Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 231914 AFDRAH AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 210 PM EST TUE DEC 23 2014 .SYNOPSIS...A COLD AIR DAMMING RIDGE WILL REMAIN LOCKED IN PLACE EAST OF THE APPALACHIANS THROUGH THIS EVENING. A WARM FRONT WILL LIFT NORTHWARD THROUGH THE AREA WEDNESDAY MORNING AS A LOW PRESSURE SYSTEM STRENGTHENS WEST OF THE MOUNTAINS. AN ASSOCIATED COLD FRONT WILL SURGE EASTWARD ACROSS THE CAROLINAS WEDNESDAY NIGHT. && .NEAR TERM /TODAY AND TONIGHT/... AS OF 1020 AM TUESDAY... MORNING UPDATE: AREAS OF FOG AND MIST CONTINUE TO PERSIST ACROSS THE CWA WITH SOME POCKETS OF LOCALLY DENSE FOG...PARTICULARLY IN THE SOUTHERN COUNTIES AT THIS TIME. WHICH VISIBILITIES HAVE IMPROVED SOMEWHAT...VERY LOW CEILINGS WILL PERSIST THROUGH THE DAY AND MOST LOCATIONS WILL HANG ON TO LIFR CONDITIONS THROUGHOUT THE NEAR TERM. WITH COLD AIR DAMMING WEDGE STILL IN PLACE OVER THE AREA...TEMPERATURES CONTINUE TO SUFFER...ESPECIALLY IN THE NORTHWEST PIEDMONT WHERE TEMPS ARE CURRENTLY IN THE UPPER 30S ARE ONLY EXPECTED TO RISE INTO THE LOWER 40S TODAY. FURTHER SOUTH THERE IS SOME IMPROVEMENT WITH TEMPS IN THE LOWER 40S BUT WOULD EXPECT MOST LOCATIONS IN THE CWA TO REMAIN IN THE 40S TODAY. OTHER THAN THE MIST...AND SOME DRIZZLE...PRECIPIATION WILL BE HELD TO A MINIMUM UNTIL AFTER 18Z OR SO WHEN PRECIPITATION CHANCES WILL STEADILY INCREASE INTO THE EVENING. SOME ELEVATED CONVECTION IS ALSO POSSIBLE THIS EVENING AS CAPE AND MID-LEVEL LAPSE RATES ARE DECENT ABOVE THE 925 MB TEMPERATURE INVERSION REPRESENTING THE COLD DOME. FURTHER DETAILS ARE PROVIDED IN THE PRIOR DISCUSSION WHICH IS BELOW. -ELLIS OVERVIEW: POTENT SHORTWAVE ENERGY IN VICINITY OF THE FOUR-CORNERS REGION WILL DIG SOUTHEAST INTO THE LOWER MIDWEST ALONG THE SOUTHWESTERN PERIPHERY OF A CUT-OFF LOW ANCHORED OVER THE CENTRAL PLAINS THIS AFTERNOON...AMPLIFYING THE LONGWAVE TROUGH IN PLACE OVER THE CENTRAL CONUS. IN RESPONSE...CYCLOGENESIS WILL OCCUR IN VICINITY OF THE TX/LA GULF COAST THIS AFTERNOON AS THE AFOREMENTIONED SHORTWAVE ROUNDS THE BASE OF THE AMPLIFYING LONGWAVE TROUGH. THE SFC LOW WILL TRACK NORTHWARD ACROSS THE DEEP SOUTH AND INTO THE TN VALLEY TONIGHT. FURTHER DOWNSTREAM...SMALL AMPLITUDE WAVES EMBEDDED WITHIN STRENGTHENING SOUTHWEST FLOW WILL TRACK NORTHEAST INTO THE CAROLINAS THIS AFTERNOON INTO TONIGHT. PRECIPITATION: PATCHY LIGHT RAIN WILL REMAIN POSSIBLE THROUGH THE MORNING...PRIMARILY IN THE WESTERN PIEDMONT VIA LOW-LEVEL WARM ADVECTION /925 MB FRONTOGENESIS/ ATOP THE PRONOUNCED COLD AIR DAMMING WEDGE LOCKED IN PLACE ACROSS THE STATE. ELSEWHERE AND OTHERWISE...FOG AND OCCASIONAL DRIZZLE WILL PERSIST INTO THE AFTERNOON. EXPECT WIDESPREAD RAIN TO DEVELOP UPSTREAM OVER GA/SC TODAY AS LOW-LEVEL FLOW /WARM ADVECTION/ STRENGTHENS IN RESPONSE TO THE ARRIVAL OF MORE POTENT SHORTWAVE ENERGY /DPVA/ IN SW FLOW ALOFT...PROGRESSING NORTHWARD INTO CENTRAL NC DURING THE LATE AFTERNOON AND EVENING HOURS. EXPECT WIDESPREAD RAIN TO PERSIST OVERNIGHT IN ASSOC/W STRENGTHENING WARM ADVECTION ATOP THE PRONOUNCED (ALBEIT INCREASINGLY SHALLOW/ERODING) COLD AIR DAMMING WEDGE. CONVECTION: ELEVATED DESTABILIZATION IS EXPECTED TO OCCUR ATOP THE CAD WEDGE THIS EVENING AS LOW-LEVEL WARM ADVECTION STRENGTHENS AND THE MID-LEVELS COOL ATTENDANT SHORTWAVE ENERGY /DPVA/ APPROACHING FROM THE SW. NAM FCST SOUNDINGS SUGGEST AS MUCH AS 250-750 J/KG OF MUCAPE WILL BE AVAILABLE AT THE LEADING EDGE OF A 925 MB WARM FRONT SURGING NORTHWARD THROUGH THE SANDHILLS/COASTAL PLAIN AND TRIANGLE BETWEEN 22-04Z. WITH THE ABOVE IN MIND...HAVE OPTED TO INTRODUCE A CHANCE OF THUNDERSTORMS IN THE AFOREMENTIONED TIME FRAME/AREA. 1-6 KM SHEAR ON THE ORDER OF 30-40 KT AND SCP VALES ~1 WILL BE SUPPORTIVE OF SUPERCELLULAR ORGANIZATION...OR PERHAPS BRIEF SUPERCELLULAR CHARACTERISTICS (I.E. SPLITTING CELLS)...SIMILAR (ALBEIT WEAKER) THAN UPSTREAM CONVECTION OBSERVED IN GA BETWEEN 10- 12Z THIS MORNING. REGARDLESS...MARGINAL MID-LEVEL LAPSE RATES WOULD MOST LIKELY PRECLUDE THE ONLY HAZARD (HAIL) ASSOC/W ELEVATED CONVECTION. TEMPERATURES: NEAR STEADY TEMPS TODAY WILL GRADUALLY RISE FROM SOUTHEAST TO NORTHWEST LATE THIS EVENING INTO TONIGHT AS A WARM FRONT ADVANCES NORTHWARD INTO THE AREA AND THE COLD AIR DAMMING WEDGE ERODES. THOUGH TEMPS WILL REMAIN IN THE 40S WHERE THE WEDGE REMAINS IN PLACE (TRIAD) THROUGH 12Z WED...TEMPS IN THE WARM SECTOR (COASTAL PLAIN...SANDHILLS AND PORTIONS OF THE TRIANGLE) WILL HAVE RISEN TO THE LOWER 60S. -VINCENT && .SHORT TERM /WEDNESDAY THROUGH WEDNESDAY NIGHT/... AS OF 200 PM TUESDAY... ...ACTIVE WEATHER DAY ON TAP WITH CAD...HEAVY RAIN AND A SMALL THREAT OF SEVERE WEATHER IN THE COASTAL PLAIN... NWP GUIDANCE IS IN GOOD AGREEMENT WITH THE OVERALL EVOLUTION OF THE WEATHER PATTERN DURING THE SHORT TERM PERIOD WITH A HIGH AMPLITUDE DIGGING TROUGH AT 500 HPA THAT EXTENDS INTO THE WESTERN GULF OF MEXICO AT DAYBREAK WEDNESDAY. THE TROUGH CLOSES OFF WITH AN UPPER LOW DURING THE MORNING AS IT LIFTS FROM NEAR KSTL TO NORTHEAST OF KDET TOWARD MIDNIGHT AS THE TROUGH AXIS SLOWLY SHIFTS EAST TO NEAR OR JSUT WEST OF THE APPALACHIANS BY DAYBREAK CHRISTMAS MORNING. AT THE SURFACE...AN AREA OF LOW PRESSURE DEVELOPING IN THE LOWER OH VALLEY RAPIDLY DEEPENS AS IT MOVES TO NORTH OF THE GREAT LAKES AFTER MIDNIGHT. THE ATTENDANT COLD FRONT REACHES THE SOUTHWESTERN APPALACHIANS AROUND MIDDAY WEDNESDAY...THE TRIANGLE TOWARD MIDNIGHT AND THE COAST BY DAYBREAK CHRISTMAS MORNING. AFTER AFTER A PREDAWN LULL...WIDESPREAD RAIN WITH EMBEDDED HEAVIER SHOWERS WILL OVERSPREAD CENTRAL NC AFTER DAYBREAK FROM THE SOUTHWEST AS A WARM FRONT LIFTS NORTH THROUGH THE REGION. THE RAIN WILL OVERWHELM MUCH OF CENTRAL NC DURING THE MORNING INTO THE EARLY AFTERNOON HOURS. GUIDANCE DIFFERS IN THE HANDLING OF THE DETAILS OF THE PRECIPITATION WITH THE CONVECTION ALLOWING MODELS (CAMS) SUGGESTING THE CONVECTION WILL OUTRUN THE COLD FRONT AND THE LAGGING UPPER TROUGH WITH THE LINE OF CONVECTION REACHING THE COASTAL PLAIN TOWARD MID/LATE AFTERNOON WITH A LULL BEHIND THE LINE. OTHER CONVECTION PARAMETERIZED MODELS SHOW A MORE OR LESS CONTINUOUS PERIOD OF PRECIPITATION THROUGH THE LATE NIGHT HOURS. FEEL THE CAMS DEPICT THE EVOLUTION OF THE EVENT MORE REALISTICALLY AND WILL USE THEM TO CONSTRUCT THE GRIDS AND HIGHLIGHT A BAND OF HEAVIER SHOWERS AND THUNDERSTORMS. PRECIPITATION CHANCES WILL DECREASE DURING THE EVENING IN A RELATIVE LULL BETWEEN THE DEPARTING SQUALL LINE AND THE APPROACHING UPPER TROUGH. GUIDANCE HAS TRENDED SLOWER WITH THE PASSAGE OF THE COLD FRONT AND THE UPPER TROUGH AND SUGGEST ANOTHER LINE OF SHOWERS WILL LIKELY REDEVELOP IN THE WESTERN PIEDMONT JUST PRIOR TO MIDNIGHT AND EXPAND IN COVERAGE AND INTENSITY AS IT MOVES EAST AND REACHES THE COASTAL PLAIN WELL AFTER MIDNIGHT. THE THREAT FOR SEVERE CONVECTION WED AFTERNOON IS LIMITED IN CENTRAL NC BUT HAS BEEN TRENDING UPWARD SLIGHTLY. THE AMOUNT OF BOUNDARY LAYER DESTABILIZATION IS STILL RATHER SMALL BUT THE MOST RECENT RUN OF THE SREF SHOWS ALL 21 MEMBERS WITH AT LEAST SOME SFC CAPE AT 18- 20Z AT KFAY WITH NEARLY HALF OF THE MEMBERS SHOWING CAPE VALUES GREATER THAN 250 UNITS. THESE ARE STILL PRETTY MEAGER VALUES BUT THEY HAVE BEEN TRENDING UPWARD AND COMBINED WITH BULK SHEAR VALUES AROUND 50KTS...A LIMITED THREAT OF SEVERE WEATHER IS PRESENT IN THE COASTAL PLAIN AND SANDHILLS DURING THE MID TO LATE AFTERNOON HOURS WEDNESDAY. FURTHER NORTH AND WEST THE BOUNDARY LAYER IS MORE STABLE SO CONVECTION IS LIKELY TO INTENSIFY NOTABLY AS IT PUSHES EAST OF U.S. 1.. MODEL CONSENSUS EXPERIMENTAL SHERB HSLC VALUES ARE ELEVATED IN THE 0.7 TO 0.9 RANGE AT 18Z ON WEDNESDAY IN THE EASTERN PIEDMONT AND SOUTHERN COASTAL PLAIN OF NC...NEAR BUT LESS THAN THE CRITICAL VALUE OF 1. THE LIMITING FACTOR IN ADDITION TO INSTABILITY IS THE LAGGING UPPER TROUGH AND ITS HEIGHTS FALLS AND INCREASED LAPSE RATES. AS IS TYPICALLY THE CASE...THE INSITU COLD AIR DAMMING WILL PERSIST IN THE NORTHWEST PIEDMONT LONGER THAN MOST OF THE NWP GUIDANCE SUGGESTS...ALTHOUGH THE HIGH RES WINDOW SIMULATIONS HOLD ONTO COOLER TEMPS LONGER COMPARED TO THE NAM/GFS/EC. EVENTUALLY THE APPROACHING COLD FRONT WILL SCOUR OUT THE CAD AIRMASS IN THE TRIAD BUT THE ASSOCIATED MIXING WILL ACTUALLY PRODUCE A WINDOW OF WARMING TEMPERATURES DURING THE EVENING THROUGH MIDNIGHT. ELSEWHERE...TEMPERATURES WILL RISE INTO THE 60S WITH SOME LOWER 70S IN THE SANDHILLS AND SOUTHERN COASTAL PLAIN BY AFTERNOON. STILL SOME CONCERN FOR THE POTENTIAL OF FLASH FLOODING AND EVENTUAL MINOR RIVER FLOODING BUT THE PRECIPITATION IS LIKELY TO ARRIVE IN MULTIPLE WAVES DURING THE PERIOD. IN ADDITION...TIME LAGGED ENSEMBLES FOR BOTH THE NAM AND GFS SUGGEST RECENT RUNS ARE TRENDING WITH LOWER QPF...GENERALLY RANGING BETWEEN 1.0 AND 1.5 INCHES. A MORE GENERAL URBAN AND SMALL STREAM THREAT SEEMS MOST PROBABLE AT THIS POINT. SEE HYDRO DISCUSSION BELOW FOR MORE DETAILS. -BLAES && .LONG TERM /THURSDAY THROUGH MONDAY/... AS OF 235 AM TUESDAY... MILD AND DRIER THURSDAY INTO SATURDAY... BREEZY WEST WINDS CHRISTMAS DAY WITH SKIES BECOMING PARTLY TO MOSTLY SUNNY. MILD HIGHS IN THE 50S TO LOWER 60S EXPECTED. WINDS SHOULD DIMINISH DURING THE MID TO LATE AFTERNOON AS THE PRESSURE GRADIENT WEAKENS AS THE SURFACE HIGH BUILDS IN FROM THE GULF COAST STATES. MILD TEMPERATURES AND MOSTLY CLEAR SKIES ARE EXPECTED FRIDAY INTO SATURDAY AS HIGH PRESSURE BUILDS OVER THE SOUTHEAST STATES. EXPECT LOWS IN THE 30S AND HIGHS IN THE 50S. MODEL SPREAD STILL INCREASES THROUGH THE WEEKEND...WITH THE EC CONTINUING MUCH DEEPER WITH THE NEXT MID/UPPER LEVEL TROUGH...RESULTING IN A STRONGER SURFACE WAVE THAT WOULD BRING YET ANOTHER WET AND UNSETTLED PERIOD SUNDAY INTO MONDAY. WHEN THE ENSEMBLES ARE CONSIDERED... A MUCH WEAKER WAVE RESULTS. THEREFORE... WE WILL SIMPLY INDICATE AN INCREASE IN CLOUDINESS THEN WITH COOLER TEMPERATURES. THERE MAY BE AN IMPORTANT STORM IF THE EC DOES INDEED VERIFY LATER SUNDAY INTO EARLY NEXT WEEK. && .AVIATION /18Z TUESDAY THROUGH SUNDAY/... AS OF 100 PM TUESDAY... 24 HOUR TAF PERIOD: HIGH CONFIDENCE OF LIFR OR WORSE CONDITIONS THROUGH THE END OF THE TAF PERIOD. CONDITIONS THIS AFTERNOON HAVE BEEN VARIABLE AS FAR AS VISIBILITIES GO...RANGING FROM 1/4 SM UP TO AS HIGH AS 5SM BUT THE COMMON DENOMINATOR HAS BEEN VERY LOW CEILINGS BELOW 500 FEET. THIS SHOULD CONTINUE THROUGH THE TAF PERIOD. WINDS THIS AFTERNOON ARE FAIRLY CALM BUT WILL BE LIGHT AND VARIABLE OVERNIGHT SHIFTING TO A MORE SOUTHERLY DIRECTION BY WEDNESDAY AFTERNOON. LATE IN THE TAF PERIOD WIND SPEEDS COULD PICK UP TO NEAR 10 KTS. NOT MUCH PRECIPITATION IN THE AREA OTHER THAN MIST OR DRIZZLE RIGHT NOW BUT THAT WILL CHANGE THIS EVENING AS RAIN WILL MOVE UP FROM THE SOUTHWEST AND WILL BEGIN TO AFFECT TAF SITES AROUND 00Z WITH KRDU AND KRWI A COUPLE OF HOURS LATER. THIS FIRST ROUND OF RAIN WILL LAST UNTIL ABOUT 6-8Z AND THEN THERE IS EXPECTED TO BE A LULL IN PRECIPIATION. DURING THIS LULL...EXPECT WIDESPREAD FOG ACROSS THE CWA WITH VISIBILITIES AS LEAST AS LOW AS 1/2 A MILE BUT WOULD COUNT ON SOME AREAS OF DENSE FOG OF LESS THAN A 1/4 MILE VISIBILITY. AROUND DAYBREAK...A SECOND ROUND OF PRECIPITATION IS PROGGED TO MOVE INTO THE AREA WHICH MAY HELP IMPROVE VISIBILITIES A LITTLE BUT CEILINGS WILL STILL BE BELOW 500 FEET. LONG TERM: PERIODS OF PRECIPITATION...AND ADVERSE AVIATION CONDITIONS WILL PERSIST THROUGH WEDNESDAY NIGHT INTO THURSDAY MORNING BEFORE MOVING OFF TO THE EAST. THE NEXT CHANCE AFTER THAT FOR ADVERSE CONDITIONS WILL BE WITH A SOUTHERN STREAM SYSTEM THAT WILL MOVE INTO CENTRAL NC FOR SUNDAY INTO MONDAY. && .HYDROLOGIC DISCUSSION /TONIGHT THROUGH THURSDAY/...
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AS OF 210 PM TUESDAY... THERE HAVE BEEN THREE FAIRLY SIGNIFICANT RAINFALL EVENTS OVER CENTRAL NORTH CAROLINA DURING THE PAST TWO WEEKS...WITH HEAVIER RAINFALL AMOUNTS (>1 INCH TOTAL) GENERALLY FROM THE TRIANGLE AND FAYETTEVILLE AREAS EAST TO THE COAST. AS A RESULT...RELATIVE SOIL MOISTURE IS UNIFORMLY HIGH THROUGH A 200 CM DEEP COLUMN...WHICH WILL PROMOTE FASTER RUNOFF AS WELL AS A HIGHER PERCENTAGE OF RAINFALL-TO- RUNOFF FROM THE UPCOMING EVENT TONIGHT THROUGH CHRISTMAS MORNING. CURRENT QUANTITATIVE PRECIPITATION FORECASTS RANGE FROM 1.25 INCHES TO NEARLY 2 INCHES...AND HAVE BEEN TRENDING DOWNWARD WITH SUCCESSIVE MODEL RUNS. AT THIS TIME...OUR PREFERRED RAINFALL FORECAST IS FOR WIDESPREAD AMOUNTS OF LESS THAN 1.5 INCHES...WHICH WILL CAUSE MAINSTEM RIVERS TO RISE SEVERAL FEET FROM CHRISTMAS DAY INTO THE WEEKEND...BUT RIVER FORECAST POINTS WOULD CREST BELOW MINOR FLOOD STAGE. WIDESPREAD RAIN NEARER THE HIGH END OF THIS RANGE (1.75 TO 2 INCHES) WOULD LIKELY PRODUCE MINOR FLOODING ON EASTERN RIVERS... PARTICULARLY THE HAW...NEUSE...AND TAR RIVERS.
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&& .RAH WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SYNOPSIS...VINCENT NEAR TERM...ELLIS/VINCENT SHORT TERM...BLAES LONG TERM...BADGETT AVIATION...ELLIS HYDROLOGY...MLM

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