Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 050352 AFDLWX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC 1052 PM EST THU FEB 4 2016 .SYNOPSIS... A WAVE OF LOW PRESSURE WILL RIDE ALONG A STATIONARY BOUNDARY PASSING BY WELL TO OUR SOUTH AND EAST TONIGHT INTO FRIDAY. HIGH PRESSURE WILL RETURN FOR SATURDAY. COASTAL LOW PRESSURE WILL PASS BY WELL TO OUR SOUTH AND EAST SUNDAY. ANOTHER LOW PRESSURE SYSTEM MAY IMPACT THE AREA EARLY NEXT WEEK. && .NEAR TERM /THROUGH FRIDAY/... NO CHANGES PLANNED TO WINTER WEATHER ADVISORY AT THIS TIME. GENERAL THEME OF THE FORECAST REMAINS ON TRACK...ALTHOUGH THE MINUTE DETAILS WITH THE MOST UNCERTAINTY CONTINUE TO LINE UP ALONG THE I-95 CORRIDOR. LOW PRESSURE DEVELOPMENT IS TAKING PLACE ALONG THE STALLED FRONT TO OUR SOUTHEAST. STRENGTHENING IS FAVORED AS IT TRACKS NE DUE TO FAVORABLE UPPER LEVEL DYNAMICS. PRECIP SHIELD WILL NUDGE NW AS THE LOW TRACKS TO THE NE...POSSIBLY ENHANCED BY MID-LEVEL FGEN. THERMAL PROFILES WILL BE MARGINAL AND WILL ONLY SLOWLY COOL. 00Z IAD SOUNDING HAD 850 MB TEMP AROUND -4C...SO ONLY LOOKING AT A RAIN-SNOW TRANSITION. SFC WET BULBS (AND THEREFORE AIR TEMPS) WILL STRUGGLE TO REACH FREEZING BY DAWN WHERE THE PRECIP IS EXPECTED...ALTHOUGH OVERALL PROFILES SUPPORT GRADUAL TRANSITION TO MIX THEN SNOW DURING THE 1-5 AM TIME FRAME. RECENT HI-RES RUNS DO NOT ADD ANY CONFIDENCE TO MAKE SIGNIFICANT CHANGES TO THE SNOW TOTALS WE DERIVED EARLIER THIS EVENING. IN SUMMARY...TOTALS ALONG I-95 HAVE LOW CONFIDENCE DUE TO WESTERN EXTENT OF PRECIP...WHILE FARTHER EAST IS DUE TO AMOUNT OF TIME AS SNOW. HIGHEST CHANCES OF A COUPLE INCHES REMAIN NEAR THE BAY THOUGH. THE SNOW SHOULD BE WINDING DOWN DURING THE MORNING COMMUTE (6-8 AM)...AND WHILE ANTECEDENT WARM CONDITIONS SHOULD HELP WITH ROADWAY ACCUMULATIONS...THERE STILL COULD BE SOME SLIPPERY CONDITIONS. IT WILL BE COLDER (BELOW FREEZING) WEST I-95...SO THE NW EXTENT OF THE PRECIP WILL HAVE TO BE WATCHED. HIGH PRESSURE WILL PUSH INTO THE REGION FRIDAY WITH CLEARING SKIES AND INCREASING WINDS. NW WINDS 10-20MPH WITH GUSTS UP TO 30MPH ARE LIKELY. && .SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY NIGHT/... HIGH PRESSURE WILL BE IN CONTROL FRIDAY NIGHT AND INTO SATURDAY AS IT MOVES TO THE SOUTH OF THE REGION. DRY AND MOSTLY SUNNY CONDITIONS EXPECTED SATURDAY WITH TEMPS REACHING THE MID 40S. A DEEP TROUGH WILL MOVE ACROSS THE REGION SUNDAY RESULTING IN CLOUDS TO INCREASE. LOW PRESSURE IS EXPECTED TO OVER THE ATLANTIC OCEAN LEAVING THE MID- ATLANTIC DRY. TEMPS ON SUNDAY WILL BE SLIGHTLY WARMER IN THE MID TO U40S TO L50S. && .LONG TERM /MONDAY THROUGH THURSDAY/... COMPUTER MODELS HAVE SOME AGREEMENT ON A COASTAL LOW FORMING ALONG OR JUST OFFSHORE THE NORTH AND SOUTH CAROLINA COAST SUNDAY NIGHT. ONE DISAGREEMENT IS THAT THE GFS MODEL KEEPS THE COASTAL LOW AN OPEN AREA OF LOW PRESSURE AND PUSHES IT FASTER OUT TO SEA SUNDAY NIGHT AND MONDAY THAN THE EUROPEAN MODEL...WHICH CLOSES OFF THE LOW AND INTENSIFIES IT BEFORE PUSHING IT OUT TO SEA. NONETHELESS...THIS COASTAL LOW WILL JUST SPREAD ADDITIONAL CLOUDS INTO OUR REGION SUNDAY WITH NO EXPECTED PRECIPITATION. A MUCH LARGER AREA OF LOW PRESSURE IN THE UPPER LEVELS IS EXPECTED TO DROP SOUTHEASTWARD FROM THE UPPER MIDWEST AND GREAT LAKES INTO THE OHIO VALLEY AND MID-ATLANTIC REGION MONDAY. A NEW SURFACE LOW PRESSURE SYSTEM COULD FORM OVER SOUTHWEST VIRGINIA ON THE SOUTHEAST FLANK OF THIS BROAD LOW AROUND MIDDAY MONDAY AND MONDAY AFTERNOON. THIS NEWLY FORMED LOW COULD HAVE A MORE DIRECT IMPACT TO OUR REGION...BRINGING A CHANCE FOR RAIN AND SNOW SHOWERS MONDAY BEFORE BECOMING MORE OF A LIGHT TO MODERATE SNOW THREAT MONDAY NIGHT WITH COLDER AIR TO SUPPORT THE NEW LOW. ONE SIMILARITY WITH THE EUROPEAN MODEL AND THE GFS MODEL WITH THIS NEW LOW IS BOTH CLOSE OFF THE NEW LOW NEAR THE MID-ATLANTIC COAST TUESDAY BEFORE PIVOTING IT NORTHWARD TOWARD NEW ENGLAND TUESDAY NIGHT. THE DIFFERENCE IN THE POSITION OF THIS CLOSED LOW WILL DETERMINE WHETHER THE MID-ATLANTIC REGION GETS HIT HARD WITH ANOTHER SNOWSTORM OR GETS A FEW RAIN AND SNOW SHOWERS WITH LITTLE TO NO ACCUMULATION. RIGHT NOW...THE EUROPEAN MODEL CLOSES THE LOW NEAR THE MOUTH OF THE CHESAPEAKE BAY AND MAKES IT A STRONGER LOW. THE GFS MODEL CLOSES THE LOW NEAR THE MOUTH OF THE DELAWARE BAY...FARTHER NORTH...AND IS NOT AS STRONG WITH THIS SYSTEM. FOR THE TIME BEING...WE HAVE A CHANCE OF RAIN OR SNOW IN OUR REGION TUESDAY AND TUESDAY NIGHT. THE MAIN UPPER LEVEL SUPPORT BEHIND THE COASTAL LOW WILL SLOWLY PIVOT FROM THE OHIO VALLEY...ACROSS THE REGION...AND OFFSHORE WEDNESDAY AND WEDNESDAY NIGHT. SOME UPSLOPE SNOW SHOWERS MAY EXIST IN THE POTOMAC HIGHLANDS WITH COLDER AIR ALOFT PUNCHING INTO THE REGION. ELSEWHERE...COLDER AND DRIER AIR WILL FILTER INTO THE REGION AS HIGH PRESSURE BUILDS IN FROM THE WEST. HIGH PRESSURE SHOULD DOMINATE THURSDAY WITH CHILLY AND DRY AIR. ANY UPPER LEVEL DISTURBANCES MOVING OVERHEAD THURSDAY WILL BE QUICK AND MAY NOT PRODUCE ANY PRECIPITATION...JUST ADDITIONAL CLOUDS. && .AVIATION /04Z FRIDAY THROUGH TUESDAY/... LOW PRESSURE WILL TRACK OFFSHORE LATE TONIGHT. PRECIP IS EXPECTED TO GRADUALLY SPREAD NW. HOWEVER IT/S STILL IN DOUBT HOW FAR NW IT TRACKS...WITH THE CUTOFF BEING NEAR I-95. THERE MAY INITIALLY BE A RA/SN MIX...WITH GREATEST CHANCE OF A PERIOD OF SNOW 08-12Z. DID NOT BRING IFR TO DCA AS AMOUNT OF SNOW IS IN DOUBT. FELT A LITTLE MORE COMFORTABLE WITH BWI/MTN BUT STILL HELD TO 2 SM. DID NOT MENTION SN AT OTHER TERMINALS FOR NOW...ALTHOUGH IAD MAY HAVE BEST CHANCE OF SEEING SOME FLAKES. SNOW SHOULD QUICKLY END BY 12-13Z FRIDAY AND VFR CONDITIONS ARE EXPECTED WITH CLEARING SKIES. NW WINDS WILL INCREASE EARLY FRIDAY MORNING. GUSTS AROUND 25KT ARE EXPECTED HOWEVER GUSTS UP TO 35KTS POSSIBLE FRIDAY MORNING. VFR CONDITIONS EXPECTED FRIDAY NIGHT THROUGH SUNDAY. VFR CONDITIONS SUNDAY NIGHT. WINDS NORTHEAST AROUND 5 KNOTS. MVFR TO IFR CONDITIONS MONDAY WITH LIGHT RAIN AND LIGHT SNOW. WINDS BECOMING SOUTHEAST 5 KNOTS GUSTS 10 TO 15 KNOTS. && .MARINE... N/NW WINDS CONTINUE ON THE WATERS AS A STATIONARY FRONT REMAINS TO THE SE. SCA-LEVEL GUSTS ARE SPORADIC BUT WILL REMAIN POSSIBLE ON ALL WATERS THROUGH TONIGHT. A COASTAL LOW WILL APPROACH THE WATERS TONIGHT WITH RAIN TRANSITIONING TO SNOW BY LATE TONIGHT. WINDS WILL INCREASE ON THE WATERS AS THE COASTAL LOW MOVES FURTHER OFF THE COAST EARLY FRIDAY MORNING. WINDS MAY APPROACH GALE FORCE FRIDAY MORNING...BUT CONFIDENCE IS STILL NOT HIGH ENOUGH TO ISSUE A WARNING AS ONCE MIXING FULLY COMMENCES...WINDS ALOFT SHOULD BE LIGHTER. HIGH PRESSURE WILL MOVE OVER THE WATERS LATE FRIDAY AND INTO THE WEEKEND. SCA CONDITIONS WILL CONTINUE THROUGH THE AFTERNOON BEFORE WINDS DIMINISH FRIDAY NIGHT ON THE MAIN CHANNEL OF THE BAY. THERE IS A CHANCE THE SCA MAY NEED TO BE EXTENDED IN TIME BY A FEW HOURS. NO MARINE HAZARDS SATURDAY THROUGH MONDAY. && .HYDROLOGY... STARTING TO WIND DOWN FROM OUR SNOWMELT FLOOD...WHICH TURNED OUT TO BE NOT AS SIGNIFICANT AS FORECAST BECAUSE WE DID NOT SEE AS MUCH RAIN IN THE POTOMAC HEADWATERS AS EXPECTED...AND WE DID NOT ENTIRELY LOSE THE SNOWPACK IN SOME AREAS WHERE IT WAS EXPECTED TO HAPPEN. MINOR FLOODING IS STILL ONGOING ON THE MONOCACY...MAINSTEM SHENANDOAH...AND MAINSTEM POTOMAC BETWEEN HARPERS FERRY AND GREAT FALLS. FLOODING WILL CONTINUE ON THE MONOCACY THROUGH TONIGHT...AND ON THE SHENANDOAH AND POTOMAC THROUGH FRIDAY. THE LIGHT PRECIPITATION EXPECTED TONIGHT WILL NOT HAVE ANY EFFECT ON STREAMS. && .TIDES/COASTAL FLOODING...
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WATER LEVELS AROUND WASHINGTON DC REMAIN 2+ FEET ABOVE ASTRO NORMS LATE THIS THU EVENING. NW WINDS SHOULD PUSH A BIT OF THE TIDAL INFLUENCE AWAY FROM THE AREA...WHICH WILL OPEN UP MORE ROOM FOR FRESHWATER WHEN IT DOES ARRIVE...WHICH WILL BE SOMETIME ON FRIDAY. FORECASTS OF THE WATER COMING DOWN THE POTOMAC HAVE FLUCTUATED UPWARD SLIGHTLY ONCE AGAIN. BASED ON THIS ALONE GEORGETOWN WOULD EASILY REACH MINOR AGAIN FRI MORNING. BASED ON EXPERIENCE...THE EXCESS FRESHWATER FANS OUT SUBSTANTIALLY BEFORE REACHING SW DC. AFTER MONITORING THE UPSTREAM RIVER FORECASTS AND THE DIFFERENCE BETWEEN THE TIDAL ANOMALIES AT DC VS THE REST OF OUR WATERS...HAVE ACHIEVED ENOUGH CONFIDENCE TO ISSUE COASTAL FLOOD ADVISORY FOR DC. I/M NOT SURE IT WILL BE THAT SIMPLE THOUGH. FOR ONE THING...IT APPEARS AS THOUGH THE FRESHWATER CONTRIBUTION WILL BE CHASING THE HIGH TIDE DOWN RIVER. THAT IS...THE TIDE WILL BE REPLACED BY THE FRESHWATER CREST. HAVE LENGTHENED THE TIME FRAME FROM THE TYPICAL SCENARIO TO ACCOMODATE FOR THIS. BASED ON THE LITTLE FALLS FORECAST...ITS POSSIBLE THAT A MARGINAL WARNING WOULD BE NECESSARY. THAT WOULD NEED TO BE A GAME TIME CHANGE.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...WINTER WEATHER ADVISORY UNTIL 10 AM EST FRIDAY FOR DCZ001. COASTAL FLOOD ADVISORY FROM 4 AM TO 11 AM EST FRIDAY FOR DCZ001. MD...WINTER WEATHER ADVISORY UNTIL 10 AM EST FRIDAY FOR MDZ006-011- 013-014-016>018-507-508. VA...WINTER WEATHER ADVISORY UNTIL 10 AM EST FRIDAY FOR VAZ054-057. WV...NONE. MARINE...SMALL CRAFT ADVISORY UNTIL 4 PM EST FRIDAY FOR ANZ535-536. SMALL CRAFT ADVISORY UNTIL 10 PM EST FRIDAY FOR ANZ530>534- 537>543.
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&& $$ SYNOPSIS...HAS NEAR TERM...ADS/HAS SHORT TERM...HAS LONG TERM...KLW AVIATION...ADS/HAS/KLW MARINE...ADS/HAS/KLW HYDROLOGY...JE TIDES/COASTAL FLOODING...HTS

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