508  
FXUS61 KLWX 221913  
AFDLWX  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC  
313 PM EDT FRI JUN 22 2018  
   
SYNOPSIS
 
 
A WARM FRONT WILL SLOWLY APPROACH THE REGION TONIGHT BEFORE  
PASSING THROUGH SATURDAY. LOW PRESSURE WILL MOVE INTO THE  
GREAT LAKES DURING THIS TIME, AND THE COLD FRONT ASSOCIATED  
WITH THE LOW WILL APPROACH SUNDAY BEFORE PASSING THROUGH  
SUNDAY NIGHT. HIGH PRESSURE WILL RETURN FOR MONDAY AND TUESDAY  
BEFORE MOVING OFFSHORE DURING THE MIDDLE PORTION OF NEXT WEEK.  
 

 
   
NEAR TERM /THROUGH TONIGHT/
 
 
A SURFACE BOUNDARY BISECTS CENTRAL VIRGINIA THIS AFTERNOON WITH  
A MOIST EAST NORTHEAST FLOW FROM THE ATLANTIC TO THE NORTH OF  
SAID BOUNDARY. THE AREA OF RAIN THAT PLAGUED LOCATIONS ALONG AND  
EAST OF THE BLUE RIDGE THIS MORNING HAS WANED A BIT AS IT  
ENTERED A MORE STABLE AIRMASS TO THE NORTH. TEMPERATURES HAVE  
HELD PRETTY STEADY THANKS TO THE CLOUD COVER AND RAIN COVERAGE,  
RANGING IN THE MIDDLE 60S TO NEAR 70 DEGREES AREA WIDE.  
 
THE AFOREMENTIONED BOUNDARY WILL CONTINUE LIFTING NORTHWARD  
THROUGH TONIGHT, AS SHOWERS AND THUNDERSTORMS DEVELOP AND PRESS  
EASTWARD THIS AFTERNOON AND EVENING. AREAS THAT HAVE EXPERIENCED  
HEAVY RAIN OVER THE LAST 24 HOURS ARE POISED TO SEE ADDITIONAL  
RAINFALL THROUGH THIS EVENING. AS SUCH, WE HAVE ISSUED A FLASH  
FLOOD WATCH UNTIL LATE TONIGHT FOR PORTIONS OF CENTRAL AND  
NORTHERN VIRGINIA, AND SOUTHERN PORTIONS OF EASTERN WEST  
VIRGINIA.  
 
THE POTENTIAL ALSO EXISTS FOR SOME STRONG THUNDERSTORMS ALONG  
THE EXTREME SOUTHERN PORTIONS OF OUR CWA, MAINLY ACROSS CENTRAL  
VA AND SOUTHERN MD THIS EVENING. MODELS DO INDICATE THIS IS  
WHERE THE BEST INSTABILITY WILL RESIDE, WITH THE NAM SHOWING  
0-6KM SHEAR VALUES 30-35KTS AND MUCAPE 1000-1500 J/KG. GFS HOLDS  
THE BOUNDARY A BIT FURTHER SOUTH, THUS KEEPS MUCH OF OUR CWA ON  
THE STABLE SIDE. REGARDLESS, THERE WILL BE SOME POTENTIAL FOR  
SOME STRONGER STORMS ACROSS THESE AREAS THIS EVENING, AND WILL  
BE DEPENDENT ON THE LOCATION OF THE BOUNDARY.  
 
THE SHOWERS AND THUNDERSTORMS THIS EVENING WILL FAVOR A MORE  
PROGRESSIVE NATURE WITH THE INCREASED FLOW ALOFT, BUT COMBINED  
WITH THE RAINFALL ALREADY RECEIVED OVER THE PAST 24 HOURS, A  
CONTINUED FLOODING THREAT REMAINS OVERNIGHT FOR THE AREAS  
HIGHLIGHTED IN THE FLASH FLOOD WATCH. HIGH RES GUIDANCE  
INDICATES A DOWNWARD TREND IN PRECIPITATION COVERAGE AFTER  
MIDNIGHT, AS ACTIVITY MOVES OFF TO THE EAST NORTHEAST. OVERNIGHT  
LOWS WILL REMAIN IN THE 60S FOR MOST.  
 

 
   
SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/
 
 
THE FRONT WILL RESIDE NEAR THE POTOMAC RIVER SATURDAY MORNING,  
LIFTING NORTHWARD AS A WARM FRONT THROUGH THE DAY. THIS WILL  
PROVIDE A WARM AND UNSTABLE AIRMASS OVER THE CWA. AT THE SAME  
TIME AND UPPER LEVEL LOW WILL BE ROUNDING THE LOWER GREAT LAKES  
WITH VORTICITY ADVECTION PASSING OVERHEAD. THIS COMBINED WITH  
THE WARM FRONT USHERING IN A WARM AND MOIST AIRMASS, SCATTERED SHOWERS  
AND THUNDERSTORMS WILL DEVELOP MID MORNING ONWARD. GIVEN THE  
AFOREMENTIONED SET UP AND INGREDIENTS, SOME OF THESE STORMS  
COULD BE STRONG.  
 
THE STALLED BOUNDARY WILL RESIDE NEARBY ON SUNDAY, WITH THE  
WARM AND HUMID AIR MASS STICKING AROUND AS WELL. ADDITIONAL  
SHOWERS AND STORMS WILL BE POSSIBLE DURING SUNDAY AFTERNOON WITH  
DAYTIME HEATING. THE FLOW ALOFT WILL FAVOR MORE OF A WESTERLY  
TRAJECTORY AS OPPOSED TO SATURDAY, WITH NOT AS MUCH FAVORABLE  
LIFT ALOFT. THE STALLED BOUNDARY LOOKS TO TRANSLATE TO A COLD  
FRONT AND MOVE THROUGH THE REGION LATER ON SUNDAY, HELPING BRING  
A BIT MORE STABLE ENVIRONMENT TO THE REGION SUNDAY NIGHT, AS  
WELL AS DRIER CONDITIONS. WITH THE THREAT FOR PRECIPITATION AND  
AMPLE CLOUD COVER, DO NOT EXPECT TEMPERATURES TO STRAY TOO FAR  
FROM NORMAL OR SLIGHTLY ABOVE THROUGH THE PERIOD.  
 

 
   
LONG TERM /MONDAY THROUGH FRIDAY/
 
 
CANADIAN HIGH PRESSURE WILL BUILD OVERHEAD FOR MONDAY THROUGH  
TUESDAY, BRINGING DRY CONDITIONS ALONG WITH SUNSHINE AND LOW  
HUMIDITY. MAX TEMPS WILL BE IN THE LOWER TO MIDDLE 80S FOR MOST  
LOCATIONS WITH MIN TEMPS IN THE 50S AND 60S.  
 
THE HIGH WILL MOVE OFFSHORE WEDNESDAY THROUGH FRIDAY AND A WEST  
TO SOUTHWEST FLOW WILL CAUSE A WARMING TREND IN TEMPERATURES  
ALONG WITH A RETURN OF MORE HUMID CONDITIONS. THE INCREASED HEAT  
AND HUMIDITY MAY LEAD TO A FEW SHOWERS AND THUNDERSTORMS EACH  
DAY, ESPECIALLY DURING THE AFTERNOON AND EVENING HOURS.  
 

 
   
AVIATION /18Z FRIDAY THROUGH WEDNESDAY/
 
 
IFR TO MVFR FORECAST THROUGH THE OVERNIGHT PERIOD WITH EPISODES  
OF LIFR CONDITIONS POSSIBLE AS WELL. THE TERMINALS WILL SEE  
SHOWERS MOVE OUT OF THE AREA EARLY THIS AFTERNOON, AS ADDITIONAL  
SHOWERS AND STORMS DEVELOP AND APPROACH FROM THE WEST SOUTHWEST.  
THE STRONGEST ACTIVITY AND GREATEST IMPACTS TO AVIATORS WILL BE  
AT CHO WHERE BETTER INSTABILITY AND HIGHER RAINFALL RATES ARE  
EXPECTED TO RESIDE. WILL MAINTAIN RAIN CHANCES AT ALL TERMINALS  
THROUGH MUCH OF THE NIGHT BEFORE TAPERING OFF AFTER MIDNIGHT  
FROM WEST TO EAST. EXPECT RESTRICTIONS TO REMAIN OVERNIGHT WITH  
AMPLE MOISTURE REMAINING OVER THE AREA.  
 
GUSTY EASTERLY BREEZES THIS AFTERNOON AND EVENING WILL SLACKEN  
OVERNIGHT, TURNING LIGHT AND SOUTHERLY SATURDAY MORNING.  
VFR/MVFR CIGS WILL HOLD ON DURING THE DAY ON SATURDAY.  
ADDITIONAL SHOWERS AND THUNDERSTORMS WILL BE POSSIBLE THROUGH  
THE REMAINDER OF THE WEEKEND, BRINGING PERIODS OF SUB-VFR  
CONDITIONS.  
 
HIGH PRESSURE WILL BRING VFR CONDITIONS MONDAY AND TUESDAY. THE HIGH  
WILL MOVE OFFSHORE WEDNESDAY, AND THIS MAY TRIGGER A FEW SHOWERS AND  
THUNDERSTORMS, ESPECIALLY DURING THE AFTERNOON AND EVENING  
HOURS.  
 

 
   
MARINE
 
 
A FRONTAL BOUNDARY SOUTH OF THE WATERS WILL CONTINUE LIFTING  
NORTHWARD TODAY AND INTO SATURDAY. GUSTY EASTERLY WINDS AHEAD OF  
THE FRONT HAVE HELPED BRING SMALL CRAFT ADVISORY CONDITIONS TO  
THE WATERS TODAY, AND THIS IS EXPECTED TO PERSIST IN TO THIS  
EVENING. A FEW STORMS MAY TRAVERSE THE LOWER TIDAL POTOMAC AND  
CHESAPEAKE BAY THIS EVENING, SOME OF WHICH MAY BE STRONG,  
REQUIRING THE ISSUANCE OF SPECIAL MARINE WARNINGS SHOULD  
CONDITIONS WARRANT.  
 
WINDS ARE FORECAST TO SLACKEN TOWARDS SATURDAY MORNING  
AS THE GRADIENT WEAKENS. SHOWERS AND STORMS MAY IMPACT THE  
WATERS SATURDAY AND SATURDAY NIGHT, POTENTIALLY PRODUCING  
LOCALLY GUSTY WINDS.  
 
HIGH PRESSURE WILL BUILD IN BEHIND A COLD FRONT MONDAY THROUGH  
TUESDAY. THE HIGH WILL MOVE OFFSHORE WEDNESDAY. A SMALL CRAFT  
ADVISORY MAY BE NEEDED FOR PORTIONS OF THE WATERS MONDAY AND MONDAY  
NIGHT...AND AGAIN FROM THE RETURN SOUTHWEST FLOW WEDNESDAY.  
 

 
   
HYDROLOGY
 
 
ONGOING FLOOD WARNINGS ARE ABOUND ACROSS CENTRAL AND NORTHERN  
VIRGINIA THIS AFTERNOON THANKS TO PERSISTENT MODERATE TO HEAVY  
RAINFALL OVERNIGHT AND EARLY THIS MORNING. THE HEAVIER RAIN HAS  
MOVED OFF TO THE NORTH AND EAST, BUT NUMEROUS ROAD CLOSURES  
REMAIN ACROSS THE AFFECTED AREAS. HEED ANY AND ALL ROAD CLOSURE  
SIGNS AND DO NOT VENTURE IN TO ANY FLOODED AREAS.  
 
WHILE THE EARLIER RAINS ARE MOVING OUT OF THE AREA TO OUR NORTH  
AND EAST, ADDITIONAL SHOWERS AND THUNDERSTORMS ARE EXPECTED TO  
DEVELOP LATER THIS AFTERNOON AND INTO THIS EVENING ACROSS THE  
SAME AREAS THAT RECEIVED THE HEAVIEST RAINFALL SINCE YESTERDAY.  
AS A RESULT, A FLASH FLOOD WATCH IS IN EFFECT LATE THIS  
AFTERNOON THROUGH TONIGHT FOR PORTIONS OF CENTRAL AND NORTHERN  
VIRGINIA, INCLUDING PORTIONS OF THE SHENANDOAH VALLEY AND  
ADJACENT EASTERN WEST VIRGINIA COUNTIES. ADDITIONAL RAINFALL  
ACCUMULATIONS OF ONE TO TWO INCHES ARE EXPECTED ACROSS THESE  
AREAS WITH LOCALIZED HIGHER AMOUNTS WHERE STRONGER SHOWERS AND  
STORMS IMPACT.  
 

 
   
TIDES/COASTAL FLOODING
 
 
WATER LEVELS RUNNING ONE TO TWO FEET ABOVE ASTRONOMICAL NORMALS  
THIS AFTERNOON. EASTERLY BREEZES WILL PROMOTE WATER LEVELS  
CONTINUING TO RUN AT OR JUST ABOVE THIS LEVEL THROUGH TONIGHT.  
A COASTAL FLOOD ADVISORY IS IN EFFECT THIS AFTERNOON AND THROUGH  
EARLY SATURDAY MORNING DURING TIMES OF HIGH TIDE AT DAHLGREN,  
THE DISTRICT OF COLUMBIA, ARLINGTON COUNTY, THE CITY OF  
ALEXANDRIA, AND THE SHORELINE IN ST. MARYS COUNTY.  
 
TONIGHTS HIGH TIDE CYCLE IS EXPECTED TO BE THE HIGHEST  
ASTRONOMICALLY, WITH LOCATIONS NEAR ANNAPOLIS AND BALTIMORE  
POTENTIALLY REACHING MINOR FLOOD STAGE OVERNIGHT. WILL ALLOW THE  
EVENING SHIFT TO FURTHER ANALYZE THIS THREAT AND FOLLOW UP WITH  
ANY NECESSARY ADVISORIES AS NEEDED.  
 
ADDITIONAL TIDAL FLOODING WILL BE POSSIBLE AGAIN SATURDAY NIGHT  
AND EARLY SUNDAY DURING THE HIGH TIDE CYCLE. AFTER WHICH, WINDS  
WILL TURN WESTERLY DURING THE DAY ON SUNDAY AND SHIFTING  
NORTHWEST MONDAY, HELPING LOWER THE THREAT FOR TIDAL FLOODING.  
 

 
   
LWX WATCHES/WARNINGS/ADVISORIES
 
 
DC...COASTAL FLOOD ADVISORY UNTIL 8 AM EDT SATURDAY FOR DCZ001.  
MD...COASTAL FLOOD ADVISORY UNTIL 4 AM EDT SATURDAY FOR MDZ016.  
COASTAL FLOOD ADVISORY FROM 6 PM THIS EVENING TO 2 AM EDT  
SATURDAY FOR MDZ017.  
VA...FLASH FLOOD WATCH THROUGH LATE TONIGHT FOR VAZ025-026-036-037-  
503-504-508.  
FLASH FLOOD WATCH FROM 6 PM EDT THIS EVENING THROUGH LATE  
TONIGHT FOR VAZ029-038>040-050-051-055>057-502-507.  
COASTAL FLOOD ADVISORY UNTIL 4 AM EDT SATURDAY FOR VAZ057.  
COASTAL FLOOD ADVISORY UNTIL 8 AM EDT SATURDAY FOR VAZ054.  
WV...FLASH FLOOD WATCH THROUGH LATE TONIGHT FOR WVZ505-506.  
MARINE...SMALL CRAFT ADVISORY UNTIL MIDNIGHT EDT TONIGHT FOR ANZ530>543.  
 

 
 

 
SYNOPSIS...BKF  
NEAR TERM...BKF  
SHORT TERM...BKF  
LONG TERM...BJL  
AVIATION...BJL/BKF  
MARINE...BJL/BKF  
HYDROLOGY...BKF  
TIDES/COASTAL FLOODING...BKF  
 
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