Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 231933 AFDRAH AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 232 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 /THROUGH TONIGHT/...
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AS OF 230 PM TUESDAY... CAD WEDGE REMAINS STRONGLY ANCHORED IN PLACE AS EVIDENT FROM THE VERY STRONG INVERSION ON THE CURRENT RAP FORECAST SOUNDINGS. THIS DOES NOT CHANGE HARDLY AT ALL OVERNIGHT AND SO THE FOG/MIST AND LOW CEILINGS WILL ALSO CONTINUE THROUGH TONIGHT. AFTER 00Z A WARM FRONT WILL BEGIN ITS NORTHWARD ADVANCEMENT THROUGH OUR CWA AND WITH IT WILL COME A PERIOD OF RAIN BEGINNING ABOUT 00Z IN THE SOUTH AND A COUPLE HOURS LATER IN THE NORTH. THIS FIRST BOUT OF RAIN WILL LAST UNTIL 6-8Z BEFORE HIGH RESOLUTION MODELS PREDICT A LULL IN PRECIPITATION. DURING THIS LULL EXPECT FOG TO BECOME AN ISSUE WITH AREAS OF 1/2 SM VISIBILITIES AND PROBABLY LESS IN SOME SPOTS. WOULD NOT BE SURPRISED TO HAVE TO ISSUE A DENSE FOG ADVISORY SOMETIME AFTER 6Z THROUGH DAYBREAK FOR THE ENTIRE AREA. A SECOND WAVE OF PRECIPITATION WILL APPROACH THE AREA FROM THE SOUTHWEST CLOSER TO DAYBREAK. THIS AGAIN IS ACCORDING TO HIGH RESOLUTION MODELS...WHEREAS THE GFS AND ECMWF HAVE A MORE CONTINUOUS PRECIPIATION PATTERN OVERNIGHT. HAVE HEDGED THE FORECAST TOWARDS THE HIGH RES MODELS. EARLIER INDICATIONS WERE THAT SOME ELEVATED INSTABILITY MIGHT CREATE ENOUGH LIFT FOR SOME THUNDERSTORMS THIS EVENING. WHILE CURRENT SHORT TERM MODELS ARE STILL SHOWING DECENT MID LEVEL LAPSE RATES FOR THIS TIME OF YEAR...THEY ARE LACKING THE INSTABILITY THAT WAS THERE IN THIS MORNINGS RUN. AS A RESULT HAVE TAKEN MENTION OF THUNDER OUT OF THE FORECAST FOR NOW. RADAR IS HOWEVER DEPICTING SOME LIGHTNING STRIKES IN THE CHARLOTTE AREA AND DOWN ALONG THE COAST SOUTH OF WILMINGTON. TRENDS IN THE ACTIVITY OVER THE PAST HOUR OR SO ARE WEAKENING SO WHILE IT IS NOT IMPOSSIBLE THAT WE WILL SEE A LIGHTNING STRIKE OR TWO...THE CHANCES ARE GETTING LESS LIKELY. TEMPERATURES WILL FOLLOW A NON-DIURNAL PATTERN THIS EVENING INTO THE OVERNIGHT HOURS AS THE WARM FRONT MOVES NORTH...EXPECT TEMPERATURES TO RISE OVERNIGHT...AND LOWS ARE MOST LIKELY TO OCCUR DURING THE EVENING HOURS TONIGHT IN THE LOW TO MID 40S BEFORE RISING OVERNIGHT. THE WARMING TREND WILL BE VERY MUTED IN THE TRIAD WHERE THE WARM AIR WILL END UP RISING UP OVER THE COLD DOME LOCKED IN BY THE CAD WEDGE. THEREFORE EXPECT LOW 40S IN THE TRIAD NEAR DAYBREAK WITH MUCH WARMER MID TO UPPER 50S IN THE SOUTH AND EAST. THE PROGRESSION OF TEMPERATURES WILL DEPEND ON THE FRONTAL ADVANCEMENT AND THEREFORE HOURLY TEMPERATURES OVERNIGHT ARE A LOW CONFIDENCE FORECAST AT BEST.
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&& .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 JUST 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 TUESDAY/... AS OF 215 PM MONDAY... THURSDAY AND THURSDAY NIGHT: THERE MAY BE A FEW SHOWERS IN THE PROCESS OF EXITING THE FAR EASTERN ZONES CHRISTMAS MORNING... OTHERWISE EXPECT A WEST TO EAST CLEARING TRENDING IN THE WAKE OF THE EXITING COLD FRONT. ADDITIONALLY...DEEPENING SURFACE LOW LIFTING NEWD INTO SE CANADA WILL RESULT IN BREEZY CONDITIONS ON CHRISTMAS DAY...WITH WEST-NORTHWESTERLY WINDS GUSTING INTO THE UPPER TEENS TO LOWER 20S...STRONGEST DURING THE FIRST HALF OF THE DAY. WEAK CAA BEHIND THE FRONT WILL STILL ALLOW FOR SEASONABLY MILD TEMPERATURES. HIGHS IN THE MID 50S NW TO LOWER 60S SE. LOWS IN THE LOWER TO MID 30S. FRIDAY THROUGH SATURDAY: DRY CONDITIONS WILL PERSIST WITH ABOVE NORMAL TEMPERATURES THROUGH SATURDAY AS HIGH PRESSURE OVER THE THE GULF COAST STATES BUILDS OVER THE AREA ON FRIDAY...WITH LIGHT SWLY RETURN FLOW ON SATURDAY AS THE HIGH CENTER SHIFTS OFFSHORE. HIGHS RANGING FROM THE UPPER 50S NW TO MID 60S SE. LOWS FRIDAY NIGHT IN THE LOWER TO MID 30S. SATURDAY NIGHT THROUGH TUESDAY: WHILE THE NEXT TROUGH/SYSTEM TO IMPACT THE REGION IS NOT COMPLETELY RESOLVED YET...THERE IS AT LEAST A GENERAL CONSENSUS THAT IT WILL BE A WET LATTER HALF OF THE WEEKEND/EARLY NEXT WEEK. ANOTHER BATCH OF PACIFIC SHORTWAVE ENERGY IS FORECAST TO EMERGE FROM THE SOUTHERN ROCKIES ON SATURDAY AND IS PROGGED TO PROGRESS EASTWARD INTO THE SOUTHERN PLAINS/ARKLATEX REGION SATURDAY NIGHT AND INTO SUNDAY...EVENTUALLY DAMPENING AS IT ROUNDS THE BASE OF THE LONG WAVE TROUGH AND MOVES INTO THE REGION LATE SUNDAY-MONDAY. IN RESPONSE...WEAK SFC CYCLOGENESIS ENSUES ON THE SOUTHERN END OF A COLD FRONT...INVOF THE LOWER MS VALLEY...THAT WILL EVENTUALLY RIDE NORTHEASTWARD TOWARDS THE SOUTHERN MID-ATLANTIC REGION...SUPPORTING WHAT COULD END BEING ANOTHER SIGNIFICANT WIDESPREAD RAIN EVENT FOR THE CAROLINAS SUNDAY-SUNDAY NIGHT. IN SHORT...THE MORE AMPLIFIED/PHASED EC SOLUTION WOULD SUPPORT A SLOWER MORE CONVECTIVE RAINFALL SCENARIO AS THE AREA OF LOW PRESSURE TRACKS THROUGH THE SOUTHERN APPALACHIANS. THE MORE PROGRESSIVE GFS DEPICTS YET ANOTHER OVERRUNNING DAMMING EVENT IN THE WAKE OF AN EARLIER NORTHERN STREAM COLD FRONTAL PASSAGE SATURDAY NIGHT AS AN AREA OF LOW PRESSURE TRACKS FROM THE NORTHERN GULF TO OFF THE SOUTHEAST COAST. GIVEN THESE LARGE MODEL DIFFERENCES..FORECAST WILL REFLECT A MODEL/ENSEMBLE BLEND WITH CHANCE POPS SUNDAY AND SUNDAY NIGHT WITH NO P-TYPE CONCERNS. TEMPERATURES DROPPING BACK TO NEAR NORMAL GIVEN WIDESPREAD RAIN AND CLOUDS. && .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/... 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. && .RAH WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SYNOPSIS...VINCENT NEAR TERM...ELLIS SHORT TERM...BLAES LONG TERM...CBL AVIATION...ELLIS HYDROLOGY...MLM/BADGETT

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