Area Forecast Discussion
Issued by NWS Sterling, VA

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957 FXUS61 KLWX 120828 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 328 AM EST Tue Dec 12 2017 .SYNOPSIS...
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An arctic cold front will pass through the area today. Weak high pressure will build overhead Wednesday. An Alberta Clipper will pass through Wednesday night through early Thursday. Developing low pressure will move up the east coast late Friday and high pressure will return for Saturday. Low pressure may impact the area early next week.
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&& .NEAR TERM /THROUGH TONIGHT/...
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Arctic front and potent upper level shortwave are poised to cross the region during the day today and into tonight, and will bring a drastic change in sensible weather. As of 3 AM, surface low pressure is moving northeastward through the eastern Great Lakes with the trailing surface front moving towards the Appalachians. Out ahead of the front, widespread cloud cover and increasing southerly flow is being observed. Echoes are increasing as well on radar ahead of the front, and some light rain showers are possible prior to frontal passage as it moves across the region. A few sleet pellets are also possible early this morning. The front will push across the region during the morning and early afternoon hours. High temperatures will be reached prior to passage, rising into the 40s to low 50s east of the Blue Ridge, with 30s to around 40F west where passage occurs earlier. Behind the front, temperatures will fall quickly and winds will turn to the northwest and increase. Along and near the Allegheny Front, this will lead to upslope snow showers/squalls and a Winter Weather Advisory remains in effect for amounts in the 3-8" range through tonight. Downwind of the Allegheny Front, with increased northwest flow and potent shortwave moving across the region, isolated to scattered snow showers and possibly even a few squalls are possible this evening. This is highlighted by the NAM12 Snow Squall Parameter as well as the surge in the Froude Number, indicative of favorable conditions for convective snow showers to make it downwind of the highest terrain. Wind gusts this evening and tonight are expected to frequently reach into the 30-40 mph range. Its possible some of the higher terrain may approach 45-50 mph, and a Wind Advisory may be needed later tonight. Overnight, temperatures will plummet into the teens to around 20F for most locations with the higher terrain dropping into the single digits. This combined with the wind will lead to wind chill values dropping into the single digits for the majority of the region, except as low as the teens below zero across the higher terrain. Therefore a Wind Chill Advisory is in effect.
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&& .SHORT TERM /6 AM WEDNESDAY THROUGH 6 PM WEDNESDAY/...
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A small axis of surface ridging will move overhead Wednesday. This will lead to a gradual relaxation of the winds through the day and any remaining upslope snow showers will come to an end. Temperatures will still be quite cold with highs possibly not breaking the freezing mark area-wide. Wind Chill Advisories run through Noon. Skies should become partly-mostly sunny, but this will be short- lived as the next system will already be on the doorstep by late in the day.
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&& .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/...
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An Alberta clipper system will pass through the Ohio Valley Wednesday evening before passing through our area overnight Wednesday into Thursday morning. This system will bring a period of snow with it...but there is still some uncertainty as to exactly where with the gradient between accumulating snow vs. little or no snow setting up over our CWA. Typically with these systems...most of the snow falls north of the track of the surface low where the frontogenetical forcing and warm advection line up best. Latest guidance has the low tracking just south of the Mason-Dixon Line. This means that the best chance for snow will be near the Mason-Dixon line into Pennsylvania...and this has been reflected in the latest forecast. However...a slight shift southward and the accumulating snow will make it into the Metro areas and possibly points south. Therefore...this will have to be monitored closely over the next couple days. One other thing to note is that an upslope flow and cold advection behind the clipper system will cause snow showers for locations along/west of the Allegheny Front and accumulating snow is likely across these areas as well. The clipper system will pull away from the area later Thursday and high pressure will briefly build into the area Thursday night...bringing dry and chilly conditions. The high will move offshore Friday and an upper-level trough will swing through the area later Friday into Friday night. Latest guidance keeps the northern stream energy and southern stream energy separate. This means that a coastal low with the southern stream energy would move out to sea. However...some southern stream moisture may still get drawn into the area ahead of the trough axis...and this would result in a period of snow or rain/snow mix. Will continue to monitor...because it may affect the evening rush across most areas. Also...should these systems phase a bit sooner than expected that would have a significant impact on the forecast with more significant snow. High pressure will build overhead Saturday before moving offshore Sunday. Low pressure may impact the area later Sunday into Monday...bringing a chance for precipitation.
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&& .AVIATION /08Z TUESDAY THROUGH SATURDAY/...
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Primarily VFR is expected for the region through Wednesday. A strong cold front will cross the region today. Strong low level southwesterly flow is overhead early this morning and this is leading to some low level wind shear with FL020 winds about 40 knots. After frontal passage, winds will shift to the northwest with gusts up to 30-35 knots developing by late in the day and tonight. Isolated to scattered snow showers and their associated restrictions are possible as well, especially MRB, but they will be brief. Winds will gradually relax on Wednesday. A clipper system may bring a period of snow Wednesday night into early Thursday. The best chance for snow will be across the northern terminals...but confidence still remains low with a tight gradient between snow vs. no snow overhead. IFR conditions are possible in any snow that develops. High pressure will return for later Thursday through Thursday night before moving offshore Friday morning. VFR conditions are expected during this time. Low pressure will move out to sea later Friday...but some moisture may get drawn into our area as an upper-level disturbance moves through. This means that a period of snow or rain/snow mix is possible later Friday into Friday night. High pressure will return for Saturday.
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&& .MARINE...
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SCA is in effect through 4 PM today with gusty southerly flow ahead of a cold front. Winds will shift markedly to the northwest following frontal passage this afternoon, and increase to Gale Force by this evening. Therefore a Gale Warning is in effect from 4 PM through 6 AM Wednesday for all waters. Winds will gradually lessen on Wednesday, but Gale/SCA winds will be around for at least a portion of the day. Low pressure will pass through the waters later Wednesday night into Thursday. A Small Craft Advisory will likely be needed for the waters during this time. High pressure will build over the waters Thursday night...but another area of low pressure may impact the waters Friday before high pressure returns Saturday. A Small Craft Advisory may be needed for the waters behind the departing low later Friday night through Saturday.
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&& .TIDES/COASTAL FLOODING...
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Anomalies have spiked up this morning to around three quarters to one foot above normal due to a southerly flow. However...the flow should gradually turn west of south later this morning through midday. Therefore...minor flooding is not expected at this time. A strong northwest flow will develop behind a cold front later this afternoon through Wednesday. Tidal blowout conditions are possible tonight through Wednesday.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...None. MD...Wind Chill Advisory from 6 PM this evening to noon EST Wednesday for MDZ501. Winter Weather Advisory from 7 AM this morning to 7 PM EST this evening for MDZ501. VA...Wind Chill Advisory from 6 PM this evening to noon EST Wednesday for VAZ503-504-507-508. Winter Weather Advisory from 7 AM this morning to 7 PM EST this evening for VAZ503. WV...Wind Chill Advisory from 6 PM this evening to noon EST Wednesday for WVZ503-506. Winter Weather Advisory from 7 AM this morning to 7 PM EST this evening for WVZ503. Winter Weather Advisory from 7 AM this morning to 7 AM EST Wednesday for WVZ501-505. Wind Chill Advisory from 4 PM this afternoon to noon EST Wednesday for WVZ501-505. MARINE...Small Craft Advisory until 4 PM EST this afternoon for ANZ531>534-537-539>543. Gale Warning from 4 PM this afternoon to 6 PM EST Wednesday for ANZ530>534-537-539>541-543. Small Craft Advisory from 6 AM to 6 PM EST Wednesday for ANZ535-536-538-542. Gale Warning from 4 PM this afternoon to 6 AM EST Wednesday for ANZ535-536-538-542. Small Craft Advisory from noon today to 4 PM EST this afternoon for ANZ535-536-538. Small Craft Advisory until 4 PM EST this afternoon for ANZ530.
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&& $$ SYNOPSIS...BJL NEAR TERM...MM SHORT TERM...MM LONG TERM...BJL AVIATION...BJL/MM MARINE...BJL/MM TIDES/COASTAL FLOODING...BJL

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