000
FXUS61 KLWX 271825
AFDLWX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC
225 PM EDT WED AUG 27 2008
.SYNOPSIS...
LOW PRESSURE OVER EASTERN TENNESSEE ASSOCIATED WITH FORMER
TROPICAL STORM FAY WILL LIFT SLOWLY TO THE NORTHEAST THROUGH THE
END OF THIS WEEK. RAINFALL WILL BE HEAVIEST WEST OF THE BLUE RIDGE
MOUNTAINS. A COLD FRONT WILL MOVE ACROSS THE AREA SATURDAY...THEN
HIGH PRESSURE BUILDS IN FOR SUNDAY.
&&
.NEAR TERM /THROUGH TONIGHT/...
RAIN IS HAVING A TERRIBLY DIFFICULT TIME MOVING NORTHEAST
TODAY...DUE TO DRY HIGH PRESSURE OVER NEW ENGLAND REFUSING TO GIVE
UP ITS GRIP AND AN AREA OF NEGATIVE VORTICITY ACROSS NORTHEAST
MARYLAND. RAIN HAS BEEN PERSISTENT ACROSS THE HIGHLANDS AND
SHENANDOAH VALLEY...WHILE ITS SIMPLY BEEN A CLOUDY DAY ACROSS DC AND
BALTIMORE.
STILL BELIEVE THE GFS HAS HAD THE BETTER HANDLE ON THE SITUATION.
12Z GFS RUN SPREADS RAIN...ALBEIT SLOWLY...NORTHEAST OVERNIGHT
POSSIBLY MAKING IT NEAR BALTIMORE IN THE WEE HOURS OF THE MORNING.
IT MAY NEED THE HELP OF ANOTHER SHORTWAVE OVER EASTERN TENNESSEE
THIS AFTERNOON TO ENHANCE PRECIPITATION FURTHER NORTHEAST. AS HAS
BEEN THE CASE...THE HIGHEST QPF...MAYBE A HALF INCH TO INCH...WILL
RESIDE ACROSS THE HIGHLANDS AND SHENANDOAH VALLEY WHILE A FEW
HUNDREDTHS OR LESS WILL OCCUR ACROSS THE IMMEDIATE DC/BALTIMORE
AREA.
&&
.SHORT TERM /THURSDAY/...
GFS INDICATES ANOTHER WAVE OR SURGE OF MOISTURE ACROSS THE
CWA. THEREFORE HAVE LIKELY TO HIGH CHANCE POPS ACROSS THE CWA FOR
THURSDAY. WENT WITH THE COOLER MAV MOS NUMBER FOR MAX T.
&&
.LONG TERM /THURSDAY NIGHT THROUGH WEDNESDAY/...
500MB HEIGHT/ANOMALY PATTERN DURING THE PAST 5 DAYS SHOWS A
TROUGH/RIDGE/TROUGH PATTERN IN SOUTHERN CANADA HAS PROGRESSED TO A
DEEPENING TROUGH ACROSS WESTERN CANADA...AND A EAST CENTRAL
CANADIAN RIDGE. WATER VAPOR LOOP SHOWS THE UPPER REMNANT CIRCULATION
OF FAY MOVING NORTHEAST INTO THE OHIO VALLEY...WITH THE RIDGE
DOMINATING THE EASTERN GREAT LAKES. SURFACE MAP EARLY THIS MORNING
PLACES A 1022MB HIGH OVER NEW ENGLAND...WITH A BAROCLINIC ZONE FROM
THE TENNESSEE VALLEY EAST ACROSS THE CAROLINAS. THE REMNANT
CIRCULATION OF FAY WAS 1008MB AND LOCATED IN EASTERN TENNESSEE.
FIRST PORTION OF MEDIUM RANGE FORECAST REVOLVES AROUND THE REMNANTS
OF FAY. PER HPC AM INCLINED TO FOLLOW EUROPEAN/GFS BLEND AS GLOBAL
MODELS SEEM TO BE HANDLING THE EVOLUTION BETTER THAN THE NAM.
COMBINATION OF WARM ADVECTION OF TROPICAL AIRMASS...ISENTROPIC
UPGLIDE ABOVE FRONTAL BOUNDARY TO THE SOUTH...AND SOUTHEASTERLY
UPSLOPE FLOW SHOULD PROVIDE AN OPPORTUNITY FOR REPLENISHING RAINS
CONTINUING THROUGH THURSDAY...WITH A FOCUS ALONG THE MOUNTAINS. ITS
IMPORTANT TO NOTE THAT ALTHOUGH POPS ACROSS THE FORECAST AREA WILL
BE HIGH...THAT THE CRITERIA IS FOR 0.01" OF RAINFALL. THERE WILL
LIKELY BE A SIGNIFICANT GRADIENT IN RAINFALL TOTALS BETWEEN THE
HEAVIER RAIN ACROSS THE WESTERN ZONES...AND FURTHER EAST ACROSS THE
METRO AREAS WHERE HIGH PRESSURE CONTINUES TO FEED A DRY AIRMASS
SOUTHWARD.
BEYOND FAY...NEXT COLD FRONTAL PASSAGE OCCURS SATURDAY...WITH HIGH
PRESSURE BUILDING IN EARLY WEEK. ALSO WILL HAVE TO KEEP AN EYE ON
THE TROPICS AS NHC CONTINUES TO MONITOR GUSTAV...AND TWO POTENTIAL
AREAS OF DEVELOPMENT IN THE ATLANTIC.
&&
.AVIATION /18Z WEDNESDAY THROUGH MONDAY/...
MAIN CHALLENGE FOR THE 18Z TAF ISSUANCE WAS TIMING THE ONSET OF
RAIN...AND ASSOCIATED CIG/VSBY RESTRICTIONS. MVFR RESTRICTIONS WILL
BECOME MORE WIDESPREAD OVERNIGHT...WITH LOCAL IFR CONDITIONS AT
KCHO. THERE IS A CHANCE THAT IFR CONDITIONS SPREAD NORTHEAST TOWARD
KIAD/KDCA THURSDAY.
03Z SREF GUIDANCE INDICATES CHANCE TO LIKELY PROBABILITIES OF MVFR
CEILINGS THROUGH THE COLD FRONTAL PASSAGE SATURDAY. IFR CEILINGS ARE
LIKELY NEAR THE APPALACHIANS MOUNTAINS...PERHAPS AS FAR EAST AS THE
BLUE RIDGE THROUGH FRIDAY MORNING. HIGH PRESSURE BUILDS IN FOR EARLY
NEXT WEEK.
&&
.MARINE...
EAST/SOUTHEAST FLOW WILL CONTINUE THROUGH THURSDAY. MODELS INDICATE
20 KNOTS OR SO THAT IS AVAILABLE TO COME DOWN...THEREFORE WILL RAISE
A SMALL CRAFT ADVISORY.
SMALL CRAFT ADVISORY CONDITIONS MAY CONTINUE INTO THURSDAY EVENING
AS THE PRESSURE GRADIENT STRENGTHENS BETWEEN HIGH PRESSURE TO THE
NORTH...AND LOW PRESSURE APPROACHING FROM THE SOUTH. A COLD FRONT
PASSES SATURDAY...WITH HIGH PRESSURE BUILDING IN FOR EARLY NEXT WEEK.
&&
.TIDES/COASTAL FLOODING...
WATER LEVELS WITHIN THE TIDAL POTOMAC RIVER AND MARYLAND
CHESAPEAKE BAY ARE A HALF FOOT TO A FOOT ABOVE ASTRONOMICAL
PREDICTIONS. WITH THE EASTERLY FLOW PERSISTING UNTIL THE COLD
FRONTAL PASSAGE SATURDAY...WATER LEVEL ANOMALIES WILL HAVE TO BE
MONITORED.
&&
.LWX WATCHES/WARNINGS/ADVISORIES...
DC...NONE.
MD...NONE.
VA...NONE.
WV...NONE.
MARINE...SMALL CRAFT ADVISORY FROM 8 PM THIS EVENING TO 6 PM EDT
THURSDAY FOR ANZ530>537.
&&
$$
SYNOPSIS...PELOQUIN
NEAR TERM...PELOQUIN
SHORT TERM...PELOQUIN
LONG TERM...ROGOWSKI
AVIATION...PELOQUIN/ROGOWSKI
MARINE...PELOQUIN/ROGOWSKI
TIDES/COASTAL FLOODING...ROGOWSKI
