Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 020759 AFDLWX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC 359 AM EDT TUE SEP 2 2014 .SYNOPSIS... BERMUDA HIGH PRESSURE WILL PERSIST TODAY AHEAD OF A WEAK COLD FRONT THAT WILL CROSS THE AREA TONIGHT. HIGH PRESSURE WILL THEN BUILD FROM THE MIDWEST THROUGH WEDNESDAY THEN PERSIST OFFSHORE THURSDAY INTO THE WEEKEND. A COLD FRONT WILL ENTER THE REGION IN THE SATURDAY NIGHT TIME FRAME. && .NEAR TERM /THROUGH TONIGHT/... ANOTHER MUGGY OVERNIGHT W/ MUCH OF THE REGIONAL OBS SHOWING DEWPOINTS IN THE L70S - PLENTY HUMID FOR AN EARLY SEPT NIGHT. SMALLER SCALE RIPPLES IN THE LOWER ATMOS CAUSED SOME SCATTERED SHOWERS/TSTMS OVER THE NERN QUADRANT OF THE CWA EARLIER IN THE OVERNIGHT WHICH HAVE SINCE DISSIPATED...BUT EVIDENCE OF THE INSTABILITY THAT CAN BE TAPPED EVEN OVERNIGHT. THE 00Z KIAD SNDG FROM LAST NIGHT SHOWS OVER 2K J/KG SFC CAPE...THOUGH THE TYPICAL NIGHTTIME DECOUPLING TOOK AWAY MUCH OF THE LL CONVERGENCE. LITTLE AIRMASS CHANGE OVER THE NEXT 12-18HRS WILL BASICALLY KEEP THE OVERHEAD ATMOS IN MAINTENANCE MODE - ALLOWING ISOLATED SHOWERS/TSTMS TO DEVELOP W/ EVEN WEAK/DISORGANIZED FORCING. A HANDFUL OF SHOWERS NOW REACHING THE SPINE OF THE APLCNS. THOUGH THIS ACTIVITY WILL LIKELY LOOK MUCH DIFFERENT OVER THE NEXT FEW HRS...THE ARM OF LOW LEVEL ENERGY CAUSING THEM MAY DEVELOP MORE ACTIVITY FURTHER EAST INTO THE MIDDAY. NEAR TERM WRF DERIVATIVES LARGELY POINT TOWARD A LATE AFTN COLLECTION OF TSTMS DEVELOPING NEAR THE BLUE RIDGE AND SLIDING E THRU THE REMAINDER OF THE EVE. A COLD FRONT WILL BE A FEW HRS BEHIND THIS ACTIVITY...SO THE AFTN/EVE TSTMS WILL LIKELY BE FORCED BY A PRE-FRONTAL TROUGH/ DIFFERENTIAL HEATING BOUNDARY. IT HAS BEEN A HOT/HUMID PAST FEW DAYS...AND TODAY WILL LIKELY BE THE APEX OF THE CURRENT STRETCH. WINDS WILL BE MAINLY WSW...BUT W/ SOME DOWNSLOPING LATER COMPONENT TO THE WINDS. THESE WINDS MAY ALSO DO WELL IN PREVENTING CONVECTION FOR A LARGE PART OF THE DAY. DRY/UNIDIRECTIONAL FLOW IS TYPICALLY NOT GOOD FOR CONVERGENCE... SO A LOW LEVEL BOUNDARY WOULD BE NECESSARY FOR EFFECTIVE INITIATION. HEATING WON`T BE AN ISSUE HOWEVER...W/ TEMPS QUICKLY RISING RIGHT BACK INTO THE 80S THRU THE MRNG HRS AND L-M90S BY THE PEAK HEATING OF THE MID AFTN. AN INFLUX OF DENSE LOW-MID CLOUD DECKS FROM THE WEST DURING THE MID AFTN MAY HELP TO CUT DOWN ON THE HIGHER TEMPS...AND SO WOULD ANY ONGOING SHOWERS/TSTMS. DEWPOINTS IN THE U60S/L70S WILL ALSO CREATE ANOTHER HOT/HUMID ENVIRONMENT W/ HEAT INDICES WELL INTO THE U90S W/ A FEW LOCALES POTENTIALLY POKING JUST ABOVE THE 100F DEG MARK IN THE URBAN AREAS. AFTER THE SHOWERS/TSTMS HAVE ROLLED THRU LATER THIS EVE...THE FRONT WILL BEGIN TO CLEAR OUT THE AREA FROM THE WEST. SKIES WILL START TO CLEAR AND WINDS WILL TURN NWLY. THIS PASSAGE THOUGH WON`T BE PARTICULARLY STRONG...W/ WINDS ONLY IN THE 5-10KT RANGE AT BEST AND ONLY ABOUT A 5F DEG DEWPOINT DROP OVERALL...SO THE HUMIDITY WILL REMAIN ELEVATED AS WE HEAD INTO WED. && .SHORT TERM /WEDNESDAY AND WEDNESDAY NIGHT/... DRY AND STABLE ON WED. NO POPS IN THE FORECAST. ANOTHER WARM AND RELATIVELY HUMID DAY...AS HIGHS WILL AGAIN APPROACH THE 90F DEG MARK ACROSS MUCH OF THE AREA. DEWPOINTS WILL STILL BE IN THE L-M60S...CREATING HEAT INDICES RIGHT AROUND THE AMBIENT TEMP LEVELS/SLIGHTLY HIGHER. WEDNESDAY NIGHT...HIGH PRESSURE CENTER OVER THE AREA SHIFTS OFFSHORE WITH A RETURN FLOW DEVELOPING IN THE OVERNIGHT HOURS. DEWPOINTS QUICKLY RAISE BACK TO NEAR 70F...MIGHT AS WELL GET AREAS OF FOG WORDING IN THE GRIDS. WITH THE HIGH PERSISTING OFFSHORE...AN UNSTABLE GULF AIRMASS BRINGS THE MID-ATLANTIC RIGHT BACK INTO AN ACTIVE PATTERN BY THURSDAY. CHANCES FOR DIURNAL DRIVEN SCATTERED THUNDERSTORMS. SHOULD LOW LEVEL FLOW BE MORE SELY THAN SLY...TERRAIN ENHANCEMENT WOULD ALSO BE A FACTOR IN THUNDERSTORM DEVELOPMENT. SIMILAR PATTERN FOR FRIDAY WITH THE NEXT COLD FRONT. MAX TEMPS THURSDAY AND FRIDAY AROUND 90F THOUGH CLOUD COVER WILL BE A FACTOR. && .LONG TERM /THURSDAY THROUGH MONDAY/... ANOTHER WEAK COLD FRONT APPROACHES FRIDAY NIGHT. LIKELY POPS FOR THUNDERSTORMS WEST FROM THE BLUE RIDGE SATURDAY AS THERE IS GOOD CONSENSUS BETWEEN THE 00Z GFS AND 00Z ECMWF. QUESTION IS HOW QUICKLY THE FRONT PUSHES THROUGH THE AREA. THE ECMWF LINGERS THE COLD FRONT OVER SERN ZONES THROUGH SUNDAY. && .AVIATION /07Z TUESDAY THROUGH SATURDAY/... AFTER SOME EARLIER SHOWER/TSTMS PASSED NEAR THE DC/BALT METRO AREAS A FEW HRS AGO...SKIES HAVE GENERALLY CLEARED OUT. MORE CLOUDS ON THE WAY FROM THE WEST...AS WELL AS A FEW SHOWERS THAT WILL LIKELY DISSIPATE TOWARD DAWN. A WSW WIND DURING THE DAYTIME HRS...W/ SCTD SHOWERS AND TSTMS EXPECTED TO DEVELOP LATE TODAY - SURROUNDING THE 00Z TIME PERIOD - W/IN A COUPLE HRS EITHER SIDE. SOME EARLIER ACTIVITY POSSIBLE...AFTER 18Z BUT TIMING/PLACEMENT CONFIDENCE IS LOW ATTM. A COLD FRONT WILL PUSH THIS ACTIVITY OFF THE COAST LATE TONIGHT AND CLEAR OUT THE AREA FOR WED. FOG WEDNESDAY NIGHT UNDER HIGH PRESSURE. LIGHT SLY RETURN FLOW THURSDAY AND FRIDAY WITH MAINLY AFTERNOON CHANCES FOR THUNDERSTORMS. COLD FRONT ENTERS THE AREA LATE SATURDAY WITH THUNDERSTORMS AHEAD OF IT. && .MARINE... A RELATIVELY LIGHT SWLY FLOW OVER THE WATERS TONIGHT...THE HIGHER GUSTS NEAR 15KT OVER THE LOWER BAY ZONES. THIS WILL CONTINUE INTO THE DAYTIME HRS...THOUGH MORE OF A WLY DIRECTION TO THE FLOW THRU- OUT THE DAY. SCTD TSTMS POSSIBLE LATE THIS AFTN/EVE...SO ANOTHER POTENTIAL FOR CONVECTIVE MARINE HAZARDS. A COLD FRONT WILL WEAKLY PASS THRU THE REGION OVERNIGHT...TURNING WINDS TO NWLY BUT STILL LOWER THAN SCA CRIT. WED WILL SEE HIGH PRES BUILD INTO THE REGION AND DROP WIND SPEEDS DOWN EVEN FURTHER. LIGHT AND VARIABLE FLOW WEDNESDAY EVENING GIVES WAY TO SLY FLOW AS HIGH PRESSURE MOVES OFFSHORE. WILL HAVE TO WATCH FOR CHANNELING AGAIN (PROBABLY THURSDAY EVENING) BUT AS OF NOW THERE ARE NO SCA HEADLINES EXPECTED. COLD FRONT APPROACHES THE WATERS SATURDAY WITH GOOD CHANCES FOR THUNDERSTORMS. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...NONE. MD...NONE. VA...NONE. WV...NONE. MARINE...NONE. && $$ SYNOPSIS...BAJ NEAR TERM...GMS SHORT TERM...GMS LONG TERM...BAJ AVIATION...GMS/BAJ MARINE...GMS/BAJ

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