Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 182028 AFDRAH AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 320 PM 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 /THROUGH TONIGHT/... AS OF 920 AM THURSDAY... AN ALREADY LOW AMPLITUDE TROUGH ALOFT OVER THE MID-MISSISSIPPI RIVER VALLEY THIS MORNING WILL CONTINUE TO DEAMPLIFY AS IT SKIRTS ACROSS THE TENNESSEE VALLEY AND VIRGINIA THROUGH TONIGHT. AN AREA OF LIGHT RAIN MOVING ACROSS EASTERN TENNESSEE...FOCUSED NEAR THE H925-H85 FRONTAL ZONE WHERE ISENTROPIC LIFT IS GREATEST...CONTINUES TO GRADUALLY WEAKEN WITH EASTWARD EXTENT. EXPECT THIS TREND TO THE CONTINUE AS THE UPPER DISTURBANCE MOVES OFF TO THE EAST-NORTHEAST AND THE FORCING DECOUPLES. SO NO PRECIP IS EXPECTED OVER CENTRAL NC. EAST OF THE MOUNTAINS...THE AIRMASS REMAINS VERY DRY WITH ALL MOISTURE ABOVE 12K FT. WHILE THERE IS A FAIR AMOUNT OF HIGH LEVEL MOISTURE UPSTREAM...GUIDANCE SUGGESTS THERE WILL PERIODS OF SUN MIXED IN WITH THE HIGH CLOUDS (POTENTIALLY OROGRAPHICALLY ENHANCED AT TIMES THIS MORNING. TEMPS SHOULD GET TO WITHIN A COUPLE DEGREES OF THE CURRENT FORECAST HIGHS OF 49-55. IF ANYTHING...THEY WILL END UP ON THE COOLER SIDE. SKIES WILL BEGIN TO CLEAR OFF BY EARLY FRIDAY MORNING WITH THE PASSAGE OF THE AFOREMENTIONED MID LEVEL TROUGH. GIVEN A RATHER WEAK MSLP GRADIENT ACROSS THE AREA WITH SURFACE HIGH PRESSURE EXTENDING INTO THE AREA FROM THE WEST... WE SHOULD SEE LIGHT AND VARIABLE TO CALM WINDS OVERNIGHT. THIS COMBINED WITH CLEAR SKIES WILL ALLOW US TO COOL QUITE NICELY TONIGHT. EXPECT LOW TEMPS WILL RANGE FROM THE UPPER 20S TO AROUND 30 (WITH MAYBE EVEN A FEW MID 20S) IN THE USUAL RURAL COLD LOCATIONS TO THE LOWER 30S ELSEWHERE. && .SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/... AS OF 255 PM THURSDAY... FRIDAY AND FRIDAY NIGHT... HIGH PRESSURE OVER THE MIDWEST WILL SLOWLY SETTLE EAST ACROSS THE CENTRAL APPALACHIANS AS THE PARENT HIGH MOVES TOWARD EASTERN CANADA. WE SHOULD BE IN A RELATIVE MIN BETWEEN MID/HIGH CLOUDS TODAY AND MOISTURE SPREADING IN AHEAD OF SATURDAY`S SYSTEM. GUIDANCE SUGGESTS DEWPOINTS WILL FALL INTO THE 20S..THOUGH WOULDNT BE SURPRISED TO SEE SOME UPPER TEENS BASED ON REGIONAL OBSERVATIONS. THICKNESSES SUPPORT HIGHS IN THE LOW TO MID 50S. LOWS FRIDAY NIGHT SHOULD BE ABLE TO FALL INTO THE LOW/MID 30S (ESPECIALLY NORTHEAST) PRIOR TO THE ARRIVAL OF MORE ROBUST CLOUD COVER. SATURDAY... THE SHORTWAVE CURRENTLY OVER THE CALIFORNIA BAJA REGION WILL LIFT NORTHEAST ACROSS THE SOUTHERN-CENTRAL US ON FRIDAY AND THEN TOWARD OUR REGION ON FRIDAY NIGHT. MODELS CONTINUE TO TREND TOWARD MORE DEAMPLIFICATION WITH THE WAVE BY THE TIME IT REACHES THE SOUTHERN APPALACHIANS SATURDAY MORNING. ACCORDINGLY...THE SURFACE LOW EXPECTED TO DEVELOP ALONG THE GULF COAST IS MUCH WEAKER AND ESSENTIALLY MOVES EAST OFF THE SOUTHEAST COAST. HIGH PRESSURE OVER EASTERN CANADA WILL EXTEND DOWN THE EASTERN SEABOARD AND INTO CENTRAL NC. WHILE NOT OVERLY STRONG OR FAR ENOUGH SOUTH... THE HIGH IS IN A MORE FAVORABLE POSITION TO SUPPLY DRY AIR INTO NC THAN FORECAST A FEW DAYS AGO. HOWEVER...WITH WEAKER DYNAMIC FORCING...THE DRY AIR WILL ALSO LIMIT PRECIP...AND THE NAM/GFS/ECMWF ARE ALL DRY OR NEARLY DRY. CAN`T HELP BUT CONTINUE TO CUT BACK ON POPS BASED ON RECENT GUIDANCE...BUT GIVEN THAT THE SHORTWAVE WILL BE INTERACTING WITH AN ASSORTMENT OF NORTHERN STREAM WAVES OVER THE NORTHERN PLAINS...A SMALL SHIFT BACK TO THE NORTH WILL BRING MEASURABLE PRECIP INTO CENTRAL NC. SHOULD NOTE THAT A SHIFT BACK TO THE NORTH COULD STILL CREATE SOME PTYPE ISSUES AS PTYPE NOMOGRAMS HOVER AROUND THE WINTRY MIX AND WETBULB PROFILES REMAIN CURIOUSLY CLOSE TO FREEZING IN THE LOWEST 10K FT. THIS IS A FINE LINE AS WELL...AS THE SHIFT WOULD ALSO BRING SOME WARM AIR BACK NORTH. WILL REDUCE POPS TO CHANCE ACROSS THE NORTH AND LOW-END LIKELY (MAINLY BETWEEN 12Z AND 00Z) NEAR THE SC BORDER...WHICH MAY STILL BE TOO HIGH BUT FOLLOWS A CONSERVATIVE TREND. WILL ALSO RAISE HIGHS A FEW DEGREES...THOUGH NORTHEASTERLY FLOW AND THICK CLOUD COVER WOULD STILL KEEP TEMPS IN THE UPPER 40S NORTHWEST TO NEAR 50 SOUTHEAST. SATURDAY NIGHT....ALL ENERGY ASSOCIATED WITH THIS SYSTEM WILL DEPART SATURDAY NIGHT...ALLOWING FOR SOME MID-LEVEL DRYING AND SCATTERING OF CLOUDS. HOWEVER...MODELS ARE QUICK TO BRING MOISTURE BACK TOWARD THE REGION FROM THE SOUTH AS ANOTHER SYSTEM GETS GOING BY LATE SUNDAY. LOWS IN THE MID/UPPER 30S. && .LONG TERM /SUNDAY THROUGH THURSDAY/... AS OF 320 PM WEDNESDAY... ...VERY ACTIVE PATTERN DEVELOPING OVER THE LONG TERM PERIOD... BEHIND THE DEPARTING SOUTHERN STREAM SHORT WAVE TROUGH...SHORT WAVE RIDGING MOVES INTO THE CAROLINAS ON SUNDAY AS HIGH PRESSURE CENTERED OVER THE EASTERN GREAT LAKES AND MID ATLANTIC EXTENDS INTO THE REGION. THE NEXT SOUTHERN STREAM WAVE WHICH MOVES ONSHORE ACROSS CA ON FRIDAY REACHES THE WESTERN GULF OF MEXICO ON SUNDAY NIGHT. LOOK FOR INCREASING AND THICKENING CLOUDS ON SUNDAY WITH HIGHS IN THE UPPER 40S TO LOWER 50S. PRONOUNCED ISENTROPIC LIFT WHICH DEVELOPS AHEAD OF THIS SYSTEM SUNDAY EVENING ACROSS THE SOUTHEAST...SPREADS INTO THE CAROLINAS AFTER MIDNIGHT AND PERSISTS FOR MUCH OF MONDAY MORNING. EXPECT A BROAD AREA OF RAIN WITH THIS SYSTEM AND HAVE INCREASED POPS. TEMPERATURES SUNDAY NIGHT WILL FALL INTO THE MID 30S ACROSS THE NORTHERN PIEDMONT AND THE UPPER 30S ELSEWHERE...THESE TEMPERATURES ALONG WITH A PRONOUNCED WARM NOSE AND WET BULB ZERO VALUES HOLDING ABOVE FREEZING SHOULD PRECLUDE AND PTYPE ISSUES. A SURFACE LOW DEVELOPING IN THE NORTHEAST GULF OF MEXICO WILL MOVE NORTHEAST ALONG THE CAROLINA COAST MONDAY AFTERNOON AND FURTHER OFFSHORE MONDAY NIGHT. PRECIPITATION SHOULD BECOME MUCH MORE SCATTERED AND LIGHT MONDAY NIGHT INTO TUESDAY AS THE STRONGEST FORCING ABATES BUT WIDESPREAD CLOUDINESS AND SMALL RAIN CHANCES WILL PERSIST AS A MOIST AIR MASS LINGERS ACROSS THE AREA. AN IMPRESSIVE NORTHERN STREAM SHORTWAVE TROUGH DIVES INTO THE MIDWEST ON TUESDAY AND TUESDAY NIGHT CARVING OUT A HIGH AMPLITUDE LONG WAVE TROUGH DEVELOPS ACROSS THE MISSISSIPPI VALLEY TUESDAY NIGHT AND BECOMES NEGATIVELY TILTED ON WEDNESDAY AS IT BROADENS AND REACHES THE EAST COAST. BOTH THE DETERMINISTIC GFS AND ECMWF ARE VERY SIMILAR WITH THIS PATTERN. AT THE SURFACE...A STRONG SURFACE CYCLONE DEVELOPS ACROSS THE GREAT LAKES ON TUESDAY INTO WEDNESDAY WITH A STRONG COLD FRONT PROGGED TO MOVE ACROSS THE CAROLINA ON TUESDAY NIGHT AND EARLY WEDNESDAY. THIS PATTERN WILL SUPPORT STRONG AND GUSTY WINDS AHEAD OF AND BEHIND THE COLD FRONT WITH A PERIOD OF WIDESPREAD RAIN AND PERHAPS A FEW THUNDERSTORMS ASSOCIATED WITH THE FRONT. THE DEEP LAYER FLOW ATTENDANT TO THE FRONT IS VERY STRONG WITH A SOUTHWEST WIND OF AROUND 40KTS PROGGED AT 1 KFT NEAR RDU ON WEDNESDAY MORNING AND A WESTERLY WIND OF 40 KTS FORECAST AT RDU AT 1.5 KFT WEDNESDAY NIGHT. BOUNDARY LAYER INSTABILITY IS LIMITED BUT BOTH THE GFS AND THE ECMWF SUGGEST A NARROW WINDOW FOR BOUNDARY LAYER DESTABILIZATION AHEAD OF THE FRONT. THE IMPRESSIVE WIND FIELD COMBINED WITH 700-500 MB LAPSE RATES APPROACHING 7 AS NOTED BY THE GFS...LEADS TO AN ELEVATED SHERB VALUE THAT SUGGESTS SOME HIGHER IMPACT HSLC CONVECTION IS POSSIBLE. THE PRECIPITATION QUICKLY ENDS BEHIND THE FRONT WEDNESDAY AFTERNOON WITH DRY WEATHER EXPECTED FOR CHRISTMAS DAY. HIGHS ON TUESDAY WILL RANGE FROM THE LOWER 50S IN THE TRIAD TO NEAR 60 ACROSS THE SOUTHERN COASTAL PLAIN. TEMPERATURES SHOULDN`T FALL MUCH TUESDAY NIGHT WITH THE CLOUDS AND STRONG SOUTHERLY FLOW. RAISED MINS ABOVE GUIDANCE TO THE MID 40S TO AROUND 50. HIGHS WEDNESDAY WILL RANGE FROM 50 IN THE TRIAD TO NEAR 60 IN THE SOUTHEAST WITH CHILLIER MAXES IN THE MID 40S TO NEAR 50 EXPECTED FOR CHRISTMAS DAY. -BLAES && .AVIATION /18Z THURSDAY THROUGH TUESDAY/... AS OF 1240 PM THURSDAY... VFR CONDITIONS WILL CONTINUE THIS AFTERNOON THROUGH THE END OF THE TAF PERIOD. A WEEK DISTURBANCE WILL CROSS VIRGINIA THIS EVENING...BUT THE ONLY IMPACT OVER NC WILL BE INCREASING MID/HIGH CLOUDS....MAINLY ACROSS THE NORTHERN HALF OF THE AREA. WINDS WILL BE LIGHT OUR OF THE NORTH-NORTHWEST...INCREASING TO 4-6KT DURING THE DAY FRIDAY. OUTLOOK...A LOW PRESSURE SYSTEM MOVING FROM THE GULF COAST TO THE CAROLINA COAST FRIDAY NIGHT AND SATURDAY WILL BRING A ROUND OF SUB- VFR CONDITIONS TO CENTRAL NC ON SATURDAY AND SATURDAY NIGHT. HOWEVER....THIS SYSTEM MAY NOT BE AS SIGNIFICANT AS ONCE EXPECTED. 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...BSD/BLS SHORT TERM...BLS LONG TERM...BLAES AVIATION...BLS

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