Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 281507 AFDRNK AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BLACKSBURG VA 1007 AM EST SUN DEC 28 2014 .SYNOPSIS... A WEAK COLD FRONT WILL DRIFT SOUTHEAST INTO THE MOUNTAINS TODAY BEFORE SLIDING SOUTH OF THE REGION LATER SUNDAY NIGHT. WEAK WAVES OF LOW PRESSURE TRACKING ALONG THE BOUNDARY TODAY INTO MONDAY MORNING WILL BRING PERIODS OF MOSTLY LIGHT RAIN TODAY THROUGH EARLY MONDAY. COLD HIGH PRESSURE WILL SLOWLY BUILD EAST INTO THE REGION BEHIND THE FRONT FOR THE MIDDLE OF NEXT WEEK. && .NEAR TERM /THROUGH TONIGHT/...
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AS OF 955 AM EST SUNDAY... A BROAD AND MOIST SOUTHWESTERLY FLOW CONTINUES ACROSS SOUTHEASTERN AND MID-ATLANTIC STATES THIS SUNDAY MORNING WITH REGIONAL RADARS SHOWING A FAIRLY DISORGANIZED SHIELD OF RAINFALL COVERING MUCH OF THAT AREA...BUT MAINLY SOUTH OF OUR FORECAST AREA. A COUPLE OF FACTORS HAVE LED TO A BACKING OFF ON RAINFALL AMOUNTS FROM THIS INITIAL SHORTWAVE. PRECIPITATION SEEMS TO BE HAVING A VERY DIFFICULT TIME OVERCOMING THE DRY AIR OVER MUCH OF THE CWA... WITH 12Z KRNK SOUNDING SHOWING SEVERAL DRY LAYERS IN THE COLUMN. HIGH- RES MODELS NMM AND ARW BOTH SHOWING THE CURRENT RADAR TRENDS FAIRLY WELL WITH THE HEAVIER RAINFALL SPLITTING NORTH AND SOUTH OF THE CWA AND THESE HAVE BEEN FOLLOWED MORE FOR TODAYS RAINFALL. ADJUSTED POPS DOWNWARD SOME TO REFLECT THIS PATTERN WHICH IS FORECAST TO PERSIST MUCH OF THE DAY BUT MANY AREAS MAY STILL PICK UP MEASURABLE RAINFALL..JUST A LOT LESS THAN WAS FORECAST EVEN YESTERDAY...MANY AREAS MAY SEE ONLY A FEW HUNDRETHS OF AN INCH TODAY. A SECOND SHORT WAVE FORECAST TO MOVE NORTHEAST OVERNIGHT MAY OFFER A BETTER CHANCE FOR MORE SUBSTANTIAL RAINFALL...POSSIBLY 0.25 TO 0.50 INCHES AND THE BULK OF THAT MAY COME WITH SECOND SHORTWAVE FORECAST TO MOVE THROUGH TONIGHT.
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&& .SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/... AS OF 130 AM EST SUNDAY... COLD FRONT SHOULD BE WELL TO THE SE OF THE AREA MONDAY WITH LOW PRESSURE OFFSHORE KEEPING AN AXIS OF DEEPER MOISTURE BACK INTO THE REGION ESPCLY EASTERN SECTIONS INTO MONDAY AFTERNOON. THIS WILL LIKELY KEEP SOME STEADIER RAIN GOING EAST OF THE BLUE RIDGE DURING THE MORNING WITH POPS TAPERING FROM WEST TO EAST DURING THE AFTERNOON AS THE WAVE EXITS. HOWEVER AS THE FINAL MAIN PIECE OF SHORTWAVE ENERGY PASSES MONDAY MORNING...WILL SEE A LEFTOVER RESIDUAL INVERTED TROUGH TAKE SHAPE OVER THE MOUNTAINS IN RESPONSE TO ANOTHER FAINT STRUNG OUT MID LEVEL WAVE RIDING UP THE APPALACHIANS. THIS MAY ACT TO KEEP SOME -RA GOING MOUNTAINS EVEN AS THE MAIN PRECIP SHIELD PULLS OUT BY EARLY AFTERNOON. COLD ADVECTION LOOKS TO SLOWLY OOZE IN BEHIND THIS FEATURE WITH PERHAPS A FEW LOCATIONS OVER THE FAR NW MOUNTAINS SEEING A CHANGE TO LIGHT SNOW. OTRW LIKELY POPS EAST EARLY AND CHANCE POPS ELSW MONDAY. WITH SLOWER COOLING AT 85H...OPTED FOR THE SLIGHTLY WARMER SIDE OF MOS WHICH RESULTS IN HIGHS MOSTLY 40S AND PERHAPS LOW 50S SE. LEFTOVER BUT SOMEWHAT DEEPER MOISTURE WILL BE THE MAIN CONCERN ESPCLY WEST MONDAY NIGHT INTO TUESDAY MORNING AS THE RESIDUAL INVERTED TROUGH LOOKS SLOW TO WEAKEN. MODELS CONTINUE TO PRINT OUT A TENTH OR TWO OF QPF ALONG WESTERN SECTIONS OF THE BLUE RIDGE WITH THE NAM BY FAR A WET OUTLIER IN DEVELOPING A BETTER SURFACE WAVE. COLDER AIR DRAINING INTO THIS WEAK CONVERGENCE ZONE WILL EVENTUALLY MAKE FOR PTYPE ISSUES ESPCLY NORTH WITH FORECAST SOUNDINGS SHOWING ENOUGH MOISTURE DEPTH FOR SNOW OR SLEET LATER MONDAY NIGHT NW...AND PERHAPS SOME FREEZING RAIN LATE AS THE MOISTURE BECOMES MORE SHALLOW. GIVEN UNCERTAINTY WITH BOTH QPF AND DEPTH OF THE COLD AIR UNDER SUCH A WEAK SYSTEM...WILL LEAVE MOSTLY -RA AND KEEP SOME SNOW/MIX MAINLY NORTH OF HIGHWAY 460 AND ALONG HIGHER ELEVATIONS INTO TUESDAY MORNING BEFORE DRIER/COLD ADVECTION ENDS PRECIP IN ALL EXCEPT THE EXTREME NORTH IN THE AFTERNOON. LIGHT SNOW ACCUMULATIONS COULD OCCUR MAINLY AT ELEVATION BUT NO MORE THAN AN INCH OR SO WITH SOME LIGHT ICING POSSIBLE WHEN/IF SURFACE TEMPS GET COLD ENOUGH AND MOISTURE SHALLOWS OUT. OTRW LOWS MONDAY NIGHT UPPER 20S/LOW 30S NORTH HALF TO AROUND 40 SOUTH. HIGHS TUESDAY MOSTLY 30S MOUNTAINS TO LOW/MID 40S EAST. LARGE DOME OF COLD HIGH PRESSURE OVER THE CENTRAL STATES WILL FINALLY RIDGE EASTWARD AND ALLOW FOR SOME CLEARING TUESDAY NIGHT BUT COLD WITH LOWS IN THE 20S AND PERHAPS SOME TEENS IN THE VALLEYS. && .LONG TERM /WEDNESDAY THROUGH SATURDAY/... AS OF 430 PM EST SATURDAY... LONG RANGE MODELS...AT LEAST FOR WEDNESDAY AND THURSDAY ARE IN AGREEMENT WITH FAIR CONTINUITY AS WE TRANSITION INTO THE NEW YEAR. BROAD UPPER LEVEL TROUGH IS PROGGED TO EXTEND FROM EASTERN CANADA...SOUTHWEST THROUGH THE CENTRAL CONUS INTO A CUTOFF AREA OF LOW PRESSURE OVER SOUTHERN CALIFORNIA. A HIGH AMPLITUDE RIDGE WILL BE POSITIONED OFF THE PAC NW COAST. THIS PATTERN FAVORS COLD TEMPERATURES PERSISTING OVER THE NORTHERN CONTINENTAL UNITED STATES (CONUS) AND SOUTHWARD OVER THE PLAINS...AND MILDER AIR CONFINED TO THE EXTREME SOUTHEAST CONUS...I.E. FLORIDA. FOR THE MID ATLANTIC WE WILL LIKELY END UP IN THE TRANSITION ZONE...OR PRETTY CLOSE TO NORMAL TO FINISH OUT THE WEEK. WORST CASE...WE MAY END UP 5 DEGREES OR SO BELOW NORMAL FOR NEW YEARS DAY...BUT NOTHING EXTREME. AFTER THURSDAY THE MODELS DIVERGE...WHICH IS TO BE EXPECTED SINCE THEY DO NOT HANDLE CUTOFF LOWS VERY WELL...AND ESPECIALLY THOSE THAT EJECT FROM THE DESERT SOUTHWEST. WITHIN THE EVOLVING MEAN PATTERN THE PRIMARY FEATURE OF INTEREST WILL BE THE UPPER LOW WHICH ALL GUIDANCE CLOSES OFF OVER/NEAR CA/AZ INITIALLY BUT THEN DIVERGE ON WHEN THE LOW OPENS UP AND MOVES EAST DURING THE FRIDAY SATURDAY TIME FRAME. THERE IS A FAIR AMOUNT OF MODEL/ENSEMBLE SPREAD REGARDING THE TIMING AND TO A SOMEWHAT LESSER DEGREE TRACK OF THE UPPER LOW. THE OPERATIONAL RUN OF THE GFS IS 12-24 HOURS FASTER THAN THE ECMWF...WHICH MAKES A DIFFERENCE IN WHETHER IT PRECIPITATES HERE ON FRIDAY VS SATURDAY. THIS HIGH SPREAD/LOW CONFIDENCE FORECAST FAVORS AN ENSEMBLE MEAN APPROACH FOR BOTH FRIDAY AND SATURDAY...THE FORECAST REFLECTING A BLEND...WITH CHC PRECIP BOTH FRIDAY AND SATURDAY...THE GREATEST POPS FAVORING FRIDAY NIGHT WHERE THE BLENDED SOLUTIONS OVERLAP. TIMING AND TRACK OF THE SYSTEM WILL ALSO DETERMINE P-TYPE. TEMPERATURES LOOK TO BE BORDERLINE CRITICAL FRIDAY...SUGGESTING AT LEAST SOME WINTRY MIX IF PRECIP ARRIVES FRIDAY MORNING. IF ONSET HOLDS OFF UNTIL LATER FRIDAY...THE P-TYPE WILL BE MOSTLY RAIN. && .AVIATION /15Z SUNDAY THROUGH THURSDAY/... AS OF 625 AM EST SUNDAY... COLD FRONT AND INITIAL SURFACE WAVE WILL CONTINUE TO SATURATE CURRENT DRY AIR WITH CEILINGS LOWERING TO MVFR AT LWB/BLF THIS MORNING...AT RNK/ROA/DAN LATER IN THE DAY...AND THEN LYH THIS EVENING. RAIN MOVES ACROSS THE REGION INTO THIS AFTERNOON THEN A BRIEF BREAK BEFORE THE NEXT WAVE BRINGS RAIN BACK INTO THE AREA OVERNIGHT. IN BETWEEN THESE WAVES...EXPECT RESTRICTIONS TO REMAIN LOW WITH FOG AND DRIZZLE. CONFIDENCE IS LOW ON INITIAL TIMING OF LOWERING CEILINGS AND RAIN. CONFIDENCE INCREASES THAT ONCE A STATION DROPS BELOW VFR...CONDITIONS LIKELY TO REMAIN OR LOWER THROUGH THE TAF PERIOD. EXTENDED DISCUSSION... THE FRONT DRIFTS OFF TO THE SE MONDAY NIGHT INTO TUESDAY WHILE LEAVING PLENTY OF RESIDUAL LOW LEVEL MOISTURE IN ITS WAKE. HIGH CONFIDENCE IN STAYING SUB-VFR MONDAY NIGHT WITH MORE FOG/DRIZZLE AROUND OVERNIGHT. SLOW DRYING TO TAKE SHAPE ON TUESDAY AS A LARGE AREA OF HIGH PRESSURE STARTS TO NOSE IN FROM THE WEST. HOWEVER MAY TAKE UNTIL AFTERNOON FOR EASTERN SITES TO RETURN TO VFR WHILE LINGERING LOW CLOUDS COULD CAUSE MVFR CIGS TO PERSIST OVER THE WEST UNTIL LATE IN THE DAY. SOME SPOTTY LIGHT RAIN OR WINTRY PRECIP MAY ALSO OCCUR EARLY TUESDAY AS A WEAK TROUGH OF LOW PRESSURE REDEVELOPS OVER THE MOUNTAINS. OTRW HIGH PRESSURE BUILDS EAST WEDNESDAY INTO THURSDAY WITH GOOD FLYING CONDITIONS RETURNING UNDER VFR. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...NONE. NC...NONE. WV...NONE. && $$ SYNOPSIS...JH/SK NEAR TERM...PC/RCS SHORT TERM...JH/PC LONG TERM...PM AVIATION...RCS

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