Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 152021 AFDRAH AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 315 PM EST MON DEC 15 2014 .SYNOPSIS...HIGH PRESSURE OVER THE AREA WILL SLIDE OFFSHORE THIS EVENING IN ADVANCE OF A COLD FRONTAL PASSAGE FROM THE WEST ON TUESDAY. HIGH PRESSURE WILL THEN BUILD INTO THE AREA THROUGH FRIDAY. && .NEAR TERM /THROUGH TONIGHT/... AS OF 220 PM MONDAY... DEEP LAYER RIDGING WILL SHIFT EAST TONIGHT AHEAD OF THE LOW PRESSURE SYSTEM CURRENTLY LIFTING THROUGH THE MID-MISSISSIPPI VALLEY REGION.A FEW AREAS OF VERY THIN CIRRUS...OUT AHEAD OF THE MORE DENSE OVERCAST ASSOCIATED WITH THE SYSTEM...WILL DRIFT OVERHEAD THIS AFTERNOON AND EVENING. HOWEVER...RADIATIONAL COOLING WILL STILL BE RATHER STRONG...ESPECIALLY IN THE EAST WHERE WINDS WILL BE LIGHTER AND DECOUPLING MORE FAVORABLE. EXPECT TEMPS TO DROP DRAMATICALLY AFTER SUNSET AND THEN LEVEL OFF AS THE NIGHT WEARS ON AS MID/HIGH CLOUDS MOVE INTO WESTERN NC. ELEVATED DEWPOINTS AND LIGHT WINDS MAY LEAD TO SOME PATCHES OF FOG IN THE EAST...WHICH IS SUPPORTED BY SREF PROBABILITIES AND NAM/GFS BUFR SOUNDINGS AT KFAY/KRWI. LOWS WILL RANGE FROM THE UPPER 30S WEST TO LOWER 30S IN THE EAST. && .SHORT TERM /TUESDAY AND THURSDAY NIGHT/... AS OF 305 PM MONDAY... THE CLOSED LOW AND OCCLUDING LOW PRESSURE SYSTEM CURRENTLY OVER THE MID-MISSISSIPPI VALLEY REGION WILL CONTINUE TO LIFT NORTHEAST TOWARD THE GREAT LAKES TONIGHT. MEANWHILE...EXPECT TO SEE THE CURRENT BAND OF SHOWERS STRETCHING FROM NASHVILLE TO BIRMINGHAM WEAKEN THIS EVENING AS IT MOVES AWAY FROM THE BETTER FORCING...THEN RE-INTENSIFY LATER TONIGHT AND INTO TUESDAY AS A TRAILING SPEED MAX/SHEAR VORTICITY ROUND THE BASE OF THE TROUGH AND LIFT ACROSS THE SOUTHERN CENTRAL APPALACHIANS. THE WARM CONVEYOR BELT ASSOCIATED WITH THIS SYSTEM IS QUITE NARROW AND MOISTURE TRANSPORT FROM THE GULF IS LIMITED. RELYING ON MAINLY UPPER FORCING FROM DCVA AND 60-100M HEIGHT FALLS...THERE MAY BE JUST ENOUGH LOW-LEVEL WARM/MOIST ADVECTION TO COINCIDE WITH 6-6.5 C/KM MID-LEVEL LAPSE RATES AND YIELD A FEW HUNDRED J/KG OF ELEVATED CAPE. NOT VERY CONFIDENT IN HOW MUCH THUNDER THERE WOULD BE GIVEN SOME OVERALL DOWNWARD TREND ON COVERAGE NOTED IN RECENT CAM FORECASTS. NO SEVERE WEATHER IS EXPECTED...SO PREFER TO LEAVE THUNDER OUT OF THE CURRENT FORECAST AND RE-EVALUATE THE MESOSCALE LATER. FORCING SHOULD BE STRONGER TO THE NORTH...SO WE WILL HAVE THE GREATEST POPS (50-60%) FROM THE TRIANGLE TO THE NORTHEAST WHERE FORCING...MOISTURE AND DIURNAL TIME SHOULD BE MAXIMIZED. QPF WILL BE LIGHT...MOSTLY BELOW A TENTH OF AN INCH FOR MOST LOCATIONS. HIGHS IN THE MID 50S NORTH TOP AROUND 60 IN THE SOUTH. THE ACTUAL COLD FRONT WILL TRAIL THE PRE-FRONTAL SHOWERS BY A FEW HOURS...CROSSING THE AREA DURING THE LATER AFTERNOON AND EVENING HOURS. DRIER AIR ALOFT AND WEAKER FORCING SHOULD KEEP THE FROPA DRY. LITTLE AIRMASS CHANGE IS EXPECTED IMMEDIATELY BEHIND THE WEAK FRONT...AND LOWS WILL ONLY FALL INTO THE MID/UPPER 30S. -BLS WEDNESDAY THROUGH THURSDAY NIGHT SHOULD FEATURE FAIR WEATHER AS THE UPPER FLOW BECOMES ZONAL BEHIND THE UPPER TROUGH THAT MOVES OFF THE COAST TUESDAY NIGHT. HIGH PRESSURE INITIALLY CENTERED OVER SOUTHERN MANITOBA WEDNESDAY MORNING SLIPS INTO THE EASTERN GREAT LAKES AND OHIO VALLEY THURSDAY NIGHT. THE LOW LEVEL FLOW IS GENERALLY LIGHT NORTHWESTERLY THROUGH THE PERIOD WITH MODEST COLD ADVECTION ON WEDNESDAY FOLLOWED BY A SECONDARY SURGE OF COOLER AIR THAT MOVES INTO THE REGION ON THURSDAY. A VERY DRY AIR MASS PERSISTS IN PLACE THROUGH THE PERIOD WITH PW VALUES RANGING AROUND A THIRD OF AN INCH. THE RESULT SHOULD BE A GOOD DEAL OF SUNSHINE BOTH DAYS WITH SOME INCREASING CIRRUS CLOUDS ON THURSDAY. HIGHS ON WEDNESDAY WIL RANGE IN THE MID 50S TO NEAR 60 WITH COOLER MAXES ON THURSDAY WITH HIGHS IN THE LOWER TO MID 50S. LOWS WILL RANGE IN THE LOWER 30S BOTH NIGHTS. -BLAES && .LONG TERM /FRIDAY THROUGH MONDAY/... AS OF 315 PM MONDAY... ...ACTIVE WEATHER SETTING UP FOR THE WEEKEND... A SHORT WAVE TROUGH MOVING ON THE CA COAST LATE TUESDAY MAKES IT TO SOUTHEASTERN TX BY FRIDAY MORNING AND INTO MS/AL BY FRIDAY NIGHT. A MODEST 1025 MB SURFACE HIGH PRESSURE SYSTEM CENTERED ACROSS THE NORTHERN GREAT LAKES DURING THIS PERIOD ONLY WEAKLY EXTENDS INTO THE MID ATLANTIC. AS THE RIDGE AXIS ALOFT MOVES EAST...EXPECT HIGH AND MID LEVEL CLOUDINESS TO INCREASE ON FRIDAY AS LIGHT PRECIPITATION BREAKS OUT OVER THE DEEP SOUTH. HIGHS ON FRIDAY WILL RANGE FROM AROUND 50 NEAR THE VA BORDER TO AROUND 55 NEAR THE SC BORDER. THE SOUTHERN STREAM SHORT WAVE AMPLIFIES ON FRIDAY NIGHT INTO SATURDAY AS A STRAIGHT SUBTROPICAL JET STRENGTHENS WITH WINDS GREATER THAN 130KTS. CYCLOGENESIS SHOULD COMMENCE ACROSS THE NORTHERN GULF OF MEXICO FRIDAY NIGHT WITH THE SURFACE LOW LIKELY MAKING SOME TURN UP THE SOUTHEAST COAST SATURDAY. THERE IS STILL A LARGE SPREAD IN SOLUTIONS FOR THIS SCENARIO WITH A KEY INGREDIENT BEING THE AMOUNT OF PHASING BETWEEN THE SOUTHERN STREAM ENERGY AND THE POLAR JET. THE OPERATIONAL EC DEVELOPS A STRONG SURFACE LOW THAT MOVES UP A GOOD PORTION OF THE EAST COAST. THE OPERATIONAL GFS HAS A MUCH FLATTER SOLUTION WITH A SET OF MUCH WEAKER SURFACE LOWS AND LIGHT PRECIPITATION. THE PARALLEL VERSION OF THE GFS TENDS TO LOOK MORE LIKE THE EC THAN THE CURRENT GFS. STILL SOME OF THE VARIOUS ENSEMBLE MEMBERS SHOW A MORE SUPPRESSED SYSTEM WITH THE LOW MOVING MORE EASTWARD OFF THE COAST. ONE DETAIL THAT IS CONSISTENT WITH ALL OF THE VARIOUS SCENARIOS IS A LACK OF A SOURCE OF COLD DRY AIR TO EASILY SUPPORT WINTRY PRECIPITATION. AT THIS POINT WITH THE SOUTHERN STREAM SYSTEM YET TO REACH THE PACIFIC COAST AND WITH LIMITED CONFIDENCE WE ARE HESITANT TO BUY INTO ANY GIVEN SOLUTION. IT IS WORTH NOTING THAT THE EC COULD SUPPORT SOME WET SNOW ACROSS THE INTERIOR PIEDMONT BUT THIS SCENARIO IS AN OUTLIER AT THIS POINT. WITH BOUNDARY LAYER TEMPERATURES JUST ABOVE FREEZING AND THE NEED FOR DYNAMICAL COOLING TO SUFFICIENTLY COOL THE PROFILE...THIS APPEARS UNLIKELY AT THIS TIME. IT IS MUCH MORE LIKELY THAT A CHILLY RAIN WITH TEMPS IN THE 30S. IT ALSO APPEARS THAT ANY THUNDERSTORM THREAT WOULD BE SUPPRESSED TO THE IMMEDIATE COASTAL AREAS. HIGHS ON SATURDAY WILL BE CHILLY AND WILL RANGE IN THE LOWER 40S ACROSS THE TRIAD TO THE LOWER 50S ACROSS THE SOUTHERN COASTAL PLAIN. LOWS SATURDAY NIGHT SHOULD RANGE JUST ABOVE FREEZING IN THE MID 30S. PRECIPITATION SHOULD MOST LIKELY END FROM SOUTHWEST TO NORTHEAST ON SUNDAY WITH HIGHS ONLY IN THE MID TO UPPER 40S. DRIER WEATHER RETURNS FOR MONDAY WITH SOME SHORT WAVE RIDGING ALOFT AND WEAK HIGH PRESSURE OVERHEAD. HIGHS WILL RANGE IN THE MID 40S TO NEAR 50. -BLAES && .AVIATION /18Z MONDAY THROUGH SATURDAY/... AS OF 1255 PM MONDAY... HIGH PRESSURE CONTINUES TO DOMINATE THIS AFTERNOON WITH VFR CONDITIONS AND A LIGHT WIND THAT WILL SLOWLY BECOME SOUTHERLY THIS EVENING. MOISTURE WILL BEGIN TO INCREASE MORE READILY TONIGHT AHEAD OF A FRONTAL SYSTEM MOVING THROUGH THE TN VALLEY. HOWEVER... CONFIDENCE IN VFR CONDITIONS IS HIGH THROUGH 06Z. AFTER 06Z...SOME MODELS INDICATE VERY SHALLOW MOISTURE ACROSS THE COASTAL PLAIN THAT MAY RESULT IN PATCHY FOG/LOW CLOUDS PRIOR TO 12Z. A BAND OF SHOWERS WILL THEN MOVE INTO THE WESTERN PIEDMONT AFTER 12Z...WHICH SHOULD RESULT IN MAINLY MVFR CEILINGS AS IT MOVES WEST TO EAST ACROSS THE AREA BETWEEN 12Z AND 21Z. OUTLOOK...ONCE A BAND OF SHOWERS MOVES EAST OF THE AREA TUESDAY AFTERNOON...A COLD FRONT WILL PUSH EAST THROUGH CENTRAL NC...WITH DRIER AIR AND VFR CONDITIONS RETURNING TUESDAY NIGHT. THE NEXT CHANCE OF WIDESPREAD SUB-VFR CONDITIONS WILL BE DURING THE UPCOMING WEEKEND. && .RAH WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SYNOPSIS...CBL NEAR TERM...BLS SHORT TERM...BLS/BLAES LONG TERM...BLAES AVIATION...BLS

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