Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 240849 AFDRNK AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BLACKSBURG VA 449 AM EDT FRI APR 24 2015 .SYNOPSIS... HIGH PRESSURE WILL BUILD ACROSS THE APPALACHIAN CHAIN TODAY... MAINTAINING DRY CONDITIONS WITH ABUNDANT SUNSHINE. LOW PRESSURE WILL TRACK EAST FROM THE SOUTHERN AND CENTRAL PLAINS TONIGHT...AND WILL PASS ACROSS OUR AREA ON SATURDAY...MAKING FOR A WET WEEKEND. && .NEAR TERM /THROUGH TONIGHT/... AS OF 345 AM EDT FRIDAY... SURFACE OBSERVATIONS THIS MORNING INDICATE THAT HIGH PRESSURE IS CENTERED OVER OHIO AND KENTUCKY...AND GRADUALLY DRIFTING EAST. NORTHWESTERLY WINDFLOW CONTINUES TO PUSH VERY DRY AIR INTO THE AREA...ALTHOUGH WINDS REMAIN GUSTY ACROSS THE MOUNTAINS DUE TO A TIGHTENED PRESSURE GRADIENT. STILL A CONCERN THAT IF WIND SPEEDS FALL TO NEAR CALM IN THE DEEPER MOUNTAIN VALLEYS THROUGH SUNRISE THIS MORNING...THEN CLEAR SKIES AND VERY LOW DEWPOINTS WILL ALLOW RADIATIONAL COOLING TO TAKE OVER...SUCH THAT TEMPERATURE MAY STILL FALL TO FREEZING IN A FEW SPOTS. AS SUCH...WILL KEEP THE FREEZE WARNING IN EFFECT...ALTHOUGH BELIEVE THAT FREEZING TEMPERATURES WILL BE SPOTTY AND LOCALIZED AT BEST. NORTHWEST FLOW WILL PERSIST THROUGH THE DAY UNDER ABUNDANT SUNSHINE...ALTHOUGH WITH HIGH CLOUDS INCREASING TOWARD SUNSET. WEATHER FORECAST MODELS SEEM TO BE UNDERESTIMATING HOW DRY THE AIR IS...AND THEREFORE HOW MUCH WARMING IS POSSIBLE IN THE DOWNSLOPE WINDFLOW. HAVE RAISED AFTERNOON TEMPERATURES A COUPLE OF DEGREES IN THE EAST...WITH HIGHS EXPECTED TO RANGE FROM THE MID/UPPER 50S ACROSS SOUTHEAST WEST VIRGINIA...TO NEAR 70 ACROSS THE SOUTHSIDE AND NORTH CAROLINA PIEDMONT. WHILE STILL A LITTLE GUSTY THIS AFTERNOON...WIND SPEEDS WILL NOT BE AS STRONG AS THOSE OBSERVED ON THURSDAY AS HIGH PRESSURE SETTLES ACROSS THE APPALACHIAN CHAIN... ALLOWING THE PRESSURE GRADIENT TO CONTINUE RELAXING THROUGH THE DAY. AS MENTIONED...CLOUDS WILL INCREASE LATE THIS EVENING AS ANOTHER LOW PRESSURE SYSTEM PASSES ACROSS THE CENTRAL PLAINS. TONIGHT WILL BE ONE OF THOSE NIGHTS WHERE TEMPERATURES COOL QUICKLY AROUND SUNSET... BUT THEN COOLING SLOWS OR STOPS ALTOGETHER AFTER MIDNIGHT AS CLOUDS THICKEN AND LOWER AS THEY BUILD IN FROM THE SOUTHWEST. WILL SEE A FEW SHOWERS ENTER THE AREA TOWARD DAWN SATURDAY...HOWEVER BELIEVE IT WILL TAKE TIME FOR MOISTURE TO OVERCOME THE DRY AIR IN PLACE...SO EXPECT THE MORE CONTINUOUS RAINFALL TO HOLD OUTSIDE OUR AREA UNTIL AFTER SUNRISE SATURDAY. && .SHORT TERM /SATURDAY THROUGH MONDAY NIGHT/... AS OF 415 AM EDT FRIDAY... SURFACE HIGH PRESSURE AND DRY AIR WILL QUICKLY GIVE WAY TO A STRONG AND HIGHLY KINEMATIC UPPER TROUGH MOVING ESE FROM THE CENTRAL PLAINS. THIS SYSTEM WILL BE THE WEATHER FOCUS FOR THIS PERIOD AS IT MOVES EAST INITIALLY...THEN BECOMES CAUGHT IN THE CIRCULATION ON THE BACK SIDE OF THE PERSISTENT NORTHEAST U.S. UPPER LOW EARLY SUN...SLIDING EAST-SOUTHEAST TO OFF THE VA/NC COAST BY SUNDAY NIGHT. AN EXTENDED PERIOD OF WET WEATHER WILL RESULT. STRONG ISENTROPIC LIFT WILL ALLOW PRECIPITATION TO SPREAD QUICKLY INTO THE DRY...COOL ANTECEDENT AIR MASS AFTER DAYBREAK SATURDAY. HOWEVER...MODELS ARE A TAD SLOWER BRINGING PRECIPITATION INTO THE CWA AT 12Z SATURDAY THAN NOTED YESTERDAY. AS THE PRECIPITATION FALLS INTO THE INITIALLY COOL/DRY SFC AIR MASS...AN INSITU WEDGE WILL QUICKLY EVOLVE AND PERSIST SATURDAY WITH MAX TEMPS STRUGGLING TO RISE INTO THE 50S. EVEN SOME AREAS ONLY IN THE UPPER 40S...SUCH AS PARTS OF THE NEW/ROANOKE/AND SHENANDOAH VALLEYS. A PERIOD OF LIGHT TO MODERATE RAINFALL IS LIKELY FROM WEST TO EAST THROUGH THE DAY. ANY CONVECTION SHOULD BE CONFINED TO FAR SOUTHWEST VA...LIKELY HITTING A WALL AS IT NEARS OUR CWA BORDER AND THE WEDGE. THERE IS A SHARP GRADIENT IN INSTABILITY/SHEAR AS YOU SHIFT WEST INTO EASTERN TN...WHERE SEVERE THUNDERSTORMS APPEAR LIKELY SATURDAY AFTERNOON. SPC HAS OUTLOOKED MUCH OF TN INTO A ENHANCED RISK AREA...WITH THE SLIGHT RISK AREA CREEPING TO NEAR OUR WESTERN CWA BORDER...AND THE MARGINAL RISK INTO THE NEW RIVER VALLEY. SPC THINKING IS THAT DEEP WESTERLY FLOW ACROSS THE TN VALLEY MAY ALLOW SOME OF THE TN STRONGER CONVECTION TO DRIFT INTO WESTERN VA BEFORE DIMINISHING IN THE STABLE WEDGE AIR MASS. FOR NOW...HAVE JUST INDICATED A SLIGHT CHC OF THUNDER IN THE FAR WEST VA COUNTIES...WEST OF I-77. OF BIGGER CONCERN...IS RAINFALL ON THE SATURATED GROUND WHICH HAS JUST BARELY RECOVERED FROM HEAVY RAINFALL EARLIER THIS WEEK. A GOOD SOAKING 3/4 TO 1 INCH OF RAIN APPEARS LIKELY IN THE WEST...1/2 TO 3/4 INCH EAST FROM THE FIRST ROUND. AS THE UPPER/SFC LOW DRIFTS EAST- SOUTHEAST INTO SUN...ADDITIONAL RAINFALL IS EXPECTED WITH INPUT FROM ATLANTIC MOISTURE. THIS COULD BRING ANOTHER 1/4 TO 1/2 INCH OF RAINFALL TO THE REGION. WPC HAS OUTLOOKED PARTS OF THE NEW RIVER/GREENBRIER/CLINCH RIVER BASIN IN OUR WESTERN COUNTIES FOR EXCESSIVE RAINFALL. GIVEN THAT RAINFALL AMOUNTS OVERALL SHOULD BE 1 INCH OR LESS...WILL NOT ISSUE ANY FLOOD WATCH HEADLINES YET...BUT SUCH HEADLINES MAY BE FORTHCOMING WITH LATER FORECAST PACKAGES. I DO NOT FEEL THAT THIS EVENT COMES ANYWHERE CLOSE TO THIS PAST SUN-MON EVENT...BUT THE CONCERN IS THAT THIS RAIN WILL BE FALLING ON SATURATED SOILS AND RIVER LEVELS ARE STILL ABOVE NORMAL IN SOME AREAS...SO RISES COULD BE QUICKER THAN NORMAL...EVEN WITH MODEST RAINFALL AMOUNTS. THE UPPER-LEVEL DISTURBANCE WILL MOVE EAST-SOUTHEAST OF THE CWA BY EARLY MON. HOWEVER...DEEP LOW PRESSURE REMAINS ANCHORED OVER THE NORTHEAST U.S. AND IS PROGGED TO SLIDE ALMOST DUE SOUTH ALONG THE MID-ATLANTIC COAST. THIS WILL KEEP OUR AREA UNSETTLED AND COOL INTO MONDAY...BUT PRECIPITATION SHOULD BECOME MORE SHOWERY AND GENERALLY CONFINED TO THE MOUNTAINS. BELOW NORMAL TEMPERATURES ARE EXPECTED THROUGH THE ENTIRE PERIOD...A GOOD 10-15 DEGREES ON MAX TEMPS SAT AND SUN...WITH LOW TEMPS CLOSER TO NORMAL. && .LONG TERM /TUESDAY THROUGH THURSDAY/... AS OF 430 AM EDT FRIDAY... THE UPPER LOW OFF THE NORTHERN MID-ATLANTIC COAST WILL BEGIN TO DRIFT EAST...BUT DEEP LOW PRESSURE ALOFT IS SLATED TO QUICKLY FILL IN BEHIND IT ACROSS THE GREAT LAKES. MEANWHILE...A CLOSED AREA OF LOW PRESSURE ALOFT IN THE CENTRAL/SOUTHERN U.S. WILL BEGIN TO DRIFT EASTWARD. GENERALLY A SPLIT FLOW IN PLACE. CONFIDENCE IN THIS PERIOD IS LOW WITH THE GFS TRENDING SOUTHWARD NOW WITH THE MID-LATE WEEK SYSTEM...AND THE ECMWF SHOWING WHAT THE GFS HAD YESTERDAY. ANOTHER PERIOD OF AT LEAST MODERATE RAINFALL APPEARS LIKELY MID TO LATE WEEK...BUT PERHAPS NOT AS HEAVY AS THOUGHT YESTERDAY. THEREAFTER...LOW PRESSURE REMAINS IN PLACE ACROSS THE GREAT LAKES/NORTHEAST WITH DEEP NORTHWEST FLOW IN PLACE ACROSS OUR REGION NORTHWARD. THIS WILL KEEP VARIABLE CLOUDINESS AND MUCH BELOW NORMAL TEMPERATURES IN PLACE AS WE GO INTO EARLY MAY WITH VERY LIMITED THREAT OF NORMAL SPRING CONVECTION. && .AVIATION /08Z FRIDAY THROUGH TUESDAY/... AS OF 130 AM EDT FRIDAY... SURFACE HIGH PRESSURE CONTINUES TO BUILD IN FROM THE WEST THROUGH THE REMAINDER OF THE NIGHT...WITH VERY DRY AIR ASSOCIATED WITH THE AIRMASS SUPPORTING VFR CONDITIONS. THE PRESSURE GRADIENT THROUGH THE NIGHT WILL REMAIN TIGHT ENOUGH TO SUPPORT GUSTY RIDGETOP WINDS...HOWEVER SUSTAINED WINDS AT TAF SITES WILL BE 10KTS OR LESS...WITH OCCASIONAL GUSTS IN THE 15KT TO 20KT RANGE FOR KROA/KBCB/KBLF/KLWB. THE HIGH PRESSURE CENTER DOES NOT PASS DIRECTLY OVER THE AREA... THEREFORE ONCE THE INVERSION BREAKS FRIDAY MORNING (AROUND 14Z)...THE WINDS WILL INCREASE TO 10-15KTS WITH A FEW GUSTS IN THE 18-22KT RANGE. DRY AIR ASSOCIATED WITH THE HIGH WILL ALLOW FOR CONTINUED VFR CONDITIONS THROUGH THE END OF THE 24/06Z TAF PERIOD...ALTHOUGH HIGH CLOUDS WILL BE ON THE INCREASE BY SUNSET. HIGH CONFIDENCE IN VFR CIGS/VSBYS THROUGH THE TAF VALID PERIOD. MEDIUM TO HIGH CONFIDENCE IN WIND SPEED/DIRECTION THROUGH THE TAF VALID PERIOD. EXTENDED AVIATION DISCUSSION... BY SATURDAY...MOISTURE APPEARS DEEP ENOUGH TO ALLOW FOR PERIODS OF RAIN AND MVFR...WITH BEST FORCING ACROSS THE CAROLINAS. COMPLEX LOW PRESSURE TO THE WEST WILL THEN PUSH THROUGH THE REGION SATURDAY NIGHT INTO SUNDAY...BEFORE EXITING OFFSHORE SUNDAY NIGHT. THIS WILL LIKELY KEEP SUB-VFR CONDITIONS IN LOW CLOUDS AND LINGERING RAINFALL INTO SUNDAY NIGHT. FLYING CONDITIONS SHOULD START TO IMPROVE SOME BY MONDAY AS THE LOW PULLS FARTHER OFFSHORE AND WEAK HIGH PRESSURE BUILDS IN FROM THE NORTHWEST. HOWEVER CONTINUED CYCLONIC FLOW LIKELY TO KEEP MORE MVFR CIGS OVER THE MOUNTAINS WITH IMPROVEMENT TO VFR ACROSS THE EAST ON MONDAY. && .EQUIPMENT... AS OF 400 AM EDT FRIDAY... SEVERAL EQUIPMENT ISSUES TO REPORT. THE WYTHEVILLE NOAA WEATHER RADIO BROADCAST REMAINS OUT OF SERVICE. TECHNICIANS WILL WORK TO RESTORE SERVICE TODAY. THE DEWPOINT AT RICHLANDS VA IN TAZEWELL COUNTY VIRGINIA IS REPORTING ERRONEOUS DEWPOINTS...SO IGNORE. THE TEMPERATURES AND DEWPOINTS REPORTED FROM THE VIRGINIA TECH AIRPORT WILL BE INTERMITTENT. THE FAA HAS BEEN NOTIFIED ABOUT THIS PROBLEM. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...FREEZE WARNING UNTIL 9 AM EDT THIS MORNING FOR VAZ007-009>020- 022>024. NC...FREEZE WARNING UNTIL 9 AM EDT THIS MORNING FOR NCZ001-002-018. WV...FREEZE WARNING UNTIL 9 AM EDT THIS MORNING FOR WVZ042>044-507- 508. && $$ SYNOPSIS...NF NEAR TERM...NF SHORT TERM...RAB LONG TERM...RAB/WP AVIATION...NF/PM EQUIPMENT...RAB

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