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 210548 AFDRAH AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 1245 AM EST SAT NOV 21 2009 .SYNOPSIS...-- Changed Discussion --SURFACE HIGH PRESSURE WILL DRIFT TO OUR NORTHEAST ON SATURDAY... IN ADVANCE OF A STORM SYSTEM WHICH WILL MOVE INTO THE REGION FROM THE SOUTHWEST AND BRING CLOUDS AND RAIN FOR SUNDAY AND SUNDAY NIGHT.-- End Changed Discussion --&& .NEAR TERM /THROUGH SATURDAY/... AS OF 845 PM FRIDAY... TONIGHT...WILL HAVE TO CONTEND WITH CIRRUS BLOW OFF FROM THE GULF CONVECTION ASSOCIATED WITH THE DEEPENING SOUTHERN STREAM VORTEX OVER SOUTHERN TEXAS. MODELS PAINT 70 TO 80 PERCENT 500-300MB MEAN LAYER RH MOISTURE OVER THE AREA FROM 06Z ON. THESE CLOUDS APPEAR SUFFICIENTLY OPAQUE ENOUGH ON IR IMAGERY TO IMPACT OVERNIGHT LOWS. WILL BUMP UP OVERNIGHT LOWS A FEW DEGREES...RANGING FROM AROUND 40 NORTH TO MID 40S SOUTH. SOME PATCHY GROUND FOG IS ALSO POSSIBLE OVERNIGHT... ESPECIALLY ACROSS THE EASTERN PORTIONS OF THE FORECAST AREA WHERE HIGHER DEWPOINTS AND LOW T/TD SPREADS RESIDE. OF COURSE....IF CIRRUS CANOPY IS AS ADVERTISED...IT WOULD CERTAINLY HELP DETER WIDESPREAD FOG DEVELOPMENT. -CBL FOR SATURDAY: THE MID LEVEL VORTEX DEEPENING OVER EASTERN TX/OK THIS AFTERNOON MOVES SLOWLY ACROSS LA/MS THROUGH SATURDAY. MID-UPPER LEVEL MOISTURE STEADILY INCREASES AND DEEPENS OVER NC... WHILE THE SURFACE HIGH TO OUR NORTH CONTINUES MOVING NORTHWARD TO A POSITION OVER LAKE ERIE... WHERE IT BEGINS TO BE SUPPORTED BY CONFLUENT FLOW ALOFT WITH NORTHERN STREAM SHORTWAVE TROUGHING OVER PA... CAUSING THE HIGH TO GRADUALLY STRENGTHEN. THIS PATTERN SHOULD ENSURE A STEADY NORTHEASTERLY FEED OF LOW LEVEL DRY AIR INTO CENTRAL NC... SETTING US UP FOR DAMMING ONCE THE SURFACE HIGH CAN GET INTO A MORE FAVORABLE POSITION (OVER THE ST LAWRENCE VALLEY) SATURDAY NIGHT WHERE IT CAN TAP INTO EVEN COOLER AIR AND STRENGTHEN FURTHER WITH THE SUPPORT ALOFT PERSISTING. EXPECT STEADILY LOWERING AND THICKENING CLOUDS THROUGH THE DAY... WITH A FEW LOWER STRATOCUMULUS OVER THE WESTERN PIEDMONT WITHIN EASTERLY UPSLOPE LOW LEVEL FLOW. BUT THE MOST ACTION WITH THIS SYSTEM ON SATURDAY WILL BE BANDS OF CONVECTION OVER AL/GA AND THE NORTHEAST GULF... AND ANY SIGNIFICANT FORCING TO GENERATE PRECIP OVER NC HOLDS OFF UNTIL AFTER SUNDOWN SATURDAY WITH NO DISCERNIBLE ISENTROPIC UPGLIDE AND UPPER DIVERGENCE HOLDING TO OUR SOUTH THROUGH THE DAY... PLUS THE SUBCLOUD LAYER AT 1-3 KM OVER NC REMAINS RATHER DRY. HIGHS 60-65 ARE SUPPORTED BY LOW LEVEL THICKNESS TRENDS AND ARE NEAR THE LATEST STATISTICAL GUIDANCE. -GIH && .SHORT TERM /SATURDAY NIGHT THROUGH MONDAY NIGHT/... AS OF 315 PM FRIDAY... THIS PORTION OF THE FORECAST WILL BE DRIVEN BY 1) A STRENGTHENING (TO NEAR 1028-1034 MB) BUT MIGRATORY CAD SURFACE HIGH... 2) THE APPROACH OF A DEAMPLIFYING BUT POWERFUL AND COMPLEX SOUTHERN STREAM TROUGH ALOFT AND ATTENDANT UPPER JET LEFT EXIT REGION.... 3) CYCLOGENESIS ALONG THE LOW LEVEL BAROCLINIC ZONE OFFSHORE THE EAST COAST SOUTHWESTWARD INTO THE WESTERN GULF OF MEXICO... AND 4) RESULTANT ISENTROPIC LIFT ATOP THE COLD DOME OVER INTERIOR NC. THE OPERATIONAL MODELS ARE ALL OVER THE PLACE REGARDING THE EVOLUTION OF THESE FEATURES AND RESULTANT TIMING OF THE UPCOMING POTENTIAL RAIN EVENT OF AROUND A THIRD TO ONE INCH... HIGHLIGHTED BY A FAST AND HEAVY OVER-THE-WESTERN PIEDMONT GFS LATE SATURDAY NIGHT... AND THE TOTALLY DRY NAM FROM THE TRIANGLE NORTHWARD THROUGH THE ENTIRE DAY SUNDAY. FORECAST CONFIDENCE IS ACCORDINGLY RELATIVELY LOW... BUT WE WILL CONTINUE TO FAVOR THE TIMING OF THE PREVIOUS FORECAST -- MOST CLOSELY RESEMBLING THE 12Z GEM -- WITH THE BULK OF THE RAINFALL SUNDAY MORNING OVER THE SOUTHWEST... AND AREA WIDE BY SUNDAY AFTERNOON AND SUNDAY NIGHT. FORECAST MAX TEMPERATURES SUNDAY IN THE UPPER 40S NORTHWEST TO UPPER 50S SOUTHEAST COULD BE A FEW DEGREES TOO COOL (ESPECIALLY ACROSS THE NORTHERN AND EASTERN PORTION OF THE CWA)... GIVEN THE AFOREMENTIONED TIMING OF THE PRECIPITATION SHIELD. ALTHOUGH THE BULK OF THE PRECIPITATION SHOULD BE WINDING DOWN AND LIFTING NORTHEASTWARD WITH THE COASTAL LOW... LOW CLOUDS AND DRIZZLE IN NORTH OR NORTHEASTERLY LOW LEVEL FLOW ARE APT TO CONTINUE THROUGHOUT THE DAY MONDAY (AND PERHAPS INTO MONDAY NIGHT). HAVE ACCORDINGLY REDUCED HIGH TEMPERATURES BY 4 TO 8 DEGREES INTO THE 56 TO 63 DEGREE RANGE FROM NORTH TO SOUTH. && .LONG TERM /TUESDAY THROUGH FRIDAY/... AS OF 105 PM FRIDAY... TUESDAY THROUGH WEDNESDAY: ELONGATED SOUTHWEST TO NORTHEAST SURFACE RIDGING... BENEATH BUILDING HEIGHTS IN SOUTHWESTERLY FLOW ALOFT (AHEAD OF AN EVOLVING CENTRAL AND EVENTUALLY EASTERN CONUS TROUGH)... SHOULD YIELD DRY AND ABOVE AVERAGE TEMPERATURES. HIGHS 62 TO 69... WARMEST TUESDAY. LOWS WITHIN A FEW DEGREES OF 45. WEDNESDAY NIGHT THROUGH FRIDAY: A SERIES OF INTENSE SHORTWAVE TROUGHS ALOFT WILL HELP AMPLIFY THE AFOREMENTIONED UPPER TROUGH ACROSS THE GREAT LAKES AND INTO THE NORTHEAST THROUGH THE REMAINDER OF THE PERIOD. CYCLOGENESIS SHOULD ENSUE OFF THE EAST COAST... AS THE UPPER FORCING APPROACHES THE BAROCLINIC ZONE JUST OFFSHORE. ACCORDINGLY EXPECT PRECIPITATION CHANCES TO REMAIN SUPPRESSED TO THE EAST OF CENTRAL NC... THOUGH THERE PROBABLY WILL BE PERIODS OF MID AND HIGH CLOUDS ASSOCIATED WITH THE APPROACHING UPPER TROUGH/JET AXIS... PARTICULARLY OVER THE EASTERN HALF OF THE STATE. TEMPERATURES TREND DOWNWARD AS THE L/W TROUGH AXIS SETTLES IN BY FRIDAY. HIGHS 60 TO 65 THURSDAY... WITH LOWER TO MIDDLE 50S EXPECTED FOR FRIDAY. LOWS INTO THE 30S THURSDAY AND FRIDAY NIGHTS. && .AVIATION /06Z SATURDAY THROUGH WEDNESDAY/...-- Changed Discussion --AS OF 1245 AM SATURDAY... OTHER THAN PERIODS OF LIMITED VSBYS AT KRWI WHERE LINGERING LOW LEVEL MOISTURE IS ALREADY CAUSING SOME SHALLOW FOG...VFR CONDITIONS ARE GENERALLY EXPECTED ACROSS CENTRAL NC THROUGH THE CURRENT TAF PERIOD. HIGH PRESSURE OVER THE OHIO VALLEY REGION WILL DRIFT SLOWLY NORTHEAST TODAY...AND SETTLE OVER NEW ENGLAND TONIGHT...KEEPING SURFACE WINDS PREDOMINATELY OUT OF THE NE. A DEVELOPING SYSTEM OVER THE WESTERN GULF COAST WILL BEGIN TO LIFT NORTH AND EASTWARD TODAY...WITH PERIODS OF CIRRUS DRIFTING OVER CENTRAL NC FROM THE SOUTHWEST. CEILINGS WILL EVENTUALLY BEGIN TO LOWER LATE TONIGHT AND INTO SUNDAY MORNING AS THE PRECIPITATION SHIELD ASSOCIATED WITH THE GULF COAST STORM SYSTEM SPREADS TO TOWARDS THE AREA...BUT ALL SITES SHOULD REMAIN VFR AND PRECIP FREE THROUGH 06Z. LOOKING BEYOND 00Z SUNDAY: A STORM SYSTEM WILL APPROACH FROM THE SOUTHWEST SUNDAY MORNING AND MAY BRING MVFR OR IFR CONDITIONS TO THE REGION FOR SUNDAY THROUGH MONDAY. LOW CIGS ARE EXPECTED TO CLEAR OUT BY MONDAY AFTERNOON BUT AREAS OF IFR/LIFR FOG MAY DEVELOP LATE MONDAY NIGHT/TUESDAY MORNING. VFR CONDITIONS ARE ANTICIPATED TUESDAY MORNING THROUGH WEDNESDAY WITH HIGH PRESSURE BUILDING IN FROM THE SOUTHWEST.-- End Changed Discussion --&& .RAH WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SYNOPSIS...SMITH NEAR TERM...HARTFIELD SHORT TERM...MWS LONG TERM...MWS AVIATION...SMITH/BSD
