Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 282026 AFDLWX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC 326 PM EST SUN DEC 28 2014 .SYNOPSIS... A COLD FRONT WILL MOVE SOUTH OF THE MID-ATLANTIC REGION THIS EVENING. WEAK DISTURBANCES WILL PASS MAINLY TO THE SOUTH OF THE AREA THROUGH TUESDAY...WITH STRONG HIGH PRESSURE BUILDING OVER MUCH OF THE COUNTRY BY MIDWEEK. THE HIGH WILL PUSH SOUTH AND EAST...AND WEAKEN...DURING THE SECOND HALF OF THE WEEK. THIS WILL ALLOW ANOTHER STORM SYSTEM TO APPROACH BY NEXT WEEKEND. && .NEAR TERM /THROUGH MONDAY NIGHT/... A COUPLE OF EARLIER FORECAST UPDATES SAW POPS TRIMMED DOWN W/ THE INCOMING PRECIP LARGELY DISSIPATING UPON REACHING THE SPINE OF THE CNTRL APLCNS. NOW IN THE MID-LATE AFTN HRS...ENOUGH MOISTURE HAS BEEN CARRIED ALONG W/ THE INCOMING UPPER WAVE TO GENERATE SOME LIGHT SHOWERS ACROSS PORTIONS OF THE AREA - NAMELY THE LOWER HALF. STILL W/ PLENTY OF THIS ACTIVITY FALLING FROM 8-12KFT CLOUD DECKS...MUCH OF IT IS DISSIPATING TO LITTLE MORE THAN SPRINKLES BY THE TIME IT REACHES THE SFC. ONLY POCKETS OF SLIGHTLY HEAVIER ACTIVITY WILL GENERATE WET GROUND ACROSS THE AREA HEADING INTO THE EVE HRS... MAINLY ALONG AND SOUTH OF I-66. A COLD FRONT FROM THE UPPER MIDWEST HAS TAKEN A BETTER PART OF THE DAY DRIFTING JUST TO OUR NORTH AND IS NOW DROPPING DOWN THRU THE NRN TIER COUNTIES. THE FRONT WILL PROVIDE LITTLE MORE THAN A WIND SHIFT AND AIR THAT IS GENERALLY A FEW DEGREES LOWER THAN THE AIRMASS AHEAD OF IT. TEMPS ATTM ARE HOVERING IN THE L-M50S - SOME LOCALES NEAR 60F ACROSS THE SRN PORTIONS OF THE AREA W/ RELATIVELY HIGHER DEWPOINTS AS WELL. THE FRONT WILL EVENTUALLY SWING ACROSS THE REMAINDER OF THE CWA...THOUGH THE UPPER WAVE OF MOISTURE WILL REMAIN PARKED OVER THE REGION INTO MONDAY. THE PRECIP AXIS WILL SHIFT SOUTHWARD AND BE LOCKED INTO PLACE FROM SRN DELMARVA SW...JUST INTO THE SRN FRINGES OF THE CWA OVERNIGHT. THIS WILL ALLOW THE NRN HALF OF THE AREA TO SEE SOME BREAKS IN CLOUD COVER AND REMAIN DRY - OUTSIDE OF A FEW ISOLATED SHOWERS. TEMPS WILL REMAIN MODERATED IN THE U30S/L40S EVEN THRU DAYBREAK MON. THE HIGHER TERRAIN OF THE APLCNS WILL SEE A FEW SUB-FREEZING TEMPS W/ A LIGHT MIX OF RAIN/SNOW POSSIBLE FOR THESE ELEVATED AREAS BUT LITTLE/IF ANY ACCUM EXPECTED. THE 12Z NAM HAS BACKED OFF LARGELY ON THE PRECIP POTENTIAL AND THE NORTHWARD EXTENT INTO THE I-66 CORRIDOR WHILE THE GFS/EURO DEPICTIONS MAINTAIN THE SRN HALF WILL SEE AT LEAST A PERIOD OF LIGHT RAIN INTO THE EVE HRS. KEPT THE LIKELIES DOWN ACROSS THE I-64 CORRIDOR AND BARELY INTO THE CHANCE RANGE INTO THE DC METRO AREAS. THESE POTENTIAL RANGES WILL CARRY ONLY LIGHT AMOUNTS W/ THEM. STAYING W/ THE GFS/EURO DEPICTIONS...POPS CONTINUE TO DROP OFF AND MOVE SWD W/ THE WAVE INTO THE LATE AFTN EVE HRS. ANOTHER LOCAL UPPER KINK CAUGHT IN THE SWLY FLOW WILL PASS OVER THE SRN APLCNS LATE MON AND MEET UP W/ SOME OVERRUNNING POTENTIAL FOR THE LOWER APLCN PORTION OF THE CWA MON NIGHT. THIS WILL DEVELOP SOME SNOW POTENTIAL FOR THESE HIGHER TERRAIN AREAS BUT ON THE LIGHT SIDE W/ LOW AMOUNTS OF MOISTURE TO WORK WITH AND MARGINALLY COLD AIR DROPPING DOWN FROM THE NORTH. && .SHORT TERM /TUESDAY THROUGH TUESDAY NIGHT/... THE MOST CHALLENGING PART OF THE FORECAST IS TUESDAY...AND THE VAST DIFFERENCES IN THE FORECAST MODELS LARGELY COME DOWN TO TWO FACTORS. THE NAM...AND SIMILAR SREF...ALONG WITH THE CANADIAN MODEL...ARE OUT ON AN ISLAND WITH A NORTHEASTERLY OR EVEN EASTERLY SFC-925 MB WIND...FAVORABLE FOR BRINGING IN MOISTURE. THIS SET OF MODELS ARE ALSO THE ONLY ONES KEEPING MID-LEVEL MOISTURE AROUND ON TUESDAY. THE REST ARE BONE DRY ALOFT. IN THE LOW-LEVELS...TYPICALLY IN THIS SETUP WE WOULD LEAN HEAVILY ON THE NAM FOR POTENTIAL COLD-AIR DAMMING AND MOISTURE RETURN. BUT THAT DOES NOT SOLVE THE DIFFERENCES ALOFT. ONE THING NOTED FROM THE 06Z MODEL SUITE IS THE 4KM NESTED NAM DID NOT SPREAD PRECIP AS FAR NORTH AS ITS 12KM RESOLUTION PARENT...NOR WAS IT AS HEAVY. THUS IT SEEMS LIKE A REASONABLE MEANS TO FACTOR IN THE POSSIBILITIES OF THE NAM/SREF WITHOUT BITING OFF ON THEM COMPLETELY. THIS YIELDS CHANCE POPS ROUGHLY SOUTH OF US 50...AND SLIGHT CHANCE FURTHER NORTH. PRECIP TYPE IS LEFT AS RAIN-OR-SNOW IN MOST AREAS...BUT LEANING IN THE DIRECTION OF SNOW FROM THE BLUE RIDGE ZONES WESTWARD. IF PRECIP DOES DEVELOP...THE HIGHS CURRENTLY IN THE FORECAST WOULD HAPPEN EARLY IN THE DAY AND TEMPS WOULD THEN FALL. THE KEY HERE IS THAT AMOUNTS LOOK LIGHT. WPC QPF FITS WELL WITH OUR FORECAST REASONING OUTLINED ABOVE AND WAS USED FOR QPF AMOUNTS. PARTIAL CLEARING EXPECTED TUESDAY NIGHT...WITH MUCH COLDER AIR PUSHING IN TO THE AREA. LOWS WILL BE MAINLY IN THE LOW/MID 20S...HIGHER IN THE URBAN AREAS AND LOWER IN THE HIGHLANDS. SOME UPSLOPE SNOW SHOWERS WILL STILL BE POSSIBLE WEST OF THE ALLEGHENY FRONT. && .LONG TERM /WEDNESDAY THROUGH SUNDAY/... COLD HIGH PRESSURE WILL BUILD ACROSS THE AREA WEDNESDAY INTO THURSDAY. TEMPERATURES WILL BE BELOW NORMAL DURING THIS TIME. DAYTIME HIGHS WILL BE IN THE 30S WITH OVERNIGHT LOWS IN THE UPPER TEENS AND LOWER 20S. DEPENDING ON HOW LIGHT WINDS GET OVERNIGHT WEDNESDAY NIGHT...TEMPERATURES MAY BE EVEN COOLER ESPECIALLY OVER NORTHWESTERN PARTS OF THE AREA WHERE DEW POINTS WILL BE IN THE SINGLE DIGITS ABOVE ZERO WITH A MAINLY CLEAR SKY. HEIGHTS BEGIN TO RISE ALONG THE EASTERN SEABOARD AS BROAD AND DEEP UPPER LOW OVER EAST-CENTRAL CANADA RETROGRADES TO THE WEST LATE IN THE WEEK...ALLOWING A CUTOFF LOW OVER THE SOUTHWESTERN CONUS TO BECOME RE-ABSORBED IN THE MEAN FLOW AND TRACK INTO THE OHIO VALLEY. AT THE SURFACE...HIGH PRESSURE /NEAR 1040 MB/ WILL PARK ITSELF OVER NORTHERN NEW ENGLAND...AND LOW PRESSURE WILL DEVELOP OVER THE SOUTHERN PLAINS/WESTERN GULF COAST STATES. MEAN 500 MB BLOCKING RIDGES OVER THE GULF OF ALASKA AND JUST EAST OF GREENLAND ARE MORE TYPICAL OF AN EAST-BASED NAO PATTERN...AND ONE WHERE SURFACE LOWS TYPICALLY TRACK WEST OF THE APPALACHIAN MOUNTAINS. ALTHOUGH THE CWA LOOKS LIKE IT WOULD BE POSITIONED ON THE EAST /WARM/ SIDE OF THIS SYSTEM...SHALLOW COLD AIR AT THE SURFACE IN A CAD SETUP DUE TO THE HIGH TO THE NORTH MAY VERY WELL LEND TO A WINTRY MIX AT LEAST AT THE ONSET. TOO EARLY TO GET INTO SPECIFICS OTHER THAN JUST MENTIONING A GENERIC MIX AT THIS POINT SINCE ENSEMBLE SPREAD IS SIGNIFICANT BOTH WITH EXACT TRACK AND TIMING AS WELL AS THE EXTENT OF COLD AIR PRECEDING THE STORM/S ARRIVAL. MODELS DIVERGE FURTHER AFTER THE INITIAL LOW PRESSURE CENTER MOVES PAST THE AREA...WITH SOME STALLING THE FRONT NEARBY AND RIDING A WAVE OR WAVES OF LOW PRESSURE ALONG IT KEEPING UNSETTLED WEATHER OVER THE AREA FOR MOST OF NEXT WEEKEND. && .AVIATION /20Z SUNDAY THROUGH FRIDAY/... SCATTERED SHOWERS CONTINUING TO STREAM INTO THE REGION AND AS MORE MOISTURE MOVES IN OVER THE MTNS...SOME OF THIS ACTIVITY NOW BEGINNING TO REACH THE SFC AFTER BEING VIRGA FOR SEVERAL HRS. A WEAK COLD FRONT MOVING THRU THE REGION...NOW JUST N OF THE DC AREA SITES WHICH WILL TURN SLY WINDS TO A MORE NWLY FLOW BEHIND IT. VFR CONDITIONS WILL CONTINUE INTO THE EVE. ONLY A BRIEF PERIOD OF LOW-END VFR/HIGH-END MVFR CIGS OVERNIGHT INTO EARLY MON. CIG/VSBY RESTRICTIONS ARE POSSIBLE TUESDAY IF PRECIP DEVELOPS AND/OR COLD-AIR DAMMING OCCURS. CLEARING EXPECTED TUESDAY NIGHT EITHER WAY WITH VFR CONDITIONS. OUTLOOK... W FLOW 10 KTS OR LESS WED-THU...VFR. && .MARINE... BEHIND A BRIEF ENHANCEMENT OF WINDS AT THE LOWER END OF THE MD BAY THIS MRNG...WINDS ACROSS THE WATERS HAVE DROPPED OFF TO NEAR CALM JUST AHEAD OF AN APPROACHING COLD FRONT. THE FRONT WILL MOVE ACROSS THE WATERS THRU THE EVE HRS...BUT ONLY MAKE WINDS TURN TO NWLY W/ SPEEDS IN THE 5-10KT RANGE. THE FRONT WILL SETTLE JUST SOUTH OF THE REGION AND ALLOW FOR PERIODS OF WEAK NLY CHANNELING TO AFFECT THE MAIN STEM OF THE BAY THRU MON EVE...W/ WINDS STAYING JUST BELOW SCA RANGE. EXPECTING N/NE WINDS AROUND 10 KNOTS TUESDAY AND TUESDAY NIGHT. SOME GUSTS OVER SCA CRITERIA ARE POSSIBLE TUESDAY. CHANCE OF RAIN TUESDAY DURING THE DAY...ESPECIALLY SOUTHERN MD WATERS. RELATIVELY WEAK GRADIENT SHOULD KEEP WINDS/SEAS BELOW SMALL CRAFT ADVISORY LEVELS DURING THE MIDDLE AND LATTER PART OF THIS WEEK. THE NEXT APPRECIABLE CHANCE FOR SCA WINDS/SEAS LOOKS TO BE LATE IN THE WEEK/EARLY NEXT WEEKEND IN SOUTHERLY CHANNELING AHEAD OF APPROACHING LOW PRESSURE. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...NONE. MD...NONE. VA...NONE. WV...NONE. MARINE...NONE. && $$ SYNOPSIS...JE NEAR TERM...GMS SHORT TERM...JE LONG TERM...DFH AVIATION...GMS/JE/DFH MARINE...GMS/JE/DFH

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