Area Forecast Discussion
Issued by NWS Sterling, VA

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000
FXUS61 KLWX 211900
AFDLWX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC
300 PM EDT THU MAY 21 2015

.SYNOPSIS...
LOW PRESSURE PASSES SOUTH OF THE REGION LATE THIS
AFTERNOON...WITH A COLD FRONT MOVING THROUGH ON FRIDAY. HIGH
PRESSURE BUILDS IN FOR THE WEEKEND WITH COOL WEATHER TO START
FOLLOWED BY NOTICEABLE WARMING LATER ON. AS A SUMMER-LIKE AIR MASS
ADVECTS INTO THE REGION EARLY NEXT WEEK...HOT TEMPERATURES AND AN
INCREASED RISK OF THUNDERSTORMS WILL BECOME THE MAIN STORY.

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
SFC LOW EXPECTED TO BE PUSHING OFF THE CAROLINA COAST BY THIS
EVENING...WITH HIGH PRESSURE BRIEFLY BUILDING IN THE WAKE. 18Z
WATER VAPOR IMAGERY SHOWING DRIER AIR WORKING INTO WESTERN
VA...WITH PCPN ALL BUT ENDING WEST OF THE BLUE RIDGE. EXPECTING
PCPN TO CONTINUE EAST OF THE BLUE RIDGE INTO THE EARLY PART OF THE
EVENING...THEN TAPERING OFF AS THE LOW PUSHES OFFSHORE. OVERCAST
CONDITIONS AND PCPN HAVE KEPT THE TEMPS FROM CLIMBING MUCH AT ALL
TODAY...SO HAVE UPDATED THE TEMPS FOR THIS AFTN...KEEPING
EVERYWHERE IN THE 50S.

ANY LINGERING PCPN MAINLY OVER SOUTHERN MD THIS EVENING WILL EXIT
BY THE SECOND HALF OF TONIGHT. DRY CONDITIONS THEN IN PLACE. NW
FLOW USHERING IN COOLER AIR WILL DROP TEMPS INTO THE 40S. SKIES
WILL BE CLEARING BY LATE...BUT WINDS SHOULD STAY ELEVATED ENOUGH
TO KEEP TEMPS FROM DROPPING MUCH MORE THAN THE LOW TO MID 40S.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH SUNDAY/...
MAINLY DRY PERIOD TO END THE WEEK AS HIGH PRESSURE BUILDS ACROSS
THE REGION. A QUICK MOVING COLD FRONT WILL PUSH THROUGH FRIDAY
BEFORE THE HIGH COMPLETELY TAKES HOLD OF THE REGION. EXPECTING
MAINLY A DRY PASSAGE WITH MOISTURE LACKING. COULD SEE ISO SHOWERS
OVER NORTHWEST CORNER OF THE CWA MAINLY DUE TO ANY TERRAIN
CIRCULATIONS ON THE NW FLOW.

TIGHTENING PRESSURE GRADIENT BETWEEN THE DEPARTING...DEEPENING
SFC LOW AND APPROACHING HIGH WILL LEAD TO GUSTS UP INTO THE 20S
DURING THE DAY FRIDAY...DIMINISHING BY FRI EVENING. HIGHS ON FRI
NEAR SEASONAL...HAVING GONE A DEGREE OR TWO HIGHER THAN THE
GUIDANCE TO ACCOUNT FOR DOWNSLOPING EFFECTS...WHICH PUSHES MOST
AREAS INTO THE UPPER 60S TO LOWER 70S. CLEARING SKIES...LIGHT
WINDS AND CAA WILL DROP LOWS FRI NIGHT BLW NORMAL. SOME SPOTS OF
FAR WESTERN POTOMAC HIGHLANDS COULD EVEN SEE TEMPS FALLING INTO
THE MID 30S...HAVE INCLUDED PATCHY FROST TO ACCOUNT FOR THIS
THOUGH LIMITED MOISTURE ON SUCH A DRY PATTERN MIGHT LIMIT ANY
DEVELOPMENT DESPITE THE TEMP.

BY SATURDAY MORNING...HIGH PRESSURE WILL BE CENTERED OVER THE
UPPER OHIO VALLEY WITH LIGHT WINDS AND MOSTLY CLEAR SKIES ACROSS
THE CWA. AFTER A COOL START FOR LATE MAY...TEMPERATURES WILL
REBOUND INTO THE LOWER 70S ACROSS MOST OF THE REGION WITH LIGHT
WINDS. ALTHOUGH STILL A BIT BELOW NORMAL...PLENTIFUL SUNSHINE WILL
HELP MAKE IT A VERY NICE DAY...IF STILL TOO COOL TO REALLY BE
CONSIDERED A BEACH DAY.

DURING SATURDAY NIGHT HIGH PRESSURE WILL SLIDE OFF THE COAST INTO
THE WESTERN ATLANTIC. WINDS WILL GRADUALLY SHIFT TOWARDS THE
SOUTHWEST...BUT THE RETURN OF WARMTH AND MOISTURE WILL NOT HAVE
REALLY STARTED JUST YET SATURDAY NIGHT SO ANOTHER GOOD RADIATIONAL
NIGHT IS EXPECTED WITH MOSTLY CLEAR SKIES...LIGHT WINDS...AND
TEMPS IN THE 40S OUTSIDE OF THE CITIES.

HIGH PRESSURE WILL CONTINUE ITS SLOW SLIDE EAST ON SUNDAY...BUT
THE RETURN FLOW WILL BE MORE NOTICEABLE WITH TEMPERATURES RISING
BACK ABOVE NORMAL. LOW 80S WILL BE COMMON ACROSS THE CWA. COMBINED
WITH THE EXPECTED DEWPOINTS RISE OF OVER 20 DEGREES (SATURDAY`S
READINGS IN THE 30S VERSUS SUNDAY`S IN THE 50S)...AND IT WILL FEEL
MUCH MORE SUMMERLIKE ON SUNDAY VERSUS SATURDAY. WHILE THIS
INCREASED WARMTH AND MOISTURE WILL SIGNAL A RETURN OF CONVECTION
NEXT WEEK...IT SHOULD NOT OCCUR JUST YET ON SUNDAY WITH THE RIDGE
AXIS ALOFT STILL WEST OF THE REGION.

&&

.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
ECMWF 500 MB RIDGE OVER MID ATLANTIC REGION WILL ASSUME A
NORTHEAST TO SOUTHWEST ORIENTATION AND WEAKEN DURING THE WORKWEEK.
STRONGEST PVA WILL STAY WEST AND NORTH OF AREA. SURFACE WARM FRONT
WILL CROSS AREA SUNDAY NIGHT THROUGH MONDAY AFTERNOON. ALTHOUGH
BEST CHANCES OF SHOWERS AND THUNDERSTORMS LOOKS TO OCCUR
MONDAY...INCREASED MOISTURE... TEMPERATURES AND WEAK PVA WILL
SUPPORT SCATTERED CONVECTIVE ACTIVITY EACH DAY.

&&

.AVIATION /19Z THURSDAY THROUGH TUESDAY/...
LIGHT PCPN THIS AFTN RESULTING IN VARYING CONDITIONS. CIGS ARE
VARYING BETWEEN IFR AND MVFR. EXPECTING ANY IFR CIGS TO IMPROVE
BETWEEN 20-22Z...WITH THE MVFR CIGS THEN LINGERING INTO THIS
EVENING UNTIL THE WINDS SWITCH TO A NW FLOW...ALLOWING FOR DRIER
AIR. VIS MAINLY MVFR OR VFR...DROPPING TO MVFR WITH THE RAIN...AND
INCREASING BACK TO VFR WITH THE PCPN ENDING. EXPECTING
PREDOMINATELY VFR VIS BETWEEN 20-21Z. ANY PCPN OCCURRING THIS AFTN
WILL TAPER OFF BETWEEN 20-22Z...WITH PSBL -DZ LASTING THRU 02-03Z.
VFR CONDITIONS THEN EXPECTED THE REMAINDER OF THE TAF PERIOD.

WINDS LIGHT/VRB THRU THE AFTN...BCMG NW 5-10 KTS OVERNIGHT. NW
FLOW WILL INCREASE FRI...WITH GUSTS 20-25 KTS BEGINNING AROUND
13-14Z AND LASTING INTO FRI EVENING.

VFR CIGS AND VIS THROUGH THE WEEKEND WITH LIGHT WINDS AS HIGH
PRESSURE REIGNS SUPREME. THE THREAT OF THUNDERSTORMS WILL BEGIN TO
INCREASE ON MEMORIAL DAY AS THE HIGH MOVES EAST AND A WARMER AND
MORE HUMID AIR MASS ADVECTS INTO THE REGION.

&&

.MARINE...
SCA IN EFFECT FOR MIDDLE CHESAPEAKE BAY WATERS NOW THRU FRI
MORNING WITH NE GUSTS 20-25 KTS ALREADY OCCURRING AND EXPECTED TO
CONTINUE THRU THE NIGHT. A BRIEF LULL IN WINDS ARE PSBL THIS
EVENING...BUT NOT LONG ENOUGH TO WARRANT ENDING THE SCA AND
STARTING LATER IN THE NIGHT. ELSEWHERE...SCA WILL GO IN EFFECT FOR
THE REMAINDER OF THE CHESAPEAKE BAY AND THE LOWER TIDAL POTOMAC
LATE TONIGHT WITH WINDS INCREASING ALONG THE TIGHTENING PRESSURE
GRADIENT. THIS SCA REMAINS UP THRU FRI MORNING AS WELL.

BY FRI AFTN...EXPECTING ALL WATERS TO BECOME SUB-SCA EXCEPT THE
NORTHERN CHESAPEAKE BAY...WITH GUSTS UP TO 20 KTS PSBL BEHIND THE
FRONT. PSBL SCA FOR THE REMAINDER OF THE WATERS FRI NIGHT...BUT
WITH THE WINDS BEING BORDERLINE...CONFIDENCE IS TOO LOW AT THIS
TIME TO EXTEND THE SCA TO ANY OTHER ZONES.

WINDS SHOULD STAY BELOW SCA LEVELS THROUGH THE WEEKEND WITH HIGH
PRESSURE IN CONTROL. NO PRECIPITATION IS EXPECTED. THE THREAT OF
SHOWERS AND THUNDERSTORMS WILL INCREASE NEXT WEEK AS A WARMER AND
MORE HUMID AIR MASS MOVES INTO THE AREA.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...NONE.
MD...NONE.
VA...NONE.
WV...NONE.
MARINE...SMALL CRAFT ADVISORY FROM 3 AM TO 8 AM EDT FRIDAY FOR ANZ531-
     540-541.
     SMALL CRAFT ADVISORY FROM 3 AM TO 10 AM EDT FRIDAY FOR ANZ532-
     533-537-542.
     SMALL CRAFT ADVISORY UNTIL 2 PM EDT FRIDAY FOR ANZ534-543.
     SMALL CRAFT ADVISORY FROM 1 PM TO 11 PM EDT FRIDAY FOR ANZ530-
     531.

&&

$$
SYNOPSIS...SEARS/RCM
NEAR TERM...SEARS
SHORT TERM...SEARS/RCM
LONG TERM...CEM
AVIATION...SEARS/RCM
MARINE...SEARS/RCM



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