Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 280840 AFDRAH AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 340 AM EST SUN DEC 28 2014 .SYNOPSIS... A COLD FRONT WILL APPROACH FROM THE NORTHWEST TODAY... THEN PUSH SLOWLY SOUTHEAST THROUGH THE AREA MONDAY AND MONDAY NIGHT. HIGH PRESSURE WILL BUILD IN FROM THE WEST LATE TUESDAY THROUGH THURSDAY. && .NEAR TERM /TONIGHT THROUGH SUNDAY/... AS OF 920 PM SATURDAY... SATELLITE CLOUD TRENDS SUPPORT EARLIER EXPECTATIONS OF CLOUDS OVERSPREADING CENTRAL NC FROM THE WEST AND SW... WITH CLOUDS STEADILY THICKENING AND LOWERING OVERNIGHT. THIS IS EXHIBITED WELL BY A COMPARISON OF THE 00Z GSO SOUNDING WITH UPSTREAM SOUNDINGS... WITH DRY AIR NEAR 500 MB AND BELOW 700 MB AND A PW NEAR 0.6 IN AT GSO... WHEREAS UPSTREAM LOCATIONS INCLUDING THE FFC AND BMX SOUNDINGS ARE MUCH WETTER THROUGH THE COLUMN... WITH PW VALUES 1.0- 1.5 IN... OVER 200% OF NORMAL... WHICH IS POISED TO ADVECT INTO NC. THE DEEPEST MOISTURE WAS OBSERVED OVER CENTRAL/SRN AL INTO CENTRAL GA... ASSOCIATED WITH A WEAK VORTICITY MAXIMUM... AND THIS IS ALSO WHERE REGIONAL RADAR SHOWS EXPANSIVE RAIN -- SOME OF IT MODERATE -- SPREADING TOWARD SOUTH CENTRAL NC... ALTHOUGH THE NE PROGRESSION OF THIS RAIN IS HINDERED BY INITIAL EVAPORATION ON ITS NE EDGE. NEVERTHELESS... RADAR TRENDS SUGGEST THAT THE SOUTHERN HALF OF THE CWA MAY SEE MEASURABLE RAIN FIRST... SIMILAR TO THE LATEST HRRR AND WRF-ARW RUNS. WE ALSO HAVE MUCH LOWER SURFACE DEWPOINTS IN THE NORTHERN CWA (UPPER 30S) AS COMPARED TO THE SOUTHERN AND EXTREME ERN CWA (LOW 40S)... AND THESE LOWER DEWPOINTS WILL TAKE LONGER TO BE OVERCOME. BASED ON THIS... HAVE MADE MINOR ADJUSTMENTS TO RAISE POPS A BIT LATE TONIGHT OVER THE SOUTH WITH A SMALL DOWNWARD TWEAK OF POPS IN THE NORTHWEST/NORTH CENTRAL... AND NO POPS NORTHEAST OF RALEIGH OVERNIGHT. MUCH OF WHAT WILL INITIALLY BE SEEN ON RADAR IN CENTRAL NC IS APT TO EVAPORATE PRIOR TO HITTING THE GROUND... BUT STILL EXPECT A LOW CHANCE OF MEASURABLE AMOUNTS TO SPREAD INTO THE SW CWA SOON AFTER MIDNIGHT... THEN EXPAND INTO THE REST OF THE WRN AND SRN CWA AFTER 3 AM. AMOUNTS WILL BE VERY LIGHT... A FUNCTION OF THE WEAK FORCING FOR ASCENT. AS ANTICIPATED EARLIER... TEMPS FELL MOST SOON AFTER NIGHTFALL AND HAVE LEVELED OFF SOMEWHAT... AND THE ENCROACHMENT OF CLOUDS WILL FURTHER BLANKET THE CWA OVERNIGHT... LIMITING TEMP FALL. EXPECT LOWS TO RANGE FROM THE LOWER 40S (NE) TO UPPER 40S (WEST). -GIH EARLIER DISCUSSION FROM 255 PM: LATEST SURFACE ANALYSIS SHOWS AN AREA OF SURFACE HIGH PRESSURE CENTERED NEAR THE NC/SC COAST ALONG THE COASTAL CAROLINAS AND VIRGINIA. A POSITIVELY TILTED DEAMPLIFYING UPPER TROUGH ACROSS THE WESTERN PLAINS AT MIDDAY TODAY WILL CONTINUE TO SHIFT SLOWLY EAST TODAY AND TONIGHT. THE NORTHERN PORTION OF THE TROUGH ASSOCIATED WITH THE NORTHERN JET STREAM WILL SHEAR EAST INTO THE EASTERN GREAT LAKES WHILE BY LATE SUNDAY AFTERNOON. THE SOUTHERN EXTENT OF THE TROUGH IS SHARPER AND MOVES SLOWLY EAST REACHING THE ARKLATEX BY SUNDAY EVENING. THE ASSOCIATED COLD FRONT WHICH EXTENDS FROM MICHIGAN SOUTHWEST TO EASTERN TX AT MIDDAY TODAY WILL MOVE EAST REACHING EASTERN PA AND NORTHWEST GA BY LATE SUNDAY AFTERNOON. MAINLY CLEAR SKIES ACROSS CENTRAL NC THIS AFTERNOON WILL GIVE WAY TO INCREASING HIGH AND MID CLOUDINESS THIS EVENING AS MOISTURE BEGINS TO INCREASE AHEAD OF THE FRONT. WHILE HIGH AND MID LEVEL CLOUDINESS WILL THICKEN OVERNIGHT... NAM/GFS/SREF BUFR SOUNDINGS ALL NOTE THAT THE LOWER THIRD OF THE TROPOSPHERE IS SLOW TO MOISTEN OVERNIGHT UNTIL LOW LEVEL MOISTURE SURGES INTO THE REGION FROM THE SOUTHWEST TOWARD DAYBREAK SUNDAY. THE PRECIPITATION IS LARGELY DRIVEN BY ISENTROPIC LIFT AROUND THE 295K SURFACE WITH THE BEST SURGE OF LIFT SKIRTING ACROSS THE NORTHWESTERN EDGE OF CENTRAL NC AND WILL ADVERTISE LOW CATEGORICAL POPS THERE. FORCING FOR ASCENT DECREASES TO THE SOUTH AND EAST ON SUNDAY WITH LOW CHANCE POPS ADVERTISED FOR THE SANDHILLS AND SOUTHERN COASTAL PLAIN. SO SUNDAY WILL FEATURE CLOUDY SKIES ACROSS CENTRAL NC IT WONT BE WASHOUT FOR EVERYONE. MAINLY SPOTTY LIGHT RAIN IS EXPECTED TO THE SOUTH AND SOUTHEAST OF RALEIGH WITH THE MOST WIDESPREAD RAIN EXPECTED NEAR AND ESPECIALLY TO THE NORTHWEST OF INTERSTATE 85. TEMPERATURES WILL BE A LITTLE TRICKY AS THEY WILL TEND TO FALL QUICKLY THIS EVENING AFTER SUNSET BUT THE OVERNIGHT TREND WILL BE COMPLICATED BY THE INCREASING MID AND HIGH CLOUDS AND THE LIGHT SOUTHERLY STIRRING AHEAD OF THE APPROACHING COLD FRONT. WILL FORECAST LOWS IN THE MID 40S WEST TO LOWER 40S EAST. HIGHS ON SUNDAY WILL BE COMPLICATED BY TE PRECIPITATION WHICH SHOULD KEEP HIGHS IN THE MID 50S ACROSS THE NORTHWEST PIEDMONT WITH MID 60S ACROSS THE SANDHILLS AND SOUTHERN COASTAL PLAIN WHERE THE RAIN WILL BE MORE SCATTERED. -BLAES && .SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY/... AS OF 330 PM SATURDAY... SUNDAY NIGHT...DEEP SW FLOW WILL MAINTAIN A MOIST AIR MASS ACROSS CENTRAL NC SUNDAY NIGHT INTO MONDAY MORNING. A STOUT DISTURBANCE ALOFT WILL APPROACH IN THE SW FLOW LATE SUNDAY AND CROSS OUR REGION EARLY MONDAY. MEANWHILE CENTRAL NC WILL BE IN THE RIGHT ENTRANCE REGION OF A 130-150KT JET. THE ENHANCED LIFT SUPPLIED BY BOTH THE JET AND THE APPROACHING DISTURBANCE INTERACTING WITH AN ABNORMALLY MOIST AIR MASS (PRECIP WATER VALUES PROJECTED TO BE 1-1.2 INCHES) WILL CAUSE RAIN TO BECOME WIDESPREAD LATE SUNDAY NIGHT INTO EARLY MONDAY. MIN TEMPS UPPER 40S FAR NORTH TO THE LOW-MID 50S FAR SOUTH. MONDAY... WIDESPREAD RAIN WILL DIMINISH IN COVERAGE AND INTENSITY BY MID DAY-EARLY AFTERNOON AS THE UPPER LEVEL DISTURBANCE AND THE JET DEPART OUR REGION. WHILE THE MID-UPPER LEVELS OF THE ATMOSPHERE WILL DRY OUT...SW FLOW IN THE 925-850MB LAYER WILL MAINTAIN SATURATION RESULTING IN OVERCAST SKIES WITH SPOTS OF RAIN AND/OR DRIZZLE. SURFACE COLD FRONT WILL DRAG SOUTHWARD ACROSS THE REGION AS THE UPPER DISTURBANCE PULLS OUT TO THE EAST-NE. A SHALLOW COLD AIR AIR MASS WILL INFILTRATE OUR REGION FROM THE NORTH. THIS SHOULD LEAD TO FALLING TEMPS BY LATE MORNING ACROSS THE NORTH...AND THE AFTERNOON ACROSS THE SOUTH. MONDAY NIGHT INTO TUESDAY...850MB TROUGH PROJECTED TO REMAIN WEST OF OUR REGION UNTIL MID DAY TUESDAY. THIS SUGGEST A CONTINUATION IN EXTENSIVE CLOUDINESS AND PATCHY LIGHT RAIN/DRIZZLE THROUGH THE MAJORITY OF MONDAY NIGHT. PASSAGE OF 850MB TROUGH AND ENSUING W-NW FLOW IN ITS WAKE ALONG WITH SUBSIDENCE WILL AID TO DISSIPATE CLOUD COVERAGE WEST-TO-EAST TUESDAY AFTERNOON. LOW LEVEL COLD AIR ADVECTION WILL RESULT IN A SMALLER THAN NORMAL DIURNAL TEMP RECOVERY. MIN TEMPS MID-UPPER 40S NORTH TO LOWER 40S SOUTH. MAX TEMPS TUESDAY MID 40S NORTH TO NEAR 50 SOUTH. && .LONG TERM /TUESDAY THROUGH SATURDAY/...
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AS OF 340 AM SUNDAY... TUE: A WELL-DEFINED SHORTWAVE TROUGH OVER WA THIS MORNING WILL AMPLIFY ACROSS THE ROCKIES THROUGH TONIGHT...THEN DEAMPLIFY FROM THE CENTRAL PLAINS TO THE MIDDLE ATLANTIC COAST TUE THROUGH TUE NIGHT. THE PASSAGE OF THIS FEATURE AND ASSOCIATED GLANCING BLOW OF THE SOUTHERN PERIPHERY OF HEIGHT FALLS ALOFT...AND ASSOCIATED FORCING FOR ASCENT...WILL PRODUCED RENEWED LIFT ATOP A STILL-SATURATED LAYER IN ROUGHLY THE LOWEST SEVERAL THOUSAND FT --DEEPEST NEAR THE VA BORDER/IN CLOSEST PROXIMITY TO THE STRONGEST HEIGHT FALLS ALOFT OVER THE MIDDLE ATLANTIC-- SUCH THAT OTHERWISE PERIODS OF DRIZZLE THROUGH EARLY TUE WILL BECOME LIGHT RAIN DURING THE MORNING TO MIDDAY HOURS. BEHIND THE PASSING SHORTWAVE TROUGH...NW 925-850 MB FLOW WILL PROMOTE BOTH DOWNSLOPE FLOW AND A DISSIPATION OF THE CAD INVERSION VIA CAA - BOTH MECHANISMS THAT SHOULD SERVE TO SCOUR THE CAD REGIME AND PROMOTE CLEARING BY THE AFTERNOON OR EVENING. THE TIMING OF THIS PROCESS REMAINS IN QUESTION OWING TO TIMING DIFFERENCES AMONGST NWP GUIDANCE REGARDING THE PASSAGE OF THE SHORTWAVE TROUGH...THOUGH THERE HAS BEEN AN OVERALL SLOWER TREND EVIDENT IN THE GUIDANCE. AS SUCH...WILL FAVOR A SLOWER CLEARING TREND FROM WEST TO EAST SUCH THAT TEMPERATURES LIKELY WILL NOT HAVE TIME TO RESPOND TO THE SETTING AFTERNOON SUN...WHICH WOULD RESULT IN HIGHS ONLY IN THE LOWER TO MIDDLE 40S. IF A FEW HOUR PERIOD OF SUN DOES MATERIALIZE... MOST LIKELY OVER THE SOUTHERN AND WESTERN PIEDMONT DURING THE AFTERNOON...TEMPERATURES WOULD RESPOND TO NEAR NORMAL VALUES IN THE UPPER 40S. TUE NIGHT-SAT: STRONG...ARCTIC HIGH PRESSURE OVER THE ROCKIES EARLY IN THE PERIOD WILL OTHERWISE SPRAWL EAST ACROSS THE MIDDLE ATLANTIC AND SOUTHEAST STATES...THROUGH THU...BENEATH GENERALLY WESTERLY FLOW BETWEEN CONFLUENT BRANCHES OF THE UPPER JET...BEFORE DRIFTING OFFSHORE AHEAD OF AN EJECTING DESERT SW (BRIEFLY) CUTOFF LOW FRI- SAT. THE RESULT WILL BE COOL CONDITIONS --MOST NOTABLY WED AND THU MORNINGS WHEN TEMPERATURES SHOULD DIP WELL INTO THE 20S (WITH UPPER TEENS POSSIBLE IN OUTLYING AREAS THAT EXPERIENCE CALM) GIVEN PROJECTED LOW LEVEL THICKNESS VALUES IN THE 1290-1300 METER RANGE. IT WILL ALSO BE CLEAR OR MOSTLY SO DURING MOST OF THE PERIOD...WITH THE EXCEPTION OF A PERIOD OF PASSING MID-HIGH CLOUDS IN ASSOCIATION WITH A STRONGLY-SHEARED AND POSITIVELY-TILTED TRAILING SHORTWAVE TROUGH DURING THE DAY WED. WHILE MEDIUM RANGE GUIDANCE DIVERGES WITH RESPECT TO THE EJECTION OF THE AFOREMENTIONED DESERT SW LOW AND ASSOCIATED APPROACH OF THE NEXT FRONTAL SYSTEM...THE OVERALL PATTERN (WITH TIMING DIFFERENCES TO BE WORKED OUT) WOULD RESULT IN A LOW TRACK TO THE NW OF CENTRAL NC AND AN ASSOCIATED WARM FRONTAL PASSAGE IN RETURN FLOW AROUND THE DEPARTING AND MODIFIED ARCTIC HIGH --TIMING MOST LIKELY FRI NIGHT- SAT FAVORED (DUE TO TYPICAL FAST BIAS IN NWP WITH SW CUTOFFS) AS DEPICTED BY THE SLOWER EC AND EC MEAN. IF SHOWERS OCCUR WITH THE PASSAGE OF THE WARM FRONT...MOST LIKELY OVER THE WESTERN PIEDMONT... AN IN-SITU DAMMING EVENT WOULD EVOLVE INTO THE EARLY PART OF THE WEEKEND...IN A PATTERN OTHERWISE SUPPORTIVE OF WARMING TEMPERATURES OVER THE SE CONUS. AS SUCH...TEMPERATURES ON SAT ARE APT TO DISPLAY A WIDER THAN AVG RANGE OVER CENTRAL NC...WITH MARKEDLY COOLER TEMPERATURES OVER THE NW PIEDMONT.
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&& .AVIATION /06Z SUNDAY THROUGH WEDNESDAY/... AS OF 115 AM SUNDAY... AN AREA OF LIGHT RAIN ASSOCIATED WITH WEAK UPPER LEVEL DISTURBANCE WILL BRING SOME SUB-VFR CONDITIONS TO CENTRAL NC TODAY. HOWEVER...GIVEN THE AMOUNT OF DRY AIR STILL IN PLACE OVER THE AREA...THERE REMAIN UNCERTAINTY REGARDING HOW MUCH RAIN WILL FALL AND HOW LOW CEILINGS WILL DROP. WHILE A FEW SPRINKLES ARE POSSIBLE THROUGH 12Z...CEILINGS WILL MOSTLY REMAIN AT OR ABOVE 10K FT. BETTER MOISTURE IS STILL EXPECTED TO SPREAD UP ACROSS THE WESTERN PIEDMONT...WHERE THERE IS A BETTER CHANCE OF MORE STEADY RAIN AND CEILINGS IN THE MVFR OR IFR RANGE IN THE 12Z TO 21Z TIME FRAME. TO THE EAST...WHERE SUPPORT FOR PRECIP IS WEAKER...CONFIDENCE IN MVFR CEILINGS IS LOWER...AND SOME GUIDANCE SUGGESTS CEILINGS MAY REMAIN VFR THROUGH MIDDAY FROM KRDU EASTWARD. EXPECT THE LIGHT RAIN TO THEN BE INTERMITTENT LATER THIS AFTERNOON AS A LULL DEVELOPS...WITH A PERIOD OF VFR CONDITIONS POSSIBLE BEFORE A COLD FRONT PUSHES INTO THE AREA FROM THE NORTH TONIGHT. WINDS WILL BE 5-10KT OUT OF THE SOUTHWEST TODAY...TURNING TO NORTHWESTERLY AND THEN NORTHERLY BEHIND THE FRONT TONIGHT. LOOKING AHEAD: CEILINGS WILL REDEVELOP MONDAY AS STRENGTHENING SOUTHERLY FLOW ATOP THE SOUTHWARD SINKING COLD FRONT LEADS TO WIDESPREAD RAIN. THESE LOW CEILINGS...ALONG WITH SOME PERIODS OF LIGHT RAIN OR DRIZZLE WILL LINGER THROUGH MONDAY NIGHT...WITH CONDITIONS EXPECTED TO IMPROVE TUESDAY && .RAH WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SYNOPSIS...HARTFIELD NEAR TERM...HARTFIELD/BLAES SHORT TERM....WSS LONG TERM...MWS AVIATION...BLS

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