Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 220054 AFDRNK AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BLACKSBURG VA 754 PM EST SUN DEC 21 2014 .SYNOPSIS... WARM AND MOIST AIR WILL OVERSPREAD COLD HIGH PRESSURE AT THE SURFACE TO BRING RAIN...ALONG WITH SOME SLEET AND FREEZING RAIN...TO THE REGION TONIGHT INTO TOMORROW. CLOUDY CONDITIONS AND LIGHT PRECIPITATION WILL THEN LINGER THROUGH TUESDAY. A STRONG COLD FRONT WILL MOVE THROUGH ON WEDNESDAY...BRINGING RAIN AND GUSTY WINDS TO THE AREA FOR CHRISTMAS EVE. HIGH PRESSURE WILL THEN BUILD OVER THE EASTERN UNITED STATES WITH FAIR WEATHER AND SEASONABLE TEMPERATURES THROUGH THE END OF THE WEEK. && .NEAR TERM /THROUGH MONDAY NIGHT/... AS OF 245 PM EST SUNDAY... UPPER FLOW WILL BE TRANSITIONING FROM ZONAL TO AMPLIFIED AS A DEEP TROF DEVELOPS OVER THE CENTRAL AND WESTERN US. THIS WILL BUCKLE THE FLOW TO A SWLY DIRECTION ALOFT...ALLOWING MOISTURE TRANSPORT TO OCCUR ALONG WITH ISENTROPIC LIFT AND WEAK SHORT WAVE ENERGY. THIS WILL ALLOW PCPN TO DEVELOP LATE TONIGHT INTO TOMORROW. MEANWHILE AT THE SURFACE...STRONG HIGH PRESSURE OVER QUEBEC WILL CONTINUE TO BUILD DOWN THE EASTERN SEABOARD AND DEVELOP A SOLID COLD AIR DAMMING WEDGE. THUS...AS THE PCPN FALLS INTO THE COLD AIR AT THE SURFACE THE RAIN WILL FALL AS FREEZING RAIN AND SLEET...ESP FROM THE NEW RIVER AND ROANOKE VALLEYS NORTH AND EASTWARD WHERE TEMPERATURES WILL BE FLIRTING WITH THE FREEZING MARK. WITH ONLY LIGHT QPF EXPECTED THIS SHOULD NOT BE ANYTHING MORE THAN A NUISANCE EVENT...BUT IT DOES NOT TAKE MUCH FREEZING RAIN TO CAUSE TROUBLE...SO WILL ISSUE A FREEZING RAIN ADVISORY FOR THE AFFECTED AREAS. TEMPERATURES WILL BE IN NO HURRY TO WARM UP TOMORROW AS THE PCPN WANES SO THE ADVISORY WILL GO UNTIL NOONTIME. THE COLD AIR WEDGE WILL REMAIN IN PLACE THROUGH MONDAY NIGHT AND EXPECT ENOUGH MOISTURE WILL BE LOCKED IN TO KEEP SOME R-/L- AROUND. && .SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/... AS OF 245 PM EST SUNDAY... UPPER TROUGH AMPLIFYING OVER THE CENTRAL U.S. WILL SUPPORT DEVELOPMENT OF A SURFACE LOW EARLY TUESDAY OVER THE LOWER TN VALLEY WHICH WILL LIFT NORTH DURING THE DAY. DEEP LAYER SOUTHERLY FLOW AHEAD OF THE TROUGH AND ASSOCIATED COLD FRONT WILL PULL ABUNDANT MOISTURE NORTHWARD AS WELL WITH PWATS SURGING FROM LESS THAN 0.50 INCHES MONDAY UP CLOSE TO 1.25 INCHES ACROSS THE CENTRAL CWA BY EARLY WEDNESDAY WHICH IS CLOSE TO 99TH PERCENTILE FOR THE DATE PER KRNK PWAT CLIMATOLOGY. ALL THICKNESS PARAMETERS AND THERMAL PROFILES SUPPORT A PURE RAIN EVENT WITH POPS INCREASING FROM WEST TO EAST THROUGH THE DAY BUT ESPECIALLY LATE TUESDAY NIGHT AS PERSISTENT LOW LEVEL MOISTURE TRANSPORT AND FAVORABLE RIGHT ENTRANCE REGION UPPER JET SUPPORT...UP TO 140 KT AT 250 MB. HIGHS TUESDAY HOLD IN THE 40S UNDER THE WEDGE AND DO NOT FALL MUCH OVERNIGHT WITH THICK OVERCAST AND SOUTHERLY FLOW. EXPECT MODERATE TO AT TIMES BRIEFLY HEAVY RAINFALL IN THE PERIOD FROM LATE TUESDAY THROUGH MUCH OF WEDNESDAY...CHRISTMAS EVE...AS A WARM FRONT LIFTS NORTH ACROSS THE AREA AND SURFACE LOW TRACKS STEADILY NORTH TOWARD THE GREAT LAKES. SEE HYDRO SECTION FOR MORE ON EXPECTED RAINFALL AND RELATED ANY IMPACTS. THE SYSTEM WILL HAVE PLENTY OF DEEP LAYER SHEAR BUT VERY LOW CAPES AS WOULD BE EXPECTED IN LATE DECEMBER WITH COMPLETE OVERCAST. SPC NOT IMPRESSED AT THIS POINT WITH THE POTENTIAL FOR A HIGH-SHEAR LOW- CAPE EVENT BUT WILL HAVE TO BE MONITORED AS THE SYSTEM EVOLVES AS SIMILAR SYSTEMS HAVE SURPRISED IN THE PAST AND CIPS ANALOGS OF 15 MOST SIMILAR EVENTS SHOWS AT LEAST LOW END PROBABILITY OF SEVERE IN THE FAR SOUTHEAST CWA. MODELS HAVE COLD FRONT CROSSING THE MOUNTAINS BETWEEN ABOUT 03-09Z EARLY CHRISTMAS MORNING AND EXIT THE EASTERN CWA BY IN 12-18Z TIME FRAME. ALL THE HEAVY PRECIPITATION WILL HAVE EXITED TO THE NORTHEAST BUT STRONG WEST- NORTHWEST WINDS WILL SURGE IN BEHIND THE FRONT AND BEGIN UPSLOPE SNOW SHOWERS IN THE USUAL AREAS BY AROUND 09Z ON CHRISTMAS MORNING AS H85 TEMPS PLUMMET FROM AROUND +10C AT 00Z TO -5C BY 12Z ON THE 25TH. && .LONG TERM /THURSDAY THROUGH SUNDAY/... AS OF 245 PM EST SUNDAY... ON CHRISTMAS DAY...A COLD FRONT WILL MOVE TO THE COAST WHILE AN UPPER LEVEL TROUGH PIVOTS OVER THE REGION. PRESSURE RISES...A TIGHT GRADIENT AND FALLING TEMPERATURES WILL MAKE FOR A BLUSTERY DAY ACROSS THE FORECAST AREA. THE ONLY AREA THAT WILL SEE SNOW FALLING ON CHRISTMAS WILL BE WESTERN SLOPES OF SE WV AND SW VA...POSSIBLE FLURRIES TO THE BLUE RIDGE. THESE BLUSTERY CONDITIONS WILL NOT LAST LONG AS HIGH PRESSURE WILL EDGE IN ALLOWING WINDS TO SUBSIDE AND ZONAL FLOW AND LOW LEVEL SOUTHWESTERLY FLOW TO USHER IN WARMER AIR ON FRIDAY. ACCORDING TO THE 12Z GFS...THE NEXT FRONT IS SCHEDULED TO ARRIVE SATURDAY. THIS FRONT WILL NOT HAVE MUCH MOISTURE AS DRY HIGH PRESSURE TRACKS ACROSS THE SOUTHERN STATES...BLOCKING GULF MOISTURE FROM RETURNING INTO THE REGION. ANY SHOWERS WITH THIS FRONT SATURDAY WILL BE ALONG WESTERN SLOPES AS WINDS BECOME WESTERLY RATHER QUICKLY. TEMPERATURES AND BOUNDARY LAYER WARM ENOUGH TO KEEP ANY PRECIPITATION AS RAIN. BY THE TIME THE COLD AIR ARRIVES...MOISTURE IS GONE. ALSO WITH A MOSTLY DRY FRONT...BREEZY AND GUSTY CONDITIONS EXPECTED SATURDAY. THE 12Z ECM STALLS THE FRONT ACROSS THE SOUTHERN APPALACHIANS SATURDAY. A DEEP UPPER LEVEL TROUGH DIGGING INTO TEXAS WILL DEVELOP A SURFACE LOW NEAR THE MOUTH OF THE MISSISSIPPI SATURDAY...TRACKING ACROSS THE CAROLINAS ON SUNDAY. THE ECM SCENARIO WILL BRING RAIN WHEREAS THE GFS IS DRY. WILL LEAN THIS WEEKEND`S FORECAST TOWARDS WPC AND KEEP POPS LOW IN THE 20-30 PERCENT RANGE. WITH EITHER MODEL...BEHIND THE FRONT SATURDAY OR SUNDAY...ANOTHER AREA OF HIGH PRESSURE WILL TRACK ALONG THE GULF STATES...KEEPING THE REGION DRY INTO EARLY NEXT WEEK. && .AVIATION /00Z MONDAY THROUGH FRIDAY/... AS OF 745 PM EST SUNDAY... HIGH PRESSURE WILL REMAIN WEDGED AGAINST THE EASTERN SIDE OF THE APPALACHIAN MOUNTAINS WHILE AN AREA OF LOW PRESSURE ADVANCES NORTHEAST ALONG THE SOUTHEAST ATLANTIC COAST. THIS AREA OF LOW PRESSURE WILL CAST MOISTURE INTO THE MID ATLANTIC REGION OVERNIGHT. MVFR CIGS WILL ADVANCE NORTHWARD FROM THE CAROLINAS...OVERSPREADING VIRGINIA AND WEST VIRGINIA DURING THE OVERNIGHT. ONSET OF ANY PRECIPITATION WILL HOLD OFF UNTIL AFTER MIDNIGHT. LIGHT RAIN IS EXPECTED TO ADVANCE NORTHWARD FROM NORTH CAROLINA AFTER MIDNIGHT. WHILE THERE IS HIGH CONFIDENCE THAT THE MAIN PRECIPITATION TYPE WILL BE RAIN...A PERIOD OF FREEZING RAIN AND SLEET IS LIKELY NORTH OF HIGHWAY 460...ALONG AND NORTH OF A BLF- BCB-ROA-LYH LINE. MODELS ARE FORECASTING ABOUT A TENTH OF AN INCH OF QPF...SO ITS NOT OUT OF THE QUESTION TO HAVE UP TO A TENTH OF AN INCH OF ICE ACCRETION ON ELEVATED SURFACES MONDAY MORNING. A FREEZING RAIN ADVISORY IS IN EFFECT FROM 5AM TO NOON MONDAY TO ACCOUNT FOR THE FREEZING RAIN. SOME OF THE FORECAST MODELS...ESP THE NAM...SUGGEST A PERIOD OF IFR MAY ACCOMPANY THE PRECIPITATION MONDAY MORNING... SO THERE IS A CHC THAT CIGS COULD DROP TO BETWEEN 400-800 FT AGL WITH VSBYS BETWN 1-3SM. THE PRECIPITATION IS EXPECTED TO COME TO AN END MONDAY AFTERNOON. LINGERING MOISTURE...HOWEVER...WILL RESULT IN A PERSISTENT LOW CEILING...MAINLY MVFR...IN ADDITION TO SOME PATCHY DRIZZLE. EXTENDED DISCUSSION... A LOW PRESSURE SYSTEM ALONG THE NORTH CAROLINA COAST WILL HEAD OFFSHORE BY MONDAY NIGHT...BUT MVFR CEILINGS AND DRIZZLE MAY LINGER INTO TUESDAY DUE TO THE WEDGE OF HIGH PRESSURE EAST OF THE APPALACHIAN MOUNTAINS. BY WEDNESDAY...A STRONG COLD FRONT WILL SWEEP OVER THE MID ATLANTIC...AND ERODE THE WEDGE. MODEL SOUNDINGS CONTINUE TO SHOW THE POTENTIAL FOR LOW LEVEL WIND SHEAR AHEAD OF THIS FRONT...WITH A POTENT LOW LEVEL JET DEPICTED. RAIN AND MVFR CONDITIONS WILL ACCOMPANY THE FRONTAL PASSAGE. BEHIND THE COLD FRONT...SURFACE WINDS SHOULD TURN FROM THE SOUTHEAST TO THE NORTHWEST...AND BECOME RATHER GUSTY BY CHRISTMAS DAY. MEANWHILE...MVFR CEILINGS AND UPSLOPE RAIN/SNOW SHOWERS SHOULD PERSIST IN THE MOUNTAINS...WHILE VFR CONDITIONS WILL RETURN IN THE PIEDMONT. HIGH PRESSURE SHOULD BUILD OVER THE SOUTHEAST BY THURSDAY NIGHT...WHICH WILL PROVIDE GOOD FLYING WEATHER WITH LIGHTER WINDS ON FRIDAY. && .HYDROLOGY... AS OF 245 PM EST SUNDAY... BIGGER STORY MAY BE THE RAINFALL WITH UPCOMING EVENT AS FORECAST STORM TOTAL QPF IS IN THE 1.00 TO 1.50 INCH RANGE ACROSS THE ENTIRE CWA. THIS WILL ACTUALLY BE WELCOME FROM A HYDROLOGIC STANDPOINT AS MUCH OF THE PIEDMONT HAS BEEN DRIER THAN NORMAL OVER SEVERAL MONTHS AND DECEMBER HAS HAD BELOW NORMAL PRECIPITATION IN MOST OF THE CWA. SHOULD NOT SEE ANY EXCESS RUNOFF ISSUES..I.E. FLOODING...ALTHOUGH SOME DITCHES WILL LIKELY FILL UP AND MAYBE A FEW PONDING ISSUES. RIVER FORECASTS FROM NORTH AMERICAN AND GEFS ENSEMBLES PORTRAY BASIN- AVERAGE QPF GENERALLY IN THE 0.75 TO 1.25 INCH RANGE WITH SOME OUTLIERS AS USUAL BUT ALL SOLUTIONS AT THIS POINT PRODUCE ONLY MINOR WITHIN-BANK RISES ON MAINSTEM RIVERS BY LATER IN THE WEEK. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...FREEZING RAIN ADVISORY FROM 5 AM TO NOON EST MONDAY FOR VAZ011- 013-014-016>020-022>024-034-035-045>047. NC...NONE. WV...FREEZING RAIN ADVISORY FROM 5 AM TO NOON EST MONDAY FOR WVZ044- 507-508. && $$ SYNOPSIS...MBS NEAR TERM...AMS/MBS SHORT TERM...PC LONG TERM...RCS AVIATION...PM/PW HYDROLOGY...PC

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