Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 281644 AFDRNK AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BLACKSBURG VA 1244 PM EDT TUE JUL 28 2015 .SYNOPSIS... A DIFFUSE FRONTAL BOUNDARY WILL LINGER ACROSS THE REGION THROUGH WEDNESDAY BEFORE ANOTHER STRONGER COLD FRONT ARRIVES FROM THE NORTHWEST THURSDAY. SCATTERED SHOWERS AND THUNDERSTORMS WILL CONTINUE IN THE FORECAST UNTIL THE SECONDARY FRONT MOVES SOUTH OF THE REGION FRIDAY. SOMEWHAT DRIER AIR SHOULD FOLLOW THE FRONT FOR THE WEEKEND AS HIGH PRESSURE BUILDS IN FROM THE NORTHWEST. && .NEAR TERM /THROUGH TONIGHT/... AS OF 930 AM EDT TUESDAY... SOUPY AIRMASS REMAINS OVER THE REGION WITH EARLY MORNING SOUNDINGS SHOWING PWATS BETWEEN 1.5 AND 2.0 INCHES. THIS COMBINED WITH VERY HIGH VALUES OF SFC-85H THETA-E AND WEAK WESTERLY STEERING SHOULD AGAIN PROMPT THE DEVELOPMENT OF SLOW MOVING SHRA/TSRA THROUGH THE AFTERNOON. HOWEVER EXACTLY WHERE THE BEST COVERAGE WILL BE...AND HOW FAST IT DEVELOPS A BIT IFFY GIVEN CURRENT EXPANSE OF LOW CLOUDS/FOG NEAR THE BLUE RIDGE...AND SUBTLE RESIDUAL OUTFLOW FROM OVERNIGHT. LATEST SHORT TERM GUIDANCE DOES SHOW A WEAK SHORTWAVE PER VAPOR LOOP SLIDING IN FROM THE NW ALONG THE SURFACE FRONT WHICH REMAINS JUST NORTH. ALSO ADVECTION OF EVEN HIGHER 85H THETA-E FROM THE SW SHOULD ENHANCE COVERAGE OVER THE SW. THUS PLAN TO HAVE THE HIGHEST CHANCE POPS INITIALLY OVER THE NORTH AS WELL AS THE SOUTHERN BLUE RIDGE WITH FILLING IN LIKELY OUT EAST LATER WHERE BEST INSTABILITY WILL RESIDE. SEVERE THREAT AGAIN APPEARS RATHER LOW BUT EVOLUTION FROM LIKELY PULSE NATURE CONVECTION TO SOME MULTICELL BANDS/CLUSTERS POSSIBLE AS STORMS MERGE AND OUTFLOW DOMINATES. OTRW HEAVY RAIN THE CONTINUED MAIN CONCERN GIVEN ONLY 10 KNOTS OR STEERING FLOW UNDER SUCH A SATURATED COLUMN. KEPT HIGH TEMPS CLOSE TO PREVIOUS EXCEPT NUDGED UP IN THE EAST WHERE WILL SEE MORE SUN TO START. PREVIOUS DISCUSSION AS OF 400 AM EDT TUESDAY... THERE HAS BEEN LITTLE CHANGE IN THE SYNOPTIC PATTERN AS AN UPPER RIDGE OVER THE EASTERN US KEEPS US IN NORTHWEST FLOW ALOFT...WHILE A DIFFUSE FRONTAL BOUNDARY REMAINS DRAPED OVER THE REGION. THIS WILL YIELD AN OVERALL SCENARIO QUITE SIMILAR TO YESTERDAY WITH SOME EARLY ISOLATED TO SCATTERED SHOWER AND THUNDERSTORM ACTIVITY MOVING THROUGH THE REGION... FOLLOWED BY REDEVELOPMENT LATER THIS AFTERNOON AS DIURNAL INSTABILITY INCREASES WITH HEATING. AND AGAIN WE WILL SEE MODEST INSTABILITY COMBINE WITH WEAK DYNAMICS AND HIGH PRECIPITABLE WATER VALUES TO BRING CONVECTION WITH SLOW MOVING VERY HEAVY DOWNPOURS WHILE THE OVERALL THREAT OF SEVERE WEATHER FROM WIND/HAIL REMAINS LOW. THE STORMS ON MONDAY WERE INDEED QUITE PROLIFIC PRECIPITATION PRODUCERS WITH LOCAL AMOUNTS OVER 3 INCHES FOUND ON SOME MESONET SITES...AND SEVERAL FLOOD ADVISORIES ISSUED. THIS WILL LIKELY BE THE CASE AGAIN TODAY AS LOCAL FLOODING FROM DOWNPOURS REMAINS A DISTINCT POSSIBILITY. ACTIVITY WILL BE ON THE DECLINE THIS EVENING WITH THE LOSS OF HEATING AFTER SUNSET...BUT EXPECT SOME ACTIVITY WILL BE STEWING AROUND UNTIL AROUND MIDNIGHT IF NOT A BIT LONGER. MORNING CLOUD COVER AND EXPECTED EARLY AFTERNOON ONSET TIME OF CONVECTION SHOULD BE ENOUGH TO KEEP TEMPERATURES TODAY WITHIN A DEGREE OR TWO OF READINGS YESTERDAY...WITH UPPER 80S EAST OF THE BLUE RIDGE TO LOW/MID 80S WEST. TONIGHT WILL STAY MUGGY WITH LOWER 70S EAST TO MIDDLE 60S WEST. && .SHORT TERM /WEDNESDAY THROUGH FRIDAY NIGHT/... AS OF 430 AM EDT TUESDAY... A WEAK AND VERY DIFFUSE FRONTAL BOUNDARY WILL REMAIN ACROSS THE REGION WEDNESDAY WITH A LARGE SUBTROPICAL RIDGE...ANCHORED ACROSS TX...STILL RIDGING INTO THE MID-ATLANTIC REGION. WEAK CONVERGENCE AND RESULTANT MOISTURE POOLING NEAR THIS BOUNDARY WILL AGAIN SUPPORT DIURNAL CONVECTION. MOST MODELS POINT TO THE PIEDMONT AND AREAS EAST OF THE BLUE RIDGE AS THE MOST FAVORED AREA FOR CONVECTION...BUT GIVEN THE OVERALL ENVIRONMENT...PULSE THUNDERSTORMS CANNOT BE RULED OUT ANYWHERE. WEAK STEERING CURRENTS WILL CONTINUE TO SUPPORT SLOW MOVING SHOWERS/THUNDERSTORMS...WITH VERY HEAVY RAINFALL IN ISOLATED LOCATIONS. OTHER SPOTS WILL RECEIVE NO RAINFALL...JUST AS HAS BEEN THE CASE THE PAST COUPLE OF DAYS. BY THURSDAY...A NEW COLD FRONT WILL MOVE INTO THE AREA FROM THE WEST...SUPPORTED BY A DEEP UPPER TROUGH SLIDING EASTWARD ACROSS CENTRAL INTO EASTERN CANADA. SUBSEQUENT RETROGRESSION AND AMPLIFICATION OF THE SUBTROPICAL RIDGE INTO THE SOUTHWEST U.S. WILL RESULT IN STRENGTHENING NORTHWEST FLOW ALOFT ACROSS THE EASTERN U.S. THIS SHOULD ALLOW THIS SECOND FRONT TO BE PUSHED FURTHER SOUTH THAN THE EARLY WEEK FRONT...POTENTIALLY REACHING GA/SC BEFORE STALLING IN THAT REGION LATE FRIDAY. INSTABILITY LOOKS TO BE A TAD BETTER WITH THE THURSDAY FRONT THAN WAS THE CASE WITH THE EARLY WEEK FRONT. THUS...WOULD EXPECT SLIGHTLY BETTER COVERAGE OF SHOWERS AND THUNDERSTORMS AS A RESULT. HOWEVER...TIMING OF THE FRONT COULD BE PROBLEMATIC...ARRIVING IN THE WESTERN AREAS AROUND 12Z THU...THEN PUSHING EAST OF THE BLUE RIDGE BY AFTERNOON. THIS COULD AGAIN RESULT IN THE BULK OF THE CONVECTION BEING ACROSS THE PIEDMONT...WITH MORNING CLOUD COVER...CONVECTIVE DEBRIS...AND OVERNIGHT LINGERING DISSIPATING SHOWERS/THUNDERSTORMS WESTERN AREAS. AS NOTED ABOVE...INSTABILITY IS BETTER THAN WE HAVE SEEN OVER THE PAST FEW DAYS...ALBEIT MID- LEVEL WIND FIELDS ARE NOT REMARKABLE AT ALL...NORTHWEST 10KTS AT BEST DURING THIS TIME FRAME. SPC HAS OUTLOOKED THE ENTIRE AREA FOR A MARGINAL RISK OF SEVERE THUNDERSTORMS...NOTING THAT THE FAVORED TYPE OF STORM WILL BE PULSE WITH ISOLATED SEVERE WIND GUSTS POSSIBLE. CONFIDENCE IN THIS OCCURRING GIVEN THE SYNOPTIC PATTERN AND VERY WEAK DYNAMICS IS LOW. THE SLIGHTLY STRONGER MID-LEVEL FLOW SHOULD...HOWEVER...REDUCE THE FLOOD POTENTIAL. FOR FRIDAY...THE MODELS HAVE COME INTO BETTER AGREEMENT NOW...AS NOTED ABOVE...THAT THE FRONT WILL PUSH SOUTH OF THE AREA...ENOUGH SO THAT POPS HAVE BEEN DROPPED BELOW 15 PERCENT FOR FRI AND FRI NIGHT. DEWPOINTS DROP ON AVERAGE 5-7 DEGREES INTO FRI...ALLOWING MIN TEMPS TO DROP BACK CLOSER TO NORMAL VALUES FOR EARLY AUGUST...MAINLY 55 TO 60 MOUNTAINS AND 60S PIEDMONT. THE MAX TEMP DROP WILL ONLY BE A COUPLE OF DEGREES OVERALL WITH LESS CLOUD COVER... LOWER DEWPOINTS...AND GREATER AMOUNTS OF INSOLATION. && .LONG TERM /SATURDAY THROUGH MONDAY/... AS OF 445 AM EDT TUESDAY... THE MAIN FOCUS FOR THE EXTENDED PERIOD WILL BE ON A BROAD UPPER TROUGH REMAINING ACROSS EASTERN CANADA AND ITS POTENTIAL DEEPENING INTO THE EASTERN U.S. ANOTHER CONCERN FOR THIS PERIOD WILL BE A WEAK AREA OF LOW PRESSURE ALOFT...MOST LIKELY INITIALLY AN EASTERLY WAVE...DEVELOPING ACROSS THE FL/GA/WESTERN GULF AREA AND COMBINING WITH THE STALLED LATE WEEK FRONTAL BOUNDARY LOCATED ACROSS THE SOUTHEAST STATES. WITH RESPECT TO THE UPPER TROUGH...THE GFS IS MUCH MORE AGGRESSIVE THAN THE OTHER MODELS IN DEEPENING THIS TROUGH ACROSS THE EASTERN U.S. CONSEQUENTLY...IT ADVERTISES A FAIRLY DRAMATIC DROP IN 850MB TEMPS FROM THE CURRENT READINGS AROUND +20C TO AS LOW AS THE +7C RANGE BY THE MIDDLE OF NEXT WEEK. THE ECMWF IS NOT ON BOARD WITH THIS AT ALL...INSTEAD SHOWING A LARGE-SCALE BROAD UPPER RIDGE REMAINING IN PLACE PRETTY MUCH ACROSS THE ENTIRE U.S. SOUTH OF 40 DEG LATITUDE. AT THE SAME TIME THE GFS HAS 850MB TEMPS IN THE SINGLE DIGITS...THE ECMWF SHOWS READINGS IN EXCESS OF +20C. FEEL THAT THE GFS IS OVERDONE WITH THE DEEPENING OF THE EASTERN U.S. UPPER TROUGH AND ITS RELATED DRAMATIC COOLING IN THE EASTERN STATES. WITH RESPECT TO THE PSEUDO-TROPICAL LOW DEVELOPING ACROSS THE EASTERN GULF...SOUTHEAST STATES...MODELS ARE IN BETTER AGREEMENT ON THIS ISSUE THAN THE EASTERN CANADIAN UPPER TROUGH...WITH THE ECMWF STILL THE MOST AGRESSIVE...AS IT HAS BEEN FOR SOMETIME...IN DEVELOPING A CLOSED SFC LOW...AND THEN TRACKING IT NORTHEASTWARD JUST INLAND FROM THE GA/SC/NC COAST INTO EARLY NEXT WEEK. A WEAK FRONTAL BOUNDARY IS SLATED TO ARRIVE WITH THE DEEPENING OF THE UPPER TROUGH...AT LEAST IN SOME FASHION...BY EARLY NEXT WEEK...WHICH COULD...DEPENDING ON TIMING...EITHER PULL THE SOUTHEAST U.S. LOW PRESSURE AREA FURTHER INLAND ACROSS THE PIEDMONT OR HELP SHOVE IT EAST OF THE AREA. AT ANY RATE...RAIN CHANCES SHOULD BE ON THE INCREASE AGAIN BY EARLY NEXT WEEK. THE BEST RAIN CHANCES WILL REMAIN ACROSS THE PIEDMONT WITH THIS SCENARIO. HOWEVER...THE WEEKEND FOR THE MOST PART LOOKS DRY WITH SEASONABLE TEMPERATURES AND HUMIDITY LEVELS. && .AVIATION /18Z TUESDAY THROUGH SUNDAY/... AS OF 1235 PM EDT TUESDAY... OVERALL SHOTGUN PATTERN TO DEVELOPING CONVECTION WILL CONTINUE THROUGH MID AFTERNOON BEFORE PERHAPS SHRA/TSRA ORGANIZES A BIT MORE AND HEADS EAST OFF THE BLUE RIDGE. HOWEVER STILL TOO IFFY TO INCLUDE PREVAILING CONVECTION AT ANY ONE TAF SITE PENDING RELEASE SO KEEPING MOSTLY VCTS GOING AT MOST LOCATIONS INTO EARLY THIS EVENING. COULD BE ENOUGH COVERAGE THIS EVENING FOR MORE OF A PREVAILING MVFR GROUP IN SHOWERS OUT EAST BUT LOW CONFIDENCE. OTRW EXPECT MOST SITES TO FINALLY IMPROVE TO VFR LEVELS OUTSIDE OF CONVECTION EARLY THIS AFTERNOON WITH LOCALIZED MVFR/IFR IF A HEAVIER SLOW MOVING SHRA/TSRA MOVES OVER OR NEAR A TAF LOCATION DURING THE NEXT FEW HOURS. SHOWERS MAY LINGER WELL PAST MIDNIGHT TONIGHT ESPCLY IN THE EAST WITH A WEAK WAVE PASSING. THIS SHOULD GRADUALLY TREND TOWARD MORE FOG IN THE VALLEYS AND WHERE HEAVIER RAINFALL OCCURRED LATE. THIS SUPPORTS IFR/LIFR IN THE VALLEY SPOTS SUCH AS KLWB/KBCB AND OVERALL MVFR ELSW WITH EVEN SOME FOG POSSIBLE AT KROA IF THEY SEE AFTERNOON/EVENING RAINFALL. WEDNESDAY WILL BE ANOTHER DAY OF MVFR TO IFR VALLEY FOG IN THE MORNING AND SCATTERED AFTERNOON AND EVENING SHOWERS AND THUNDERSTORMS. HOWEVER COVERAGE MAY BE LESS AND MORE OROGRAPHIC/OUTFLOW DRIVEN WEDNESDAY AFTERNOON WITH AGAIN LOCAL SUB-VFR IN AND NEAR ANY CONVECTION. EXTENDED AVIATION FORECAST... MORE WIDESPREAD SHOWERS AND THUNDERSTORMS ARE EXPECTED ON THURSDAY AS A COLD FRONT CROSSES THROUGH REGION FROM THE NORTHWEST. THIS FRONT SHOULD PUSH SOUTH OF THE AREA AND ALLOW FOR DRIER AND MOSTLY VFR CONDITIONS FRIDAY THROUGH SUNDAY. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...NONE. NC...NONE. WV...NONE. && $$ SYNOPSIS...JH/RAB NEAR TERM...JH/MBS SHORT TERM...RAB LONG TERM...KK/RAB AVIATION...AMS/JH/MBS/RAB

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