706 FXUS62 KRAH 021916 AFDRAH AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 215 PM EST TUE DEC 2 2008 .SYNOPSIS... HIGH PRESSURE WILL BUILD INTO THE REGION FROM THE SOUTHWEST TONIGHT... BRINGING QUIET WEATHER AND MODERATING TEMPERATURES THROUGH WEDNESDAY NIGHT. A COLD FRONT EXPECTED TO SWEEP THROUGH LATE THURSDAY WILL USHER IN ANOTHER SHOT OF COLDER AIR FOR FRIDAY INTO THE WEEKEND. && .NEAR TERM /THROUGH WEDNESDAY/... AS OF 125 PM TUESDAY... TONIGHT: CURRENT PATCH OF CUMULUS AND STRATOCUMULUS SHOULD DISSIPATE WITH LOSS OF HEATING AFTER SUNSET. A DRY... UPSTREAM SHORTWAVE TROUGH ALOFT -- CENTERED OVER SOUTHERN OHIO AT 18Z -- WILL DIG SOUTHEASTWARD ACROSS CENTRAL NC THIS EVENING. DEEP LAYER SUBSIDENCE BEHIND THE TROUGH WILL FURTHER DRY THE COLUMN (PRECIPITABLE WATER VALUES FORECAST TO DIP BELOW A QUARTER INCH) AND HELP BUILD THE SURFACE HIGH OVER THE GULF COAST THIS AFTERNOON NORTHEASTWARD OVER THE CAROLINAS TONIGHT. THUS IDEAL CLEAR AND CALM RADIATIONAL COOLING CONDITIONS ARE EXPECTED OVERNIGHT... WITH LOW TEMPERATURES RANGING FROM 22 TO 27 DEGREES. WEDNESDAY: SHORTWAVE RIDGING ALOFT AND ASSOCIATED HIGH PRESSURE AT THE SURFACE WILL SHIFT OFFSHORE... BUT CLEAR... STABLE... AND DRY CONDITIONS (WITH A PRONOUNCED SUBSIDENT INVERSION CENTERED AROUND H8) ARE EXPECTED TO REMAIN. THERE MAY... HOWEVER... BE AN INCREASE IN CIRRUS FROM THE SOUTHWEST LATE IN THE AFTERNOON AND INTO WED EVENING... AS BACKING FLOW ALOFT TRANSPORTS HIGH LEVEL MOISTURE INTO OUR AREA AHEAD OF AN UPSTREAM TROUGH ALOFT. RETURN FLOW AROUND THE LOW LEVEL RIDGE WILL RESULT IN A MODERATING TREND... WITH AFTERNOON FORECAST LOW LEVEL THICKNESSES IN THE 1315 TO 1325 METER RANGE. CORRESPONDING HIGH TEMPERATURES OF 53 TO 57 DEGREES EXPECTED. && .SHORT TERM /WEDNESDAY NIGHT THROUGH FRIDAY/... AS OF 200 PM TUESDAY... FOR WEDNESDAY NIGHT: THE SURFACE RIDGE WILL SHIFT EAST OF THE AREA... IN ADVANCE OF A COLD FRONT APPROACHING FROM THE W THAT IS ASSOCIATED WITH BROAD TROUGHING DIGGING OVER THE NORTHERN U.S. BOTH LOW AND MID LEVEL FLOW INCREASES FROM THE SW OVERNIGHT... PRODUCING A STRONG SURFACE BASED INVERSION AS 850 MB TEMPS WARM ABOVE +8C BY THURSDAY MORNING. HOWEVER DESPITE THE LOW LEVEL FLOW OUT OF THE GULF... DEEP MOISTURE IS NONEXISTENT AS THE PRECURSOR AIR MASS OVER THE GULF IS QUITE DRY THROUGH A DEEP LAYER. WE SHOULD SEE INCREASING HIGH CLOUDS AS THE COLD FRONT... IN CONJUCTION WITH SHEARING MID LEVEL SHORTWAVE ENERGY... MOVES IN FROM THE WEST... AND THE SURFACE INVERSION MAY BE STRONG ENOUGH TO LEAD TO AREAS OF ADVECTION FOG BY DAYBREAK. LOWS 32-36. FOR THURSDAY THROUGH FRIDAY: THE 12Z NAM AND GFS AGREE WELL ON A SURFACE FRONTAL PASSAGE DURING THE LATE AFTERNOON THROUGH EVENING... AND PRECIP CHANCES AHEAD OF THE FRONT ARE STILL LOOKING RATHER SMALL. A POTENT (BUT DIMINISHING) LOW LEVEL JET OF 30-40 KTS SHIFTS INTO THE CWA... BUT THE LOW LEVEL WINDS BECOME WESTERLY AHEAD OF THE FRONT RESULTING IN DOWNSLOPE DRYING... AND FORECAST LOW LEVEL MOISTURE TRANSPORT IS MEAGER AND DECREASING AS THE FRONT NEARS. THE MID LEVEL SHORTWAVE TROUGH HOLDS WELL TO OUR NW AND SHEARS OUT... THE UPPER JET CORE REMAINS WELL TO OUR NORTH... AND THE COLUMN REMAINS PRETTY STABLE... WHICH ALL RESULT IN LIMITED LIFT MECHANISMS. WILL HOLD POPS AT NO BETTER THAN 18% THURSDAY AFTERNOON. THICKNESSES CLIMB ABOVE SEASONAL NORMALS... BUT MUCH OF THIS IS REALIZED IN THE 925-850 MB LAYER... RATHER THAN AT THE SURFACE. NEVERTHELESS THERE SHOULD BE ENOUGH HEATING TO ALLOW US TO HIT HIGHS IN THE MID 50S NW TO MID 60S SE. EXPECT CLEARING SKIES AND A COOLING AND FURTHER-STABILIZING COLUMN THURSDAY NIGHT. WITH THICKNESSES PLUNGING... HAVE TRIMMED A COUPLE OF DEGREES OFF LOWS... GOING AROUND 28-36. SPRAWLING POLAR-SOURCE HIGH PRESSURE BUILDS IN FROM THE WEST... AND JUST A FEW HIGH CLOUDS ARE EXPECTED UNDER BROAD CYCLONIC FLOW ALOFT. HIGHS 46-54 ARE IN LINE WITH GUIDANCE TRENDS AND LOW LEVEL THICKNESSES. -GIH && .LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/... AS OF 200 PM TUESDAY... FOR FRIDAY NIGHT/SATURDAY: HIGH PRESSURE AT THE SURFACE SPANS THE TN VALLEY AND MIDATLANTIC REGION FRIDAY NIGHT... BUT HIGH CLOUDS SHOULD BE ON THE INCREASE AS THE AXIS OF THE LONGWAVE TROUGH APPROACHES FROM THE WEST. BY MIDDAY SATURDAY BOTH THE GFS AND ECMWF SHOW AT LEAST A 130 KT JET DIVING IN FROM THE NW... THE LEFT EXIT REGION OF WHICH PASSES OVER CENTRAL NC AS A CLIPPER COLD FRONT APPROACHES FROM THE NW. THIS POSSIBLE JET-INDUCED FORCING FOR ASCENT ALONG WITH INCOMING MID LEVEL POSITIVE VORTICITY ADVECTION SHOULD BE ENOUGH TO PRODUCE SIGNIFICANT MID AND HIGH CLOUDS FOR SATURDAY AFTERNOON INTO EARLY EVENING. FORECAST SOUNDINGS DO SHOW A RATHER DEEP MOIST LAYER EXTENDING FROM AROUND 750 MB UP THROUGH 400 MB PARTICULARLY ACROSS THE NORTH... AND THE VERTICAL WET BULB PROFILE IN THIS AREA IS ALL AT OR BELOW FREEZING. BUT THESE SOUNDINGS ALSO INDICATE A HIGH CLOUD BASE (OVER 7000 FT) SO EVEN WITH DEPARTING HIGH PRESSURE AT THE SURFACE... THE SUBCLOUD LAYER IS LIKELY TO BE TOO DRY FOR ANY PRECIPITATION GIVEN THE INDICATED FORCING MECHANISMS. BUT... IF THE MODELS TREND EVEN MORE MOIST AND/OR DEVELOP STRONGER LIFT IN LATER RUNS... A FEW FLURRIES MAY NEED TO BE ADDED OVER THE NORTHERN FORECAST AREA ON SATURDAY. EXPECT LOWS OF 26-32 AND HIGHS OF 45-53... IN LINE WITH PROJECTED THICKNESSES THAT ARE 15-20 M BELOW NORMAL AND FACTORING IN THE CLOUDS AND REDUCED INSOLATION. FOR SATURDAY NIGHT/SUNDAY: THE LONGWAVE TROUGH AXIS SHIFTS OFFSHORE SATURDAY NIGHT AS THE SURFACE CLIPPER FRONT DROPS THROUGH... AND DECREASING CLOUD COVER IS EXPECTED. AS SURFACE PRESSURES LOWER OVER THE INTERIOR NORTHEAST STATES WHILE ARCTIC HIGH PRESSURE DROPS INTO THE MID MISSISSIPPI VALLEY... THE RESULTANT MSLP GRADIENT SHOULD KEEP WINDS UP A BIT SATURDAY NIGHT... AND WITH GOOD MIXING ON SUNDAY... IT COULD BE RATHER BLUSTERY. WITH TEMPS LIKELY TO TOP OUT ONLY IN THE UPPER 40S TO LOWER 50S SUNDAY... WIND CHILLS MAY STRUGGLE TO GET OUT OF THE 30S IN SOME SPOTS. FOR SUNDAY NIGHT THROUGH TUESDAY: MID LEVEL TROUGHING DEEPENS OFF THE NEW ENGLAND COAST THROUGH MONDAY AS THE CHILLY SURFACE HIGH BUILDS INTO AND OVER THE MIDATLANTIC REGION. THICKNESSES REMAIN WELL BELOW NORMAL AND TEMPS SHOULD FOLLOW SUIT... WITH LOWS IN THE MID-UPPER 20S AND HIGHS AGAIN IN THE UPPER 40S TO LOWER 50S. THEN FOR LATE MONDAY ONWARD... THE MODELS REALLY START TO DIVERGE WITH THE TIMING AND AMPLITUDE OF MID LEVEL TROUGH THAT DIGS OVER CENTRAL THEN EASTERN NOAM. THE 12Z/02 GFS KEEPS A CLOSED VORTEX SITTING OFF THE SRN CA COAST AND IS LESS AMPLIFIED AND FASTER WITH THE NORTHERN PORTION OF THE TROUGH... BRINGING IT THROUGH THE GREAT LAKES INTO THE WESTERN SLOPES OF THE APPALACHIANS BY LATE TUESDAY WITH EXPANSIVE PRECIP SPREADING OVER NC. IN CONTRAST THE 00Z/02 ECMWF AND THE 06Z/02 GFS KEPT THE TROUGH FULL-LATITUDE AND PHASED AND THUS WAS MUCH MORE AMPLIFIED AND SLOWER. THE NON-PHASED SOLUTION SEEMS MUCH MORE LIKELY CLIMATOLOGICALLY SPEAKING... BUT WITH THIS MUCH DIFFERENCE AMONG THE OPERATIONAL MODELS AND WITH THE GEFS SHOWING A LARGE SPREAD IN SOLUTIONS AROUND ITS MEAN TROUGH POSITION... WILL GO FOR A SLOW INCREASE IN CLOUDS MONDAY THROUGH TUESDAY... AND A CHANCE OF RAIN STARTING TUESDAY... MAINLY WEST. WILL HOLD ONTO HIGHS IN THE UPPER 40S TO LOWER 50S MONDAY... RISING TO THE LOW-MID 50S FOR TUESDAY. THE SURFACE RIDGE WILL BE EXITING TO THE NE BUT IF IT DEPOSITS RESIDUAL DRY AND COOL SURFACE AIR OVER THE PIEDMONT AS THE RAIN ARRIVES... AN IN SITU DAMMING EVENT COULD RESULT AND TEMPS COULD BE MUCH COOLER IN THE WESTERN CWA ON TUESDAY. -GIH && .AVIATION /18Z TUESDAY THROUGH SATURDAY/... AS OF 120 PM TUESDAY... VFR CONDITIONS EXPECTED THROUGH THE 24-HOUR TAF PERIOD. SCATTERED TO BROKEN DIURNAL CU/STRATOCU -- WITH BASES BETWEEN 5 AND 7 THSD FT -- WILL DISSIPATE AFTER SUNSET AND RESULT IN CLEAR SKIES OVERNIGHT. ASIDE FROM SOME CIRRUS IN WSW FLOW ALOFT LATE WEDNESDAY INTO WEDNESDAY NIGHT... SKIES WILL REMAIN CLEAR THROUGH EARLY THURSDAY. A LIGHT WEST TO WSW WIND THIS AFTERNOON IS EXPECTED TO GO CALM AS SURFACE HIGH PRESSURE BUILDS OVERHEAD TONIGHT... THEN BECOME LIGHT S TO SSW AS THE RIDGE SHIFTS OFFSHORE WEDNESDAY AFTERNOON. LOOKING AHEAD... A NOCTURNAL LOW LEVEL JET... CHARACTERIZED BY SOUTHWESTERLY WINDS OF 30 TO 40 KTS AT 2000 FT... IS FORECAST TO DEVELOP OVER CENTRAL NC LATE WEDNESDAY NIGHT. IF THE SURFACE WINDS DECOUPLE AND GO (NEAR) CALM AS EXPECTED... THEN LOW END LLWS COULD BE A CONCERN IN THE 08 TO 12Z TIME FRAME EARLY THURSDAY MORNING. A COLD FRONT WILL MOVE THROUGH CENTRAL NC THURSDAY AFTERNOON OR EVENING... AND RESULT IN GENERALLY VFR... 3 TO 7 THSD FT CEILINGS AND A FEW SPOTTY SHOWERS. VFR CONDITIONS ARE EXPECTED TO PREVAIL THROUGH SUNDAY... THOUGH FORECAST UNCERTAINTY INCREASES BY THIS WEEKEND... WHEN STORMINESS OVER THE NORTHEASTERN US MAY THREATEN. && .RAH WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SYNOPSIS...HARTFIELD NEAR TERM...MWS SHORT TERM...HARTFIELD LONG TERM...HARTFIELD AVIATION...MWS