Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 250229 AFDRAH AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 1029 PM EDT TUE MAR 24 2015 .SYNOPSIS...CANADIAN HIGH PRESSURE WILL BUILD SOUTH AND FORCE A WEAK BACKDOOR COLD FRONT THROUGH CENTRAL NC TONIGHT. THE FRONT WILL THEN BECOME QUASI-STATIONARY OVER CENTRAL NC LATE WEDNESDAY AND WEDNESDAY NIGHT...BEFORE RETREATING NORTH OF THE AREA ON THURSDAY. A COLD FRONT WILL FOLLOW FOR THURSDAY NIGHT. && .NEAR TERM /THROUGH TONIGHT/... AS OF 1025 PM TUESDAY... SURFACE HIGH PRESSURE WILL CONTINUE TO RIDGE INTO CENTRAL NC OVERNIGHT AS IT SLIDES SOUTHWARD ALONG THE MID ATLANTIC COAST. LOW CLOUDS WILL CONTINUE TO MOVE INTO THE NORTHERN PORTIONS OF THE AREA...PARTICULARLY IN THE TRIAD...AND PERSIST THROUGH THE NIGHT. THERE IS STILL A CHANCE FOR SOME DRIZZLE OR LIGHT RAIN OVERNIGHT BUT COVERAGE WILL BE LIMITED. EXPECT LOWS RANGING FROM THE LOW 40S NE TO LOW 50S SW. && .SHORT TERM /WEDNESDAY AND WEDNESDAY NIGHT/... AS OF 315 PM TUESDAY... IN SITU CAD WEDGE WILL BE IN PLACE FOR MUCH OF THE DAY ON WEDNESDAY AND THIS WILL SET UP A DIFFERENTIAL HEATING GRADIENT WITH HIGHS ONLY TOPPING OUT IN THE UPPER 50S TO LOWER 60S IN THE TRIAD BUT POSSIBLY REACHING THE LOWER 70S IN THE SOUTHEAST. THIS DIFFERENCE IN TEMPERATURE COULD COME IN TO PLAY LATER IN THE DAY IN REGARDS TO DEVELOPING INSTABILITY AS HIGH PRESSURE OFF THE COAST WILL GRADUALLY SLIDE OUT TO SEA...THE WEDGE FRONT WILL BREAK DOWN AND IN ITS PLACE SOUTHERLY RETURN FLOW WITH A LOT OF LOW LEVEL MOISTURE TRANSPORT OFF OF THE SOUTHWEST ATLANTIC. AS FAR AS PRECIPITATION IS CONCERNED...ONLY LIGHT RAIN AND DRIZZLE ARE EXPECTED EARLY IN THE WEST UNDER THE CAD REGIME...BUT LATER IN THE EVENING AS THE HIGH SLIDES OUT TO SEA...MORE CONVECTIVE SHOWERS AND THUNDERSTORMS WILL BE POSSIBLE WEDNESDAY EVENING INTO WEDNESDAY NIGHT. CAPE IS TALL AND SKINNY AND MOISTURE IS VERY LIMITED TO THE LOW LEVELS BUT ANY CAPPING THAT IS IN PLACE INITIALLY AROUND THE 700 MB LEVEL WILL EVENTUALLY ERODE OVERNIGHT...SETTING UP MORE FAVORABLE CONDITIONS FOR CONVECTION ON WEDNESDAY NIGHT AND INTO THE DAY ON THURSDAY. WIND SHEAR WILL BE VERY FAVORABLE FOR CONVECTION AS 40 KTS OR SO OF BULK SHEAR IS EXPECTED. RAIN CHANCES WILL INCREASE SIGNIFICANTLY AFTER 6Z THURSDAY. A NON-DIURNAL TEMPERATURE TREND IS EXPECTED AS THE CAD BREAKS DOWN AND A SURFACE WARM FRONT WORKS NORTHWARD THROUGH THE AREA. THEREFORE NEAR STEADY TO SLIGHTLY INCREASING TEMPERATURES AFTER 00Z THURSDAY WILL MORE RAPIDLY INCREASE EARLY THURSDAY MORNING. LOWS LISTED IN THE LOW TO UPPER 50S BUT AGAIN THIS WILL OCCUR EARLIER WEDNESDAY NIGHT AS OPPOSED TO THURSDAY MORNING. && .LONG TERM /THURSDAY THROUGH TUESDAY/... AS OF 320 PM TUESDAY... A DEEPENING MIDWEST TROUGH ASSOCIATED WITH A STRENGTHENING NORTHERN STREAM FLOW WILL MOVE SLOWLY EAST REACHING THE WESTERN APPALACHIANS BY FRIDAY MORNING. NWP GUIDANCE SUGGESTS A STREAM OF SCATTERED SHOWERS FROM OFF THE SC COAST NORTHWARD INTO EASTERN NC OWING TO LOW LEVEL ISENTROPIC LIFT SHOULD BE ONGOING AT THE START OF THE PERIOD. THIS PRECIPITATION WILL SHIFT EAST DURING THE LATE MORNING RESULTING IN A GENERAL MINIMUM OF PRECIPITATION ACROSS CENTRAL NC DURING THE AFTERNOON HOURS AND EVENING HOURS THURSDAY AS THE REGION WILL BE IN BETWEEN THE DEPARTING WARM ADVECTION SHOWERS IN THE EAST AND AN APPROACHING COLD FRONT ALONG WITH A LAGGING UPPER TROUGH TO THE WEST. THE RESULT SHOULD BE LIMITED PRECIPITATION CHANCES DESPITE A GOOD DEAL OF CLOUDINESS. BREEZY SOUTHWESTERLY WINDS ARE EXPECTED WITH GUSTS TO AROUND 20 MPH AT TIMES. WARM ADVECTION AND THICKNESS VALUES RISING INTO THE 1370-1380M RANGE SUPPORT HIGHS IN THE 75-82 RANGE. IT SHOULD BE NOTED HOWEVER THAT GUIDANCE HAS TRENDED TOWARD A LITTLE MORE INSTABILITY FOR THURSDAY AFTERNOON AND EVENING. THE NAM IS GENERALLY MORE UNSTABLE THAN THE GFS WITH COMPROMISE INSTABILITY VALUES BETWEEN THE TWO IN THE 500 TO 1000 J/KG RANGE ACROSS THE COASTAL PLAIN AND EASTERN PIEDMONT WITH LESS INSTABILITY TO THE WEST. THE LOW LEVEL FLOW IS UNIDIRECTIONAL BUT RATHER VIGOROUS WITH 0-6KM SHEAR VALUES IN THE 30-40KT RANGE ACROSS THE SOUTHEAST NEAR KFAY AND AROUND 40 KTS IN THE NW PIEDMONT NEAR KGSO. IF CONVECTION IS ABLE TO FIRE...IT INCREASINGLY HAS THE POTENTIAL TO BE STRONG...ESPECIALLY IN THE EAST. IN SHORT HAVE INCLUDED CHANCES OF SHOWERS AND THUNDERSTORMS FOR THURSDAY. LATER SHIFTS WILL NEED TO MONITOR THIS. CHANCES OF SHOWERS AND THUNDERSTORMS IN THE EAST INCREASE ON THURSDAY NIGHT INTO EARLY FRIDAY AS A COLD FRONT MOVES ACROSS THE MOUNTAINS THURSDAY LATE AFTERNOON AND THEN ACROSS CENTRAL NC BEFORE MOVING OFFSHORE ON FRIDAY MORNING. THE PRECIPITATION NEAR AND BEHIND THE FRONT WILL BE SUPPORTED BY THE TRAILING UPPER TROUGH AND THE EXIT REGION OF A STOUT 150KT JET. COOLER AIR WILL MOVE INTO CENTRAL NC LATE THURSDAY NIGHT WITH LOWS EVENTUALLY FALL INTO THE 48-56 RANGE. EXPECT LITTLE DAYTIME WARMING ON FRIDAY WITH COLD ADVECTION AND THE ARRIVAL OF THE UPPER TROUGH AXIS. WITH COLD AIR ALOFT...LOW LEVEL LAPSE RATES STEEPEN. THE GFS AND NAM FORECAST SOUNDINGS SHOW SATURATION ABOVE THE MIXED LAYER WHICH WOULD SUPPORT A GOOD DEAL OF CLOUD COVER. WITH THE APPROACHING TROUGH THERE SHOULD BE ENOUGH FORCING TO WARRANT LOW CHANCE POPS. CLOUDS AND A COOLER AIRMASS SUPPORT HIGHS IN THE UPPER 50S TO LOWER 60S. DRIER WEATHER ARRIVES BY FRIDAY NIGHT WITH CHILLY LOWS IN THE MID 30S. HIGHS ON SATURDAY WILL ONLY RANGE IN THE UPPER 40S TO LOWER 50S WITH LOWS SATURDAY NIGHT EVEN COLDER IN THE 30-35 RANGE. HEIGHTS BEGIN TO RISE ON SUNDAY AS THE UPPER TROUGH EXITS THE REGION. THIS SHOULD ALLOW TEMPERATURES TO MODERATE INTO THE MID 50S TO NEAR 60 ON SUNDAY AND IN THE MID 60S ON MONDAY. FORECAST UNCERTAINTY INCREASES ON MONDAY INTO TUESDAY AS A FAST MOVING SHORT WAVE TROUGH IS PROGGED TO DROP INTO THE MID ATLANTIC. THIS SYSTEM WILL BE MOISTURE STARVED BUT RECENT GFS AND ECMWF RUNS...WHILE DIFFERING ON TIMING...SUGGEST SOME LIGHT PRECIPITATION IS POSSIBLE MONDAY OR MONDAY NIGHT. GIVEN THE UNCERTAINTY WILL OPT FOR A DRY FORECAST FOR NOW BUT INCREASE CLOUD COVER. -BLAES && .AVIATION /00Z WEDNESDAY THROUGH SUNDAY/... AS OF 845 PM TUESDAY... INITIALLY VFR CONDITIONS THIS EVENING WILL BECOME MAINLY MVFR WITH THE DEVELOPMENT OF STRATUS BETWEEN 06-09Z...IN RESPONSE TO SLOPED ASCENT ATOP A BACKDOOR COLD FRONT THAT WILL SETTLE SOUTH ACROSS OUR AREA TONIGHT. WHILE ASSOCIATED CEILINGS WILL AVERAGE MVFR THROUGHOUT THE TAF PERIOD IN A RESULTANT COLD AIR DAMMING REGIME...THERE IS 40- 50 PERCENT CHANCE OF THESE CEILINGS LOWERING TO IFR RANGE AND WITH PATCHY LIGHT RAIN AT TRIAD TERMINALS...PARTICULARLY INT...DURING THE MORNING HOURS ON WED. IN ADDITION... SOME LIFTING AND SCATTERING OF THE MVFR CEILINGS TO HIGH MVFR (NEAR 3000 FT) OR VFR...WILL BE POSSIBLE LATE IN THE AFTERNOON AT RWI AND FAY. OUTLOOK: NOCTURNAL COOLING WILL CAUSE CEILINGS TO LOWER TO IFR OR LIFR WED NIGHT...WITH SOME ASSOCIATED SUB-VFR VISIBILITY RESTRICTIONS ALSO PROBABLE...MAINLY AT TRIAD TERMINALS. THE CHANCE OF SHOWERS AND EVEN A FEW THUNDERSTORMS WILL ALSO INCREASE THU MORNING...FOLLOWED BY A GRADUAL LIFTING AND SCATTERING TO VFR THROUGH THU AFTERNOON - SLOWEST AT TRIAD TERMINALS. WIDESPREAD SHOWERS AND ISOLATED THUNDERSTORMS WILL THEN ACCOMPANY A COLD FRONT THAT WILL CROSS OUR AREA THU NIGHT. && .RAH WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SYNOPSIS...RAH NEAR TERM...KC SHORT TERM...ELLIS LONG TERM...BLAES AVIATION...RAH

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