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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