Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 310709 AFDRAH AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 310 AM EDT THU JUL 31 2014 .SYNOPSIS...THE ATMOSPHERE ACROSS CENTRAL NC WILL BECOME VERY MOIST AND SLIGHTLY UNSTABLE. THIS WILL LEAD TO AN UNSETTLED WEATHER PATTERN FRIDAY...LASTING INTO THE WEEKEND. && .NEAR TERM /THROUGH TONIGHT/... AS OF 310 AM THURSDAY... RECENT TRANQUIL WEATHER PATTERN WILL BECOME INCREASINGLY UNSETTLED THE NEXT FEW DAYS. ISOLATED-SCATTERED SHOWERS IN THE SANDHILLS EARLY THIS MORNING ASSOCIATED WITH WEAK LOW LEVEL MOISTURE ADVECTION COUPLED WITH AN AREA OF LIFT ASSOCIATED WITH THE NOSE OF THE UPPER LEVEL JET. EXPECT THESE SHOWERS TO WANE IN COVERAGE AND INTENSITY PRIOR TO DAYBREAK AS LIFT WEAKENS THOUGH COULD SEE A REGENERATION OF SHOWER ACTIVITY FARTHER WEST ACROSS THE SOUTHERN PIEDMONT AROUND DAYBREAK IN ASSOCIATION WITH MODEST UPSLOPE FLOW. MID-UPPER LEVEL FLOW WILL BACK IN RESPONSE TO A L/W TROUGH THAT DIGS SOUTHWARD INTO THE TN VALLEY-DEEP SOUTH THROUGH FRIDAY. THIS BACKING FLOW WILL ADVECT PLENTY OF WARM MOIST AIR RESIDING OVER TEH GULF INTO CENTRAL NC. THIS WILL LEAD TO CONSIDERABLE CLOUDINESS OVER THE REGION TODAY AND CONTINUING THROUGH TONIGHT. A SERIES OF UPPER LEVEL DISTURBANCES WILL INTERACT WITH THIS AVAILABLE MOISTURE TO CAUSE SCATTERED SHOWERS ALONG WITH A FEW T- STORMS. FIRST SUCH DISTURBANCE BEING HANDLED WELL BY THE ECMWF...CURRENTLY EXITING THE MS DELTA REGION AT 06Z. THIS FEATURE SHOULD BE NEAR THE UPSTATE OF SC BY 18Z...THEN TRACK E-NE ALONG THE NC/SC BORDER. BASED ON FAVORABLE TIMING AND DEEPER MOISTURE RESIDING OVR THE WEST VERSUS THE EAST...WILL MAINTAIN THE CHANCE POPS THIS AFTERNOON ACROSS THE WESTERN-SOUTHERN PIEDMONT TO SLIGHT CHANCE ACROSS THE NE. WILL NEED TO MONITOR UPSTREAM CONDITIONS AS COULD SEE A HIGHER SHOWER THREAT OVER THE WEST-SW THAN CURRENTLY ADVERTISED. PRESENT INDICATIONS ARE THAT ANY STORMS THAT DEVELOP SHOULD REMAIN BELOW SEVERE CRITERIA AS ATMOSPHERE PROJECTED TO BE RELATIVELY STABLE FOR LATE JULY. DID NOTE THE BULK SHEAR OF 35-40KTS (PER GFS) ACROSS THE SOUTH-SE THIS AFTERNOON-EARLY EVENING. THIS WOULD SUGGEST SOME POTENTIAL FOR SOME ORGANIZED CONVECTION IF ENOUGH INSTABILITY WAS ACHIEVED. MAX TEMPS THIS AFTERNOON WILL LIKELY BE HELD DOWN A FEW DEGREES FROM NORMAL VALUES DUE TO CONSIDERABLE CLOUD COVER. MAX TEMPS GENERALLY IN THE MID 80S. TONIGHT...POTENTIAL TO SEE AN INCREASE IN SHOWER ACTIVITY...ESPECIALLY ACROSS THE WEST...AS ANOTHER UPPER LEVEL DISTURBANCE PROJECTED TO LIFT E-NE ACROSS THE REGION. THIS OCCURS WHILE LOW LEVEL FLOW AND ITS UPSLOPE COMPONENT INCREASES...PROVIDING LOW LEVEL LIFT IN THE INCREASINGLY MOIST AIR MASS. PLAN TO MAINTAIN FORECAST TREND OF INCREASING POPS OVERNIGHT...ESPECIALLY AFTER MIDNIGHT IN THE WEST. IF MODEL TRENDS REMAIN CONSISTENT...COULD ARGUE FOR HIGH CHANCE/LOW END LIKELY IN VICINITY OF THE YADKIN RIVER LATE TONIGHT. MIN TEMPS 65-70. && .SHORT TERM /FRIDAY AND FRIDAY NIGHT/... AS OF 310 AM THURSDAY... MID-UPPER LEVEL FLOW BACKS IN RESPONSE TO THE DIGGING L/W TROUGH OVER THE TN VALLEY/DEEP SOUTH. THIS PLACES CENTRAL NC IN THE FAVORABLE RIGHT ENTRANCE REGION OF UPPER JET PLUS TAPS ABUNDANT MOISTURE RESIDING OVER THE GULF. A RATHER POTENT VORT MAX CURRENTLY ASSOCIATED WITH CONVECTION ACROSS THE SOUTHERN PLAINS/RED RIVER VALLEY PROJECTED TO CROSS OUR REGION FRIDAY. LIFT ASSOCIATED WITH THIS FEATURE ALONG WITH JET DYNAMICS AND AVAILABLE DEEP MOISTURE SUGGEST LIKELY POPS FOR FRIDAY-FRIDAY EVENING. LOCALLY HEAVY RAIN MAY OCCUR ACROSS THE WEST WHERE TRAINING OF SHOWERS/STORMS APPEARS MORE PROBABLE. CONVECTIVE PARAMETERS OVER THE SANDHILLS AND COASTAL PLAIN ARE A BIT MORE SUPPORTIVE OF AN ISOLATED SEVERE STORM AS MLCAPE APPROACHES 1000J/KG AND BULK SHEAR 40-50KTS NOTED. CONSIDERABLE CLOUDINESSS AND NUMEROUS SHOWERS-STORMS SHOULD HOLD TEMPS DOWN SEVERAL DEGREES BELOW NORMAL. MAX TEMPS WILL LIKELY NOT BREAK 80 DEGREES OVER PARTS OF THE PIEDMONT. MAX TEMPS FRIDAY MID 70S NW TO LOWER 80S SE. IF SHOWER COVERAGE TURNS OUT TO BE MORE SCATTERED THAN NUMEROUS...THEN MAX TEMPS MAY END UP BEING 3-4 DEGREES WARMER. ANTICIPATE A BRIEF LULL IN CONVECTIVE ACTIVITY SOMETIME FRIDAY NIGHT AS ONE VORT MAX LIFTS OUT OF THE REGION BUT ANOTHER FOLLOWS BEHIND IT...BUT DIFFICULT TO SAY FOR CERTAIN IF/WHEN THIS OCCURS. PLAN TO SHOW A GRADUAL DECREASE IN POPS FRIDAY NIGHT WITH CHANCE POPS (40 PERCENT) FOR MOST LOCATIONS AFTER MIDNIGHT. MIN TEMPS 65-70. && .LONG TERM /SATURDAY THROUGH WEDNESDAY/... AS OF 310 AM THURSDAY... EARLY IN THIS LONG-TERM PERIOD...A SOUTHWEST FLOW ALOFT IS FORECAST OVER CENTRAL NORTH CAROLINA WITH A SERIES OF MID-LEVEL SHORTWAVES MOVING THROUGH THE AREA AHEAD OF A TROUGH AXIS EXTENDING FROM THE GREAT LAKES THROUGH THE OHIO AND TENNESSEE VALLEYS. WHILE THERE ARE TIMING DIFFERENCES...THE LONG-TERM GUIDANCE IS FAIRLY CONSISTENT IN LIFTING THE TROUGH NORTH BY EARLY NEXT WEEK WITH A MODEST INCREASE IN HEIGHTS ALOFT...ALTHOUGH A WEAK SOUTHWEST FLOW ALOFT TENDS TO PERSIST OVER CENTRAL NORTH CAROLINA AHEAD OF A WEAKNESS IN THE MID- LEVEL HEIGHTS TO OUR SOUTHWEST IN VICINITY OF MISSISSIPPI AND ALABAMA. EXTENDING TO THE NORTHEAST OF THIS WEAK LOW ALOFT MAY BE A SHEAR AXIS NEAR OR OVER CENTRAL NORTH CAROLINA AT LEAST THROUGH EARLY NEXT WEEK. ON SATURDAY...THE NAM MOVES THE SURFACE FRONT FARTHER INLAND THAN THE GFS AND ECMWF...SUCH AS THAT FORMER MODEL HAS MUCH HIGHER QPF TOWARD THE TRIAD THAN THE OTHERS. NEVERTHELESS...MOS GUIDANCE POPS ARE HIGH THROUGHOUT CENTRAL NORTH CAROLINA SATURDAY...AND GIVEN THE INLAND PROGRESSION OF THE FRONT...MID-LEVEL WAVES MOVING THROUGH THE AREA IN THE SOUTHWEST FLOW...AND AMPLE MOISTURE...WILL CONTINUE WITH THE LIKELY CHANCES FOR SHOWERS ALONG WITH SCATTERED THUNDERSTORMS. INSTABILITY ON THE NAM AND ECMWF IS NOT AS GREAT AS ON THE GFS... WHICH ACTUALLY PAINTS A SOMEWHAT CONCERNING PICTURE SATURDAY WITH SURFACE-BASED CAPE NEAR 1000J/KG...0-3KM HELICITY NEAR 100M2/S2... AND SHEAR IN THE 20S KNOTS. AS LCL HEIGHTS SHOULD BE LOW GIVEN THE DEGREE OF LOW-LEVEL MOISTURE...DEPENDING ON THE INSTABILITY A ROTATING CELL OR TWO IN VICINITY OF THE SURFACE FRONT WOULD BE A POSSIBILITY IF THE GFS VERIFIES. FOR NOW WILL FOCUS ON THE RAINFALL BUT TRENDS WILL NEED TO BE MONITORED. BASIN-AVERAGE RAINFALL AMOUNTS LATE IN THE WEEK INTO THE WEEKEND COULD BE AROUND 1.5 INCHES. WHILE THE SURFACE TROUGH SLOWLY WEAKENS...ITS POSITION RELATIVE TO CENTRAL NORTH CAROLINA FOR NOW DEMONSTRATES LITTLE CHANGE THROUGH THE REST OF THE LONG-TERM PERIOD. GOOD CHANCES FOR SHOWERS AND AT LEAST ISOLATED THUNDERSTORMS CONTINUE INTO SUNDAY...THEN AS THE MID- LEVEL TROUGH LIFTS NORTH AND HEIGHTS INCREASE MODESTLY ALOFT... CHANCES FOR SHOWERS AND THUNDERSTORMS SHOULD START TO TAKE ON MORE OF A TYPICAL DIURNAL NATURE...AIDED BY WEAK SURFACE CONVERGENCE... LINGERING MOISTURE...AND A LIKELY SHEAR AXIS NEARBY OR OVERHEAD. THE PRECIPITATION CHANCES FOR MONDAY THROUGH WEDNESDAY WILL TEND TO BE LOWEST IN THE NORTHWEST PIEDMONT AND HIGHEST TOWARD THE INTERSTATE 95 CORRIDOR...THE LATTER WHERE THE DEEPER MOISTURE AND BETTER INSTABILITY SHOULD BE EACH DAY. WHILE THE ECMWF REALLY TRENDS TOWARD A TYPICAL DIURNAL PATTERN OF CHANCES DURING THE DAY AND VIRTUALLY DRY OVERNIGHT...THE LATEST GFS FORECASTS PERIODIC WAVES MOVING SOUTHWEST-TO-NORTHEAST ALONG THE WEAK SURFACE BOUNDARY WHICH KEEPS SOME CHANCES EVEN INTO THE OVERNIGHT HOURS. FOR NOW...WILL FORECAST FOR EARLY NEXT WEEK LOWER POPS OVERNIGHT BUT ONLY SHOW BELOW SLIGHT CHANCE POPS IN THE NORTHWEST PIEDMONT OVERNIGHT TUESDAY WHEN BOTH THE GFS AND THE ECMWF AGREE IN A RELATIVE MINIMUM OF PRECIPITATION THERE THEN. 1000-850MB THICKNESSES AND EXPECTED CLOUD COVER WILL RESULT IN HIGHS LEANING TOWARD THE COOLER END OF GUIDANCE...AND IN MANY CASES A LITTLE BELOW. SOME OF THE BIAS-CORRECTED GUIDANCE HAS BEEN DOING WELL OF LATE AND WILL LEAN TOWARD THOSE WHICH ARE CONSISTENTLY ON THE COOL END OF GUIDANCE. SOME AREAS COULD START TO RETURN CLOSE TO 90 BY WEDNESDAY. && .AVIATION /06Z THURSDAY THROUGH MONDAY/... AS OF 120 AM THURSDAY... WHILE VFR CONDITIONS WILL PERSIST THROUGH THIS AFTERNOON...THERE MAY BE A BRIEF INSTANCE OF MVFR/IFR FOG BETWEEN 09Z-12Z IN THE VICINITY OF KRDU...KFAY AND KRWI. IN ADDITION...THERE MAY BE A PASSING SHOWER AND ASSOCIATED MVFR VISIBILITY IN THE KFAY VICINITY THROUGH 09Z OTHERWISE EXPECT MOSTLY CLOUDY SKIES ACROSS THE REGION THROUGH THURSDAY AFTERNOON THOUGH CLOUD BASES WILL REMAIN VFR. SURFACE WINDS WILL BE LIGHT AND VARIABLE. THERE WILL BE ISOLATED TO SCATTERED SHOWERS AND POSSIBLY A THUNDERSTORM DUE TO SURFACE HEATING AND A MARGINALLY MOIST AIR MASS. THE HIGHEST THREAT FOR THIS SCATTERED CONVECTION WILL BE SOUTH OF THE TRIAD TERMINALS AND WEST OF KFAY. FOR TONIGHT INTO MONDAY...EXPECT PERIODS OF ADVERSE AVIATION WEATHER CONDITIONS TO BEGIN LATER TONIGHT AS THE ATMOSPHERE OVER CENTRAL NC BECOMES INCREASINGLY MOIST AND UNSTABLE. A SERIES OF UPPER LEVEL DISTURBANCE WILL TRAVERSE THE REGION THROUGH MONDAY...CAUSING SCATTERED TO NUMEROUS SHOWERS AND THUNDERSTORMS. IN ADDITION...THERE WILL BE AN INCREASED PROBABILITY FOR PROLONGED MVFR-IFR CEILINGS. && .RAH WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SYNOPSIS...WSS NEAR TERM...WSS SHORT TERM...WSS LONG TERM...DJF AVIATION...WSS

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