Dense Fog in Maryland, September 23, 2011

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC
819 PM EDT THU SEP 22 2011

.SYNOPSIS...
A COLD FRONT OVER EASTERN OHIO WILL APPROACH THE AREA TONIGHT. MOIST
FLOW AHEAD OF THE FRONT WILL PROVIDE UNSETTLED CONDITIONS. THE
FRONT WILL STALL OUT NEAR THE MID ATLANTIC COAST OVER THE WEEKEND.
MEANWHILE...AN UPPER LOW WILL REMAIN OVER THE OHIO VALLEY THROUGH
EARLY NEXT WEEK.

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
UPPER-LEVEL LOW CONTINUES TO SPIN OVER THE GREAT LAKES WHILE AN
UPPER-LEVEL RIDGE REMAINS OVER THE ATLANTIC OCEAN. A SOUTHWEST
FLOW IN BETWEEN THESE TWO SYSTEMS CONTINUES TO USHER IN TROPICAL
MOISTURE ACROSS THE MID-ATLANTIC. MOISTURE PLUME CAN BE SEEN WELL
ON THE WATER VAPOR IMAGERY.

THE COLD FRONT ASSOCIATED WITH THE UPPER-LEVEL LOW WILL REMAIN
OVER THE OHIO VALLEY TONIGHT WHILE AN INVERTED TROUGH REMAINS
ALONG THE SOUTHEAST AND MID-ATLANTIC COAST. A LACK OF LIFTING
MECHANISMS TONIGHT MEANS THAT ANY SHOWER ACTIVITY WILL REMAIN
ISOLATED TO SCATTERED ACROSS MOST LOCATIONS.

THE ONE EXCEPTION MAY BE ACROSS LOWER SOUTHERN MARYLAND AND
CENTRAL VIRGINIA TOWARDS MORNING. AN UPPER- LEVEL DISTURBANCE
EMBEDDED WITHIN THE SOUTHWEST FLOW ALOFT IS CURRENTLY LOCATED OVER
THE SOUTHEASTERN CONUS THIS EVENING. THIS SYSTEM WILL QUICKLY MOVE
UP THE MID-ATLANTIC COAST TOWARD OUR AREA BY FRIDAY MORNING.
SHOWERS/ISOLATED T-STORMS ASSOCIATED WITH THIS SYSTEM MAY MOVE
INTO LOWER SOUTHERN MARYLAND/CENTRAL VIRGINIA TOWARD FRIDAY
MORNING. HOWEVER...THE HEAVIEST PRECIPITATION IS EXPECTED TO HOLD
OFF UNTIL FRIDAY.

MIN TEMPS WILL RANGE FROM THE 50S IN THE POTOMAC AND ALLEGHENY
HIGHLANDS TO THE LOWER 70S IN DOWNTOWN WASHINGTON AND BALTIMORE.
DESPITE PLENTY OF HIGH AND MID-LEVEL CLOUDS...ABUNDANT LOW-LEVEL
MOISTURE WILL LIKELY CAUSE LOW CLOUDS AND AREAS OFF FOG LATE
TONIGHT.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/...
A WET DAY LOOKS TO BE ON TAP FOR FRI. THE UPPER-LEVEL LOW WILL
REMAIN OVER THE WESTERN GREAT LAKES WHILE THE UPPER-LEVEL HIGH
CONTINUES TO SPIN OVER THE ATLANTIC OCEAN. TROPICAL MOISTURE WILL
REMAIN OVER OUR REGION WITH PRECIPITABLE WATERS PROGGED TO BE OVER
2 INCHES FOR LOCATIONS EAST OF THE BLUE RIDGE MOUNTAINS.

A VORT MAX ROTATING AROUND THE UPPER-LEVEL LOW WILL CAUSE THE MID-
LEVEL TROUGH AXIS TO DEEPEN OVER THE MIDWEST. AN INCREASING
GRADIENT BETWEEN THIS SYSTEM AND THE UPPER-LEVEL HIGH OVER THE
ATLANTIC WILL CAUSE THE SOUTHWEST FLOW TO STRENGTHEN. A
STRENGTHENING LOW-LEVEL JET WILL ACT AS A LIFTING MECHANISM TO
TRIGGER SHOWERS AND ISOLATED THUNDERSTORMS. AN UPPER-LEVEL
DISTURBANCE IN THE SOUTHWEST FLOW WILL ALSO BE PASSING THROUGH THE
MID-ATLANTIC ON FRIDAY. LOCALLY HEAVY RAIN IS EXPECTED NEAR THIS
SYSTEM AS IT COMBINES WITH THE TROPICAL MOISTURE ALREADY IN PLACE.
THE BEST CHANCE FOR THE HEAVY RAINFALL WILL LIKELY BE NEAR THE
I-95 CORRIDOR AND ALSO ACROSS LOWER SOUTHERN MARYLAND. A FLASH
FLOOD WATCH REMAINS IN EFFECT FOR THESE AREAS FRIDAY. RAINFALL
AMOUNTS AROUND ONE TO THREE INCHES ARE EXPECTED WITH LOCALLY
HIGHER AMOUNTS OVER 4 INCHES POSSIBLE.

&&

.LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/...
CDFNT/SFC TROF PERSISTS ACRS RGN FRI NGT...GRADUALLY BCMG STATIONARY
NEAR CWAS ERN BORDER. BEST CHC FOR HVY RN WILL MOVE N/E OF RGN
OVNGT...BUT WIDESPREAD SHWRS SHUD CONT. RISK FOR FLOOD/FLASH FLOOD
WILL DEPEND ON XTNT OF EARLIER PRECIP AMOUNTS...AS LATEST GUIDANCE
PROGRESS ADDITIONAL RNFL FRI NGT/SAT GENERALLY BELOW ONE INCH.
AFOREMENTIONED SFC FEATURES WILL RMN PARKED ALONG CHESAPEAKE
BAY/DELMARVA THIS WKEND...AS ALREADY STRONG UPR TROF DEEPENS FURTHER
AND CLOSES OFF ACRS OH VLY. DRIER M/ULVL AIR MAY INHIBIT HVY
PRECIP...BUT SIGNIFICANT LIFT WARRANT HIGH CHC/LOW LIKELY POPS ALL
WKEND.

UPR LOW MAY EVEN RETROGRADE INTO MIDWEST CONUS BEFORE MODELS HINT AT
ITS LIFTING TWD GREAT LAKES ERY NEXT WK. POPS ACRS MID-ATLC PERSIST
UNTIL STRONGER PUSH OF COOLER/DRY AIR MOVES THRU THE AREA. ATTM
LOOKS LIKE IT WILL BE UNTIL MIDWEEK WHEN FRNT FINALLY IS ABLE TO
BRING THIS CHANGE.

GUIDANCE TRENDING WARMER LAST FEW RUNS...SO XTDD FCST NOW REFLECTS
SEASONAL MAXIMA WITH ABV NRML MINIMA. DIURNAL TEMP TRENDS WILL BE
MINIMIZED BY ABUNDANT CLOUDS/PRECIP.

&&

.AVIATION /00Z FRIDAY THROUGH TUESDAY/...
LOW CLOUDS AND AREAS OF FOG WILL CAUSE IFR CIGS/VSBYS LATE TONIGHT
INTO FRIDAY MORNING. AN UPPER-LEVEL DISTURBANCE WILL TRIGGER
LOCALLY HEAVY RAINFALL WHICH WILL REDUCE CIGS AND VSBYS
FRIDAY...ESPECIALLY ACROSS THE EASTERN TERMINALS.

SATURATED LLVLS AND LGT WINDS WILL SUPPORT IFR VSBY/CIGS BOTH FRI
NGT/SAT MRNG AND SAT NGT/SUN MRNG...PROVIDED SHWRS DONT MIX
FG/LOWEST STRATUS OUT. SOME IMPROVEMENT...ESPECIALLY WITH
CIGS...DURG DAYTIME HRS SAT/SUN...BUT SCT/NUM SHWRS STILL FCST...AND
MVFR CIGS CUD PREVAIL. WITH POPS CONTG THRU MON NGT...FURTHER
RESTRICTIONS ARE PSBL ERY NEXT WEEK.

&&

.MARINE...
WINDS TO RMN BLO SCA LVLS TNGT AND FRI. HVY RAIN PSBL FRIDAY WL
CAUSE SIG REDUCTIONS TO VSBY.

SELY FLOW XPCD FRI NGT/SAT...FOLLOWED BY L/V WINDS SAT NGT THRU MON
NGT AS SFC BNDRY STALLS NEAR WATERS. WIDESPREAD SHWRS FRI NGT
GRADUALLY DIMINISH TO SCATTERED BY SAT NGT...WITH CHC POPS CONTG
INTO ERY NEXT WEEK. HVY PRECIP ASSOCIATED WITH THESE SHWRS CUD CAUSE
LWRD VSBYS.

&&

.TIDES/COASTAL FLOODING...
WATER LVLS TO RMN ABT ONE HALF TO THREE QUARTERS OF A FOOT ABOVE
NORMAL TONIGHT. THE ONSHORE FLOW IS NOT EXPECTED TO STRENGTHEN
OVERNIGHT SO ANOMALIES WILL LIKELY REMAIN THE SAME. THIS WILL KEEP
WATER LEVELS JUST BELOW THRESHOLDS FOR MINOR FLOODING IN SENSITIVE
AREAS...PARTICULARLY IN ALEXANDRIA AND ANNAPOLIS. WILL CONTINUE TO
MONITOR OVERNIGHT. ANOMALIES MAY INCREASE FRIDAY WHICH WILL CAUSE
A THREAT FOR MINOR COASTAL FLOODING OF SENSITIVE AREAS.
HOWEVER...FRIDAYS HIGH TIDE CYCLE WILL BE THE LOWER OF THE TWO
HIGH TIDES.

WITH SE FLOW CONTG FRI NIGHT/SAT AND LOPRES IN VICINITY...MINOR
COASTAL FLOODING IS PSBL...AND MAY CONT INTO SUN DURG PERIODS OF
HIGH TIDE.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...FLASH FLOOD WATCH FROM 6 AM EDT FRIDAY THROUGH FRIDAY
     AFTERNOON FOR DCZ001.
MD...FLASH FLOOD WATCH FROM 6 AM EDT FRIDAY THROUGH FRIDAY
     AFTERNOON FOR MDZ004>007-009>011-013-014-016>018.
VA...FLASH FLOOD WATCH FROM 6 AM EDT FRIDAY THROUGH FRIDAY
     AFTERNOON FOR VAZ042-052>055-057.
WV...NONE.
MARINE...NONE.

&&

PRODUCTS...LASORSA/WOODY!/SBK

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