Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 240347 AFDRAH AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 1047 PM EST TUE DEC 23 2014 .SYNOPSIS...
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HIGH PRESSURE AND ASSOCIATED COLD AIR DAMMING WILL REMAIN LOCKED IN PLACE EAST OF THE APPALACHIANS TONIGHT. A WARM FRONT WILL RETREAT NORTH THROUGH THE AREA WEDNESDAY...AS A LOW PRESSURE SYSTEM STRENGTHENS WEST OF THE MOUNTAINS. AN ASSOCIATED COLD FRONT WILL SURGE EAST ACROSS THE CAROLINAS WEDNESDAY NIGHT.
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&& .NEAR TERM /TONIGHT/...
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AS OF 1045 PM TUESDAY... A DEEP PLUME OF MOISTURE IN PERTURBED SW FLOW ALOFT...DRIVEN BY COUPLED UPPER JETS AND CONTAINING A MYRIAD OF MCV/S OVER THE SE US...HAS CAUSED PRECIPITABLE WATER VALUES TO SURGE WELL IN EXCESS OF 200% OF NORMAL OVER NC PER 00Z RAOB DATA FROM BOTH GSO AND MHX. THIS REGIME WILL HOLD...WITH ASSOCIATED PERIODS OF HEAVY RAIN...UNTIL A COLD FRONT CROSSES THE REGION WED. DESPITE THE ALREADY-OBSERVED AND FORECAST HEAVY RAIN...FLASH FLOOD GUIDANCE VALUES REMAIN ABOVE FORECAST QPF...SO ANY ASSOCIATED FLOODING SHOULD CONSEQUENTLY BE OF THE URBAN AND RIVER/CREEK VARIETY. A LEAD MCV OVER SE NC...AND ASSOCIATED SHIELD OF HEAVY RAIN AND EMBEDDED ELEVATED THUNDER SUPPORTIVE OF 3-HOUR PRECIP AMOUNTS OF A HALF AN INCH TO AN INCH OVER EAST-CENTRAL NC...WILL CONTINUE TO TO LIFT NE DURING THE NEXT FEW HOURS. MEANWHILE...TWO OTHER AREAS OF PRECIPITATION ARE LIKELY TO IMPACT CENTRAL NC OVERNIGHT...THE FIRST FROM ANOTHER MCV FORECAST TO TRACK FROM EASTERN GA TO EASTERN NC - MOST LIKELY TO BRING ADDITIONAL RAIN TO THE I-95 CORRIDOR - AND ANOTHER IN THE FORM OF THE SQUALL LINE FROM NORTHERN GA TO THE FL PANHANDLE. THIS LATTER FEATURE WILL SPREAD...IN A WEAKENING STATE OWING TO A MORE STABLE ENVIRONMENT OVER THE CAROLINAS...INTO THE SOUTHERN AND WESTERN PIEDMONT BETWEEN 08-12Z...THEN CONTINUE NE ACROSS THE REMAINDER OF CENTRAL NC EARLY WED. DESPITE THE OVERALL WEAKENING TREND...FORECAST MUCAPE OF UP TO 500 J/KG OR SO FOR PARCELS ORIGINATING AT THE NOSE OF A SHALLOW CAD INVERSION...WILL SUPPORT ISOLATED THUNDER. THE CAD AIR MASS...HOWEVER SHALLOW PER 00Z-OBSERVED RAOB DATA AT GSO AND MHX...WHICH REVEALED A 20 DEGREE DIFFERENTIAL BETWEEN THE SURFACE AND JUST A THOUSAND FT OR SO AGL...REMAINS FIRMLY ENTRENCHED. IN FACT...AT 03Z THE COASTAL FRONT WAS ANALYZED STILL OFFSHORE THE NC COAST TO NORTH FL...AND THE WEDGE FRONT FROM THE COASTAL CAROLINAS TO NORTH-CENTRAL GA. IN OTHER WORDS...THESE BOUNDARIES STILL HAVE A LONG WAY TO GO TO GET TO CENTRAL NC...SO LITTLE CHANGE IN AIR MASS CAN BE EXPECTED THROUGH EARLY WED. AREAS OF FOG...SOME LOCALLY DENSE...WILL BE POSSIBLE IN THE SUBSIDENT/DRY WAKE OF THE AFOREMENTIONED MCV/S OVERNIGHT...WITH A BETTER PROBABILITY OF DENSE FOG AS THE WARM FRONT FINALLY BEGINS A RETREAT INTO CENTRAL NC DURING THE MID-MORNING TO EARLY AFTERNOON HOURS WED.
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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 /00Z WEDNESDAY THROUGH SUNDAY/... AS OF 700 PM TUESDAY... COLD AIR DAMMING CONTINUES TO PLAGUE THE REGION WITH WIDESPREAD LIFR CONDITIONS RESULTING FROM BOTH LOW CLOUDS AND FOG. A LARGE AREA OF RAIN MOVING INTO NC FROM THE SOUTH WILL CAUSE VSBYS TO BOUNCE AROUND A BIT THROUGH 06Z...BUT ONCE THE HEAVIER AREAS OF RAIN MOVE OFF TO THE NORTHEAST... THERE IS HIGH CONFIDENCE THAT THESE CONDITIONS WILL BE PREDOMINATE THROUGH THE OVERNIGHT HOURS. S LOW PRESSURE SYSTEM MOVING FROM THE DEEP SOUTH TO THE GREAT LAKES REGION WILL SEND A COLD FRONT TOWARD OUR REGION ON WEDNESDAY. MEANWHILE...A COASTAL/WARM FRONT WILL SLOWLY RETREAT THROUGH CENTRAL NC. DUE TO THE STRENGTH OF THE COLD AIR DAMMING...CONFIDENCE IN THE TIMING OF LOW CLOUD/FOG EROSION IS LOW. MODELS ALL INDICATE THAT WINDS WILL EVENTUALLY BECOME SOUTHEASTERLY AND THEN SOUTHERLY DURING THE DAY ON WEDNESDAY. THE CURRENT TAF WILL SHOW THIS TREND BUT DELAY IT BEYOND WHAT MODELS SHOW. ONCE WINDS DO SHIFT...THEY WILL BE RELATIVELY STRONG WITH SUSTAINED 12-15KT EXPECTED. THIS IS MORE LIKELY FROM KRDU TO KRWI/KFAY. DESPITE THE SOUTHERLY FLOW... LOW LEVEL MOISTURE WILL REMAIN HIGH AND CEILINGS ARE LIKELY TO REMAIN AT LIFR/IFR LEVELS. SHOWERS WILL ALSO BECOME MORE NUMEROUS DURING THE DAY...WITH SOME ISOLATED THUNDERSTORMS POSSIBLE FROM KRWI/KFAY TOWARD THE COAST. LONG TERM: A COLD FRONT WILL MOVE THROUGH THE AREA WEDNESDAY NIGHT AND BRING AN END TO THE SUB-VFR CONDITIONS. WESTERLY WINDS WILL BE GUSTY ON THURSDAY BEHIND THE FRONT. 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...MWS NEAR TERM...MWS SHORT TERM...BLAES LONG TERM...CBL AVIATION...BLS HYDROLOGY...MLM/BADGETT

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