Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 311421 AFDRAH AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 1021 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 1015 AM THURSDAY... FOR THE MID MORNING UPDATE...FORECAST IS ON TRACK FOR INCREASING CLOUD COVER AND LIGHT PRECIPITATION FROM THE SOUTHWEST INTO THE NORTHWEST PIEDMONT. THIS WILL BE AIDED BY AN EXITING UPPER LEVEL 80 KT JET STREAK AND A COUPLE OF MID-LEVEL DISTURBANCES...THE FIRST OF WHICH WILL BE A WEAKER ONE...MOVING THROUGH THE AREA BY LATE AFTERNOON...FOLLOWED BY A STRONGER VORT MAX OVERNIGHT WHICH WILL MOVE THROUGH THE WESTERN PIEDMONT BY EARLY FRIDAY MORNING. WHILE THESE DISTURBANCES COULD INITIATE A FEW THUNDERSTORMS...INSTABILITY IS MODEST AT BEST AND SHEAR VALUES OF 35-40 KTS MAY BE TOO FAR EAST TO INTERACT WITH INCREASING MOISTURE WHICH SHOULD FIRST ARRIVE IN THE WEST...THUS MAKING SEVERE THUNDERSTORMS FAIRLY UNLIKELY. CLOUD COVER SHOULD KEEP MAX TEMPS THIS AFTERNOON IN THE MID 80S DESPITE THICKNESS VALUES SUGGESTIVE OF NEAR 90 DEGREES. THE STRONGER DISTURBANCE THIS EVENING...WHILE DIURNALLY UNFAVORABLE FOR CONVECTION...WILL HAVE SOME HELP WITH FORCING FROM AN INCREASING UPSLOPE COMPONENT AND THUS CONTINUED CLOUD COVER AND RAIN CHANCES WILL CONTINUE THROUGH THE NIGHT...ESPECIALLY IN THE WESTERN PIEDMONT WHERE THE WAVE IS ANTICIPATED TO TRACK. INCREASED CLOUDS WILL HELP TO MODERATE LOW TEMPS INTO THE MID 60S TO NEAR 70 DEGREES. && .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 /12Z THURSDAY THROUGH MONDAY/... AS OF 715 AM THURSDAY... POCKETS OF MVFR/IFR FOG WILL LIFT/DISSIPATE PRIOR TO 14Z. OTHERWISE EXPECT CONSIDERABLE CLOUDINESS TODAY THOUGH CLOUD BASES WILL REMAIN VFR. SURFACE WINDS WILL BE LIGHT (6KTS OR LESS) AND VARIABLE. THERE WILL BE ISOLATED TO SCATTERED SHOWERS AND POSSIBLY A THUNDERSTORM THIS AFTERNOON 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...ELLIS SHORT TERM...WSS LONG TERM...DJF AVIATION...WSS

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