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...
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.

&&

.NEAR TERM /THROUGH TONIGHT/...
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.

&&

.SHORT TERM /MONDAY THROUGH TUESDAY/...
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.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
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.

&&

.AVIATION /07Z SUNDAY THROUGH THURSDAY/...
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.

&&

.MARINE...
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.

&&

.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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