Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 220231 AAA AFDLWX AREA FORECAST DISCUSSION...UPDATED NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC 931 PM EST SUN DEC 21 2014 .SYNOPSIS...
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HIGH PRESSURE WILL MOVE OFF THE COAST OVERNIGHT...AHEAD OF A WEAK COASTAL LOW PRESSURE SYSTEM OVER THE SOUTHERN ATLANTIC. THE LOW WILL TRACK UP THE MID-ATLANTIC COAST MONDAY INTO TUESDAY. A STRONG COLD FRONT WILL APPROACH THE AREA WEDNESDAY BEFORE PASSING THROUGH WEDNESDAY NIGHT. BREEZY CONDITIONS ON THURSDAY...THEN HIGH PRESSURE WILL BUILD BACK INTO THE REGION TO CLOSE OUT THE WEEK.
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&& .NEAR TERM /THROUGH MONDAY/...
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HIGH TEMPS ACROSS AREA WERE ON PAR W/ AVG VALUES FOR THIS...THE WINTER SOLSTICE. HOWEVER...AFTER ONLY REACHING THE LOW-MID 40S EARLIER THIS AFTN MOST AREAS ARE BACK BELOW FREEZING...WELL AHEAD OF ANY INCOMING PRECIP. CLEAR SKIES AND CALM SFC WINDS EASILY ALLOWING TEMPS TO DROP DOWN TOWARD CURRENT DEWPOINTS VALUES IN THE LOWS 20S. SLOW AND STEADY DEVELOPMENT OF A COASTAL LOW PRES SYSTEM OVER THE SRN ATLC STATES IS THE NEXT FEATURE TO MONITOR...AS DENSE CLOUD COVER AND LIGHT/SCATTERED SHOWERS ARE EXPECTED TO MOVE UP THE EASTERN SEABOARD OVERNIGHT. MANY SMALLER-SCALE ELEMENTS ARE COMING TOGETHER OVER A MUCH LARGER PICTURE AND WILL CONTINUE TO DO SO WELL INTO THE MIDDLE OF THE CURRENT WEEK. NEARLY TWO-THIRDS OF THE CONUS WILL BE AFFECTED IN SOME PART OF THESE ELEMENTS OR A LARGER COMBINATION OVER THE NEXT FEW DAYS. THIS FIRST PIECE IS THE INITIAL GROUPING OF A SHORTWAVE OVER THE SRN ATLC COAST AND A TROUGH STILL WELL BACK TO THE WEST. THE TROUGH WILL FORCE THE ATLC MOISTURE UP THE COAST...THIS FIRST BATCH AFFECTING OUR AREA MONDAY INTO EARLY TUE - W/ THE MORE MOISTURE-LADEN SYSTEM MOVING THRU THE REGION ON THU. SUCH A COMPLEX SYSTEM OF WAVES...TROUGHS...SYNOPTIC FEATURES INTERACTING W/ EACH OTHER OVER A MULTI-DAY PERIOD CAN SHOW VERY VARIED DEPICTIONS AMONGST THE GUIDANCE SUITE. HOWEVER...MOST MEMBERS IN BOTH THE ENSEMBLE AND DETERMINISTIC FIELDS ARE IN FAIRLY GOOD AGREEMENT EARLY ON W/ THE INCOMING PRECIP. THE NAM STILL THE COLDER MODEL IN TERMS OF TEMPS/DEWPOINTS BUT IS NOW COMING INTO BETTER AGREEMENT W/ GFS/EURO ON PRECIP TIMING. THE I-95 CORRIDOR WHICH IS NORMALLY A DIVIDING LINE OF NEAR- FREEZING TEMPS W/ INCOMING PRECIP IS NO DIFFERENT FOR THIS INCOMING WAVE MON MORNING. HOWEVER...HI-RES AND NEAR TERM GUIDANCE DOES NOT HAVE THE FIRST BATCHES OF LIGHT PRECIP ARRIVING UNTIL POST-DAWN AND ACTUALLY MORE TOWARD MID-LATE MRNG...ESPECIALLY FURTHER NORTH. TEMPS WILL STILL BE BORDERLINE FREEZING/SUB- FREEZING ACROSS THE PIEDMONT/FOOTHILLS - WHERE THE FREEZING RAIN ADZY IS IN EFFECT - BUT THE I-95 CORRIDOR MAY ONLY SEE A BRIEF ONSET PERIOD OF A FZRA/SLEET MIX...THEN TEMPS WILL RISE EVEN A COUPLE MORE DEGREES HEADING INTO THE LATE MRNG/EARLY AFTN. THE SHEN VLY AND BLUE RIDGE WILL LIKELY BE ON THE WRN/NRN PERIPHERY OF THE PRECIP BUT WILL BE MOST PRONE TO HOLDING ONTO THE SUB-FREEZING CONDITIONS WELL INTO THE MRNG HRS AND EVEN THRU THE AFTN POTENTIALLY. THE AMOUNTS AND POTENTIAL FOR PRECIP WILL BE THE MAIN QUESTION BUT THERE IS GOOD CONFIDENCE THAT THESE AREAS WILL HAVE TROUBLE SEEING TEMPS ABOVE FREEZING FOR VERY LONG TMRW. LINGERING POCKETS OF DRIZZLE AND LIGHT/SCATTERED RAIN WILL BE THE REASON FOR AN EXPANSION/EXTENSION OF THE ADZY FOR ALL/MOST OF THE SHEN VLY/POTOMAC HIGHLANDS/BLUE RIDGE.
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&& .SHORT TERM /MONDAY NIGHT THROUGH TUESDAY NIGHT/... LIGHT PRECIPITATION WILL CONTINUE MONDAY NIGHT INTO TUESDAY MORNING. THE BIG QUESTION WILL BE HOW LONG THE CAD WEDGE WILL REMAIN IN PLACE. THERE IS STILL POTENTIAL FOR TEMPERATURES TO REMAIN AT OR BELOW FREEZING MAINLY WEST OF THE BLUE RIDGE LEAVING THE POTENTIAL FOR LIGHT FREEZING RAIN TO CONTINUE. TO THE EAST OF THE BLUE RIDGE AND ACROSS THE METRO AREAS...WARMER AIR WILL LIKELY WIN OUT KEEPING PRECIPITATION IN THE FORM OF RAIN. A BRIEF RESPITE FROM THE PRECIPITATION IS POSSIBLE TUESDAY AFTERNOON/EVENING BEFORE THE NEXT ROUND OF RAINFALL MOVES IN FROM THE WEST LATER TUESDAY NIGHT. && .LONG TERM /WEDNESDAY THROUGH SUNDAY/... STRONG COLD FRONT CROSSING THE MID ATLANTIC REGION WEDNESDAY INTO THURSDAY. LIQUID PRECIPITATION SHOULD ACCOMPANY THE FRONT EXCEPT ALONG THE ALLEGHENY FRONT WHERE ARRIVAL OF COLDER AIR WILL CAUSE THE RAIN TO CHANGE OVER TO SNOW BEFORE PRECIPITATION ENDS THURSDAY. PASSAGE OF SHORTWAVE WILL SUPPORT DEVELOPMENT OF A COASTAL LOW LATE SATURDAY AND SUNDAY. PRECIPITATION MAY START AS A WINTRY MIX AT HIGHER ELEVATIONS. && .AVIATION /01Z MONDAY THROUGH FRIDAY/...
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VFR CONDITIONS INTO THE OVERNIGHT...W/ MID CLOUD DECKS INCREASING TOWARD THE PREDAWN MON. PRECIPITATION WILL OVERSPREAD THE TERMINALS MONDAY MORNING - MOST LIKELY POST-DAWN - FROM SOUTHWEST TO NORTHEAST. FREEZING RAIN IS EXPECTED ACROSS KCHO AND POSSIBLY KMRB AND KIAD FOR A RELATIVELY BRIEF PERIOD...W/ TEMPS HOVER NEAR FREEZING DURING THE MID-MRNG HRS. FREEZING RAIN MAY MIX WITH SLEET AND SNOW AT THE ONSET. RAIN IS EXPECTED ACROSS THE REST OF THE TERMINALS. RAIN MAY MIX WITH SLEET AT THE ONSET. CIGS/VSBYS WILL DETERIORATE TO MVFR LEVELS MONDAY WITH POSSIBLE IFR LEVELS. SUB VFR CIGS AND VSBYS EXPECTED MONDAY NIGHT INTO TUESDAY. LIGHT FREEZING RAIN WILL BE POSSIBLE WEST OF THE BLUE RIDGE WITH RAIN TO THE EAST. A PRECIPITATION RESPITE MAY OCCUR TUESDAY AFTERNOON BEFORE RAIN MOVES BACK IN LATER TUESDAY NIGHT. A STRONG COLD FRONT WILL APPROACH THE TERMINALS WEDNESDAY BEFORE PASSING THROUGH WEDNESDAY NIGHT. MORE RAIN AND SUBVFR CONDITIONS ARE EXPECTED.
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&& .MARINE... HIGH PRESSURE WILL REMAIN OVER THE WATERS TONIGHT. LOW PRESSURE WILL TRACK UP THE MID-ATLANTIC COAST MONDAY. A SMALL CRAFT ADVISORY IS IN EFFECT FOR THE MIDDLE PORTIONS OF THE BAY AND LOWER TIDAL POTOMAC RIVER. WINDS ARE EXPECTED TO REMAIN BELOW SCA CRITERIA MONDAY NIGHT THROUGH TUESDAY NIGHT. A STRONG COLD FRONT WILL APPROACH THE WATERS WEDNESDAY BEFORE PASSING THROUGH WEDNESDAY NIGHT. A SMALL CRAFT ADVISORY WILL LIKELY BE NEEDED FOR THE WATERS WEDNESDAY AND THURSDAY. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...NONE. MD...NONE. VA...FREEZING RAIN ADVISORY FROM 6 AM TO NOON EST MONDAY FOR VAZ025- 026-029-036>040-050-051-501>504-507-508. WV...NONE. MARINE...SMALL CRAFT ADVISORY FROM 10 AM TO 6 PM EST MONDAY FOR ANZ534- 537-543. && $$ UPDATE...GMS PREV DISCUSSION...BJL/KRW/CEM

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