Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 220359 AFDRAH AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 1059 PM EST SUN DEC 21 2014 .SYNOPSIS...
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HIGH PRESSURE OVER THE NORTHEAST WILL WEDGE IN TO THE AREA THROUGH MONDAY...AS A LOW PRESSURE SYSTEM MOVES ALONG THE COAST.
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&& .NEAR TERM /THROUGH MONDAY NIGHT/...
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AS OF 1055 PM SUNDAY... TONIGHT: A PERTURBED SOUTHWESTERLY UPPER JET...WITH A SERIES OF WELL- DEFINED IMPULSES ON WV VAPOR IMAGERY FROM THE WESTERN GOM TO THE MID-ATLANTIC COAST AND WHICH INCLUDES A 250 MB 150 KT JET STREAK OBSERVED AT KWAL...WILL PROMOTE AN ACCELERATION OF THE UNDERLYING LOW LEVEL FLOW ACROSS THE CAROLINAS TONIGHT. AN ASSOCIATED 925-850 MB WARM FRONT...ANALYZED AT 00Z JUST INLAND OF THE GULF COAST TO THE SAVANNAH BASIN...WILL CONSEQUENTLY RETREAT NORTHWARD; ASSUME AN INCREASINGLY SW TO NE ORIENTATION; AND BISECT CENTRAL NC BY 12Z... PER A WELL AGREED UPON CONSENSUS OF NWP GUIDANCE. THE LIFT AND ASSOCIATED MOISTURE TRANSPORT - STRONGEST IN AN AXIS BOUNDED ROUGHLY BY I-95 AND US HWY 1 - WILL CAUSE RAIN TO INCREASE IN COVERAGE AND INTENSITY AS IT SPREADS/DEVELOPS NORTHWARD FROM SC. INDEED HOURLY RAINFALL AMOUNTS OVER SC HAVE INCREASED FROM A FEW HUNDREDTHS OR LESS FOR MUCH OF THE AFTERNOON AND EVENING...TO A TENTH TO .15 DURING THE PAST COUPLE OF HOURS. THE LATEST NMM AND RAP...AND 21Z SREF MEAN SUPPORT THIS IDEA OF INCREASING RAINFALL RATES AND AMOUNTS DEVELOPING NORTH INTO OUR AREA...WITH AN AXIS OF A QUARTER TO HALF INCH PROBABLE CENTERED SQUARELY OVER CENTRAL NC THROUGH 12Z. LOCALLY HIGHER AMOUNTS WILL BE POSSIBLE WERE EMBEDDED HEAVIER RAIN OCCURS OWING TO POCKETS OF WEAK ELEVATED INSTABILITY CHARACTERIZED BY MUCAPE VALUES OF UP TO A COUPLE OF HUNDRED J/KG. ASIDE FROM OVER THE FAR NE PIEDMONT AND NORTHERN COASTAL PLAIN... WHERE LOWS IN THE MID 30S HAVE ALREADY BEEN ATTAINED FROM A BRIEF PERIOD OF RELATIVE CLEAR AND RADIATIONAL COOLING EARLY THIS EVENING...TEMPERATURES WILL HOLD STEADY OR EVEN RISE A DEGREE OR TWO WITH THICKENING CLOUD COVER...BEFORE FALLING VIA EVAPORATIONAL COOLING AS THE RAIN OVERSPREADS THE AREA AND CAUSES TEMPERATURES TO TREND TO SURFACE WET BULB TEMPERATURES IN THE MIDDLE 30S TO LOWER 40S. -MWS AS OF 220 PM SUNDAY... HIGH PRESSURE OVER THE MID ATLANTIC STATES IS SLOWLY SHIFTING EASTWARD THIS AFTERNOON AND INTO A FAVORABLE POSITION FOR A HYBRID COLD AIR DAMMING EVENT TO BEGIN. THIS IS KEEPING MOISTURE PENNED UP SOUTH OF THE AREA TODAY BUT A SOUTHERN STREAM SHORTWAVE WILL BEGIN TO CREATE SOME TROUGHING ALONG THE COAST OVERNIGHT TONIGHT INTO MONDAY WHICH WILL HELP TO SHARPEN UP THE WEDGE FRONT ACROSS CENTRAL NC AND SET THE STAGE FOR A WET COUPLE OF DAYS. MONDAY AND MONDAY NIGHT: RAIN IS EXPECTED TO PERSIST THROUGH THE DAY ON MONDAY AS THE SURFACE LOW MOVES NORTHWARD JUST OFF THE NC COAST. LOW CEILINGS AT IFR LEVELS ARE EXPECTED THROUGHOUT THE DAY. WHILE RAIN IS EXPECTED TO BE CONTINUOUS...THE QPF AMOUNTS WITH THIS SYSTEM ARE FAIRLY LOW. ONLY EXPECT A TOTAL OF A QUARTER TO A HALF OF AN INCH OF RAIN. WITH THE WEDGE LOCKED IN TO THE WEST AND THE COASTAL TROUGH TO THE EAST...THERE SHOULD BE A FAIRY DECENT GRADIENT FROM NORTHWEST TO SOUTHEAST IN THE HIGH TEMPERATURE SPREAD ON MONDAY. HIGHS IN THE NORTHWEST PIEDMONT WILL STRUGGLE TO GET TO 40 DEGREES WHEREAS HIGHS CLOSER TO 50 WILL BE COMMON IN THE SOUTHEAST. FOR THE MOST PART PRECIPIATION IS EXPECTED TO END AFTER 00Z TUESDAY BUT CONTINUED MOISTURE IN THE AREA WILL LEND ITSELF TO CONTINUING DRIZZLE THROUGH THE NIGHT ON MONDAY. THIS WET AND COOL PERIOD WILL ALSO LEND ITSELF TO PERIODS OF PATCHY FOG WITH THE MOST LIKELY TIMES BEFORE SUNRISE ON TUESDAY. LOWS IN THE MID 30S TO LOWER 40S.
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&& .SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/... AS OF 300 PM SUNDAY... TUESDAY: MID-LEVEL DRY AIR AND THE ABSENCE OF DISCERNIBLE FORCING...ASIDE FROM WEAK SOUTHERLY UPGLIDE...DOWNSTREAM OF THE DEEPENING TROUGH WILL KEEP MEASURABLE RAIN CHANCES LOW ACROSS CENTRAL NC DURING MUCH OF THE DAY ON TUESDAY. DESPITE A LACK OF PRECIP...SHROUD OF LOW CLOUDS WILL HELP TO KEEP THE LINGERING COLD AIR WEDGE LARGELY IN PLACE ACROSS CENTRAL NC. MEASURABLE RAIN CHANCES WILL RETURN BY THE LATE AFTERNOON/EVENING WITH THE RESURGENCE OF WARM MOIST ADVECTION AND LOW-LEVEL CONVERGENCE ALONG THE NORTHWARD RETREATING WEDGE FRONT. TEMPERATURES TUESDAY DURING THE DAY WILL REMAIN THE 40S THROUGHOUT MUCH OF THE DAY...THE EXCEPTION WILL BE ACROSS THE FAR SE ZONES WHERE NEARBY WEDGE/WARM FRONT COULD ALLOW TEMPERATURES TO WARM INTO THE LOWER 50S LATE IN THE DAY. TUESDAY NIGHT THROUGH WEDNESDAY NIGHT: SOGGY...BREEZY...AND POSSIBLY STORMY CONDITIONS... AHEAD OF THE AMPLIFYING UPSTREAM LONGWAVE TROUGH AND ATTENDANT SFC COLD FRONT...DEEP GULF MOISTURE FEED WILL TRANSPORT ANOMALOUSLY HIGH PWATS OF 1.5-1.7" INTO THE REGION...WHICH WOULD RANK IN THE 99TH PERCENTILE(HIGHEST OBSERVED PWAT DURING THE MONTH OF DEC AT KGSO BASED ON THE RADIOSONDE DATABASE WHICH DATE BACKS TO 1948 IS 1.59"- 12/21/1991). STRENGTHENING RIGHT ENTRANCE JET REGION COUPLED WITH THIS STRONG MOISTURE FLUX CONVERGENCE WILL RESULT IN CATEGORICAL POPS AND A WIDESPREAD SOAKING RAIN OF 1-2"...HEAVY AT TIMES TUESDAY NIGHT AND INTO CHRISTMAS EVE MORNING. ONCE THE WARM FRONT LIFTS NORTHWARD INTO THE VIRGINIA...THERE WILL LIKELY BE A LULL IN PRECIP UNTIL THE ARRIVAL OF THE COLD FRONT AND ATTENDANT BAND OF SHOWERS AND POSSIBLY EMBEDDED THUNDER THROUGH THE AREA BETWEEN 00-06Z THURSDAY...WHICH IS A BIT SLOWER THAN YESTERDAYS TIMING. SEVERE POTENTIAL: EVALUATING SEVERE POTENTIAL IN A LOW CAPE-HIGH SHEAR ENVIRONMENT IS ALWAYS DIFFICULT...EVEN AT CLOSER TIME SCALES OF 6-12 HOURS...MUCH LESS AT THE CURRENT TIME SCALE OF 3 TO 4 DAYS. THE LULL IN PRECIP DURING MUCH OF THE DAYTIME HOURS...MAY ALLOW FOR SOME MODEST HEATING...ESPECIALLY FROM THE US 1 CORRIDOR EASTWARD...WHICH COULD LEAD TO BETTER DESTABILIZATION THAN MODEL CURRENTLY SHOW(100-200J/KG MUCAPE). WEAK LAPSE RATES ALOFT OF 5.0- 5.5 C/KM ALONG WITH THE MAIN DYNAMICS PASSING OVER THE GREAT LAKES...WILL LIMIT WIDESPREAD SEVERE POTENTIAL...BUT GIVEN IMPRESSIVE DEEP LAYER SHEAR CANNOT RULE OUT ISOLATED SEVERE STORM AT THIS TIME. TEMPERATURES VERY CHALLENGING WITH HIGH BUST POTENTIAL...DEPENDENT ON EVOLUTION/TIMING/LOCATION OF THE RETREATING WEDGE FRONT INTO THE CLIMATOLOGICALLY FAVORED AREA ACROSS THE FAR NW PIEDMONT TUESDAY NIGHT AND INTO THE DAY ON WEDNESDAY. EXPECT TEMPERATURES TO STEADILY WARM AFTER MIDNIGHT TUESDAY NIGHT...WITH THE EXCEPTION BEING ACROSS THE NW PIEDMONT...WHERE TEMPS COULD HOLD NEARLY STEADY. LOWS RANGING FROM LOWER 40S NW TO MID 50S SE. WARM SECTOR SHOULD OVERSPREAD MUCH OF THE AREA BY WEDNESDAY AFTERNOON...WARMING INTO THE 60S ACROSS THE PIEDMONT...AND POSSIBLY LOWER 70S ACROSS THE SANDHILLS/SOUTHERN COASTAL PLAIN. RAPID CLEARING AND COOL AIR ADVECTION WILL COMMENCE CHRISTMAS EVE EVENING/NIGHT WITH LOWS CHRISTMAS MORNING IN THE UPPER 30S NW TO LOWER/MID 40S SE. && .LONG TERM /THURSDAY THROUGH SUNDAY/... AS OF 255 PM SUNDAY... DEEPENING SURFACE LOW LIFTING NEWD INTO SE CANADA WILL RESULT IN BREEZY CONDITIONS ON CHRISTMAS DAY. OTHERWISE DRY WITH SEASONABLY MILD TEMPERATURES CONTINUING INTO FRIDAY AS HIGH PRESSURE BUILDS IN FROM THE NORTHERN GULF. MODEL SPREAD BEGINS TO INCREASE DRAMATICALLY LATE FRIDAY THROUGH THE WEEKEND...WITH THE EC MUCH DEEPER WITH THE NEXT MID/UPPER LEVEL TROUGH...RESULTING IN YET ANOTHER WET AND UNSETTLED PERIOD NEXT WEEKEND. MEANWHILE...THE LESS AMPLIFIED GFS INDICATES QUASI-ZONAL FLOW ACROSS SOUTHEAST US WITH DRY CONDITIONS THROUGH THE WEEKEND. WILL INDICATE SLIGHT CHANCE POPS UNTIL BETTER AGREEMENT IS REACHED. && .AVIATION /00Z MONDAY THROUGH FRIDAY/... AS OF 710 PM SUNDAY... THERE IS HIGH CONFIDENCE THAT AVIATION CONDITIONS WILL DETERIORATE TONIGHT AS A LOW PRESSURE SYSTEM BEGINS TO DEVELOP OFF THE SOUTHEAST COAST. WHILE A FEW PATCHES OF LIGHT RAIN WILL BE POSSIBLE IN THE NEXT FEW HOURS... EXPECT THE LIGHT RAIN CURRENTLY OVER SC TO EXPAND AND SPREAD INTO NC AFTER 06Z...WITH CEILINGS EVENTUALLY LOWERING TO MVFR AND IFR...FIRST AT KFAY/KGSO/KINT BY 06-08Z AND THEN KRDU/KRWI BY 09Z. ONCE IFR CEILINGS DEVELOP...FORECAST SOUNDING SHOW VERY LITTLE IMPROVEMENT...IF ANY...DURING THE DAY ON MONDAY. THE RAIN WILL TRANSITION TO JUST DRIZZLE AS THE DAY PROGRESSES...BUT POOR AVIATION CONDITIONS WILL PERSIST THROUGH THE END OF THE TAF PERIOD. LONG TERM: POOR CONDITIONS WITH LOW CEILINGS/VSBYS AND PERIODS OF RAIN WILL CONTINUE THROUGH TUESDAY AND INTO WEDNESDAY. A COLD FRONT WILL CROSS THE AREA ON WEDNESDAY...BRINGING STRONG WINDS AND A CHANCE OF A FEW THUNDERSTORMS. CONDITIONS SHOULD RETURN TO VFR FOR CHRISTMAS DAY WITH SOME AFTERNOON GUSTY WINDS POSSIBLE. && .RAH WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SYNOPSIS...SEC NEAR TERM...MWS/ELLIS SHORT TERM...CBL LONG TERM...CBL AVIATION...BLS

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