Area Forecast Discussion
Issued by NWS Blacksburg, VA

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219 FXUS61 KRNK 181734 AFDRNK AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BLACKSBURG VA 134 PM EDT MON MAY 18 2015 .SYNOPSIS... HIGH PRESSURE OFFSHORE WILL MAINTAIN A MOIST SOUTHERLY FLOW OF AIR ACROSS THE REGION TODAY. A COLD FRONT WILL SLOWLY APPROACH FROM THE WEST TONIGHT...MOVING THROUGH THE REGION TUESDAY. COOLER...DRIER AIR WILL FILTER INTO THE REGION BY THE MID PART OF THE WEEK BEHIND THE FRONT. && .NEAR TERM /THROUGH TONIGHT/... AS OF 1100 AM EDT MONDAY... MORNING RNK/GSO SOUNDINGS WERE MOIST WITH PWATS NEAR 1.50 INCHES...2-3 STANDARD DEVIATIONS ABOVE NORMAL. AIRMASS IS UNSEASONABLY JUICY FOR THE DATE AND PRETTY MUCH SETS THE TONE FOR THE FORECAST WHICH ENTERTAINS SLOW MOVING SHOWERS AND THUNDERSTORMS WITH POTENTIAL FOR HEAVY RAIN. FORECAST AREA REMAINS UNDERNEATH A VERY SHARP UPPER LEVEL RIDGE...WHICH IS PROMOTING VERY LIGHT STEERING WINDS...0-8KM MEAN WIND OF ONLY 11 KTS...FROM 290 DEGREES. THIS UPPER RIDGE IS EXPECTED TO WEAKEN AS WE TRANSITION THROUGH THE DAY...WITH THE RIDGE SHOWING SIGNS OF FLATTENING AS A DEEP LOW PASSES ACROSS THE NORTHERN PLAINS. AT THE SURFACE...A COLD FRONT WILL BE ENTERING THE OHIO VALLEY FROM THE UPPER MID WEST. AS THE FRONT MOVES ACROSS THE OHIO VALLEY TODAY IT WILL BEGIN TO FOCUS THE DEEPER MOISTURE FROM THE OH/TN VALLEYS INTO THE WEST SIDE OF THE APPALACHIANS. THIS SQUEEZE PLAY OF DEEPER MOISTURE IN ADDITION TO HEATING AND INSTABILITY OVER THE MOUNTAINS SHOULD RESULT IN GREATER COVERAGE OF SHOWER AND THUNDERSTORM ACTIVITY FOR OUR CWA FOR THIS AFTERNOON AND EVENING. SHOWERS WILL LIKELY DEVELOP EARLY AFTERNOON...BEGINNING ALONG THE RIDGE LINES WHERE OROGRAPHICS WILL PROVIDE THE BEST LIFT...COVERAGE THEN INCREASING AS WE TRANSITION THROUGH THE AFTERNOON. WESTERLY WIND FLOW WILL HELP PUSH SOME OF THESE SHOWERS AND THUNDERSTORMS EAST ACROSS THE PIEDMONT DURING LATE AFTERNOON AND THE EVENING. MODEL CAPES OF 1500-2000 J/KG ARE FORECAST...SO STORMS SHOULD BE A LITTLE MORE ROBUST THAN WHAT WE EXPERIENCED OVER THE WEEKEND...BUT BELIEVE WE WILL STILL LACK THE DYNAMICS FOR ANYTHING ORGANIZED WITH THE MAIN THREAT BEING HEAVY RAIN AND THE POTENTIAL FOR LOCALIZED FLASH FLOODING. COVERAGE AND INTENSITY OF SHOWER AND THUNDERSTORM ACTIVITY WILL DECREASE DURING THE EVENING WITH THE LOSS OF DAYTIME HEATING. HOWEVER...WITH THE COLD FRONT CONTINUING ITS APPROACH OVERNIGHT...DO NOT EXPECT THE RAINFALL THREAT TO GO AWAY ALTOGETHER. HAVE THEREFORE KEPT MENTION OF SHOWER AND THUNDERSTORM ACTIVITY IN THE FORECAST FOR THE OVERNIGHT TONIGHT. HELD PRETTY CLOSE TO MET/MAV GUIDANCE...WHICH WERE SIMILAR FOR TEMPERATURES TODAY AND TONIGHT. EXPECT UPPER 70S AND LOW 80S ACROSS THE MOUNTAINS FOR AFTERNOON HIGHS...WHILE TEMPERATURES ACROSS THE PIEDMONT WILL RISE INTO THE MID AND UPPER 80S. LOW TO MID 60S ARE EXPECTED AREAWIDE TONIGHT. && .SHORT TERM /TUESDAY THROUGH THURSDAY NIGHT/... AS OF 415 AM EDT MONDAY... LONG AWAITED FRONTAL SYSTEM WILL BE ON OUR DOORSTEP AT 12Z TUESDAY. THIS IS A LATER ARRIVAL TIME THAN ADVERTISED BY THE MODELS A FEW DAYS BACK AS UPPER SUPPORT/LOW PRESSURE TROUGH IS WANING...SHEARING OUT ACROSS THE GREAT LAKES/NORTHEAST. AS A RESULT...HAVE RETAINED POPS LONGER AND FURTHER WEST THAN IN PREVIOUS FORECAST PACKAGES. WOULD STILL EXPECT A GOOD CHANCE OF CONVECTION...EVEN IN THE MORNING HOURS TUESDAY...AS THE BETTER DYNAMICS DRIFT INTO THE REGION WITH THE FRONT AND UPPER TROUGH. BY AFTERNOON...THE FOCUS FOR CONVECTION SHOULD SHIFT EAST OF THE BLUE RIDGE...WHERE THUNDERSTORMS REMAIN POSSIBLE. SHEAR REMAINS WEAK...BUT INSTABILITY REMAINS HEALTHY...SO A FEW STRONG STORMS REMAIN POSSIBLE DURING THE AFTERNOON EAST OF THE BLUE RIDGE. SPC HAS CURRENTLY BACKED OFF THE MARGINAL RISK AREA THAT PREVIOUSLY EXTENDED DOWN INTO CENTRAL VA...PULLING IT BACK FURTHER NORTH INTO THE NORTHERN MID-ATLANTIC/NEW ENGLAND. SIGNIFICANT SVR VALUES ARE SLIGHTLY LESS THAN ADVERTISED BY THE MODELS YESTERDAY. BY 00Z WED...THE CHANCE OF ANY REMAINING CONVECTION SHOULD BE LIMITED TO THE FAR SOUTHEAST COUNTIES OF THE CWA. TEMPERATURES WILL STILL BE QUITE WARM...ESPECIALLY IN THE EAST TUESDAY...WHICH SHOULD HELP FUEL CONVECTION ACROSS THAT REGION...BUT BY AFTERNOON...DRIER/SLIGHTLY COOLER AIR SHOULD FILTER INTO THE WESTERN/NORTHERN AREAS. WEDNESDAY APPEARS TO OFFER A BRIEF WINDOW OF OPPORTUNITY FOR A BREAK FROM THE CONVECTION. HOWEVER...THIS WINDOW KEEPS GETTING SMALLER WITH EACH MODEL RUN. THE KEY FEATURE THROUGH THIS TIME FRAME IS A DEEPENING UPPER LOW ACROSS SOUTHEAST CANADA INTO THE CANADIAN MARITIMES...ALMOST A WINTERLIKE PATTERN. THIS LEAVES AN UNSTABLE NORTHWEST FLOW ACROSS OUR REGION...WHICH AT THIS TIME OF YEAR CAN QUICKLY GIN UP CONVECTION. FOR SEVERAL DAYS NOW...THE MODELS HAVE BEEN ADVERTISING A NORTHWEST FLOW SHORT WAVE TO TRACK ACROSS THE REGION THURSDAY. THE LATEST MODEL RUNS ALL DEPICT THE SHORT WAVE...BUT WITH WIDELY VARYING DEGREES OF INTENSITY AND RESULTANT MOISTURE/PRECIPITATION. THE SPECTRUM RANGES FROM THE FLAT/WEAK/DRY GFS TO THE DEEPER/WETTER NAM/CANADIAN. THE ECMWF SEEMS TO BE THE BEST COMPROMISE AND IS GENERALLY THE WAY WPC HAS LEANED WITH THIS SYSTEM AS WELL. THEREFORE...HAVE EXPANDED POPS NORTHWARD AND INTO THE MID-CHANCE RANGE WED NIGHT IN THE WEST AND ACROSS THE CWA THU. MODELS VARY CONSIDERABLY WITH THE NORTHWARD EXTENT OF THIS PRECIPITATION...SO FOR NOW HAVE GENERALLY BROAD BRUSHED POPS ACROSS THE CWA...WITH THE HIGHER CHANCES ACROSS THE SOUTHERN HALF WHERE MOISTURE IS EXPECTED TO BE A LITTLE DEEPER. THIS SHORT WAVE WILL EXIT TO THE SOUTHEAST THU NIGHT AS SFC HIGH PRESSURE SLIDES FROM THE PLAINS TO THE GREAT LAKES. OVERALL...THE AIR MASS APPEARS TO BE TOO STABLE AND DRY IN THE LOWER LEVELS TO SUPPORT THUNDER WITH THIS EVENT. HOWEVER...A NOSE OF INSTABILITY DOES POKE UP INTO THE SOUTHERN PARTS OF THE PIEDMONT WITH DAYTIME HEATING THU AFTERNOON...SO HAVE INTRODUCED A SLIGHT CHANCE OF THUNDER GENERALLY SOUTH OF U.S. 460 AND EAST OF THE BLUE RIDGE THU AFTERNOON. THE MOST NOTABLE DIFFERENCE WED-THU WILL BE IN THE TEMPERATURE/HUMIDITY DEPARTMENT...WITH DEWPOINTS DROPPING FROM CURRENT READINGS WELL INTO THE 60S DOWN INTO THE 40S WEST AND 50S EAST. MAX TEMPERATURES WILL DROP BACK INTO THE 60S ACROSS THE MOUNTAINS...70S THROUGH MUCH OF THE CWA...TO NEAR 80 ACROSS THE SOUTHEAST PART OF THE CWA...WITH LOWS FALLING INTO THE 40S MOUNTAINS AND 50S PIEDMONT. THIS WILL BE A PLEASANT CHANGE. && .LONG TERM /FRIDAY THROUGH SUNDAY/... AS OF 430 AM EDT MONDAY... MOISTURE...WARMTH...AND HUMIDITY ARE POISED TO RETURN FAIRLY QUICKLY THROUGH THE WEEKEND AND INTO EARLY NEXT WEEK AS YET ANOTHER DEEP UPPER LOW BUILDS INTO THE SOUTHWEST U.S...THE NORTHEAST U.S. TROUGH SHIFTS EASTWARD INTO THE NORTHERN ATLANTIC...AND AN ELONGATED RIDGE BETWEEN THESE TWO LOW PRESSURE AREAS DRIFTS INTO OUR REGION. THE MID-WEEK FRONTAL BOUNDARY WILL BEGINS TO RETURN NORTHWARD BY SATURDAY AS CANADIAN HIGH PRESSURE SHIFTS OFF THE MID-ATLANTIC COAST. SOME MODELS ARE ADVERTISING A WEDGE DEVELOPING ACROSS THE REGION OVER THE WEEKEND WITH OVERRUNNING PRECIPITATION DEVELOPING FROM THE WEST. THIS IS EVIDENT VIA SOME FAIRLY HEALTHY SFC PRESSURE RISES ACROSS THE NORTHERN INTO THE CENTRAL MID-ATLANTIC REGION SAT-SUN. HOWEVER...THERE IS ALSO A TENDENCY WITH TIME FOR THE SFC HIGH TO SHIFT FURTHER SOUTH...PUTTING THE CWA INTO A WARMER/SOUTHWEST FLOW REGIME. THE UPPER RIDGE WILL SHIFT SOUTHEAST BY MONDAY...ALLOWING AN ACTIVE BAROCLINIC ZONE TO SHIFT SOUTH AND EAST INTO THE REGION FROM THE OH/TN VALLEY. DID NOT MAKE ANY SIGNIFICANT CHANGES TO THE WEEKEND GRIDS AT THIS TIME...WITH SAT STILL LOOKING MOSTLY DRY...AND CHC POPS CREEPING BACK INTO THE WEST BY SUN. TEMPERATURES APPEAR SEASONABLE WITH A SLOW WARMING TREND AND SLOWLY INCREASING HUMIDITY AS WELL. && .AVIATION /18Z MONDAY THROUGH FRIDAY/...
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AS OF 200 PM EDT MONDAY... A VERY MOIST AIR MASS REMAINS ACROSS THE REGION STUCK UNDER AN UPPER RIDGE AS THE PAST SEVERAL DAYS. HOWEVER...THE PATTERN IS FINALLY ABOUT TO CHANGE. THE UPPER LOW THAT HAS PLAGUED THE CENTRAL U.S. THE PAST SEVERAL DAYS IS SHEARING OUT AND MOVING TOWARD THE GREAT LAKES WHERE IT WILL AID IN THE DEVELOPMENT OF A DEEP UPPER LOW ACROSS SOUTHEAST CANADA BY MIDWEEK. THIS WILL PUSH A FRONT INTO OUR REGION BY EARLY TUESDAY...THEN EAST OF THE REGION BY TUE EVENING. FOR THIS AFTERNOON AND EVENING...SCATTERED TO NUMEROUS SHOWERS AND THUNDERSTORMS ARE EXPECTED IN ADVANCE OF THE UPPER TROUGH/FRONT. THE ACTIVITY WILL EXTEND FROM THE OH/TN VALLEYS EASTWARD INTO THE MID ATLANTIC...AND FARTHER EAST INTO THE PIEDMONT AS COMPARED TO THE PAST FEW AFTERNOONS. DURING THE AFTERNOON/EVENING...THUNDERSTORMS SHOULD BE MOST NUMEROUS WEST OF THE BLUE RIDGE...SOME WITH LOCALLY HEAVY RAINFALL. THE STRONGER STORMS WILL CONTAIN LOCALIZED OUTFLOW WINDS OF 25-30KTS IN ADDITION TO MVFR CIGS AND BRIEF PERIODS OF MVFR-IFR VSBYS. LACK OF ORGANIZATION MAKES IT DIFFICULT TO PINPOINT TIMING AT ANY OF THE TERMINALS...ESP THOSE EAST OF THE BLUE RIDGE. AS SUCH...WILL CONTINUE TO MENTION VCNTY SHRA/TSRA AND AMEND TAFS PER RADAR TRENDS. COVERAGE OF THE SHOWERS/THUNDERSTORMS WILL DECREASE AFTER SUNSET...BUT GIVEN THE SYNOPTIC PATTERN WOULD EXPECT SOME ACTIVITY TO LINGER ALL NIGHT ACROSS MUCH OF THE CWA. WITH WINDS BECOMING NW...UPSLOPE MVFR CIGS APPEAR LIKELY ACROSS EASTERN WV INTO FAR SOUTHWEST VA...WITH MODELS SUGGESTING PERIOD OF IFR CIGS PRIOR TO DAYBREAK TUESDAY AT KBLF/KLWB. MVFR VSBYS WILL ALSO PREVAIL IN MANY AREAS OVERNIGHT IN A NEAR SATURATED BOUNDARY LAYER WITH MOIST/WET GROUND AND LIGHT WSW-WNW WINDS. ANY LOW CIG/VSBY ELEMENTS SHOULD QUICKLY IMPROVE AFTER 13Z/9AM TUESDAY WITH RETURN OF VFR FOR THE AFTERNOON. SURFACE FRONT WILL PASS THROUGH ALL TERMINALS TUESDAY WITH SKIES CLEARING AND WINDS BECOMING NORTHWEST 6 TO 12 KTS. EXTENDED AVIATION DISCUSSION... DRIER/COOLER CONDITIONS ARE EXPECTED TUESDAY NIGHT AND WEDNESDAY. AN UPPER DISTURBANCE WILL LIKELY BRING SOME MVFR CIGS AND -SHRA BACK INTO THE AREA THU...MOVING OUT FRIDAY WITH VFR CONDITIONS RETURNING FRI-SAT.
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&& .RNK WATCHES/WARNINGS/ADVISORIES... VA...NONE. NC...NONE. WV...NONE. && $$ SYNOPSIS...NF NEAR TERM...NF/PM SHORT TERM...RAB LONG TERM...RAB/SK AVIATION...PM

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