Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 171845 AFDLWX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC 245 PM EDT SUN AUG 17 2014 .SYNOPSIS... A WEAK COLD FRONT WILL MOVE INTO THE AREA TONIGHT AND THEN STALL OVER THE AREA THROUGH MONDAY. LOW PRESSURE OVER THE OHIO VALLEY WILL TRACK ALONG THE BOUNDARY...PASSING THROUGH OUR AREA MONDAY NIGHT INTO TUESDAY. THE BOUNDARY WILL REMAIN STALLED OVER THE AREA DURING THE MIDDLE AND LATTER PORTIONS OF THE WEEK. && .NEAR TERM /THROUGH TONIGHT/... AT 18Z...SFC WINDS HAVE TURNED WESTERLY THROUGHOUT THE AREA. THE AREAS WITH LESS SKY COVER HAVE REACHED THE MID 80S...ALLOWING THE WINDS TO MIX DOWN. WIND GUSTS IN THE UPPER TEENS ARE BEING NOTED ACROSS THE LAND AREAS THIS AFTERNOON. THE SKY COVER HAS TRENDED DOWN IN MOST AREAS TODAY. HAVE DECREASED POPS TO ISOLATED TRW AS SPC MESOANALYSIS AT 17Z HAS SFC BASE CAPES AROUND 1000 J/KG2 OVER MUCH OF CENTRAL AND WESTERN MD...AND INTO THE NORTHERN SHENANDOAH VALLEY. WITH DEWPOINTS REACHING THE MID 60S...WILL BE EXPECTING PATCHY FOG TONIGHT IN OUR VALLEY LOCATIONS. LINE OF SHRA IN SW PA HAVING HARD TIME GETTING TO GARRETT COUNTY MD. IF IT DOES MAKE IT...COVERAGE WILL BE VERY LIMITED AS IT IS ALIGNED WITH THE SFC WIND FIELD. ONE THING TO NOTE...PCPN ATTM ACROSS CNTRL KY/TN APPEARS TO BE RUNNING WAY OUT AHEAD OF THE 12Z ECMWF QPF MAP...SO AM A LITTLE CONCERNED ABOUT SHRA GETTING TO WESTERN PART OF CWA LATE TONIGHT INSTEAD OF FIRST THING MONDAY MORNING AS IN PRESENT FCST. && .SHORT TERM /MONDAY/... MONDAY MORNING WILL START OUT DRY EAST OF THE BLUE RIDGE...WITH SCT/NUM SHRA OVER THE POTOMAC HIGHLANDS AND CENTRAL/NORTHERN SHENANDOAH VALLEY IN RESPONSE TO THE SHORT WAVE AND CLOSED SFC LOW APPROACHING FROM THE OH VALLEY. THE 12Z ECMWF KEEPS THE PCPN MAINLY TO THE SOUTHERN PARTS OF CWA MONDAY...WHEREAS THE 12Z GFS ACTUALLY SPLITS THE ENERGY AND TAKES PART OF IT UP INTO PA. WILL GO WITH THE ECMWF FOR NOW AS IT HAS BEEN CONSISTENT WITH THIS PCPN PATTERN FOR SEVERAL CYCLES. IN THE AFTERNOON THERE WILL BE A CHC OF TSTMS ACROSS THE ENTIRE CWA. TEMPS WILL BE SIMILAR TO SUNDAYS HIGHS. I DO NOT ANTICIPATE A SVR THREAT AS THE SFC LOW WILL BACK WINDS TO THE SOUTHEAST WITH GFS SFC CAPES ONLY AROUND 1000 J/KG2. && .LONG TERM /MONDAY NIGHT THROUGH SUNDAY/... GUIDANCE REMAINS CONSISTENT SHOWING A SHORT WAVE CROSSING A SURFACE BOUNDARY WHICH WILL BE ZONALLY BISECTING THE EASTERN US TO BEGIN THE WORKWEEK. THIS WILL BRING A FAIRLY COMPACT YET POTENT LOW THROUGH THE MID-ATLANTIC MONDAY NIGHT AND INTO TUESDAY MORNING. A RIBBON OF MID-LEVEL SUBTROPICAL MOISTURE WILL BRING PWATS NEAR 2 INCHES...ALLOWING FOR PERIODS OF HEAVY SHOWERS WITH THE PASSAGE OF THE LOW. THE PRIMARY UNCERTAINTY WITH THIS SYSTEMS REMAINS THE LOCATION OF THE HIGHEST RAINFALL AMOUNTS. GUIDANCE APPEARS TO BE FOCUSING IN ON SOUTHERN VIRGINIA UP TO THE MIDDLE PIEDMONT AND LOWER SHENANDOAH VALLEY REGIONS...SO POPS AND QPF WERE ADJUSTED ACCORDINGLY WITH HIGHEST VALUES AND LONGEVITY ACROSS THE SOUTHERN PORTION OF THE CWA. RELATIVELY DRIER AIR WILL WORK IN ON THE BACK SIDE OF THE LOW AND GREATLY REDUCE THE COVERAGE OF THE PRECIP BY TUESDAY AFTERNOON. THIS MAY ALLOW FOR SOME BREAKS IN THE CLOUDS AND ANY SOLAR HEATING THAT DOES OCCUR TUESDAY AFTERNOON COULD...IN CONJUNCTION WITH THE NORTHWEST FLOW ALOFT ON THE UPSTREAM SIDE OF THE SHORTWAVE COULD LEAD TO SOME MODEST INSTABILITY (AROUND 1000 J OF CAPE OR LESS). WITH THE SURFACE BOUNDARY IN THE AREA AND THE POTENTIAL FOR DIFFERENTIAL HEATING BOUNDARIES TO FORM IN THE AFTERNOON TO PROVIDE THE FORCING FOR CONVECTION...HAVE SCATTERED THUNDERSTORMS IN THE FORECAST FOR TUESDAY EVENING...THOUGH COVERAGE WILL LIKELY BE RATHER LIMITED AND SEVERE THREAT WILL BE LOW. ANY THUNDERSTORM ACTIVITY SHOULD DIMINISH SHORTLY AFTER SUNSET WITH THE LOSS OF SURFACE HEATING. GIVEN THE INCREASED SOIL MOISTURE FOLLOWING MONDAY NIGHTS RAINFALL AND THE CALM LOW LEVEL WINDS...FOG MAY BE A CONCERN LATE TUESDAY NIGHT/WEDNESDAY MORNING. THE REMAINDER OF THE LONG TERM WILL CONTINUE THE ACTIVE PATTERN...THOUGH CONFIDENCE IN TIMING AND LOCATION OF FEATURES IS LOWER AT THIS RANGE. A SERIES OF SHORT WAVES TRAVERSING THE LARGE SCALE TROUGH ACROSS THE EASTERN US AND WESTERN ATLANTIC WILL BRING PERIODS OF RAINFALL WEDNESDAY...THURSDAY AND FRIDAY. CONVECTIVE POTENTIAL AND STORM COVERAGE WILL LARGELY BE INFLUENCED BY THE TIMING OF THE SHORTWAVE PASSAGES IN RELATION TO THE ENHANCED INSTABILITY FROM DAYTIME HEATING. GUIDANCE CURRENTLY SHOWS THE FIRST SHORTWAVE ARRIVING WEDNESDAY AFTERNOON AND EVENING WITH THE STRONGEST LIFT FOCUSED ALONG THE MASON DIXON LINE. THUS MODERATE TO HIGH END CHANCE POPS AND THUNDERSTORMS FOCUSED ACROSS MARYLAND AND WEST VIRGINIA NORTH OF THE METRO AREAS FOR WEDNESDAY AND WEDNESDAY NIGHT. BEYOND WEDNESDAY CONFIDENCE IS LOW IN THE TIMING OF THE ARRIVAL OF SHORTWAVES...SO MODERATE TO LOW END POPS AND THUNDERSTORMS ARE IN THE FORECAST THROUGH THE END OF THE WEEK...WITH SLIGHTLY HIGHER POPS DURING THE EVENINGS. HIGH PRESSURE WILL NOSE DOWN THE EASTERN SEABOARD BEGINNING SATURDAY. THIS...WITH THE ASSISTANCE OF LOW PRESSURE WELL OFF THE MID-ATLANTIC COAST...WILL BRING A COOLER...MORE STABLE LOW LEVEL AIR MASS INTO THE AREA. EXPECT HIGHS IN THE MID 80S FOR THE MIDDLE AND END OF THE WORKWEEK TO GIVE WAY TO HIGHS NEAR 80 FOR THE WEEKEND AND MOSTLY CLOUDY SKIES GRADUALLY TRANSITION TO PARTLY CLOUDY TO MOSTLY SUNNY SKIES THROUGH THE SAME PERIOD. && .AVIATION /18Z SUNDAY THROUGH FRIDAY/... MVFR CONDITIONS FOR VSBYS BETWEEN 06Z AND 12Z FOR MRB...CHO...AND MAYBE IAD. SFC WIND GUSTS WILL SUBSIDE EARLY THIS EVENING WITH THE LOSS OF DAYTIME HEATING. MONDAY WILL HAVE CHC OF TSTMS IN THE AFTERNOON ALL TAF LOCATIONS BUT AT A LOW PROBABILITY NOT TO HAVE IN TAF ATTM. IFR CONDITIONS ARE EXPECTED AS A WAVE OF LOW PRESSURE WILL BRING LOW CEILINGS TO THE AREA THROUGH THE NIGHT MONDAY AND INTO TUESDAY MORNING. A COOL MOIST MARINE AIRMASS AND LIGHT WINDS MAY COMBINE TO ENCOURAGE FOG FORMATION EARLY WEDNESDAY MORNING FOR THE NORTHEASTERN PORTION OF THE CWA INCLUDING IAD DCA BWI AND MTN. PERIODS OF SHOWERS AND THUNDERSTORMS MAY ALSO EFFECT AVIATION WEDNESDAY AFTERNOON AND EVENING. && .MARINE... NO MARINE HEADLINES EXPECTED THROUGH THE SHORT TERM. CHC TSTMS MON AFTERNOON AND LOWERING VSBYS MON NIGHT. CONFIDENCE IS LOW IN THE OCCURRENCE OF ANY MARINE HAZARDS THROUGH THE LONG TERM. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...NONE. MD...NONE. VA...NONE. WV...NONE. MARINE...NONE. && $$ SYNOPSIS...LEE NEAR TERM...LEE SHORT TERM...LEE LONG TERM...CEB AVIATION...LEE/CEB MARINE...LEE/CEB

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