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 220307 AFDRAH AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 955 PM EST SAT NOV 21 2009 .SYNOPSIS... A LOW PRESSURE SYSTEM IN THE NORTHWEST GULF OF MEXICO WILL MOVE SLOWLY NORTHEAST TOWARD THE FLORIDA GULF COAST TONIGHT...BEFORE WEAKENING AND REFORMING ALONG THE CAROLINA COAST SUNDAY NIGHT. THIS WILL BRING INCREASING CLOUD COVER AND A GOOD CHANCE OF RAIN TO CENTRAL NORTH CAROLINA BY SUNDAY NIGHT. && .NEAR TERM /THROUGH SUNDAY/...-- Changed Discussion --AS OF 955 PM SATURDAY... A 1024 MB SURFACE HIGH WAS LOCATED OVER WESTERN NY STATE THIS EVENING... WITH RIDGING DOWN THE EASTERN SEABOARD THROUGH NC/SC INTO GA. WEAK LOW PRESSURE WAS CENTERED OVER SE LA. CIRRUS CLOUDS CONTINUED TO INCREASE FROM THE SW THIS EVENING... WITH MID CLOUDS STILL HOLDING BACK OVER CENTRAL GA INTO SC. THE CLOSEST MEASURABLE RAIN THIS EVENING EXTENDED FROM BIRMINGHAM TO TALLAHASSEE. LITTLE CHANGE APPEARS NEEDED TO THE OVERNIGHT FORECAST. THE LATEST 00Z/22 NOVEMBER UPPER AIR DATA INDICATED VERY DRY AIR IN THE LOW AND MID LEVELS... EVEN AS FAR SOUTH AS NORTHERN ALABAMA AND GEORGIA... SUGGESTING THAT MUCH OF THE NIGHT WILL CONTINUE TO HAVE ONLY SCATTERED TO BROKEN CIRRUS OVER CENTRAL NC. THIS WILL HELP LOWS DIP A FEW DEGREES LOWER THAN CURRENTLY FORECAST. STILL... A GRADUAL INCREASE IN THE CIRROSTRATUS IS EXPECTED LATE.... WITH LOWS OF 35-40. SUNDAY: LATEST 12Z MODEL GUIDANCE CONTINUES TO SHOW THE 500 MB LOW DEAMPLIFYING INTO A SHORTWAVE DURING THE DAY SUNDAY AS IT MOVES N/NE INTO NORTHERN AL/GA AND EASTERN TN. THE ATTENDANT GULF COAST SFC LOW IS PROGGED TO WEAKEN AS IT REMAINS CENTERED NEAR THE FL/AL COAST. AN INVERTED SFC TROUGH/COASTAL FRONT IS EXPECTED TO SHARPEN OFF THE SOUTHEAST/CAROLINA COAST DURING THE DAY SUN IN ASSOC/W WEAK UPPER FORCING ASSOC/W THE DEAMPLIFYING SOUTHERN STREAM WAVE. SFC HIGH PRESSURE OVER THE MID-ATLANTIC IS PROGGED TO STRENGTHEN AND SHIFT NORTHEAST INTO NEW ENGLAND ON SUNDAY...WITH A PROMINENT SFC RIDGE AXIS EXTENDING SOUTH INTO THE CAROLINAS. ALTHOUGH UPPER LEVEL LIFT WILL BE RATHER ANEMIC SUNDAY...ISENTROPIC LIFT IN THE LOWER LEVELS WILL BE STRENGTHENING FROM THE SOUTH/SW THROUGHOUT THE DAY AS THE 925-850MB WARM FRONT STALLS OUT JUST TO THE SOUTH IN SC AND THE MSLP/HEIGHT GRADIENT TIGHTENS BETWEEN THE SFC RIDGE EXTENDING SOUTH INTO THE AREA AND WEAK LOW PRESSURE OVER THE DEEP SOUTH AND CAROLINA/SOUTHEAST COAST. BASED ON THE 12Z MODEL RUNS...THE TIMING OF THE ARRIVAL OF PRECIP IS QUITE DELAYED COMPARED TO PREVIOUS RUNS...WITH THE BEST CHANCE FOR RAIN STARTING IN THE SOUTH/SW IN THE AFTERNOON...SPREADING NORTHWARD INTO THE TRIANGLE AREA BY OR SHORTLY AFTER SUNSET. WILL SHOW HIGHEST POPS (CATEGORICAL/LIKELY) IN THE SW PIEDMONT AND SANDHILLS...TAPERING DOWN TO SLIGHT CHANCE N/NE OF THE TRIANGLE. RAINFALL AMOUNTS SHOULD BE NO HIGHER THAN 0.10-0.25" IN THE SW PIEDMONT/SANDHILLS. TEMPERATURE FCST SOMEWHAT DIFFICULT ON SUN...DEPENDING PRIMARILY ON THE ARRIVAL OF PRECIP. LATEST MOS GUIDANCE HAS COME IN WARMER IN ALMOST ALL LOCATIONS...RANGING FROM THE MID/UPPER 50S TO THE LOWER 60S...AND THAT LOOKS PRETTY REASONABLE BASED ON THE DELAYED ONSET OF PRECIP. -VINCENT-- End Changed Discussion --&& .SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY/... AS OF 135 PM SATURDAY... FOR SUNDAY NIGHT/MONDAY: THE PARENT SURFACE HIGH STRENGTHENS FURTHER TO AROUND 1033 MB AND DRIFTS FROM OVER MAINE ACROSS THE CANADIAN MARITIMES THROUGH MONDAY. THE INVERTED SURFACE TROUGH JUST WEST OF THE SOUTHERN APPALACHIANS WEAKENS GRADUALLY WHILE THE SECONDARY SURFACE CYCLOGENESIS BEGINS OFF THE SC COAST WITH ITS CENTER THEN PASSING NEAR THE OUTER BANKS THROUGH MONDAY. THE SUBCLOUD LAYER OVER CENTRAL NC FINALLY BEGINS TO SATURATE FROM SOUTHWEST TO NORTHEAST SUNDAY EVENING WITH INCREASING MOIST ISENTROPIC UPGLIDE AT 290K-300K AND INCREASING LOW LEVEL MASS CONVERGENCE AND MOISTURE TRANSPORT. HOWEVER... DEEP FORCING FOR ASCENT REMAINS SOMEWHAT LIMITED. THE DPVA DECREASES RAPIDLY LATE SUNDAY THROUGH SUNDAY NIGHT AS THE DAMPENING MID LEVEL SHORTWAVE TROUGH SWINGS UP THROUGH EASTERN TN AND WESTERN NC... AND UPPER DIVERGENCE (WHILE MAXIMIZED OVER NC AT 06Z SUNDAY NIGHT) TRENDS WEAKER AS WELL. AND BY THE TIME THE LOWEST 1-2 KM BECOMES SATURATED... WE LOSE THE MOISTURE ALOFT AS THE VERY DRY AIR NOW OVER NM/WEST TX/OK PUNCHES IN ON THE BACK SIDE OF THE SHORTWAVE TROUGH. THESE FACTORS SHOULD RESULT IN RATHER LOW QPF... ALTHOUGH GIVEN THAT THE LOW LEVEL FORCING FEATURES LINE UP WELL... WE STILL ANTICIPATE WIDESPREAD LIGHT RAIN AND DRIZZLE FROM SUNDAY NIGHT THROUGH MONDAY... AND WILL RETAIN CATEGORICAL POPS... MAINLY FROM LATE EVENING SUNDAY THROUGH MONDAY MORNING BEFORE TAPERING OFF POPS TO A CHANCE OF LIGHT RAIN/DRIZZLE NORTH TO SOUTH THROUGH MONDAY AFTERNOON. LOWS 46-51. THEN WITH THICK OVERCAST PERSISTING THROUGH MONDAY... LOW LEVEL COLD AIR ADVECTION... AND THE COASTAL LOW HOLDING OFFSHORE... TEMPS SHOULD RISE VERY LITTLE MONDAY. HIGHS 49-59... AND EVEN THIS MAY BE OPTIMISTIC WITHIN THE HEART OF THE COLD DOME IN THE NORTHERN PIEDMONT. FOR MONDAY NIGHT/TUESDAY: THE COASTAL LOW IS EXPECTED TO TRACK SLOWLY FROM THE NORTHERN OUTER BANKS TO OFF THE NJ COAST BY LATE TUESDAY. THE PARENT HIGH CONTINUES TO SHIFT EAST OFF THE CANADIAN MARITIMES AND LOSES ITS GRIP ON THE WEDGE AIR MASS OVER CENTRAL NC... HOWEVER IT LEAVES BEHIND A FORMIDABLE COOL POOL. FORECAST SOUNDINGS FROM BOTH THE NAM AND GFS SHOW STABLE AIR AND PLENTY OF MOISTURE IN THE LOWEST 2 KM THROUGH MONDAY NIGHT... AND THE GFS HOLDS ONTO LOW LEVEL ISENTROPIC UPGLIDE IN THIS LAYER AROUND 295K. A WEAK SHORTWAVE PASSES BY MONDAY NIGHT BUT WITH NO MOISTURE ALOFT AND STABLE/WARMING MID LEVELS... IT SHOULD HAVE NO SENSIBLE IMPACT IN THIS AREA. WITH WEAK AND LIGHTLY DIFFLUENT MAINLY NORTHERLY SURFACE WINDS THROUGH THE NIGHT AND SATURATED LOW LEVELS... WE SHOULD SEE AREAS OF LIGHT DRIZZLE AND FOG THROUGH TUESDAY MORNING. HAVE BROUGHT LOWS UP SLIGHTLY TO 45-49. ON TUESDAY... THE GFS KEEPS THE LOW LEVELS NEARLY ISOTHERMAL AND SATURATED WHEREAS THE NAM WANTS TO START DRYING THIS LAYER OUT. THE MODELS OFTEN HAVE DIFFICULTLY CORRECTLY DEPICTING EROSION OF A DAMMING AIR MASS THAT HAS LOST LARGE SCALE SUPPORT AND NO LONGER HAS ANY CONNECTION TO A COOL DRY LOW LEVEL AIR SOURCE. DESPITE HEATING FROM ABOVE WHICH MAY START TO BURN OFF THE UPPER PORTION OF THIS MOIST LAYER... GIVEN THE DRY SUBSIDENCE LAYER ALOFT CAPPING OFF THIS SURFACE-BASED LAYER AND WITH WEAK WINDS IN THE LOWEST 1 KM INHIBITING MIXING... FAVOR A LEAN TOWARD THE GFS (WHICH HAS SUPPORT FROM THE CANADIAN) WHICH MEANS HOLDING ONTO CONSIDERABLE CLOUDINESS THROUGH TUESDAY. WITH THIS GREATER CLOUD COVER... HIGHS WILL ACCORDINGLY BE LOWERED... TO 58-65. -GIH && .LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...-- Changed Discussion --AS OF 240 PM FRIDAY... FOR TUESDAY NIGHT: THE GFS/CANADIAN DEPICT A WEAK 850 MB WARM FRONT SWINGING NORTHEASTWARD LEADING TO LOW LEVEL WINDS BECOMING MORE UNIFORM (YET STILL LIGHT) FROM THE WEST-SOUTHWEST BY MORNING. GFS FORECAST SOUNDINGS SHOW A DISSOLUTION OF THE LOW CLOUDS ESPECIALLY IN THE WESTERN AND SOUTHERN CWA. WILL SHOW A TREND TOWARD FEWER CLOUDS IN THIS AREA OVERNIGHT BUT BASED ON UNCERTAINTY WITH SUCH A FEATURE THIS FAR OUT... WILL NOT COMPLETELY CLEAR OUT THE CLOUDS. LOWS 43-48. FOR WEDNESDAY THROUGH SATURDAY: MODEL CONSENSUS SHOWS DEEP TROUGHING DOMINATING THE EASTERN HALF OF THE NATION THROUGH AT LEAST FRIDAY... WITH THE SURFACE BAROCLINIC ZONE HOLDING OFF THE SOUTHEAST COAST. THE GFS/ECMWF/CANADIAN AGREE FAIRLY WELL ON ONE MID LEVEL VORTEX OVER THE MIDWEST EARLY WEDNESDAY SWINGING EAST THEN NORTH THROUGH QUEBEC THROUGH FRIDAY... WHILE ANOTHER VORTEX DROPS FROM NEAR LAKE WINNIPEG THROUGH THE MIDWEST TO THE EASTERN GREAT LAKES REGION AND FINALLY TRACKS UP ALONG THE ST LAWRENCE VALLEY INTO SATURDAY. THIS CERTAINLY LOOKS TO BE AN ANOMALOUSLY DEEP PATTERN AS THE GEFS MEAN MID LEVEL HEIGHTS DROP TO 1-3 STANDARD DEVIATIONS BELOW NORMAL BY FRIDAY OVER THE SOUTHEAST. SURFACE DETAILS ARE TOUGH TO PINPOINT WITHIN SUCH A FAST FLOW ALOFT BUT IT APPEARS THAT WE`LL SEE A LARGELY DRY COLD FRONT PASSING THROUGH WEDNESDAY NIGHT OR EARLY THANKSGIVING DAY... FOLLOWED BY FALLING LOW LEVEL THICKNESSES AND DRY BUT BRISK NORTHWESTERLY LOW LEVEL FLOW AS A POLAR-SOURCE HIGH PRESSURE RIDGE BUILDS IN SLOWLY FROM THE WEST-SOUTHWEST. WILL KEEP A SLIGHT CHANCE OF RAIN MAINLY ACROSS THE NORTH WEDNESDAY NIGHT INTO THANKSGIVING DAY... THEN INDICATE DRY AND COLDER WEATHER FOR THURSDAY NIGHT THROUGH SATURDAY. EXPECT ABOVE-NORMAL TEMPS WEDNESDAY... SLIGHTLY COOLER (NEAR NORMAL) THANKSGIVING DAY... THEN THE BULK OF THE COLD AIR ARRIVES THURSDAY NIGHT WITH LOW LEVEL THICKNESSES PLUNGING BELOW 1310 M ON THE GFS AND CANADIAN BY FRIDAY MORNING. WILL HOLD TEMPS ABOUT ONE TO TWO CATEGORIES BELOW NORMAL FRIDAY THROUGH SATURDAY... WITH HIGHS IN THE LOWER-MID 50S. -GIH-- End Changed Discussion --&& .AVIATION /00Z SUNDAY THROUGH THURSDAY/... AS OF 720 PM SATURDAY... VFR CONDITIONS ARE EXPECTED THROUGH THE TAF PERIOD FOR INT/GSO/RDU/FAY...HOWEVER...KRWI MAY EXPERIENCE MVFR CONDITIONS AROUND SUNRISE. HIGH PRESSURE WILL START TO RETREAT TO THE NORTH AS A LOW PRESSURE SYSTEM FROM THE SOUTH MOVES INTO THE AREA. UPPER LEVEL CLOUD COVER WILL CONTINUE TO INCREASE IN ADVANCE OF THE STORM SYSTEM TONIGHT. LIGHT WINDS AND LOW DEWPOINT DEPRESSIONS (< 2F)MAY LEAD TO LOW STRATUS AND FOG AT KRWI. THE PRESSURE GRADIENT WILL TIGHTEN THROUGHOUT SUNDAY AS A COASTAL TROUGH DEVELOPS OFF THE SE COAST...INCREASING THE WINDS FROM 5-10 KTS. AS THE SYSTEM MOVES NORTH...CEILINGS SHOULD BEGIN TO LOWER DURING THE DAY SUNDAY. ANY MVFR/IFR CONDITIONS SHOULD HOLD OFF UNTIL AFTER 00Z MON. LOOKING BEYOND 00Z MONDAY: A STORM SYSTEM MOVING INTO THE AREA MAY BRING MVFR OR IFR CONDITIONS TO THE REGION FOR SUNDAY NIGHT THROUGH TUESDAY. LOW CIGS ARE EXPECTED TO CLEAR OUT BY TUESDAY MORNING. IFR/LIFR CONDITIONS MAY BE AN ISSUE IF CLEARING OCCURS EARLY TUESDAY NIGHT. VFR CONDITIONS ARE EXPECTED FROM TUESDAY THROUGH WEDNESDAY AS HIGH PRESSURE BUILDS IN FROM THE SOUTHWEST. && .RAH WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SYNOPSIS...VINCENT NEAR TERM...VINCENT/BADGETT SHORT TERM...HARTFIELD LONG TERM...HARTFIELD AVIATION...LEP/CBL
