Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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729 FXUS62 KRAH 051958 AFDRAH AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 255 PM EST FRI FEB 5 2016 .SYNOPSIS... IN THE WAKE OF A LOW PRESSURE SYSTEM DEPARTING THE MID- ATLANTIC COAST...COOL HIGH PRESSURE WILL BUILD IN FROM THE WEST THROUGH THE WEEKEND. A SURFACE LOW WILL PASS BY THE CAROLINA COAST ON SUNDAY...FOLLOWED BY AN ARCTIC COLD FRONT THAT WILL ARRIVE EARLY NEXT WEEK. && .NEAR TERM /THROUGH TONIGHT/...
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AS OF 255 PM FRIDAY... BENEATH A BRIEF PERIOD OF SHORTWAVE RIDGING ALOFT...AN EXPANSIVE 1032 MB HIGH TO OUR WEST WILL SPREAD EAST TONIGHT AND BE NEARLY OVERHEAD BY SATURDAY MORNING. GIVEN CLEAR SKIES AND CALM CONDITIONS...AS WELL AS P DOWN TO 0.15"...STRONG RADIATIONAL COOLING AND FORECAST THICKNESSES OF 1285-1290M BY ALL GUIDANCE WOULD SUPPORT LOWS IN THE LOW/MID 20S
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&& .SHORT TERM /SATURDAY THROUGH SUNDAY/...
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AS OF 2255 PM FRIDAY... SPLIT FLOW OVER THE CONUS WILL LEAD TO A CUT OFF SOUTHERN STREAM WAVE THAT SWINGS ACROSS THE SOUTHEAST US SATURDAY AND THEN LIFTS ALONG THE EAST COAST ON SUNDAY...WITH A RAPIDLY DEEPENING SURFACE LOW(TO 980MB) OVER THE GULF STREAM AND A SHARP CUTOFF OF PRECIP ON ITS WESTERN SIDE. HIGHS SATURDAY WILL BE IN THE UPPER 40S AND LOWER 50S...WITH CLEAR SKIES EARLY SATURDAY NIGHT AND STRONG COOLING FOR THE FIRST HALF OF THE NIGHT. THE SURFACE LOW IS FORECAST TO BEGIN TO DEVELOP AFTER 06S SUNDAY...WITH MOISTURE INCREASING RAPIDLY OVER THE SOUTHERN COASTAL PLAIN BY 12Z. TEMP PROFILES (OR WETBULB PROFILES) WILL BE SUB-FREEZING DOWN TO ABOUT A 1000 FT OR SO FROM THE SURFACE...AND MODELS INDICATE SUFFICIENT MOISTURE IN THE ICE NUCLEATION ZONE FOR CRYSTAL GROWTH AS LIFT INCREASE TOWARD 12Z SUNDAY. HOWEVER...BASED ON THE CURRENT FORECAST TRACK OF THE SURFACE LOW...RH CROSS-SECTIONS FROM MULTIPLE MODELS SHOW VERY DRY AIR NEAR AND WEST OF I-95 THAT WILL BE HARD TO OVERCOME. SIMILARLY...AND ESPECIALLY EAST OF I-95...THE BIG QUESTIONS SEEMS TO BE WHETHER OR NOT PRECIP RATES WILL BE HEAVY ENOUGH TO OVERCOME THE LOW-LEVEL ABOVE FREEZING LAYER AND SUSTAIN SNOW AS THE PREDOMINATE PTYPE. OUTSIDE OF THE STORM INDUCED NORTHERLY FLOW...THERE IS NO REAL COLD AIR SUPPORT TO OUR NORTH....AS THE INITIAL COLD HIGH OVER US WILL WEAKEN AND DEPART TO THE NORTHEAST ON SATURDAY. THE GFS HAS TRENDED WESTWARD AND WETTER IN THE PAST 2 OR 3 RUNS..BUT IS A WESTERN OUTLIER. THE CONSENSUS SEEMS TO BE A TENTH TO A THIRD OF AN INCH OF QPF FROM SCOTLAND COUNTY TO WILSON COUNTY. THESE LIQUID AMOUNTS WOULD NOT LIKELY CAUSE MANY PROBLEMS AS FAR AS SNOW ACCUMULATION GIVEN THAT PRECIP RATES WOULDNT BE THAT HIGH AND SURFACE TEMPS WOULD BE IN THE MID/UPPER 30S AT WORST. THAT IS HOW THE CURRENT FORECAST APPEARS TO PLAY OUT...BUT THERE IS STILL ENOUGH UNCERTAINTY IN THE TRACK OF THE SURFACE LOW...AS WELL AS THE ORIENTATION OF THE UPPER WAVE AND THE ROLE OF THE KICKER SHORTWAVE THAT DIVES THROUGH THE CENTRAL PLAINS BEHIND IT. THE LATTER WILL BE BETTER SAMPLED TONIGHT. SMALL CHANGES IN THESE FEATURES...IE THE MAIN SHORTWAVE BECOMING NEGATIVELY TILTED SOONER AND/OR THE SURFACE LOW TRACKING A LITTLE FURTHER WEST COULD HAVE A GREATER IMPACT. NOT TO LEAVE OUT THE WESTERN CWA...BUT MOSTLY CLEAR TO PARTLY CLOUDY SKIES WEST OF THE TRIANGLE SHOULD MAKE FOR A DECENT DAY ON SUNDAY WITH HIGHS IN THE UPPER 40S TO AROUND 50.
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&& .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... AS OF 250 PM FRIDAY... SHORTWAVE ENERGY MOVING ONSHORE BRITISH COLUMBIA TONIGHT WILL RESULT IN RAPID AMPLIFICATION OF A LONGWAVE TROUGH OVER THE EASTERN US SUN NIGHT/MONDAY AS IT DIGS SOUTHEAST THROUGH THE UPPER MIDWEST INTO THE OH VALLEY AS AN UPPER LOW...ON THE HEELS OF A NEGATIVELY TILTED SHORTWAVE PROGRESSING OFFSHORE THE SOUTHEAST COAST. THE RESULTING LONGWAVE TROUGH IS EXPECTED TO PERSIST OVER THE EASTERN CONUS THROUGH MID-WEEK...OR UNTIL UNIDIRECTIONAL FLOW ALOFT BECOMES ESTABLISHED THU/FRI. LIGHT PRECIP IS EXPECTED TO OCCUR IN ASSOC/W POTENT SHORTWAVE ENERGY ROUNDING THE SOUTHEAST PERIPHERY OF THE AFOREMENTIONED UPPER LOW MONDAY AFT/EVE. GIVEN SOUTHWEST FLOW THROUGHOUT THE COLUMN AND THE PRESENCE OF LOW-LEVEL WARM ADVECTION...NO PTYPE CONCERNS ARE ANTICIPATED. EXPECT BELOW NORMAL TEMPERATURES TUE/WED IN ASSOCIATION WITH STRONG/PERSISTENT COLD ADVECTION AS THE LONGWAVE TROUGH SHIFTS EAST INTO THE MID- ATLANTIC/CAROLINAS. FROM A PATTERN RECOGNITION STANDPOINT...I.E. UPPER TROUGH AXIS NEAR/OVER THE REGION AND WESTERLY FLOW THROUGHOUT THE LOW/MID LEVELS...DRY CONDITIONS ARE GENERALLY EXPECTED TO PREVAIL. ONE EXCEPTION WILL BE A POTENTIAL FOR ISOLD/SCT DIURNAL CONVECTION TUE AFTERNOON GIVEN STEEP LAPSE RATES/VERY COLD TEMPS ALOFT AND OCCASIONAL DPVA ASSOC/W SHORTWAVE ENERGY ROUNDING THE BASE OF THE TROUGH /UPPER LOW/ OVER THE MID-ATLANTIC. WHETHER OR NOT CONVECTION DEVELOPS WILL HIGHLY DEPEND ON THE TIMING/MAGNITUDE OF DPVA...AND EVEN IF CONVECTION WERE TO DEVELOP...A VERY DRY SUB-CLOUD LAYER WOULD HAVE TO BE OVERCOME...THUS CONFIDENCE REMAINS LOW. IF CONVECTION WERE TO DEVELOP...A SHALLOW ABOVE FREEZING LAYER IN THE LOWEST 500-1000 FT AGL WOULD SUGGEST RAIN SHOWERS...THOUGH SOME POTENTIAL FOR A RATE-DRIVEN CHANGEOVER TO SNOW SHOWERS WOULD BE PRESENT. -VINCENT && .AVIATION /18Z FRIDAY THROUGH WEDNESDAY/... AS OF 1245 PM FRIDAY... NORTH-NORTHWESTERLY WINDS WILL CONTINUE TO GUST INTO THE 20-25KT RANGE THIS AFTERNOON...WITH VFR CONDITIONS OTHERWISE PREVAILING. THE PRESSURE GRADIENT WILL WEAKEN QUICKLY TONIGHT AS HIGH PRESSURE BUILDS IN FROM THE WEST...ALLOWING WINDS TO BECOME NEARLY CALM. VFR WILL CONTINUE. LOOKING BEYOND 18Z SATURDAY...VFR CONDITIONS WILL PREVAIL ON SATURDAY. A SURFACE LOW PASSING BY THE CAROLINA COAST ON SUNDAY MAY BRING LIGHT PRECIP AND A PERIOD SUB-VFR CONDITIONS FROM KFAY TO KRWI. AN UPPER LEVEL DISTURBANCE WILL BRING ANOTHER CHANCE OF SUB- VFR CONDITIONS AND PATCHY PRECIP TO ALL SITES MON AFTERNOON INTO TUE. && .RAH WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SYNOPSIS...SMITH NEAR TERM...SMITH SHORT TERM...SMITH LONG TERM...VINCENT AVIATION...SMITH

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