Issued by NWS Raleigh/Durham, NC
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-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --000 FXUS62 KRAH 211147 AFDRAH AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 645 AM EST SAT NOV 21 2009 .SYNOPSIS... HIGH PRESSURE TO OUR NORTH WILL BRING NORTHEAST FLOW TO NORTH CAROLINA INTO SUNDAY. THE NEXT STORM SYSTEM WILL MOVE INTO THE REGION FROM THE SOUTHWEST AND BRING CLOUDS AND RAIN FOR SUNDAY AND SUNDAY NIGHT. && .NEAR TERM /THROUGH TONIGHT/... AS OF 315 AM... SURFACE HIGH PRESSURE NORTH OF NORTH CAROLINA WILL KEEP NORTHEAST FLOW OVER CENTRAL NORTH CAROLINA INTO TONIGHT. EXPECTING ENOUGH SUNSHINE THROUGH THE MID AND HIGH CLOUDS TO BRING TEMPERATURES TO JUST SHORT OF FULL SUN VALUES FOR NORTHEAST FLOW. AFTERNOON HIGHS UPPER 50S TO LOWER 60S NORTH... AND 62 TO 65 SOUTH. AN UPPER LEVEL SYSTEM DROPPING INTO TEXAS IS GENERATING SURFACE LOW PRESSURE IN THE NORTHWEST GULF OF MEXICO AT THIS TIME. BOTH THESE LOWS FORECAST TO ROUGHLY MOVE EAST NORTHEAST INTO SUNDAY. THE NAM 700 MILLIBAR PRESSURE FIELD IS PERHAPS TOO WEAK AND PREFER THE STRONGER MID LEVEL FRONT OF THE CANADIAN AND GFS MODELS. THE GFS AND CANADIAN DEPICT A 700 MILLIBAR WARM FRONT... BARELY LIFTING NORTH INTO NORTH CAROLINA BY SUNRISE SUNDAY. ISENTROPIC LIFT IS QUITE WEAK BELOW THIS FRONT SO ANY RAINFALL SHOULD BE LATE... WELL UNDER ONE TENTH INCH AND NOT STRAY TOO FAR NORTH BY SUNRISE. INCREASED LOWER CLOUDINESS AND MORE OF A BREEZE THAN FRIDAY NIGHT SHOULD YIELD LOW TEMPERATURES IN THE MID TO UPPER 40S. && .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... AS OF 315 AM... A 700 MILLIBAR WARM FRONT NEAR THE SOUTH CAROLINA BORDER SUNRISE SUNDAY WILL EXIT TO THE NORTH AFTER SUNSET. GIVEN THE EXPECTED NORTHWARD PROGRESS OF THE FRONT AND MODERATE TO WEAK ISENTROPIC LIFT NEAR 850 MILLIBARS... AT LEAST THE SOUTHWEST THREE QUARTERS SHOULD SEE MEASURABLE RAIN BEFORE SUNSET SUNDAY. AS THE UPPER SYSTEM LIFTS NORTHEAST THROUGH THE MOUNTAINS SUNDAY NIGHT... LOW PRESSURE DEVELOPMENT OFF THE CAROLINAS OCCURS AND BECOMES THE DOMINATE SURFACE LOW. GOOD MID LEVEL ISENTROPIC LIFT IN THE MID LEVELS SHOULD KEEP RAIN CHANCES HIGH INTO SUNDAY NIGHT. MOST MODELS DAMP OUT THE PARENT LOW PRESSURE TROUGH AS IT MOVES NORTHEAST AND KEEP THE SURFACE LOW WEAKER THAN THE CANADIAN AS IT APPROACHES CAPE HATTERAS. RAINFALL PERHAPS TWO TENTHS INCH NORTHWEST TO FOUR TENTHS SOUTHEAST SUNDAY NIGHT. WINDS WEAKEN LATE SUNDAY NIGHT INTO MONDAY AND HEAVIER PRECIPITATION MAY MISS US TO THE EAST MONDAY. PRECIPITATION MAY LINGER LONGEST IN THE NORTHEAST WITH THE EXITING SURFACE LOW BUT RAINFALL AMOUNTS OF ONE TENTH INCH OR LESS EAST... AND MUCH LESS TO THE WEST. OF COURSE IF THE LOW TRACKS CLOSER TO THE COAST... THESE AMOUNTS ARE UNDERDONE. ONLY A SLIGHT RAIN CHANCE IN THE NORTHEAST WITH THE EXITING LOW MONDAY NIGHT. OVERNIGHT LOWS 45 TO 50 SUNDAY NIGHT... AND MOSTLY MID 40S TUESDAY NIGHT. SUNDAY HIGH TEMPERATURES NEAR 50 NORTHWEST TO UPPER 50S SOUTHEAST. IF A COLD AIR MASS DAMMING WEDGE SETS UP AS EXPECTED MONDAY...A LOW OVERCAST WITH LIGHT RAIN OR DRIZZLE POSSIBLE. THE CURRENT FORECAST TEN DEGREE DIURNAL RISE TO MONDAY AFTERNOON WOULD NEED TO BE DROPPED TO THREE TO SIX DEGREES IF CLOUDS ARE ABLE TO PERSIST INTO EARLY MONDAY EVENING. && .LONG TERM /TUESDAY THROUGH FRIDAY/... AS OF 315 AM FRIDAY... ANY LIGHT PRECIP LINGERING INTO MONDAY NIGHT SHOULD BE GONE BY TUESDAY MORNING...AS SOUTHWESTERLY MID LEVEL FLOW USHERS IN DRIER AIR ABOVE THE REMAINING CAD AIRMASS. THE POSSIBILITY LINGERING LOW CLOUDS COULD COMPLICATE THE TEMPERATURE FORECAST ON TUESDAY AS SOME MOISTURE REMAINS BELOW A SUBSIDENCE INVERSION BENEATH DEVELOPING SHORTWAVE RIDGING ALOFT. WILL STICK WITH MOS VALUES WITH HIGHS 64-70. THE FORECAST GETS QUITE A BIT MORE COMPLICATED BEYOND TUESDAY...WITH THE HANDLING OF A SHORTWAVE EXPECTED TO ENTER THE PACIFIC NORTHWEST ON MONDAY AND EMERGE OVER THE NORTH CENTRAL PLAINS BY WED. THE GENERAL TREND IS FOR THE DEVELOPMENT OF A HIGH AMPLITUDE LONGWAVE TROUGH BY THE END OF THE WEEK...AND WHILE BOTH THE GFS AND ECMWF RESOLVE SUCH A SOLUTION...THEY CURRENTLY GET THE SIMILAR ANSWER THROUGH SOMEWHAT DIFFERENT SCENARIOS. THE GFS SENDS THE AFOREMENTIONED SHORTWAVE ACROSS THE UPPER MIDWEST AND THE LIFTS THE SYSTEM INTO ONTARIO BY THURSDAY...PUSHING A DRY COLD FRONT INTO CENTRAL NC WED NIGHT. THIS IS FOLLOWED BY A SECOND SHORTWAVE THAT DIVES SOUTH THROUGH THE MIDWEST...BUT WITH NO MORE TO WORK WITH....POINTS EAST OF THE MOUNTAINS STAY DRY EVEN AS A SURFACE WAVE IS INDUCED OFFSHORE THURSDAY MORNING. THE ECMWF ON THE OTHER HAND IS MUCH SLOWER WITH THE FIRST SHORTWAVE...ALLOWING MORE MOISTURE TO BE PULLED NORTH OVER A PERSISTENT SURFACE RIDGE ALONG THE EASTERN SEABOARD. CYCLOGENESIS ALONG THE COAST SUBSEQUENTLY OCCURS CLOSER TO THE COAST THURSDAY MORNING WITH LESS EASTWARD PROGRESSION OF THE SURFACE FRONT...AND THUS THE ECMWF PRODUCES PRECIP OVER THE EASTERN THIRD OF NC. GIVEN THESE CONTINUED MODEL INCONSISTENCIES...FEEL THE BEST COURSE OF ACTION IS TO SHOW BELOW NORMAL TEMPS FOR THE END OF THE WEEK...CONTINUING WITH HIGHER SLIGHT CHANCE POPS IN THE EAST UNTIL SOME SORT OF MODEL CONVERGENCE BEGINS. && .AVIATION /12Z SATURDAY THROUGH THURSDAY/...-- Changed Discussion --AS OF 645 AM SATURDAY... WITH MODELS TRENDING SLOWER ON THE ARRIVAL OF A STORM SYSTEM CURRENTLY OVER THE WESTERN GULF COAST...VFR CONDITIONS WILL PREVAIL THROUGH THE VALID TAF FORECAST. SCT TO BKN CIRRUS AND EVENTUALLY CIRROSTRATUS WILL DRIFT ACROSS THE REGION TODAY AND INTO TONIGHT...ALL AHEAD OF A STORM SYSTEM SET TO IMPACT CENTRAL NC FROM THE SOUTHWEST DURING THE DAY SUNDAY. AT THE SURFACE...HIGH PRESSURE TO OUR NORTH WILL SETTLE OVER NEW ENGLAND...EXTENDING A RIDGE OF DRY AIR DOWN THE EAST COAST....AND PROVIDING A LIGHT NE WIND. AS THE STORM SYSTEM APPROACHES FROM THE SOUTHWEST SUNDAY MORNING CEILINGS WILL EVENTUALLY LOWER TO MVFR OR IFR LEVELS SUNDAY THROUGH MONDAY....WITH PERIODS OF RAIN POSSIBLE AT ALL TAF SITES. AT LEAST MVFR CEILINGS ARE EXPECTED TO PERSIST THROUGH LATE MONDAY...BEFORE A COASTAL LOW MOVES AWAY FROM NC AND PULLS DRIER AIR INTO THE AREA. CONDITIONS ARE EXPECTED TO REMAIN VFR FOR THE MOST THROUGH THE BALANCE OF THE WEEK...ALTHOUGH THERE IS THE POTENTIAL FOR ANOTHER WEATHER SYSTEM TO EFFECT MAINLY EASTERN TERMINALS BY THURSDAY.-- End Changed Discussion --&& .RAH WATCHES/WARNINGS/ADVISORIES... NONE. && $$ LONG TERM...SMITH AVIATION...SMITH
