Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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156 FXUS62 KRAH 171108 AFDRAH AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 708 AM EDT SUN MAY 17 2015 .SYNOPSIS...
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A SURFACE TROUGH WILL STRENGTHEN ACROSS THE PIEDMONT OF VA AND THE CAROLINAS THROUGH TUESDAY. SUB-TROPICAL HIGH PRESSURE WILL OTHERWISE REMAIN OVER THE SOUTHEASTERN U.S. UNTIL A COLD FRONT CROSSES OUR REGION TUESDAY NIGHT.
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&& .NEAR TERM /THROUGH TONIGHT/...
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AS OF 350 AM SUNDAY... A SUB-TROPICAL RIDGE...321 DM AT 700 MB CENTERED ALONG THE SE U.S. COAST...IS FORECAST TO DRIFT SOUTHWESTWARD TO THE NE GULF OF MEXICO BY MON MORNING. THE SLOW RETROGRESSION AND ASSOCIATED SLIGHTLY LOWERING HEIGHTS ALOFT OVER THE MIDDLE ATLANTIC STATES WILL CAUSE THE MID LEVEL FLOW OVER CENTRAL NC TO ASSUME A MORE W TO WNW COMPONENT AND ALLOW UPSTREAM SHORTWAVE ENERGY FROM THE LOWER MS VALLEY TO ROUND THE RIDGE AXIS AT A LOWER LATITUDE THAN RECENT DAYS...WHICH WILL ALSO CONSEQUENTLY ALLOW THE DEEP MOIST AXIS FROM THE LOWER MS VALLEY TO THE MIDDLE ATLANTIC COAST THIS MORNING TO EDGE SOUTH INTO NORTHERN NC BY THIS EVENING. THE COMBINATION OF THE IMPROVED DEEP MOISTURE AND AFOREMENTIONED VEERING OF THE MID LEVEL STEERING FLOW TO W OR WNW WILL ALLOW SCATTERED AFTERNOON AND EVENING CONVECTION DIRECTED ACROSS CENTRAL VA TO PERHAPS GRAZE NC COUNTIES ADJACENT TO THE VA STATE LINE/OVER THE NORTHERN NC PIEDMONT...WHERE A SLIGHT CHANCE OF SHOWERS AND STORMS WILL BE RETAINED. OTHERWISE... CONTINUED SUBSIDENCE WARMING BENEATH THE SUB-TROPICAL RIDGE WILL CAUSE THE AIR MASS TO FURTHER WARM...ON THE ORDER OF ABOUT A CATEGORY ABOVE THE LOWER TO MIDDLE 80S EXPERIENCED ON SAT. A LINGERING SHEAR AXIS OVERHEAD TONIGHT...ON THE NORTHERN PERIPHERY OF THE RETROGRESSING SUB-TROPICAL RIDGE...MAY ACT WITH LINGERING OUTFLOW OVER THE NORTHERN PIEDMONT AND PROVIDE A FOCUS FOR A LINGERING SHOWER OR TWO EARLY TONIGHT...BUT NOCTURNAL STABILIZATION AND A LACK OF ANY MORE APPRECIABLE FORCING MECHANISMS SHOULD KEEP MOST AREAS DRY. LOWS IN THE MIDDLE TO UPPER 60S...WITH VARIABLE MID- HIGH CLOUDINESS.
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&& .SHORT TERM /MONDAY AND MONDAY NIGHT/... AS OF 415 AM SATURDAY... THE SUB-TROPICAL RIDGE WILL BE FURTHER FLATTENED BY THE NE PROGRESSION OF THE UPPER TROUGH OVER THE PLAINS THIS MORNING AND A SHORTWAVE TROUGH FORECAST TO AMPLIFY NE OF BERMUDA. THE LATTER FEATURE WILL CAUSE A BACKDOOR COLD FRONT TO SETTLE DOWN THE MIDDLE ATLANTIC COAST...BUT WELL SHORT OF CENTRAL NC. MOISTURE AT BOTH THE SURFACE AND ALOFT...CHARACTERIZED BY SFC DEWPOINTS IN THE LOWER- MIDDLE 60S AND PRECIPITABLE WATER VALUES NEAR ONE AND THREE QUARTER INCHES...RESPECTIVELY...WILL SUPPORT A CHANCE OF AFTERNOON AND EVENING STORMS...FOCUSED ALONG BOTH A STRENGTHENING LEE TROUGH FORECAST TO DRIFT EAST ACROSS THE PIEDMONT...AND THE SEA BREEZE FROM EASTERN VA TO THE EASTERN CAROLINAS. FURTHER MODIFICATION/WARMING OF THE AIR MASS...INCLUDING A CONTRIBUTION FROM INCREASINGLY NW DOWNSLOPE FLOW IN THE 925-850 MB LAYER...SHOULD SUPPORT HIGH TEMPERATURES IN THE UPPER 80S TO AROUND 90 DEGREES...WITH LOWS AGAIN IN THE MIDDLE TO UPPER 60S. && .LONG TERM /TUESDAY THROUGH SATURDAY/... AS OF 400 AM SUNDAY... THE PERSISTENT UPPER RIDGE OVER THE SOUTHEASTERN US WILL BE SUPPRESSED A BIT STARTING TUESDAY AS A SHORTWAVE CROSSING THE GREAT LAKES LEADS TO WEAK HEIGHT FALLS OVER THE MID-ATLANTIC STATES. THE ATTENDANT COLD FRONT WILL MOVE INTO THE OHIO VALLEY REGION BUT REMAIN WEST OF OUR AREA UNTIL AT LEAST TUESDAY NIGHT. AHEAD OF THE FRONT...A SHARP LEE TROUGH IS FORECAST TO SHIFT EAST ACROSS THE PIEDMONT AND SET UP EAST OF US 1 TUESDAY AFTERNOON. WEST OF THE TROUGH...DEWPOINTS SHOULD MIX OUT INTO THE LOW/MID 60S AND LIMIT INSTABILITY...WHILE UPPER 60 DEWPOINTS ACROSS THE COASTAL PLAIN WOULD SUPPORT 1000-1500 J/KG OF MLCAPE BASED ON FORECAST HIGHS IN THE UPPER 80S TO LOWER 90S. WHILE STEEP LOW-LEVEL LAPSE RATES SUPPORT SOME CHANCE OF AN ISOLATED SEVERE STORMS...THE OVERALL SEVERE THREAT WILL BE LOW DUE TO WEAK MID-LEVEL FLOW. CONVECTION WILL DEPART TO THE EAST AND DIMINISH TUESDAY EVENING AS THE AFOREMENTIONED FRONT MOVES IN FROM THE NORTHWEST OVERNIGHT. THE GFS CONTINUES TO BE NOTICEABLY FASTER MOVING THE FRONT THROUGH THE AREA AND ALL THE WAY TO THE SAVANNAH RIVER BASIN BY WEDNESDAY...WHILE THE ECMWF HANGS THE FRONT UP NEAR THE NC/SC BORDER. ITS HARD TO SEE THE FRONT MAKING IT AS FAR SOUTH AS THE GFS GIVEN FLAT FLOW ALOFT...BUT THE FRONT SHOULD PUSH SOUTH OF CENTRAL NC AND RESULT IN DRIER/MORE STABLE AIR OVER CENTRAL NC. THE DIFFERENCES IN WHERE THE FRONT ENDS UP LEADS TO A FAIR AMOUNT OF MODEL SPREAD IN THICKNESSES FOR WEDNESDAY AND THURSDAY. PREFER THE WARMER ECMWF BUT NOT THE EXTENT OF THE LOWER 90S ITS GIVES ACROSS THE SOUTH. WILL TEMPER HIGHS BACK A LITTLE MORE TOWARD NORMAL...WITH LOWER 80S NORTH TO NEAR 90 IN THE SOUTH. ANOTHER SHORTWAVE IS FORECAST TO SWING ACROSS THE MID-ATLANTIC STATES LATE THURSDAY AND FRIDAY...WHICH SHOULD TRIGGER A ROUND OF MAINLY SHOWERS THURSDAY NIGHT INTO FRIDAY WITH THE FRONTAL ZONE STILL TO OUR SOUTH. HIGH PRESSURE WILL THEN BUILD ACROSS NEW ENGLAND OVER THE WEEKEND AS RIDGING ALOFT AND A WARMING TREND REDEVELOPS. && .AVIATION /12Z SUNDAY THROUGH THURSDAY/...
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AS OF 705 AM SUNDAY... SUB-TROPICAL RIDGING WILL RESULT IN GENERALLY VFR CONDITIONS AND LIGHT SSW FLOW THROUGH THE TAF PERIOD...WITH THE EXCEPTION BEING A VERY SMALL CHANCE OF A SHOWER AT INT AND GSO LATE THIS AFTERNOON AND EVENING...AND A SIMILARLY LOW PROBABILITY OF A BRIEF OCCURRENCE OF FOG AT RWI AND FAY AROUND DAYBREAK MON. INITIALLY SCT TO BKN CIRRUS THIS MORNING WILL BE JOINED BY BOTH DIURNAL CUMULUS CLOUDS WITH BASES BETWEEN 3500-6000 FT...AND A LAYER OF MID CLOUDS IN BETWEEN MAINLY AT NORTHERN TAF SITES...BY THIS AFTERNOON. OUTLOOK: ASIDE FROM A LOW PROBABILITY OF BRIEF LOW STRATUS OR FOG EACH MORNING THROUGH TUE...MAINLY AT FAY AND RWI...THE ONLY OTHER POSSIBLE INTERRUPTION TO VFR CONDITIONS WILL BE IN SCATTERED SHOWERS AND STORMS...WITH THE RELATIVE GREATEST CHANCE TUE AFTERNOON AT RWI AND FAY...WITH CHANCES GENERALLY ONLY 25 PERCENT OR LESS ELSEWHERE AND OTHERWISE.
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&& .RAH WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SYNOPSIS...26 NEAR TERM...26 SHORT TERM...26 LONG TERM...22 AVIATION...26

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