Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 241031 AFDRAH AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 630 AM EDT THU JUL 24 2014 .SYNOPSIS...A COLD FRONT WILL PUSH SOUTHEASTWARD THROUGH THE REGION LATE TODAY THROUGH TONIGHT. THIS FRONT WILL STALL OUT NEAR THE SOUTH CAROLINA BORDER ON FRIDAY... AS WEAK HIGH PRESSURE BUILDS IN FROM THE NORTH. THIS FRONT WILL THEN WEAKEN AND HEAD BACK NORTH OF THE AREA OVER THE WEEKEND. && .NEAR TERM /THROUGH TONIGHT/... AS OF 320 AM THURSDAY... IT`S LOOKING LIKE THE BROAD BROKEN BAND OF PREFRONTAL SHOWERS AND STORMS WILL MOVE INTO AND THROUGH THE CWA FAIRLY EARLY TODAY... DURING THE MORNING THROUGH MID AFTERNOON... WHICH COULD CUT DOWN ON OUR SEVERE THREAT OVER ALL BUT THE SE AND EXTREME ERN CWA. THE LATEST SURFACE MAP SHOWS THE INCOMING SURFACE FRONT STRETCHING ACROSS THE OH VALLEY... WITH A PREFRONTAL TROUGH EXTENDING THROUGH CENTRAL VA AND THE NC FOOTHILLS. WE`RE ALREADY STARTING TO SEE A FEW SHOWERS FORMING ESE OF THIS CONVECTION OVER THE EAST SLOPES... AND THE HRRR SHOWS THIS MORE EASTERLY ACTIVITY EXPANDING AND BECOMING THE DOMINANT PREFRONTAL CONVECTION THIS MORNING AS IT HEADS INTO THE TRIAD REGION... EVENTUALLY SHIFTING EAST THROUGH THE PIEDMONT THROUGH THE NOON HOUR. THE LARGER SCALE MODELS SUGGEST THIS AS WELL... DEPICTING PRETTY GOOD COVERAGE OF PRECIP OVER NEARLY THE ENTIRE CWA BY 18Z. GIVEN THIS TREND IN BOTH RADAR IMAGERY AND MODEL OUTPUT... WILL FOLLOW THIS PATH AND BRING IN HIGH CHANCE TO LIKELY POPS EARLIER THIS MORNING OVER THE WRN/NRN CWA... HIGHEST ACROSS THE NRN HALF OF THE FORECAST AREA FROM LATE MORNING INTO EARLY AFTERNOON. THE FOCUS OF THE GREATEST CONVECTION COVERAGE SHOULD THEN SHIFT TO THE COASTAL PLAIN AND SANDHILLS FROM MID AFTERNOON TO EARLY EVENING AS THE PREFRONTAL TROUGH PUSHES SOUTHEASTWARD THROUGH THE CWA. BUT ADDITIONAL SCATTERED SHOWERS/STORMS ARE EXPECTED JUST AHEAD OF AND WITH THE COLD FRONT ITSELF... WHICH IS EXPECTED TO REACH THE SPINE OF THE APPALACHIANS BY 00Z THIS EVENING... PROPELLED BY RELATIVELY COOL HIGH PRESSURE CENTERED NEAR CHICAGO. THE SOUTHEAST MOVEMENT OF THIS FRONT IS APT TO BE SLOWED BY THREE FACTORS: THE EXTRA EFFORT IT WILL TAKE THE DENSER HIGH PRESSURE AREA TO GET OVER THE MOUNTAINS... A STEERING FLOW THAT WILL BE NEARLY PARALLEL TO THE SURFACE FRONT... AND EXPECTATIONS OF SURFACE LOW PRESSURE ENHANCEMENT OVER THE SOUTHERN FOOTHILLS TONIGHT. GIVEN THE POTENTIAL FOR A DELAY IN THE ARRIVAL OF THE DRIER AND MORE STABLE COLUMN FROM NW TO SE UNTIL THE OVERNIGHT HOURS... WILL HOLD CHANCE POPS THROUGH THE MID-LATE EVENING AREAWIDE BEFORE TRENDING THEM DOWN AND OUT NW TO SE LATE TONIGHT. REGARDING THE RISK OF STRONG/SEVERE STORMS... AS NOTED ABOVE... THIS RISK LOOK GREATLY LIMITED ACROSS THE NRN AND FAR WRN CWA DUE TO CLOUD COVER AND EARLY PRECIP ARRIVAL. CHANCES ARE BETTER IN THE SOUTHEAST AND EXTREME EAST... WHERE THE NAM DEPICTS MLCAPE NEAR 1500 J/KG LATE MORNING INTO EARLY AFTERNOON. WE`VE GOT IMPROVED DEEP LAYER SHEAR TO 25-35 KTS AS WELL. BUT THE ENTIRE TIMING OF THIS SYSTEM (WELL BEFORE MAX HEATING) AND EARLY CLOUD COVER SHOULD KEEP INSTABILITY DOWN. BUT THE NRN/WRN CWA IS NOT OUT OF THE WOODS AS WE MAY ACTUALLY HAVE A BIT MORE CONCERN LATE THIS AFTERNOON THROUGH EARLY EVENING AHEAD OF THE FRONT... IF WE CAN GET PARTIAL CLEARING BEHIND THIS MORNING ACTIVITY. THE GFS SHOWS INITIALLY SMALL MUCAPE IN THE MORNING IN THE NW CWA REBOUNDING ABOVE 1000 J/KG LATE TODAY... NOT A HUGE NUMBER OF COURSE BUT ENOUGH TO RETAIN AT LEAST SOME CONCERN FOR A STRONG WIND-PRODUCING STORM OR TWO AREAWIDE. WHILE CELL MOTIONS ARE EXPECTED TO BE GREATER THAN IN PREVIOUS DAYS... WE MAY SEE TRAINING CELLS REPEATEDLY OVER THE SAME AREA... AND A RISK OF LOCALLY HIGH RAIN TOTALS WILL PERSIST. TEMPS SHOULD INITIALLY BE HELD DOWN BY CLOUDS AND PATCHY PRECIP THIS MORNING... BUT ANY THIN SPOTS IN THE CLOUDS BETWEEN THIS PREFRONTAL CONVECTION AND SHOWERS/STORMS IMMEDIATELY AHEAD OF THE FRONT WILL PUSH TEMPS UP QUICKLY. SO STILL EXPECT HIGHS FROM 86 TO 92. LOWS TONIGHT 66-72... STILL MUGGY ACROSS THE SE. -GIH && .SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/... AS OF 320 AM THURSDAY... FOR FRI/FRI NIGHT: LITTLE CHANGED NEEDED TO EXISTING FORECAST. STILL EXPECT THE FRONT TO HANG UP ACROSS THE SE CWA AND ALONG THE CENTRAL/WRN NC/SC BORDER... AS THE 850 MB TROUGH HOLDS TO ITS NW ACROSS FAR NRN NC. THIS WILL KEEP ENOUGH LOW LEVEL MOISTURE WHICH... IN CONJUNCTION WITH LINGERING PW VALUES NEAR 1 INCH... NECESSITATES A MENTION OF ISOLATED TO SCATTERED SHOWERS AND STORMS OVER THE FAR SE CWA... FOCUSED ON THE TYPICAL AFTERNOON THROUGH EARLY EVENING TIME FRAME. MUCH LOWER PW OVER THE REMAINDER OF THE CWA AND MODEST RISE IN HEIGHTS AS THE MID LEVEL TROUGH WEAKENS AND STARTS TO PULL AWAY WILL LEAD TO MAINLY DRY WEATHER ELSEWHERE... ALTHOUGH ONE LAST SHOT OF DPVA THROUGH THE EXITING TROUGH BASE WILL DELAY THE MOST SUBSTANTIAL NW-TO-SE CLEARING UNTIL LATE IN THE DAY. HIGHS 85-90... WITH THICKNESSES SLIPPING TO 10-15 M BELOW NORMAL. LOWS 65-71. -GIH SAT AND SAT NIGHT: THE REMNANT FRONT FROM EARLY IN THE PERIOD WILL BECOME INCREASINGLY DIFFUSE AS IT RETREATS NORTH ACROSS THE MIDDLE ATLANTIC STATES ON SAT...WITH A PRIMARY IMPACT FOR CENTRAL NC BEING THE DEVELOPMENT OF A WARMING SOUTHERLY FLOW - HIGH TEMPERATURES GENERALLY BETWEEN 89 AND 93 DEGREES. ALOFT...MODEL GUIDANCE IS IN GOOD AGREEMENT REGARDING THE PROGRESSION OF A MID LEVEL TROUGH FROM THE APPALACHIAN SPINE SAT MORNING TO THE COAST BY EVENING...BUT ASSOCIATED FORECAST VERTICAL MOTION WITH THIS FEATURE IS WEAK...AND NAM FORECAST BUFR SOUNDINGS ARE AN UNSTABLE AND UNCAPPED OUTLIER RELATIVE TO THE GFS AND SREF ENSEMBLE MEAN. THE NAM QPF MAXIMUM FROM SE VA THROUGH CENTRAL NC IS ALSO AN OUTLIER RELATIVE TO OTHER GUIDANCE...WHICH OTHERWISE CONFINES ANY SHOWERS/STORMS TO THE SEA BREEZE. WILL CONSEQUENTLY MAINTAIN JUST A SLIGHT CHANCE POP OVER THE SOUTHERN AND CENTRAL COASTAL PLAIN...WHERE WEAK LIFT AHEAD OF THE AFOREMENTIONED TROUGH ALOFT WILL BE DIURNALLY MAXIMIZED AND COINCIDENT WITH LOW LEVEL FORCING INVOF THE SEA-BREEZE/SUBSEQUENT OUTFLOW DURING THE MID-LATE AFTERNOON. SHORTWAVE RIDGING WILL BRIEFLY FOLLOW FOR SAT NIGHT...WITH DRY CONDITIONS AND LOWS MAINLY IN THE LOWER 70S. && .LONG TERM /SUNDAY THROUGH WEDNESDAY/... AS OF 335 AM THURSDAY... THE MEDIUM RANGE WILL BE CHARACTERIZED BY THE DEVELOPMENT OF ANOTHER UNSEASONABLY HIGH AMPLITUDE TROUGH ALOFT FROM SOUTHEASTERN CANADA INTO THE EASTERN US...WITH AN ASSOCIATED NEGATIVE HEIGHT OF NEARLY 3 STANDARD DEVIATIONS BELOW AVERAGE FROM THE LOWER GREAT LAKES/OHIO VALLEY TO THE CENTRAL APPALACHIANS. PRECIPITATION CHANCES DURING THE PERIOD WILL BE HIGHEST SUN NIGHT-MON...AS THE TROUGH AMPLIFIES SOUTHEASTWARD AND SPREADS THE ASSOCIATED MID LEVEL HEIGHT FALLS AND UNDERLYING LO LEVEL FRONTAL ZONE ACROSS NC. IN FACT...THE DAY SUNDAY SHOULD BE MAINLY DRY...AND HOT AS THE MID LEVEL RIDGE OVER THE DESERT SW AND PLAINS BUILDS BRIEFLY EAST IN ADVANCE OF THE AMPLIFYING TROUGH ALOFT. H85 TEMPERATURES ARE FORECAST TO SURGE...ALBEIT BRIEFLY...INTO THE 22-24 DEGREE RANGE OVER THE CAROLINAS...SUPPORTIVE OF HIGH TEMPERATURES SOLIDLY IN THE MIDDLE 90S. UPSTREAM PRE-FRONTAL CONVECTION AND ASSOCIATED OUTFLOW WILL THEN LIKELY DRIFT...IN A GRADUALLY WEAKENING STATE OWING TO NOCTURNAL COOLING...INTO THE NORTHERN AND WESTERN PIEDMONT DURING THE EVENING AND EARLY NIGHTTIME HOURS. THE PATTERN ON THE SYNOPTIC SCALE...CHARACTERIZED BY THE APPROACHING STRONG TROUGH AND DOWNSTREAM JET ENTRANCE DIVERGENCE ALOFT...AND LOW LEVEL FRONTAL FORCING...FAVORS CONSIDERABLE THUNDERSTORM COVERAGE MON...INCLUDING SOME SEVERE OWING TO UNSEASONABLY STRONG MID LEVEL FLOW ON THE ORDER OF 40-50 KTS. HOWEVER...PREDICTABILITY ASSOCIATED WITH MESOSCALE INFLUENCES (IE. OUTFLOW AND CLOUD COVER) FROM EARLIER CONVECTION SUN NIGHT COMPLICATE THE FORECAST SO PROBABILITY OF PRECIPITATION WILL BE KEPT IN THE HIGH CHANCE RANGE FOR NOW...WITH THE EXPECTATION THAT THESE PROBABILITIES WILL BE INCREASED OVER AT LEAST A PORTION OF THE FORECAST AREA (LIKELY THE SOUTHEAST HALF). THE MIDDLE OF THE WEEK SHOULD BE DRY...AND COOLER...AS POST-FRONTAL HIGH PRESSURE BUILDS INTO THE CENTRAL APPALACHIANS...AND PRECIPITABLE WATER VALUES FALL BELOW AN INCH OVER CENTRAL NC. HIGHS AND LOWS ONLY IN THE LOWER TO MIDDLE 80S AND LOWER TO MIDDLE 60S...RESPECTIVELY...TUE-WED. && .AVIATION /12Z THURSDAY THROUGH MONDAY/... AS OF 630 AM THURSDAY... VFR CONDITIONS ARE LIKELY TO DOMINATE TODAY AT ALL TAF SITES... ALTHOUGH SCATTERED CLOUDS BASED AT 600-900 FT WILL PASS OVER OR NEAR RDU/RWI/FAY THROUGH 14Z. CURRENTLY THE VAST MAJORITY OF CLOUD COVER OVER CENTRAL NC IS LIMITED TO HIGH CLOUDS BASED ABOVE 10 000 FT AGL... AND WHILE CIGS WILL BE DROPPING AHEAD OF THE COLD FRONT APPROACHING FROM THE NW... THEY SHOULD STAY VFR. THE EXCEPTION WILL BE IN/NEAR SCATTERED TO NUMEROUS SHOWERS AND STORMS THAT ARE OCCURRING NEAR INT/GSO... WHERE CIGS/VSBYS MAY DROP TO MVFR... ALONG WITH ENHANCED WIND GUSTS. THESE SHOWERS/STORMS ARE THEN EXPECTED TO AFFECT RDU AND RWI STARTING AROUND 13Z OR 14Z... AND FAY STARTING AT 17Z OR 18Z. ADDITIONAL SCATTERED STORMS ARE POSSIBLE MAINLY AT INT/GSO/RDU DURING THE MID-LATE AFTERNOON... BUT UNCERTAINTIES REGARDING COVERAGE AND TIMING PRECLUDE MENTIONING THESE IN THE TAFS AT THIS TIME. TOUGH CALL ON WHEN THE STORM THREAT WILL EXIT TAF SITES... BUT VFR CONDITIONS SHOULD RETURN SOON AFTER 22Z AT GSO... 00Z AT RDU/RWI... AND PROBABLY NOT UNTIL AFTER 04Z AT FAY. SURFACE WINDS WILL REMAIN FAIRLY LIGHT LATE TONIGHT BUT SHIFT AROUND TO MAINLY NNE AS THE FRONT PUSHES SOUTHEAST THROUGH NC. AS THESE SURFACE WINDS SHIFT AROUND TO LIGHT FROM THE NE OR LIGHT/VRBL LATE TONIGHT... A SHORT PERIOD OF IFR CIGS/VSBYS IS POSSIBLE AT ALL SITES FROM 09Z-12Z. LOOKING BEYOND 12Z FRI MORNING... THE FRONT WILL LIKELY SLOW DOWN ENOUGH OVER SE NC TO KEEP A CHANCE OF SUB-VFR SHOWERS/STORMS AT FAY THROUGH FRI AFTERNOON... BUT OTHERWISE VFR CONDITIONS ARE EXPECTED FRI-SAT AS HIGH PRESSURE NOSES IN FROM THE NORTH. THIS FRONT WILL SHIFT BACK NORTH AND DISSIPATE OVER THE WEEKEND WITH A SURFACE TROUGH REFORMING OVER CENTRAL NC. AN UPPER LEVEL DISTURBANCE AND ACCOMPANYING COLD FRONT WILL MOVE IN FROM THE NORTHWEST ON SUNDAY... POTENTIALLY BRINGING ANOTHER ROUND OF SHOWERS/STORMS LATE SUNDAY INTO MONDAY. -GIH && .RAH WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SYNOPSIS..HARTFIELD NEAR TERM...HARTFIELD SHORT TERM...HARTFIELD/MWS LONG TERM...MWS AVIATION...HARTFIELD

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