREASED POPS IN ACCORDANCE WITH THE BETTER MODEL AGREEMENT AND SURROUNDING OFFICES. HAVE NOT INTRODUCED THUNDER AT THIS POINT...BUT COULD NOT TOTALLY RULE OUT SOME THUNDER BECAUSE OF RATHER COLD AIR ALOFT. FOLLOWING THIS DISTURBANCE COMES THE BONAFIDE COLDER AIR WHICH PROMISES TO PUSH 850MB TEMPS DOWN TO 0C OR BELOW FOR MUCH OF THE REMAINDER OF THE WEEK ACROSS THE NORTHERN AREAS. THE UPPER TROUGH GRADUALLY SHIFTS MORE TOWARD THE MID-ATLANTIC LATE IN THE PERIOD...STRENGTHENING THE NORTHWEST FLOW ACROSS THE REGION...AND ALLOWING 850MB TEMPS TO BOTTOM OUT ACROSS THE NORTH AROUND -5C BY SATURDAY. THIS PATTERN WILL CONTINUE TO RESULT IN A SIGNIFICANT TEMPERATURE DIFFERENCE FROM THE WESTERN MOUNTAINS WITH CLOUDS AND VARIABLE CLOUDS/SCATTERED SHOWERS AND MORE SUNNY/DOWNSLOPE NORTHWEST FLOW ACROSS THE PIEDMONT. OVERALL...TEMPERATURES SHOULD AVERAGE BELOW NORMAL THROUGH THE PERIOD...PERHAPS MUCH BELOW IN THE WEST. MEXMOS SHOWS SEVERAL DAYS WITH HIGHS ONLY IN THE 50S/60S WEST AND LOWS IN THE UPPER 30S TO LOWER 40S. WIND AND CLOUDS SHOULD PREVENT ANY FROST/FREEZE CONCERN AT THIS POINT. && .AVIATION /02Z SUNDAY THROUGH THURSDAY/...
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AS OF 735 PM EDT SATURDAY... ISOLATED SHOWERS ALONG THE BLUE RIDGE SHOULD DISSIPATE SHORTLY WITH LOSS OF HEATING. MODELS CONTINUE TO BE CONSISTENT WITH CLOUDS AND THEN RAIN WILL SPREAD INTO THE AREA FROM SOUTHWEST TO NORTHEAST TONIGHT. EXPECT CEILINGS TO LOWER TO MVFR ON THE 06-09Z TIME RANGE. A STRONG SOUTHEAST LOW LEVEL JET WILL DEVELOP JUST AFTER MIDNIGHT WHICH MAY BRING WIND GUSTS TO 30 KTS AT KLBF EARLY SUNDAY AND CONTINUE THROUGH 18Z. WITH A WEDGE OF HIGH PRESSURE IN PLACE ALONG AND EAST OF THE BLUE RIDGE...THESE STRONG WINDS ARE NOT EXPECTED TO REACH THE SURFACE AND KBCB AND KLWB...BUT WILL CREATE POTENTIALLY HAZARDOUS LOW LEVEL WIND SHEAR CONDITIONS. KROA WILL BE NEED TO BE CAREFULLY MONITORED TO POSSIBLE INCLUSION OF LL WIND SHEAR. CIGS WILL CONTINUE TO DROP AND RAIN WILL BECOME WIDESPREAD DURING THE DAY SUNDAY...AS A STRONG UPSLOPE FLOW TRANSPORTS ABUNDANT MOISTURE INTO THE REGION NORTH OF A NEARLY STATIONARY FRONT. WIDESPREAD IFR CONDITIONS ARE FORECAST IN THE MOUNTAINS AND FOOTHILLS...WITH MVFR CIGS AT KLYH AND KDAN LIKELY DETERIORATING AFTER 18Z SUNDAY WHEN THE HEAVIEST PRECIPITATION ARRIVES. EXTENDED AVIATION... A COLD FRONT APPROACHES LATER MONDAY WITH SOME STORMS POSSIBLE AHEAD OF AND ALONG IT...ESPECIALLY EAST OF THE BLUE RIDGE. THIS IS LIKELY TO KEEP PERIODS OF MVFR AND BRIEF IFR IN STORMS UNTIL THE FRONT CLEARS THE REGION MONDAY EVENING VFR RETURNS LATER MONDAY NIGHT AND TUESDAY WITH HIGH PRESSURE BUILDING IN BEHIND THE FRONT WITH A DRIER AIRMASS. THEN THE NEXT CHANCE OF MVFR CONDITIONS AND PRECIPITATION WILL BE IN THE MOUNTAINS ON WEDNESDAY.
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&& .HYDROLOGY...
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AS OF 330 PM EDT SATURDAY... ANTECEDENT CONDITIONS ARE STILL MOIST ACROSS THE NW NC MOUNTAINS/FOOTHILLS INTO THE SOUTHERN BLUE RIDGE OF VA WITH THE PERCENT OF NORMAL PRECIP OVER THE PAST WEEK EXCEEDING OVER 150%. THIS ALSO IS SHOWN IN RIVER STAGE LEVELS ACROSS THIS REGION FROM THE USGS WITH STREAM LEVELS IN MANY LOCATIONS RUNNING GREATER THAN THE 75TH PERCENTILE. WITH 6 HOUR FLASH FLOOD GUIDANCE VALUES BELOW 3 INCHES IN THE REGION...AND FORECAST PW VALUES 2-3 STANDARD DEVIATIONS ABOVE NORMAL...EXPECT FLOODING OF CREEKS AND STREAMS. WHILE THE SREF MODEL ENSEMBLE RIVER FORECAST ONLY IS FORECASTING A SMALL CHANCE FOR RIVERS TO REACH ACTION STAGE...WOULD NOT BE SURPRISED TO SEE A FEW SITES REACH MINOR FLOODING CONSIDERING THE MODEL TRENDS OF INCREASING QPF WITH EACH RUN.
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&& .RNK WATCHES/WARNINGS/ADVISORIES... VA...FLOOD WATCH FROM 4 AM EDT SUNDAY THROUGH SUNDAY EVENING FOR VAZ015>017-032. NC...FLOOD WATCH FROM 4 AM EDT SUNDAY THROUGH SUNDAY EVENING FOR NCZ001>003-018>020. WV...NONE. && $$ SYNOPSIS...PH NEAR TERM...PH/RCS SHORT TERM...RAB LONG TERM...RAB AVIATION...PC/PH HYDROLOGY...PH

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