Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 290740 AFDRAH AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 240 AM EST THU JAN 29 2015 .SYNOPSIS...HIGH PRESSURE WILL MOVE OFFSHORE THURSDAY AND GIVE WAY TO A COLD FRONT THAT WILL CROSS THE AREA THURSDAY NIGHT INTO FRIDAY MORNING. HIGH PRESSURE WILL BUILD IN FROM THE WEST ON FRIDAY...WHICH WILL HELP TO KEEP CONDITIONS DRY THROUGH MUCH OF THE WEEKEND. && .NEAR TERM /THROUGH FRIDAY NIGHT/... AS OF 230 AM THURSDAY... OPTIMAL RADIATIONAL COOLING FOR THE NEXT COUPLE OF HOURS WHILE HIGH PRESSURE OVERHEAD CALMS WINDS AND SKIES REMAIN CLEAR. THIS WONT LAST FOR TOO LONG HOWEVER AS HIGH CLOUDS ARE BEGINNING TO MOVE IN FROM THE WEST ASSOCIATED WITH A FRONTAL SYSTEM WITH PARENT MOVING INTO THE CHICAGO AREA AT THIS TIME. THE ATTENDANT COLD FRONT...WHICH IS WEAK IS PROGGED TO CROSS CENTRAL NC LATER TONIGHT AND INTO FRIDAY MORNING. UNTIL THEN...SOUTHERLY RETURN FLOW WILL PICK UP ON THE BACK SIDE OF THE SURFACE HIGH AFTER SUNRISE WITH SOME GUSTING AROUND 15 KTS POSSIBLE...ESPECIALLY IN THE WEST WHICH IS FURTHER AWAY FROM THE CENTER OF THE HIGH. USING DRY ADIABATIC METHODS OR LOW LEVEL THICKNESSES SUGGEST TEMPERATURES IN THE MID TO UPPER 50S BUT WITH THE INCREASING CLOUD COVER...ESPECIALLY DURING PEAK HEATING THESE NUMBERS WILL BE CUT BACK TO THE UPPER 40S TO MID 50S. CANT RULE OUT SOME HIGHER TEMPS IN THE SOUTHEAST...WHERE THE EFFECTS OF THE APPROACHING SYSTEM WILL BE MOST DELAYED. AS FAR AS PRECIPITATION CHANCES ARE CONCERNED WITH THE ACTUAL FRONT...CONDITIONS ARE DRY BUT THE LATEST VERSION OF THE MODELS IS NOT QUITE AS DRY AS YESTERDAY WITH PW VALUES INCREASING TO NEAR 0.7 INCHES AS OPPOSED TO 0.5 YESTERDAY. A FIRST LOOK AT HIGH RESOLUTION CONVECTIVE MODELS ALSO SHOWS SOME VERY LIGHT PRECIPITATION IN THE NORTHWEST PIEDMONT JUST AFTER 21Z AND FADING OUT AS IT HEADS EASTWARD BEFORE PICKING UP AGAIN OFF THE COAST. FORECAST SOUNDINGS SHOW A SIMILAR STORY WITH THE BEST SATURATION IN THE TRIAD WITH LESS LIKELY CHANCES FOR RAIN SOUTH AND EAST. DESPITE BETTER CHANCES IN THE TRIAD...SATURATION IN THE MODEL SOUNDINGS IS BRIEF AND CONFINED TO THE 850-700 MB RANGE WITH A LOT OF DRY AIR UNDERNEATH...SUGGESTIVE OF SPRINKLES OR VIRGA PERHAPS IF MOISTURE CANT REACH THE SURFACE. THE RESULT IS STILL VERY INCONSEQUENTIAL AS QPF AMOUNTS WOULD BE A COUPLE OF HUNDREDTHS OF AN INCH AT THE VERY BEST BUT MORE LIKELY TRACE AMOUNTS OR NOTHING. BETTER UPPER LEVEL DYNAMICS COME IN AFTER 00Z FRIDAY BUT THE SYSTEM JUST LOOKS TO DRY TO SUSTAIN ANYTHING AT THAT TIME. AFTER FRONTAL PASSAGE AROUND 6Z FRIDAY...WINDS WILL SHIFT BACK TO NORTHWESTERLY AND COLD AIR ADVECTION WILL ONCE AGAIN ENSUE. LINGERING CLOUD COVER BEHIND THE FRONT WILL KEEP LOWS WARMER...IN THE LOW TO UPPER 30S NW TO SE. AS DRY AIR WORKS IN BEHIND THE FRONT...CLOUDS MAY CLEAR OUT FOR THE LATE MORNING PERIOD...BUT THEN WRAP AROUND MOISTURE ON THE BACK SIDE OF THE EXITING SYSTEM WILL MOVE IN FROM THE NORTH AND CUT DOWN ON INSOLATION DURING PEAK HEATING. THIS MAY CAUSE A TEMPERATURE GRADIENT FROM THE NORTH TO SOUTH RESULTING IN HIGHS IN THE MID 40S TO LOW 50S. DRY CONDITIONS ARE EXPECTED AND INCREASING WINDS OF 15-20 KTS SUSTAINED WITH GUSTS OF UP TO 25 KTS COULD CAUSE SOME FIRE WEATHER CONCERNS ON FRIDAY AFTERNOON. AT THIS TIME MIN RH VALUES WILL HOVER AROUND 30%. WINDS WILL DIE OFF AFTER SUNSET AND COUPLED WITH CLEAR SKIES WILL CAUSE OPTIMAL RADIATIONAL COOLING RESULTING IN LOW TEMPERATURES NEAR 20 DEGREES FRIDAY NIGHT INTO SATURDAY MORNING. && .SHORT TERM /THURSDAY AND THURSDAY NIGHT/... AS OF 255 PM WEDNESDAY... THURSDAY...HIGH PRESSURE OVER THE AREA EARLY THURSDAY MORNING WILL WEAKEN AND SHIFT SOUTHEAST OF THE SC COAST BY MIDDAY. MEANWHILE...A PAIR OF SHORTWAVES... ONE EMERGING OFF THE CENTRAL ROCKIES AND ANOTHER DIVING SOUTH OUT OF THE CANADIAN ROCKIES WILL IMPACT THE REGION THURSDAY AFTERNOON/NIGHT. WITH THE LEAD WAVE...FORCING IS FOCUSED ALOFT AS THE BRUNT OF THE DCVA PASSES TO OUR NORTH. MOISTURE IS FOCUSED IN THE MID-LEVELS...PER NAM/GFS BUFR SOUNDINGS...AND DECREASES RAPIDLY WITH SOUTHWARD EXTENT. WHILE THE ECMWF PRODUCES A FEW HUNDREDTHS AREAWIDE BETWEEN 18-00Z...THE NAM/GFS/SREF SUGGEST JUST A PASSING SPRINKLE AT BEST DURING THE AFTERNOON...AND THIS LOOKS MOST REASONABLE. SOUTHERLY WIND WILL HELP HIGHS REBOUND THURSDAY ESPECIALLY IN THE WEST...UPPER 40S NORTHEAST TO LOWER 50S SOUTHWEST. THURSDAY NIGHT...ON THE HEELS OF THE LEAD WAVE...THE TRAILING SHORTWAVE...AND ASSOCIATED WEAK COLD FRONT...WILL CROSS THE CWA IN THE 03-09Z TIME FRAME. THE FORCING FOR ACCENT IS MUCH BETTER WITH STRONGER DCVA AND WEAK LOW-LEVEL FRONTAL CONVERGENCE...ALL WITHIN THE LEFT EXIT REGION OF A STRENGTHENING SPEED MAX OVER THE MID- MISSISSIPPI VALLEY REGION. HOWEVER...MOISTURE MAY AGAIN BE TOO LIMITED AS THE DEEPER LAYER FLOW BECOMES WESTERLY. A FEW ISOLATED LIGHT SHOWERS OR SPRINKLES ARE POSSIBLE WITH FRONT...BUT MAJORITY OF THE AREA SHOULD BE DRY. MAY SEE A FEW WIND GUSTS BEHIND THE FRONT WITH THE INITIAL COLD ADVECTION...AS LOWS FALL INTO THE LOW TO MID 30S. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... AS OF 335 PM WEDNESDAY... FRI THROUGH SAT NIGHT: THE MODELS ARE IN GOOD AGREEMENT REGARDING THE AMPLIFICATION OF A PAIR OF STRONG NORTHERN STREAM SHORTWAVE TROUGHS INTO AND OFFSHORE THE COAST OF THE MIDDLE ATLANTIC STATES ON FRIDAY. THE ASSOCIATED SFC COLD FRONT SHOULD BE NEAR THE COAST AT THE START OF THE PERIOD/12Z FRI...WITH STRONG CAA - OFFSET TO SOME DEGREE BY 30 KTS OF NW DOWNSLOPE FLOW - FOR THE REMAINDER OF THE DAY. THE RESULT WILL BE BLUSTERY NW WINDS TO AROUND 30 MPH WITHIN A DEEPLY MIXED (~5 K FT) BOUNDARY LAYER...AND HIGHS IN THE MIDDLE 40S TO AROUND 50 DEGREES. 1030 MB CONTINENTAL POLAR HIGH PRESSURE WILL BUILD EAST OF THE APPALACHIANS FRI NIGHT. CONSENSUS LL THICKNESS VALUES AROUND 1280 METERS SUPPORT LOWS MOSTLY IN THE LOWER 20S...THOUGH UPPER TEENS WILL BE POSSIBLE WHERE AND IF DECOUPLING/CALM OCCUR TOWARD DAYBREAK. THE SFC RIDGE WILL CREST OVERHEAD SAT...THEN DRIFT OFFSHORE SAT NIGHT. MEANWHILE ALOFT...INITIALLY NW FLOW WILL TRANSITION TO INCREASINGLY ZONAL FLOW DOWNSTREAM OF BOTH A NORTHERN STREAM S/W TROUGH APPROACHING FROM WESTERN CANADA AND AN EVOLVING UPPER LOW NEAR THE BAJA. THE ZONAL FLOW WILL ALLOW ABUNDANT MID-HIGH LEVEL MOISTURE TO STREAM EAST OVER CENTRAL NC LATE IN THE DAY AND PARTICULARLY OVERNIGHT SAT. GIVEN THE INCREASING MOISTURE ALOFT...LOWS WILL BE NOT BE AS COLD - MOSTLY IN THE LOWER (TO PERHAPS MIDDLE) 30S. WHILE THE INCREASING MID-HIGH LEVEL MOISTURE WILL LIKELY RESULT IN WIDESPREAD VIRGA BY EARLY SUN...A DRY SUB- CLOUD LAYER IN EXCESS OF 7-8 THOUSAND FT PER MODEL CROSS SECTIONS SUGGEST IT WILL REMAIN DRY AT THE SFC. *IF* LIFT AND MOISTURE WERE TO INCREASE MORE QUICKLY THAN CURRENTLY FORECAST...PARTIAL THICKNESS AND WET BULB PROFILE VALUES WOULD SUPPORT SNOW AT THE ONSET...BUT PROSPECTS OF THAT HAPPENING ARE VERY LOW. SUN THROUGH MON: THE MODELS HAVE TRENDED FLATTER WITH THE NORTHERN STREAM S/W TROUGH (REFERENCED ABOVE) AS IT AMPLIFIES INTO THE MIDDLE ATLANTIC STATES THROUGH SUN NIGHT OR MON. THE ASSOCIATED SFC LOW REFLECTION IN RECENT NWP RUNS IS CONSEQUENTLY MUCH WEAKER AND MORE SUPPRESSED THAN EARLIER FORECASTS. REGARDLESS OF WHETHER OR NOT THE GUIDANCE TRENDS BACK TOWARD A STRONGER SYSTEM LIFTING UP WEST OF THE APPALACHIANS...THE HIGHEST RAIN CHANCES WILL STILL BE SUNDAY NIGHT IN EITHER SCENARIO. BEFOREHAND...SUNDAY SHOULD BE A CLOUDY DAY FOR CENTRAL NC...WITH RAIN CHANCES INCREASING FROM TOP-DOWN SATURATION OVER MAINLY THE WESTERN AND NORTHERN PIEDMONT...AND WITH HIGHS WELL BELOW FULL SUN VALUES - MOSTLY IN THE MIDDLE 40S TO AROUND 50 DEGREES. THE FLATTER/WEAKER SFC SOLUTIONS DO HAVE RAMIFICATIONS ON PRECIPITATION CHANCES AND TEMPERATURES FOR MON...HOWEVER...SUCH THAT THE BULK OF THE DAY WOULD BE DRY BEHIND THE SYSTEM GIVEN THE LATEST MODEL TRENDS...WITH CAA-DRIVEN TEMPERATURES IN THE MIDDLE 40S TO AROUND 50 DEGREES. MON NIGHT THROUGH WED: BENEATH HEIGHT RISES ALOFT BEHIND THE DEPARTING NORTHERN STREAM S/W TROUGH...POST-FRONTAL...CONTINENTAL POLAR HIGH PRESSURE WILL BUILD OVERHEAD MON NIGHT-TUE...THEN DRIFT OFFSHORE TUE NIGHT-WED. MEANWHILE ALOFT...THE EVOLVING UPPER LOW NEAR THE BAJA IS FORECAST TO EJECT AND TRACK ACROSS THE SOUTHERN US...WITH ACCOMPANYING SFC LOW DEVELOPMENT FROM THE GOM TO THE SOUTHEAST US COAST...THROUGH MID WEEK. THE RESULT WILL BE INCREASING PRECIPITATION CHANCES IN CENTRAL NC...INTO THE SLIGHT TO CHANCE RANGE...BY EARLY WED. PROJECTED PARTIAL THICKNESS VALUES WOULD SUPPORT JUST A CHILLY RAIN. && .AVIATION /06Z THURSDAY THROUGH MONDAY/... AS OF 1235 AM THURSDAY... 24 HOUR TAF PERIOD: MEDIUM HIGH CONFIDENCE OF VFR CONDITIONS THROUGH THE TAF PERIOD AS A DRY FRONTAL SYSTEM CROSSES THE AREA NEAR THE END OF THE PERIOD. CLEAR AND CALM CONDITIONS WILL PREVAIL INITIALLY...BUT GRADUALLY HIGH CLOUDS WILL SPREAD IN FROM THE WEST. BROKEN CEILINGS WILL SET IN BY 18Z OR SO BUT THESE SHOULD BE IN THE 8-9 KFT RANGE. THERE IS A SLIGHT CHANCE THAT CEILING COULD APPROACH MVFR LEVELS AROUND 3Z BUT CONFIDENCE IN THIS IS LOW. AS FAR AS WINDS ARE CONCERNED...EXPECT SOUTHERLY WINDS AT 5-10 KTS AFTER DAYBREAK...GUSTING 15-20 KTS BEFORE RELAXING AGAIN AFTER SUNSET. LONG TERM: SUB-VFR CONDITIONS WILL BE POSSIBLE WITH A LOW PRESSURE SYSTEM THAT WILL CROSS THE AREA LATE SUNDAY INTO MONDAY. VFR CONDITIONS WILL PREVAIL FOR MUCH OF THE WEEK AFTER THAT. && .RAH WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SYNOPSIS...ELLIS NEAR TERM...ELLIS SHORT TERM...BLS LONG TERM...MWS AVIATION...ELLIS

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