Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 151737 AFDRAH AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 135 PM EDT MON SEP 15 2014 .SYNOPSIS...WEAK HIGH PRESSURE WILL EXTEND SOUTHWARD INTO CENTRAL NC TODAY... THEN MOVE EAST TONIGHT. A COLD FRONT WILL PUSH SOUTHEAST ACROSS THE AREA LATE TUESDAY. COOL HIGH PRESSURE WILL BUILD INTO THE REGION ON WEDNESDAY. && .NEAR TERM /THROUGH TONIGHT/... AS OF 1045 AM MONDAY... EVIDENCE POINTS TO THIS BEING A DRY DAY FOR THE VAST MAJORITY OF CENTRAL NC. THE SURFACE FRONTAL ZONE REMAINS SOUTH OF NC WITH COOL HIGH PRESSURE CENTERED OVER NE PA NOSING SOUTHWARD. UPPER AIR ANALYSES REVEAL PLENTIFUL MOISTURE FROM 925 MB UP THROUGH 700 MB PARTICULARLY EAST... BUT WITH DRIER AIR PUSHING IN AT ALL OF THESE LEVELS FROM THE WEST. WE`RE ALREADY SEEING SUNSHINE BREAKING OUT OVER THE WRN PIEDMONT... AND THIS PARTIAL CLEARING SHOULD PROGRESS EASTWARD OVER THE NEXT SEVERAL HOURS... ALBEIT WITH LESSER IMPACT ACROSS THE SRN CWA NEARER THE FRONTAL ZONE... SOUTH OF THE MAIN SLUG OF DRIER AIR. A 850-700 MB ANTICYCLONE OVER FL WILL HELP ENHANCE LOW LEVEL CONFLUENT FLOW AND POSITIVE MOISTURE TRANSPORT OVER NRN MS/AL/GA... AND AS THIS TRANSLATES EASTWARD THIS AFTERNOON COUPLED WITH VERY SMALL INSTABILITY (BARELY OVER 500 J/KG OVER THE SW CWA LATE TODAY)... A FEW SHOWERS MAY BREAK OUT OR PROPAGATE OVER THE SW AND FAR SRN CWA LATE... AND WILL LEAVE IN ISOLATED POPS HERE FOR THE REST OF THE DAY... WITH DRY WEATHER ELSEWHERE... WITHIN AN AREA OF WEAK SUBSIDENCE AND LACK OF INSTABILITY. AS THE NIGHT SHIFT NOTED... TEMPS ARE TRICKY AND DEPENDENT WHOLLY ON HOW MUCH SUNSHINE IS FELT. HAVE ADJUSTED HIGHS BASED ON CURRENT TRENDS OF TEMP/SKY COVER AND HIGH RES GUIDANCE TO PLACE THE WARMEST TEMPS IN THE NW AND COOLEST IN THE ERN CWA WHERE THICKER CLOUDS WILL BE LATE TO DISLODGE. HIGHS FROM 76 EAST TO NEAR 80 WEST. -GIH EARLIER DISCUSSION FROM 325 AM: UPSTAIRS...WLY FLOW ALOFT WILL ADVANCE A VORT MAX CURRENTLY CROSSING THE SOUTHERN PLAINS EARLY THIS MORNING ACROSS THE TN VALLEY THIS AFTERNOON AND INTO OUR REGION TONIGHT. THIS FEATURE PROJECTED TO SLOWLY WEAKEN AS IT GETS CLOSER TO OUR AREA. WHEN IT ARRIVES...IT WILL HAVE ENOUGH MOISTURE TO WORK WITH AS PRECIPITABLE WATER VALUES SHOULD RANGE 1.3/1.4 INCHES ACROSS THE NORTH TO 1.75 INCHES ACROSS THE SOUTH. ASIDE FROM THE MID/UPPER LEVEL DISTURBANCE...FORCING IS WEAK IN THE LOWER HALF OF THE ATMOSPHERE. THUS...CANNOT SEE POP ANY HIGHER THAN ABOUT 1-IN-3 CHANCE...AND MAINLY ACROSS THE WEST AND SOUTH SECTIONS DURING THE EVENING HOURS. SO...EXPECT WIDESPREAD CLOUDINESS THIS MORNING WITH AN ISOLATED SHOWER POSSIBLE SOUTH HALF. BY MID AFTERNOON...EXPECT SOME PARTIAL SUNSHINE. THIS HEATING MAY FUEL A FEW MORE SHOWERS AND POSSIBLY A THUNDERSTORM...AGAIN MAINLY ACROSS THE SOUTH. THE APPROACH OF THE UPPER LEVEL DISTURBANCE FROM THE WEST SHOULD MAINTAIN A THREAT FOR A FEW SHOWERS AND POSSIBLY A THUNDERSTORM...MAINLY ACROSS THE WEST AND SOUTH...WITH THE THREAT DECREASING FROM THE NW WELL AFTER MIDNIGHT. TEMPS TODAY DEPENDENT UPON WHEN/IF CLOUDS DIMINISH TO ALLOW SOME SUN. USED A BLEND OF MOS GUIDANCE WITH A SLIGHT ADJUSTMENT TOWARD THE COOLER END. MAX TEMPS UPPER 70S NORTH TO LOWER 80S SOUTH. MIN TEMPS TONIGHT MID-UPPER 60S. -WSS && .SHORT TERM /TUESDAY AND TUESDAY NIGHT/... AS OF 325 AM MONDAY... WHILE MID LEVEL DISTURBANCE CROSSES OUR REGION LATE TONIGHT...ANOTHER (THOUGH STRONGER) S/W WILL BE CROSSING THE GREAT LAKES INTO SOUTHERN NEW ENGLAND. THIS FEATURE WILL PROPEL A SURFACE COLD FRONT ACROSS CENTRAL NC TUESDAY. DUE TO WEAK SUBSIDENCE IN THE WAKE OF THE DEPARTING INITIAL DISTURBANCE...FORCING ALONG THE SURFACE COLD FRONT IS RELATIVELY WEAK. THUS...EXPECT ANY CONVECTION ASSOCIATED WITH THE COLD FRONT TO BE NO WORSE THAN SCATTERED...MAINLY OCCURRING ACROSS THE SOUTH HALF OF THE FORECAST AREA. PASSAGE OF A MID LEVEL TROUGH TUESDAY NIGHT WILL MAINTAIN A SLIGHT CHANCE POP TUESDAY NIGHT ACROSS THE FAR SOUTH. DOWNSLOPE FLOW IN THE WAKE OF THE DEPARTING INITIAL DISTURBANCE WILL AID TO MIX OUT ANY LOW LEVEL CLOUDINESS AND RESULT IN SOME SUBSIDENCE WARMING. MOS GUIDANCE APPEARS TOO WARM BY ABOUT A CATEGORY AS CLOUDINESS MAY HOLD ON LONGER THAN PROJECTED BY THE MODELS PLUS DOWNSLOPE FLOW RATHER WEAK. FAVOR MAX TEMPS 2-4 DEGREES COOLER THAN GUIDANCE (LOW-MID 80S). MIN TEMPS TUESDAY NIGHT NEAR 60 FAR NORTH-NW TO THE MID 60S SOUTH. && .LONG TERM /WEDNESDAY THROUGH SUNDAY/... AS OF 330 AM MONDAY... THE MEDIUM RANGE WILL FEATURE A LONGWAVE TROUGH THAT WILL AMPLIFY OVER THEN MOVE OFFSHORE THE EAST COAST MID-LATE WEEK. THE TROUGH WILL THEN LIFT OUT AND YIELD BRIEFLY TO A FLAT SUB-TROPICAL RIDGE FORECAST TO BUILD EAST FROM THE SOUTHERN PLAINS AND LOWER MS VALLEY EARLY THIS WEEKEND...BEFORE RENEWED TROUGHING AMPLIFIES FROM CENTRAL CANADA TO THE EAST COAST SUN-MON. MEANWHILE AT THE SFC...A LEAD SFC HIGH WILL BUILD FROM THE OH VALLEY TO THE MIDDLE ATLANTIC COAST WED...THEN BE OVERWHELMED BY A STRONGER (1028-1030 MB) ONE FORECAST TO BUILD FROM CENTRAL CANADA TO THE CANADIAN MARITIMES...AND RIDGE SOUTH/SOUTHWESTWARD IN A DAMMING ORIENTATION EAST OF THE APPALACHIANS...THU THROUGH THE WEEKEND. THE RESULT WILL BE A RELATIVELY PROLONGED PERIOD OF EAST TO NORTHEAST LOW LEVEL FLOW AND ASSOCIATED BELOW NORMAL TEMPERATURES...TEMPERED FURTHER BY PERIODS OF BKN MULTI-LAYERED CLOUD COVER. THE RELATIVE GREATEST CLOUD COVERAGE WILL OCCUR WED-THU...WHEN A SLIGHT-LOW CHANCE OF SHOWERS WILL ALSO BE INTRODUCED...AS WEAK DISTURBANCES RIPPLE THROUGH THE AFOREMENTIONED LONGWAVE TROUGH AMPLIFYING OVER THE EASTERN STATES. IN FACT...WED MAY WELL BE A RATHER DREARY DAY THROUGH AT LEAST EARLY AFTERNOON...AS ISENTROPIC LIFT IN RETURN FLOW AROUND A 925-850 MB RIDGE ALONG THE COASTAL CAROLINAS AUGMENTS THE WEAK FORCING ATTENDING THE PASSING MID LEVEL DISTURBANCES. SOME PARTIAL CLEARING FROM THE WEST WILL BE POSSIBLE BY MID LATE AFTERNOON...WHEN THE FLOW BECOMES INCREASINGLY WESTERLY/DOWNSLOPE AS THE AFOREMENTIONED LOW LEVEL ANTICYCLONE DRIFTS OFFSHORE. AS SUCH...HIGH TEMPERATURES SHOULD CLIMB ONLY INTO THE 69 TO 76 DEGREE RANGE...COOLEST OVER THE NE PIEDMONT AND NORTHERN COASTAL PLAIN. OTHERWISE...THE INCREASING INFLUENCE OF THE RIDGING AT THE SFC AND ALOFT SHOULD PROMOTE DRY CONDITIONS FRI-SUN...THOUGH WITH TEMPERATURES MODERATING ONLY SLOWLY THROUGH THE 70S. && .AVIATION /18Z MONDAY THROUGH SATURDAY/... AS OF 135 PM MONDAY... A SURFACE FRONTAL ZONE WILL HOLD JUST SOUTH OF NC THROUGH TONIGHT AS HIGH PRESSURE NOSES IN FROM THE NNE. MOIST FLOW FROM THE GULF AND ATLANTIC RIDING UP AND OVER THE COOLER SURFACE AIR WILL KEEP ABUNDANT CLOUD COVER OVER CENTRAL NC THROUGH TONIGHT... AS AN UPPER LEVEL DISTURBANCE AND WEAK SURFACE TROUGH CROSS THE REGION. MVFR CIGS AND VFR VSBYS DOMINATE AT CENTRAL NC TERMINAL SITES CURRENTLY... AND THESE CIGS ARE EXPECTED TO BRIEFLY LIFT TO VFR BETWEEN 18Z AND 21Z. AFTER SUNSET HOWEVER... WE EXPECT MVFR CIGS TO REDEVELOP... MOST LIKELY BETWEEN 03Z AND 06Z. A PERIOD OF IFR CIGS IS POSSIBLE AS WELL BETWEEN 06Z AND 12Z... AS LOW LEVEL MOISTURE INCREASES AHEAD OF THE DISTURBANCE. ONCE THIS WAVE ALOFT AND SURFACE TROUGH PASS TO OUR EAST LATE TONIGHT INTO EARLY TUE MORNING... CIGS SHOULD LIFT BACK UP TO VFR BETWEEN 10Z AND 13Z... WITH ALL SITES LIKELY TO HAVE VFR CONDITIONS AND LIGHT SURFACE WINDS FROM THE NW AFTER 13Z TUE. MOST TAF SITES WILL STAY DRY... ALTHOUGH AN ISOLATED SHOWER WITH BRIEF MVFR VSBYS IS POSSIBLE MAINLY NEAR FAY TONIGHT INTO TUE MORNING. LOOKING BEYOND 18Z TUE... FAIR SKIES AND VFR CONDITIONS ARE EXPECTED TO HOLD THROUGH TUE AFTERNOON... ALTHOUGH A SECONDARY FRONT MOVING IN FROM THE WEST WILL LEAD TO A RE-FORMATION OF VFR CLOUDS TOWARD SUNSET. THESE CLOUDS ARE THEN EXPECTED TO LOWER TO IFR WITH MVFR-IFR FOG AND A FEW SHOWERS AFTER 06Z TUE NIGHT. SUB-VFR CONDITIONS ARE EXPECTED TO LAST THROUGH MUCH OF WED -- AND POTENTIALLY INTO THU -- AS UPPER LEVEL DISTURBANCES CROSS THE AREA... ROUNDING THE BASE OF A LARGE UPPER TROUGH OVER THE EASTERN US... WHILE COOL AIR NOSES IN FROM THE NORTH AT THE SURFACE. IMPROVEMENT TO VFR APPEARS LIKELY ON FRI... LASTING THROUGH SAT. -GIH && .RAH WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SYNOPSIS...WSS NEAR TERM...HARTFIELD/WSS SHORT TERM...WSS LONG TERM...MWS AVIATION...HARTFIELD

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