Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 222109 AFDRAH AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 410 PM EST SAT NOV 22 2014 .SYNOPSIS...
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HIGH PRESSURE WILL CONTINUE TO SHIFT OFFSHORE TONIGHT. A STRONG LOW PRESSURE MOVING FROM THE SOUTHERN PLAINS INTO THE GREAT LAKES SUNDAY WILL SPREAD MOISTURE OVER THE REGION DURING THE DAY ON SUNDAY. ANOTHER COLD FRONT WILL MOVE EAST ACROSS MUCH OF THE AREA MONDAY...WITH LOW PRESSURE MOVING ALONG THE SOUTHEAST COAST TUESDAY INTO WEDNESDAY.
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&& .NEAR TERM /THROUGH TONIGHT/... AS OF 305 PM SATURDAY... HIGH PRESSURE OFFSHORE EXTENDS EAST TO WEST ACROSS THE STATE TODAY ....WHILE A FEW PATCHES OF CIRRUS DRIFT OVERHEAD IN THE SOUTHWESTERLY FLOW ALOFT. OTHERWISE...REGIONAL VIS SAT IMAGERY SHOWS VERY LITTLE CLOUD COVER UPSTREAM...WITH MOISTURE JUST NOW BEGINNING TO SURGE INLAND FROM THE GOMEX ACROSS SOUTHERN GA/AL. EXPECT SKIES WILL BE MOSTLY CLEAR FOR AT LEAST THE FIRST HALF OF NIGHT...BEFORE THICKER CIRRUS AND CIRROSTRATUS BEGIN TO ADVECTION IN FROM THE SOUTH. IN ADDITION...MOISTURE RETURN AROUND THE SURFACE HIGH MAY LEAD TO SOME STRATUS LATE TONIGHT....WITH SREF PROBABILITIES SHOWING THE BEST CHANCE OVER THE WESTERN PIEDMONT...WHICH SEEMS REASONABLE GIVEN THE AMOUNT OF DRY AIR CURRENTLY OVER CENTRAL AND EASTERN NC. LOWS ACROSS THE NORTH...WHERE RADIATIONAL COOLING WILL BE BEST FOR MOST OF THE NIGHT...MAY DIP INTO THE LOWER 30S...WHILE AREAS TO THE SOUTH WILL SEE MID AND UPPER 30S. && .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
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AS OF 410 PM SATURDAY... A SHORTWAVE TROUGH CURRENTLY OVER SOUTHWEST TX WILL EJECT NORTHEASTWARD SUNDAY MORNING...DEAMPLIFYING AS IT LIFTS TOWARD THE GREAT LAKES REGION. WHILE THE SURFACE LOW WILL ALSO LIFT WELL WEST OF THE MOUNTAINS...A 50-60KT LLJ WILL BRING PW OF OVER 1.75 INCHES WITHIN THE ASSOCIATED WARM CONVEYOR BELT LATE SUNDAY EVENING/NIGHT. BASED ON A CONSENSUS OF MODEL ISENTROPIC FIELDS...RAIN WILL BEGIN TO SPREAD INTO THE AREA FORM THE SOUTH MID-MORNING... THOUGH PRECIP WILL INITIALLY HAVE TO OVERCOME THE RATHER DRY AND ONLY SLOW MOISTENING AIRMASS OVER NC. THE HEAVIER PRECIP ACCOMPANYING THE SURFACE/850MB WARM FRONTS WILL BEGIN TO SPREAD INTO THE AREA AFTER 21Z...AND IT LOOKS AS THOUGH THE 00-06Z TIME FRAME WILL BE THE BEST FOR THE HEAVIEST RAIN. TEMPS... WITH DEWPOINTS IN THE 30S IN THE WESTERN PIEDMONT EARLY SUNDAY...EVAPORATIVE COOLING WILL LIKELY LEAD TO AT LEAST A WEAK...STABLE WEDGE AIRMASS OVER THE NORTHWEST PIEDMONT. HIGHS IN THE TRIAD MAY STRUGGLE TO GET OUT OF THE UPPER 40S...WHILE TEMPS WILL HAVE LITTLE MORE TIME TO WARM INTO THE 50S TO LOWER 60S TO THE EAST. TEMPS ARE THEN EXPECTED TO HOLD STEADY OR CONTINUE RISING SLOWLY INTO THE UPPER 50S TO MID 60S OVERNIGHT AS THE WARM FRONT SURGES NORTH THROUGH THE AREA. WHETHER OR NOT THE WEDGE AIRMASS IS ERODED IN THE NORTHWEST PIEDMONT...AS MODELS INDICATE...IS UNCERTAIN. BUT FOR MANY AREAS...HIGHS WILL LIKELY OCCUR WELL AFTER SUNSET. SEVERE THREAT... STRONG WARM ADVECTION WILL LEAD TO SOME WEAK DESTABILIZATION...ELEVATED TO THE NORTHWEST AND NEAR SURFACE BASED IN THE SOUTHEAST. MODELS HAVE TRENDED A LITTLE FURTHER INLAND WITH THE BETTER PUSH OF THE MARITIME AIR...WITH THE NAM SHOWING AS MUCH AS 200-600 J/KG MLCAPE AT KFAY AS DEWPOINTS RISE INTO THE MID 60S. THIS APPEARS TO BE THE HIGHEST MLCAPE GENERATED BY ANY OPERATIONAL NWP...WITH THE GFS/SREF SHOWING 100-300 J/KG...AND NOCTURNAL STABILIZATION WILL FURTHER LIMIT CAPE. EFFECTIVE DEEP LAYER SHEAR INCREASING TO 50-70KT SHOULD SUPPORT SOME LOW-TOPPED SUPERCELLULAR STORMS EMBEDDED WITHIN THE BANDS OF SHOWERS AND STORMS CROSSING SC AND SOUTHEASTERN NC. IN ADDITION...THERE WILL BE A 4-6 HOUR WINDOW LATE SUNDAY EVENING WHEN THE WEAK INSTABILITY WILL LINE UP WITH STRONG CYCLONICALLY CURVED LOW-LEVEL HODOGRAPHS...AND LOW-END TORNADO THREAT MAY EXIST ALONG AND EAST OF I-95. FURTHER TO THE NORTHWEST...ELEVATED INSTABILITY WILL ALSO BE WEAK..BUT DPVA FROM THE VORT LOBE ATTENDANT TO THE MAIN SHORTWAVE MAY ALSO BE ENOUGH FOR AN ISOLATED STORM EVEN BACK TOWARD THE TRIAD. -BLS MON AND MON NIGHT: THE AXIS OF A 50 KT LLJ...IN DEEP SW FLOW ALOFT...WILL CROSS CENTRAL NC DURING THE DAY MONDAY...IN ADVANCE OF THE SURFACE COLD FRONT THAT IS FORECAST TO CROSS THE AREA LATE MON AND MON EVENING - A SOLUTION BEST REPRESENTED BY THE SLOWER 12Z ECMWF/NAM GUIDANCE. THE PASSAGE OF THE LLJ AND ASSOCIATED MOISTURE TRANSPORT...THEN LATER FRONTAL FORCING/LOW LEVEL CONVERGENCE...WILL SUPPORT SCATTERED SHOWERS - AND ISOLATED THUNDER IN AN AXIS OF LOWER TO MIDDLE 60S SURFACE DEWPOINTS IN THE COASTAL PLAIN - ENDING FROM WEST TO EAST DURING THE AFTERNOON AND EVENING HOURS. STRONG AND INCREASINGLY UNIDIRECTIONAL DEEP LAYER FLOW...AFTER THE EARLY MORNING WARM FRONTAL PASSAGE...WILL SUPPORT A CONDITIONAL RISK OF SEVERE WEATHER IN THE COASTAL PLAIN. IN ADDITION...MIXING OF THE STRONG LLJ WILL RESULT IN A VERY BREEZY SW WIND...SUSTAINED AROUND 20 MPH...WITH GUSTS BETWEEN 30 AND 40 MPH PROBABLE PER ~35 KT MIXED LAYER WIND FORECASTS. HIGHS GENERALLY IN THE LOWER TO MIDDLE 70S. AFTER PERHAPS A LINGERING SHOWER IN THE COASTAL PLAIN DURING THE EVENING...COLD AND DRY ADVECTION...AND CLEARING...WILL DEVELOP EASTWARD AND RESULT IN LOWS RANGING FROM AROUND 40 WEST TO LOWER 50S EAST. -MWS
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&& .LONG TERM /TUESDAY THROUGH SATURDAY/... AS OF 350 PM SATURDAY... TUE-WED: 1025 MB SURFACE HIGH PRESSURE WILL BUILD SOUTHEAST ACROSS THE HIGH PLAINS TUE...THEN EASTWARD INTO THE MIDDLE ATLANTIC STATES TUE NIGHT. MEANWHILE...THE APPROACH OF A POSITIVELY-TILTED TROUGH ALOFT FROM THE MS VALLEY WILL INDUCE COASTAL CYCLOGENESIS...ALONG AN OLD FRONTAL ZONE OFF THE SOUTH EAST...REFERENCED IN THE SHORT TERM DISCUSSION ABOVE. THERE REMAINS CONSIDERABLE MODEL SPREAD WITH THE DEVELOPMENT AND TRACK OF THIS COASTAL LOW AS IT LIFTS UP THE SOUTHEAST AND MIDDLE ATLANTIC COASTS THROUGH MID WEEK...RANGING FROM THE WELL OFFSHORE AND TOTALLY DRY-FOR-CENTRAL-NC CANADIAN SOLUTIONS OF RECENT DAYS...TO THE CLOUDY AND WET SOLUTIONS DEPICTED BY THE PAST FEW ECMWF AND GFS RUNS. GIVEN THE POSITIVE TILT AND ONLY SLOW EASTWARD PROGRESSION OF THE TROUGH ALOFT...THE MORE WESTWARD (AND WET) SOLUTIONS OF THE OFFSHORE FRONT AND INCIPIENT COASTAL LOW ARE FAVORED...WITH A LOW TRACK MOST PROBABLE ALONG THE WESTERN WALL OF THE GULF STREAM. THE RESULT WOULD BE RAIN DEVELOPING FROM THE SOUTH LATE TUE AND ESPECIALLY TUE NIGHT...WITH A COLD RAIN LIKELY AND TEMPERATURES IN THE UPPER 30S TO MIDDLE 40S...ON WED. THOUGH THE AFOREMENTIONED PARENT POLAR HIGH IS FORECAST TO BE RELATIVELY WEAK (AROUND 1025 MB) AND POSITIONED FARTHER SOUTH (OVER THE MIDDLE ATLANTIC STATES) THAN IS TYPICAL FOR SNOW IN CENTRAL NC...THE TRACK OF THE COASTAL LOW AND RESULTANT PARTIAL THICKNESSES ON THE WEST/NW SIDE SUGGEST AT LEAST A THREAT OF A LITTLE WEST SNOW MIXING IN WITH THE RAIN OVER THE PIEDMONT. GIVEN THE MARGINALLY COLD THERMODYNAMIC PROFILES...AND FREEZING LEVELS AROUND 1000 FT OVER THE PIEDMONT PER GFS BUFR FORECAST SOUNDINGS...ANY SUCH SNOW WOULD LIKELY BE DRIVEN BY PRECIPITATION RATES...AND THE EXCEPTION RATHER THAN THE RULE. WED NIGHT-SAT: MEDIUM RANGE GUIDANCE HAS COME INTO SOMEWHAT BETTER AGREEMENT (THAN RECENT DAYS) REGARDING THE DEVELOPMENT OF A BROAD TROUGH ALOFT FROM NORTH-CENTRAL CANADA TO THE MIDDLE ATLANTIC COAST LATE WEEK INTO NEXT WEEKEND. SUCH A PATTERN WOULD FAVOR THE RE- ESTABLISHMENT OF ARCTIC HIGH PRESSURE ACROSS THE NORTHERN HALF OF THE CONUS EAST OF THE ROCKIES (JUST TO THE NORTH OF NC)...AND MILDER/MORE MODIFIED HIGH PRESSURE OVER THE SOUTHEAST STATES. THIS PATTERN OF NW TO WNW FLOW ALOFT WOULD RESULT IN DRY AND COOL CONDITIONS OVER CENTRAL NC. && .AVIATION /18Z SATURDAY THROUGH THURSDAY/... AS OF 100 PM SATURDAY... VFR CONDITIONS TO CONTINUE THIS AFTERNOON AND EVENING AS HIGH PRESSURE SETTLES JUST OFFSHORE. RETURN FLOW...ESPECIALLY ACROSS THE WESTERN PORTIONS OF NC MAY BRING SOME STRATUS ACROSS THE TRIAD LATE TONIGHT. CONFIDENCE IS ONLY MEDIUM AT THIS POINT DUE TO SOME DIFFERENCES IN FORECAST SOUNDINGS. USING THE SREF AS A GUIDE...MVFR CEILINGS (~2500 FT) MAY DEVELOP AROUND 09Z AND LAST THROUGH 13-14Z. THE TRAJECTORY OF MOISTURE WOULD SUGGEST LESS OF A CHANCE OF CEILINGS AT KRDU AND EVEN LESS AT KRWI. AT KFAY...INCREASING DEWPOINTS AND LIGHT WINDS MAY CAUSE MAINLY MVFR VSBYS BY 12Z. VFR CONDITIONS ARE EXPECTED TO REDEVELOP BY MID-MORNING...BUT ADVERSE AVIATION CONDITIONS WILL BEGIN TO DEVELOP FROM SOUTH TO NORTH DURING THE AFTERNOON AS A LOW PRESSURE SYSTEM APPROACHES FROM THE SOUTHWEST. OUTLOOK...CONDITIONS WILL DETERIORATE RAPIDLY SUNDAY EVENING AS RAIN...HEAVY AT TIMES...SPREADS ACROSS THE AREA. A STRONG 50-60KT LOW LEVEL JET MAY CAUSE SOME LOW-LEVEL WIND SHEAR...ESPECIALLY AT KGSO/KINT...WHILE SOME 30-40KT WIND GUSTS MAY BE POSSIBLE OVER EASTERN NC WHERE THUNDERSTORMS DEVELOP. CONDITIONS WILL IMPROVE SOME ON MONDAY AS DRIER AIR ALOFT MOVES IN FROM THE WEST...THOUGH LOW CEILINGS MAY LINGER AT EASTERN TERMINALS. BEYOND MONDAY..ANOTHER SYSTEM WILL IMPACT THE REGION BY MIDWEEK...THOUGH THERE IS SOME UNCERTAINTY AS TO HOW MUCH OF THE AREA WILL BE IMPACTED BY PRECIPITATION AND ADVERSE AVIATION CONDITIONS AS A SURFACE LOW MOVES ALONG THE EAST COAST. && .RAH WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SYNOPSIS...DJF NEAR TERM...BLS SHORT TERM...BLS/MWS LONG TERM...MWS AVIATION...BLS

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