Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 170754 AFDLWX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC 354 AM EDT SUN AUG 17 2014 .SYNOPSIS...
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A WEAK COLD FRONT WILL APPROACH THE AREA TODAY BEFORE STALLING OUT OVERHEAD TONIGHT INTO 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.
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&& .NEAR TERM /THROUGH TONIGHT/...
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CLEAR AND CALM CONDITIONS ARE BEING SLOWLY REPLACED BY PATCHES OF MID-LEVEL STRATUS AND A LIGHT SW WIND. LOW LEVEL MOISTURE ALSO INCREASING AFTER A RELATIVELY DRY AFTN ON SAT...W/ DEWPOINTS NOW CLIMBING BACK INTO THE L60S. THESE VALUES ARE EXPECTED TO SLOWLY CLIMB A FEW MORE DEG INTO THE M-U60S LATER THIS AFTN. DESPITE THE INCREASED CLOUD COVER TODAY...TEMPS ARE EXPECTED TO AGAIN CLIMB BACK INTO THE 80S - L80S FOR MUCH OF THE AREA W OF THE BLUE RIDGE AND M80S ACROSS THE PIEDMONT AND I-95 CORRIDOR. COUPLE THESE VALUES W/ THE HIGHER DEWPOINTS AND THE HEAT INDICES WILL APPROACH THE M-U80S FOR MUCH OF THE REGION DURING PEAK HEATING THIS AFTN. A BLEND OF THE BIAS-CORRECTED MAV/MEX USED FOR TODAY`S FORECAST MAX TEMPS. COVERAGE OF SHOWER ACTIVITY HAS INCREASED IN RECENT HRS OFF TO OUR NORTH...ASSOCIATED W/ A SLOW-MOVING COLD FRONT. ANOTHER AREA OF ENHANCED SHOWER ACTIVITY HAS DEVELOPED ALONG A LEE TROUGH AXIS BACK ACROSS THE WINDWARD SIDE OF THE CNTRL APLCNS/WRN OHIO VLY REGION. BOTH AREAS WILL CONTINUE TO APPROACH THE WRN/NRN SECTIONS OF THE CWA INTO THE PREDAWN HRS...BUT ONLY A FRACTION IS EXPECTED TO CLEAR THE HIGHER TERRAIN IN THE COMING HRS. THE COLD FRONT IS AN ELONGATED BOUNDARY ORIENTED WEST-TO-EAST AND STRETCHES BACK TO A PARENT LOW WELL OFF TO OUR NE. THE FRONT MAY NOT EVEN CLEAR THE AREA...INSTEAD STAYING JUST N OF THE MASON-DIXON LINE AND STRADDLING THE STATE LINES THE DYNAMICS OF THE SYSTEM TAKE OFF TO THE NE BUT COULD BE THE FOCUS FOR SOME SCATTERED SHOWERS/TSTMS LATER THIS AFTN. MARGINAL INSTABILITY EXPECTED FOR POTENTIAL CONVECTION THIS AFTN. THE NAM IS HIGHER ON THE POTENTIAL THAN THE GFS. THE LATEST RUNS HAVE BOTH CUT DOWN ON THEIR EARLIER CAPE DEPICTIONS...AND THIS INSTABILITY WILL BE CONDITIONAL - WHERE AREAS CAN STAY CLOUD-FREE LONGER. INSTABILITY WILL BE MUCH LOWER TOWARD THE MASON-DIXON LINE AND THE FOCUS OF MUCH OF THE PRECIP ACTIVITY DEPICTED IN MANY OF THE LOCAL WRF MODELS IS OVER THE BLUE RIDGE TOWARD THE KCHO AND SURROUNDING AREAS. AFTER A HANDFUL OF SHOWERS DOT THE MAP DURING THE MRNG HRS...THE SCOPE MAY FILL-UP A BIT MORE DURING THE AFTN HRS W/ SCATTERED SHOWERS AND ISOLATED TSTMS WHERE THE BEST CLEARING AND LOW LEVEL CONVERGENCE OCCUR. THESE DIURNALLY-DRIVEN SHOWERS/TSTMS ARE EXPECTED TO DISSIPATE THIS EVE...W/ SKIES REMAINING MOSTLY CLOUDY OVER THE WRN HALF OF THE AREA AND NERN MD SEEING MORE CLEARING. ANOTHER RELATIVELY WARM OVERNIGHT PERIOD AS DEWPOINTS REMAIN ELEVATED IN THE 60S AND MID CLOUD DECKS INSULATE THE AREA. BY THE PREDAWN HRS...SHOWERS ALONG THE LEADING EDGE OF ANOTHER LOW PRESSURE SYSTEM WILL BE APPROACHING THE WINDWARD SIDE OF THE APLCNS IN ERN WV.
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&& .SHORT TERM /MONDAY THROUGH TUESDAY/...
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THIS NEXT LOW PRESSURE SYSTEM HAS BEEN DEPICTED OVER THE PAST FEW DAYS BY MOST GUIDANCE MEMBERS AS A POTENT AND COMPACT UPPER SHORTWAVE TAPPING INTO A THIN CORRIDOR OF SUBTROPICAL AIR BY THE TIME IT REACHES THE MID-ATLC. THOUGH SOME UNCERTAINTY STILL REMAINS IN TERMS OF THE PLACEMENT AND TIMING OF POTENT PRECIP AND CONVECTIVE ELEMENTS AS THE FEATURES PASSES BY...TRENDS ARE LOOKING LESS AGGRESSIVE FOR OUR AREA. THOUGH LOCALIZED HEAVY RAIN WILL BE POSSIBLE AS THE LOW CROSSES OVER THE AREA LATE MON/EARLY TUE...THE INITIAL STAGES OF THE LOW`S ARRIVAL LOOKS LIKE A HIGH-POP/LOW QPF TYPE OF SCENARIO MON AFTN/EVE. THE MOST FAVORED AREAS WILL BE THE APLCNS AND INTO THE LOWER PIEDMONT REGION. DRIER CONDITIONS OVER NERN MD...THE BALT METRO DOWN INTO PARTS OF THE DC AREA THOUGH A FEW ISOLATED SHOWERS NOT OUT OF THE QUESTION. A POTENT SHORTWAVE AND ITS ASSOCIATED SURFACE LOW WILL TRACK ALONG A STATIONARY BOUNDARY...PASSING THROUGH OUR AREA MONDAY NIGHT INTO TUESDAY MORNING. MOISTURE WILL BE DEEP AHEAD OF THIS SYSTEM...WITH PWATS AROUND 2 INCHES. THE FORCING FROM THE SHORTWAVE AND THE SURFACE LOW WILL TRIGGER SHOWERS AND PERHAPS A FEW THUNDERSTORMS ACROSS MUCH OF THE AREA DURING THIS TIME. CONFIDENCE FOR FLASH FLOODING STILL REMAINS LOW...BUT MODELS HAVE CONVERGED ON A SOLUTION THAT KEEPS THE STRONGEST FORCING FROM THIS SYSTEM TO OUR SOUTH. THIS WOULD PREVENT A SIGNIFICANT THREAT FOR FLASH FLOODING...BUT WITH DEEP MOISTURE AROUND LOCALLY HEAVY RAIN AND LOCALIZED FLASH FLOODING IS STILL POSSIBLE. THE SHORTWAVE AND SURFACE LOW WILL MOVE OFF TO THE EAST TUESDAY AFTERNOON...BUT THE BOUNDARY WILL REMAIN OVERHEAD. A FEW SHOWERS AND THUNDERSTORMS CANNOT BE RULED OUT...BUT COVERAGE WILL BE SCATTERED AND MUCH OF THE TIME MAY ACTUALLY TURN OUT DRY. HUMID AND SEASONABLY WARM CONDITIONS ARE EXPECTED WITH MAX TEMPS IN THE LOWER TO MIDDLE 80S ACROSS MOST LOCATIONS.
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&& .LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
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A SUBTROPICAL RIDGE WILL BUILD OVER THE SOUTHEAST CONUS DURING THE MIDDLE PORTION OF THE WEEK BEFORE EXTENDING NORTH INTO THE CENTRAL CONUS BY FRIDAY. THIS WILL CAUSE A NORTHWEST FLOW ALOFT TO DEVELOP OVER OUR AREA. A STATIONARY BOUNDARY WILL REMAIN NEARBY WITH MOST OF OUR AREA REMAINING IN A WARM AND HUMID AIRMASS. SHOWERS AND THUNDERSTORMS ARE POSSIBLE EACH DAY...ESPECIALLY DURING THE AFTERNOON AND EVENING HOURS. MAX TEMPS EACH DAY WILL BE IN THE 80S ACROSS MOST LOCATIONS. UPPER-LEVEL LOW PRESSURE MAY DEVELOP OFF THE EAST COAST NEXT WEEKEND WHILE HIGH PRESSURE BUILDS INTO NEW ENGLAND. THIS WOULD CAUSE A BACKDOOR COLD FRONT TO MOVE THROUGH OUR AREA...ALLOWING FOR COOLER MARINE AIR TO MOVE IN.
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&& .AVIATION /07Z SUNDAY THROUGH THURSDAY/...
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NEARLY ALL OBS FOR THE AREA WERE CALM AND CLEAR STARTING OUT THE NIGHT...BUT NOW A FAIRLY DENSE MID/UPPER COLLECTION OF CLOUD DECKS HAVE MADE THEIR WAY OVER THE AREA. LIGHT SW FLOW HAS ALSO PICKED-UP A BIT...W/ THIS STORY CONTINUING TO PLAY-OUT FOR THE NEXT SEVERAL HRS. ONLY ISOLATED SHOWERS POSSIBLE FOR THE FIRST HALF OF THE TAF PERIOD...W/ MORE SCATTERED ACTIVITY - AND POSSIBLY A QUICK TSTM LATER THIS AFTN. OTHERWISE...VFR CONDS THRU THE CURRENT PERIOD AND DISSIPATION IN THIS ACTIVITY AGAIN LATE TONIGHT. LOW PRESSURE WILL PASS THROUGH THE TERMINALS MONDAY NIGHT INTO TUESDAY...LIKELY TRIGGERING SHOWERS AND FEW THUNDERSTORMS. SUBVFR CONDITIONS ARE POSSIBLE IN CONVECTION. AN UNSETTLED PATTERN IS EXPECTED THROUGH THE MIDDLE PORTION OF NEXT WEEK...WITH SHOWERS AND THUNDERSTORMS POSSIBLE EACH DAY...MAINLY DURING THE AFTERNOON AND EVENING HOURS. A WARM AND HUMID AIRMASS MAY LEAD TO LOW CLOUD AND AREAS OF FOG DURING THE OVERNIGHT AND MORNING HOURS EACH DAY.
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&& .MARINE...
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A LIGHT BUT STEADY SW WIND CONTINUES TO MOVE ACROSS THE WATERS OVERNIGHT...CONTINUING THROUGHOUT THE DAY. PERIODS OF SLIGHTLY HIGHER GUSTS NEAR 15KT POSSIBLE AT TIMES...BUT THAT WILL BE THE EXCEPTION. A SOLID 10KT BREEZE THIS AFTN WILL DROP OFF AGAIN LATE TONIGHT...AS WINDS SWITCH AROUND BRIEFLY TO NLY. THE TAIL END OF A COLD FRONT IS EXPECTED TO SLIDE DOWN THE BAY W/ SOME LIGHT NLY CHANNELING...THEN A WEAK ONSHORE FLOW DURING THE DAYTIME HRS ON MON. LOW PRESSURE WILL PASS THROUGH THE WATERS LATE MONDAY NIGHT INTO TUESDAY. WINDS MAY APPROACH SCA CRITERIA...BUT THE GRADIENT APPEARS TO BE WEAK SO GUSTS HAVE BEEN CAPPED AT 15 KNOTS. A STATIONARY BOUNDARY WILL REMAIN NEAR THE WATERS FOR THE MIDDLE AND LATTER PORTIONS OF THE WEEK. THIS MAY TRIGGER A FEW SHOWERS AND THUNDERSTORMS...MAINLY DURING THE AFTERNOON AND EVENING HOURS EACH DAY.
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&& .LWX WATCHES/WARNINGS/ADVISORIES... DC...NONE. MD...NONE. VA...NONE. WV...NONE. MARINE...NONE. && $$ SYNOPSIS...BJL NEAR TERM...GMS SHORT TERM...BJL/GMS LONG TERM...BJL AVIATION...BJL/GMS MARINE...BJL/GMS

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