Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 180542 AFDRAH AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 1240 AM EST THU DEC 18 2014 .SYNOPSIS...HIGH PRESSURE OVER THE MISSISSIPPI RIVER VALLEY WILL EXTEND EASTWARD INTO THE MID ATLANTIC AND CAROLINAS ON THROUGH FRIDAY. A LOW PRESSURE SYSTEM WILL DEVELOP ALONG THE TEXAS COAST ON FRIDAY AND TRACK EAST ALONG THE GULF COAST ON SATURDAY...THEN MOVE OFFSHORE THE SOUTHEAST COAST SATURDAY NIGHT INTO SUNDAY. && .NEAR TERM /OVERNIGHT/... AS OF 1045 PM WEDNESDAY... THE LATEST SATELLITE IMAGERY AND RAPID REFRESH MODEL DATA INDICATE THAT SMALL AMPLITUDE WAVES EMBEDDED IN WESTERLY FLOW ALOFT (IN CENTRAL/WESTERN TN AT 02Z) WILL TRAVERSE THE REGION OVERNIGHT. MEANWHILE...A SFC-925 MB TROUGH EXTENDING SOUTHWESTWARD FROM THE MID- ATLANTIC INTO THE CAROLINAS THIS EVENING WILL SHIFT OFFSHORE THE SOUTHEAST COAST BY 12Z THU AS 1028 MB HIGH PRESSURE (CENTERED OVER IL AT 02Z) EXTENDS EAST/SE THROUGH THE OH VALLEY INTO VA/NC. THOUGH THE LOWER LEVELS WILL REMAIN DRY...MID/UPPER LEVEL MOISTENING (AS EVIDENCED BY 12/00Z GSO RAOB DATA) WILL PERSIST OVERNIGHT AS MOISTURE ADVECTS DOWNSTREAM OF PRECIP ASSOC/W A SHORTWAVE TRACKING EASTWARD FROM THE CENTRAL PLAINS TO THE CENTRAL MS RIVER VALLEY. THIS WILL DO LITTLE TO ALTER THE DRY/STABLE AIRMASS CURRENTLY IN PLACE ACROSS THE CAROLINAS...HOWEVER...GIVEN THE DRY ANTECEDENT AIRMASS CURRENTLY IN PLACE AND THE ABSENCE OF LOW-LEVEL WARM ADVECTION. WITH THE ABOVE IN MIND...EXPECT CONTINUED DRY CONDITIONS AND ABSOLUTELY NO CHANCE FOR PRECIPITATION. GIVEN THE ONGOING/ CONTINUED INFLUX OF CIRRUS (CEILINGS ~25 KFT) FROM THE WEST...THE PRIMARY FORECAST CONCERN WILL BE THE `DEGREE` TO WHICH UPPER LEVEL CEILINGS ATTENUATE RADIATIONAL COOLING...AND THE IMPACT THEREOF ON LOW TEMPERATURES. WITH TEMPS RANGING FROM THE MID 30S TO LOWER 40S PER 02-03Z OBSERVATIONS...EXPECT OVERNIGHT LOWS IN THE 30S EVERYWHERE... LOWER 30S WHERE CIRRUS IS LESS OPAQUE/THINNER AND MID 30S WHERE CIRRUS IS THICKER. CIRRUS...LATIN FOR `CURL`...ARE HIGH ALTITUDE CLOUDS COMPOSED ENTIRELY OF ICE CRYSTALS WHICH...WHEN PRESENT AT NIGHT UNDER MOONLIGHT (OPTIMALLY A FULL MOON)...PLAY A KEY ROLE IN THE FORMATION OF AN OPTICAL PHENOMENON KNOWN AS THE 22 DEGREE HALO OR `MOON HALO`. MOON HALO ENTHUSIASTS MAY ULTIMATELY BE DISAPPOINTED...HOWEVER...DUE TO AN ILL-FATED JUXTAPOSITION WITH THE WANING CRESCENT PHASE IN WHICH <20% OF MOON LIGHT WILL BE PRESENT IN COMPARISON WITH THAT OF A FULL MOON (CITATION NEEDED). -VINCENT && .SHORT TERM /THURSDAY AND THURSDAY NIGHT/... AS OF 215 PM WEDNESDAY... MID LEVEL RIDGE AXIS APPROACHES EARLY ON THURSDAY AND GIVES WAY TO A SOUTHERN STREAM SHORTWAVE TROUGH THAT SLIDES BY ACROSS THE OH VALLEY AND THE MID ATLANTIC THURSDAY NIGHT. EXPECT FAIR WEATHER DURING THE PERIOD WITH A FAIRLY THICK VEIL OF HIGH AND ESPECIALLY MID LEVEL CLOUDINESS. GUIDANCE...ESPECIALLY THE GFS HAS BEEN HINTING AT SOME LIGHT PRECIP MAKING IT ACROSS THE MOUNTAINS INTO THE WESTERN PIEDMONT ON THURSDAY. RECENT RUNS HAVE BACKED OFF THIS AND WE EXPECT A DRY PERIOD AND HAVE LOWERED POPS. WITH A GOOD DEAL OF CLOUD COVER...HIGHS ON THURSDAY WILL SLOWLY REACH THE 48 TO 55 RANGE. LOOK FOR LESS VARIABILITY IN LOWS ON THURSDAY NIGHT WITH LINGER MID AND HIGH CLOUDS...LOWS SHOULD RANGE BETWEEN 32 AND 36 DEGREES. -BLAES && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... AS OF 254 PM WEDNESDAY... SURFACE HIGH PRESSURE ACROSS THE GREAT LAKES WILL EXTEND IN TO THE AREA... WHILE WE REMAIN UNDER CONFLUENT FLOW ALOFT. A SOUTHERN STREAM SHORTWAVE OVER THE SOUTHERN PLAINS ON FRIDAY WILL LIFT NORTHEASTWARD INTO THE OHIO VALLEY ON SATURDAY. SURFACE LOW PRESSURE WILL DEVELOP ALONG THE NORTHERN GOM LATE FRIDAY.. AND MOVE OFF THE SOUTHEAST COAST ON SUNDAY. THIS SYSTEM WILL BEGIN TO SPREAD RAIN IN TO THE REGION BY DAYBREAK SATURDAY... AND RAIN WILL REMAIN ACROSS THE AREA THROUGH THE EVENING BEFORE TAPERING OFF FROM WEST TO EAST SUNDAY MORNING. RAIN STILL LOOKS TO BE THE MAIN PRECIP TYPE... HOWEVER THERE CONTINUES BE THE POSSIBILITY OF FEW WET SNOWFLAKES MIXING IN ACROSS THE TYPICAL NORTHWEST PIEDMONT AREA. TEMPERATURES WILL BE BELOW NORMAL ON SATURDAY AND SUNDAY. WEAK HIGH PRESSURE WILL CONTINUE TO WEDGE INTO THE AREA MONDAY... AND CLOUDS AND BELOW NORMAL TEMPERATURES WILL CONTINUE WITH LOW LEVEL NORTHEASTERLY FLOW. MODELS BRING A STRONG SHORTWAVE INTO THE CENTRAL US ON TUESDAY... AND DIG A DEEP TROUGH ACROSS THE PLAINS AS FLOW BECOMES HIGHLY AMPLIFIED. AS THIS SYSTEM MOVES EASTWARD... UNSETTLED CONDITIONS LOOK LIKELY MIDWEEK. && .AVIATION /06Z THURSDAY THROUGH MONDAY/... AS OF 1235 AM THURSDAY... 24-HOUR TAF PERIOD: HIGH CONFIDENCE THAT VFR CONDITIONS AND LIGHT YET VARIABLE WINDS WILL DOMINATE THE 24 HOUR TAF PERIOD AS HIGH PRESSURE EXTENDS INTO THE AREA. SCATTERED TO BROKEN MID AND HIGH CLOUDS WILL CONTINUE TO TRAVERSE THE AREA AS A MID LEVEL DISTURBANCE APPROACHES THE REGION FROM THE WEST AND MOVES ACROSS THE AREA TONIGHT. LOOKING AHEAD: VFR CONDITIONS ARE EXPECTED TO CONTINUE INTO SATURDAY MORNING. AVIATION CONDITIONS ARE EXPECTED TO DETERIORATE ON SATURDAY AFTERNOON AS A STORM SYSTEM IS EXPECTED TO FORM OVER THE TEXAS GULF COAST LATE FRIDAY...WITH LOW PRESSURE TRACKING EAST ACROSS THE SOUTHERN GULF STATES SATURDAY...TO ALONG OR JUST OFF THE SOUTHEAST COAST SAT NIGHT AND SUN. WIDESPREAD RAIN WITH SUB-VFR CONDITIONS APPEAR LIKELY FROM SATURDAY AFTERNOON THROUGH SATURDAY NIGHT. VFR CONDITIONS ARE EXPECTED TO BRIEFLY RETURN FOR SUNDAY... BEFORE ANOTHER DISTURBANCE LIFTS NORTHEASTWARD OUT OF THE GULF... SPREADING RAIN INTO CENTRAL NC SUNDAY NIGHT INTO MONDAY... WITH SUB-VFR CONDITIONS. && .RAH WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SYNOPSIS...BSD/VINCENT NEAR TERM...VINCENT SHORT TERM...BLAES LONG TERM...SEC AVIATION...BSD/KC

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