Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 010513 AFDRAH AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 115 AM EDT FRI AUG 1 2014 .SYNOPSIS...A PROLONGED PERIOD OF UNSETTLED WEATHER CAN BE EXPECTED THROUGH EARLY NEXT WEEK AS AN UPPER LEVEL TROUGH SLOWLY APPROACHES FROM THE WEST. && .NEAR TERM /OVERNIGHT/... AS OF 830 PM THURSDAY... CYCLONIC FLOW ALOFT WILL PREVAIL OVER THE REGION THROUGH TONIGHT. UPPER LEVEL FORCING WILL BE PRESENT IN THE FORM OF CYCLONIC SHEAR VORTICITY IN ADDITION TO POCKETS OF MORE CONCENTRATED DPVA ATTENDANT TRANSIENT SMALL AMPLITUDE DISTURBANCES IN CYCLONIC FLOW ALOFT. AT THE SAME TIME...SOUTHEASTERLY RETURN FLOW WILL ADVECT RICH LOW-LEVEL MOISTURE INTO THE REGION...WITH 925 MB DEWPOINTS OF 18-19C IN COASTAL SC (23Z SPC MESO) ADVECTING INTO UPSTATE SC AND WESTERN NC OVERNIGHT. ALTHOUGH WEAK MID-LEVEL LAPSE RATES (5.0-5.5 C/KM) WILL PERSIST OVER THE REGION...RICH LOW-LEVEL MOISTURE/WARM ADVECTION WILL MAINTAIN 500-1000 J/KG OF MUCAPE OVERNIGHT...IN THE FORM OF ELEVATED INSTABILITY ATOP A SHALLOW RAIN-COOLED AIRMASS IN THE LOWEST 25-50 MB. WITH THE ABOVE IN MIND...EXPECT STRATIFORM RAIN WITH EMBEDDED ELEVATED CONVECTION TO PERSIST ALONG/WEST OF HWY 1 OVERNIGHT...PERHAPS INCREASING IN COVERAGE AND INTENSITY OVER THE FAR WESTERN PIEDMONT AND FOOTHILLS AFTER MIDNIGHT VIA LOW-LEVEL MOISTURE POOLING ALONG/EAST THE CREST OF THE APPALACHIANS AND OROGRAPHIC ASCENT IN PROXIMITY THEREOF. EXPECT LOWS DRIVEN LARGELY BY CLOUDS/PRECIP (EVAP COOLING)...IN THE MID/UPPER 60S. GIVEN THE PRESENCE OF LOWER/UPPER LEVEL FORCING...AS MUCH AS 500- 1000 J/KG MUCAPE...AND ~40 KNOTS OF DEEP LAYER SHEAR...IT IS POSSIBLE THAT ISOLATED UPDRAFTS COULD ACQUIRE MID-LEVEL ROTATION AND BRIEFLY EXHIBIT SUPERCELLULAR CHARACTERISTICS (I.E. SPLITTING/ DEVIANT MOTION) IN THE FAR WESTERN PIEDMONT/FOOTHILLS. GIVEN THE ELEVATED NATURE OF ANY CONVECTION AND THE PRESENCE OF EXTREMELY MARGINAL MID-LEVEL LAPSE RATES...DO NOT ANTICIPATE A POTENTIAL FOR SEVERE WEATHER. -VINCENT && .SHORT TERM /FRIDAY AND FRIDAY NIGHT/... AS OF 300 PM THURSDAY... ON FRIDAY THE UPPER TROUGH WILL CONTINUE TO HOVER WEST OF THE AREA. AT THE SURFACE...THERE WILL BE STRONG SURFACE CONVERGENCE OVER CENTRAL NC AS DISTURBANCES MOVE THROUGH THE TROUGH ACROSS THE WESTERN PIEDMONT AND AT THE SAME TIME A SEPARATE PLUME OF MOISTURE WILL APPROACH CENTRAL NC FROM THE SSE. THE ONSLAUGHT OF MOISTURE WILL BEGIN IN EARNEST AS PRECIPITABLE WATER VALUES CLIMB OVER 2 INCHES ACROSS THE AREA. MODEL FORECAST SOUNDINGS AS SEEN IN BUFKIT ARE VERY WET FROM TOP TO BOTTOM BUT THERE STILL REMAINS A QUESTION OF HOW MUCH INSTABILITY WILL BE PRESENT. THE NAM IS VERY BULLISH AND THE GFS NOT AS MUCH BUT THEY BOTH AGREE THAT THE SOUTHEAST WILL CARRY THE MOST CAPE. PLENTY OF FORCING WILL BE PRESENT WITH 30-40 KTS OF BULK SHEAR ACROSS THE EAST AND POTENTIALLY SOME DECENT 0-3 KM HELICITY VALUES. SPC OUTLOOK NOT CALLING FOR A SLIGHT RISK AT THIS POINT BUT WOULD NOT BE SURPRISED TO SEE A FEW STRONGER OR ROTATING CELLS IN THE SOUTHEAST FRIDAY AFTERNOON. ALL THIS ASIDE THE MAIN STORY WILL STILL BE THE RAINFALL WITH A HALF OF AN INCH TO AN INCH (LOCALLY HIGHER) EXPECTED ACROSS THE AREA WITH HEAVIER TOTALS IN MORE CONVECTIVE CELLS IN THE EAST. MAX TEMPS IN THE UPPER 70S TO LOWER 80S NW TO SE. CHANCES FOR RAIN WILL CONTINUE THROUGH FRIDAY NIGHT WITH THE EAST EXPECTED TO SEE MORE OF THE ACTIVITY MAINLY IN THE FORM OF SHOWERS AS CONDITIONS BECOME SLIGHTLY MORE STABLE OVERNIGHT. FOG AND/OR LOW STRATUS SHOULD BE PREVALENT ACROSS THE AREA BEFORE DAYBREAK ON SATURDAY. LOWS 65-70 DEGREES. && .LONG TERM /SATURDAY THROUGH THURSDAY/... AS OF 300 PM THURSDAY... ...AN UNSETTLED AND ACTIVE PERIOD THROUGH AT LEAST THE WEEKEND WITH SCATTERED TO NUMEROUS SHOWERS AND THUNDERSTORMS WITH LOCALLY HEAVY RAIN... A BROAD UPPER LEVEL TROUGH PERSISTS JUST WEST OF THE APPALACHIANS INTO EARLY NEXT WEEK. AT THE SAME TIME A SURFACE TROUGH SETS UP ACROSS EASTERN NC WITH A FLOW OF DEEP MOISTURE MOVING ACROSS THE AREA. THIS WILL RESULT IN A PERIOD OF UNSETTLED WEATHER AND THE POTENTIAL FOR SOME SIGNIFICANT RAIN. THE TROUGH WEAKENS EARLY TO MIDDLE PORTION OF NEXT WEEK AS PART OF IT LIFTS ACROSS NEW ENGLAND AND ANOTHER PORTION BREAKS OFF OVER THE LOWER MISSISSIPPI VALLEY. THIS RESULTS IN A DECREASE IN PRECIPITATION COVERAGE AND A WARMING TEMPERATURES TO NEAR OR OR JUST ABOVE AVERAGE TOWARD THE MIDDLE OF THE WEEK. GUIDANCE SUGGESTS THAT ONE SHORT WAVE TROUGH WILL BE DEPARTING THE REGION EARLY SATURDAY MORNING WITH ANOTHER WAVE POISED TO MOVE ACROSS THE AREA SATURDAY NIGHT...BUT TRYING TO TIME THESE FEATURES TOO PRECISELY IS FOOLHARDY. IN ADDITION... GUIDANCE VARIES WITH THE INLAND PROGRESSION OF THE COASTAL TROUGH. THE END RESULT THOUGH SHOULD BE A FAIRLY ACTIVE CONVECTIVE PATTERN SATURDAY WITH THE BULK OF THE SHOWERS AND THUNDERSTORMS OCCURRING DURING THE AFTERNOON THROUGH THE EVENING INTO THE EARLY OVERNIGHT HOURS. THE NAM GENERATES A LOT MORE PRECIPITATION THAN THE GFS/ECMWF DURING THE DAY AND IS ACCORDINGLY COOLER AND MORE STABLE THAN THE GFS WHICH GENERATES MLCAPE VALUES UP TO 1000-1500 J/KG ACROSS THE EASTERN PIEDMONT AND COASTAL PLAIN. IT WILL BE DIFFICULT GETTING A BETTER READ ON THE INSTABILITY UNTIL A BETTER UNDERSTANDING OF THE EVOLUTION OF THE DIURNAL CONVECTIVE CYCLE ON FRIDAY IS COMPLETE. GREATER CONFIDENCE IS IN THE AMOUNT OF SHEAR WITH THE GFS NOTING STRONGER DEEP LAYER FLOW WITH BULK SHEAR VALUES OF 20-30KTS WHICH WOULD SUPPORT SOME STORM ORGANIZATION. THE NAM HAS WEAKER FLOW...AROUND 20KTS OR SO WITH THE SREF ENSEMBLE DISTRIBUTION SPREAD FROM 20-30 KTS. THE GREATEST SHEAR SHOULD BE LOCATED FROM THE TRIAD NORTH AND WEST WITH THE GREAT INSTABILITY TO THE SOUTHEAST. STILL ANY STORM ELEMENTS THAT DEVELOP NEAR THE SURFACE BOUNDARY WILL NEED TO BE MONITORED. FOR NOW THE MOST LIKELY THREAT IS FROM EFFICIENT RAIN MAKING STORMS THAT REPEATEDLY TRACK OVER THE SAME LOCATION RESULTING IN POTENTIAL FLOODING ISSUES. ACTIVE WEATHER CONTINUES ON SUNDAY AS MULTIPLE SHORT WAVE TROUGHS APPROACH THE REGION WITH A VERY MOIST AIR MASS IN PLACE. THE RESULT SHOULD BE AN ACTIVE CONVECTIVE DAY ONCE AGAIN WITH LIKELY POPS WARRANTED AND THE GREATEST PRECIPITATION COVERAGE ACROSS THE EASTERN PORTION OF THE FORECAST AREA. INSTABILITY PARAMETERS ARE WEAKER THAN SUNDAY WITH MID LEVEL LAPSE RATES PROGGED TO BE MORE THAN 0.5 C DEG LESS AS THE MID LEVELS WARM SOMEWHAT WITH THE DEPARTING TROUGH. THE PATTERN TRANSITIONS INTO A MORE TYPICAL SUMMERTIME APPEARANCE ON ON MONDAY AND ESPECIALLY TUESDAY THROUGH WEDNESDAY AS THE UPPER TROUGH WEAKENS AND THE UPPER FLOW BECOMES NOTICEABLY MORE WESTERLY. RAIN CHANCES ACCORDINGLY DROP DURING THE EARLY TO MID WEEK PERIOD...ESPECIALLY DURING THE A-TYPICAL DIURNAL HOURS. LOW CHANCE TO SLIGHT CHANCE POPS WILL BE INCLUDED IN THE FORECAST FOR PRIMARILY THE AFTERNOON AND EVENING HOURS. TEMPERATURES AT THE BEGINNING OF THE LONG TERM WILL BE QUITE COOL AS DICTATED BY THE COOL AIR MASS WITH MORNING 1000-850MB THICKNESSES VALUES IN THE 1385-1395 RANGE ALONG WITH LOTS OF CLOUDS AND PRECIPITATION. HIGHS ON SATURDAY AND SUNDAY SHOULD RANGE IN THE UPPER 70S IN THE NORTHWEST PIEDMONT TO LOWER 80S IN THE SOUTHERN COASTAL PLAIN. HIGHS WILL MODERATE BEGINNING TUESDAY AND WARM INTO THE UPPER 80S TO AROUND 90 BY THURSDAY. -BLAES && .AVIATION /06Z FRIDAY THROUGH TUESDAY/... AS OF 115 AM FRIDAY... HIGH CONFIDENCE OF A PROLONGED PERIOD OF SUB VFR CONDITIONS ACROSS CENTRAL NC...STARTING TODAY AND PERSISTING THROUGH THE WEEKEND. A MOIST EAST-SOUTHEAST FLOW NEAR THE SURFACE COUPLED WITH A MOIST SW FLOW ALOFT WILL MAINTAIN A NEARLY SATURATED AIR MASS THROUGH SUNDAY. THIS WILL RESULT IN WIDESPREAD MVFR CEILINGS WITH PERIODS OF IFR/LIFR CEILINGS. THE IFR/LIFR SHOULD MAINLY OCCUR IN THE LATE NIGHT-EARLY MORNING HOURS. A SERIES OF UPPER LEVEL DISTURBANCES IN THE FLOW ALOFT WILL TRIGGER AREAS OF SHOWERS AND A FEW THUNDERSTORMS. THE MOST WIDESPREAD COVERAGE ANTICIPATED TODAY WITH THE PASSAGE OF A POTENT UPPER LEVEL SYSTEM...AND AGAIN SATURDAY AFTERNOON-SATURDAY NIGHT WITH THE APPROACH/PASSAGE OF ANOTHER DISTURBANCE. A FEW STRONG STORMS MAY OCCUR IN THE VICINITY OF KFAY AND KRWI THIS AFTERNOON AND AGAIN SATURDAY AFTERNOON AS LOW LEVEL INSTABILITY WILL BE BETTER IN THE SANDHILLS AND COASTAL PLAIN COMPARED TO THE PIEDMONT. AVIATION CONDITIONS ARE EXPECTED TO SLOWLY IMPROVE MONDAY INTO TUESDAY AS THE AIR MASS SLOWLY DRIES OUT AND THE BETTER LIFT FOR SHOWERS AND THUNDERSTORMS DRIFTS EAST TOWARD THE IMMEDIATE COASTLINE. STILL...THERE WILL BE PERIODS OF MVFR/IFR CEILINGS DUE TO RESIDUAL MOISTURE...MAINLY IN THE LATE NIGHT-EARLY MORNINGS HOURS. && .RAH WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SYNOPSIS...VINCENT NEAR TERM...VINCENT SHORT TERM...ELLIS LONG TERM...BLAES AVIATION...WSS

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