Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 200812 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 412 AM EDT Tue Mar 20 2018 .SYNOPSIS... Low pressure will press eastward across the Tennessee Valley and the Carolinas this morning before redeveloping off the Mid Atlantic coast this afternoon. A secondary area of low pressure will then develop off the Outer Banks overnight, strengthening east of the Delmarva Peninsula during the day on Wednesday. Canadian high pressure will build over the region for the latter half of the work week. && .NEAR TERM /THROUGH TONIGHT/... With high pressure anchored north of the Great Lakes this morning, low pressure is moving eastward through the Tennessee River Valley. Temperatures across the region are generally in the upper 30s to near 40 degrees after a little taste of spring warmth yesterday. Clouds have reigned much of the night with precipitation in the form of light rain being detected by the local radar. This area of precipitation has extended northward to around the I-66 corridor, but with all observations holding well above freezing, all reports are of the liquid variety. The aforementioned area of low pressure will continue its eastward track this morning before a coastal low develops near the Virginia Tidewater area this morning. Through the morning hours, precipitation will continue to spread northward as a result of this low pressure track, and as the coastal low begins to develop we will see colder air dragged in from our north. This will allow a transition precipitation to a rain/snow/sleet mix starting from north and progressing south and east. With temperatures still quite warm at this hour, do think much of the morning commute for the D.C. area will remain mostly rain with snow/sleet mixing in towards mid morning. The transition will occur a bit earlier for the Baltimore metro, likely after daybreak. Locations north and west of the metros are seeing dewpoints in the upper teens to low 20s this morning, with air temps near 40. So there is some dry air to overcome in these locations before precipitation gets going. That being said, once it does get going and temperatures drop, do expect a quick transition across eastern WV and western MD. As such, we have extended our Winter Storm Warning a bit further east to include Washington County in MD and Morgan, Berkeley, and Frederick counties in WV. The coastal low will push east in to the western Atlantic by this evening, which will result in a period of lighter precipitation across the area. A majority of model guidance is on board with this scenario. A secondary area of low pressure at this time will develop off the Outer Banks late this evening and overnight, while another low pressure area at the surface and aloft will reside to our west southwest. Precipitation fills back in overnight as low pressure strengthens off the coast with support from upper level shortwave swinging by to our south. Thermal profiles overnight will be less of a concern with regards to p-type as we expect mostly snow across the area. Areas to the south and east of the District, namely southern MD and near the Bay will see a mix of snow/sleet before a likely transition to all snow by daybreak. Lows overnight will hold in the upper 20s to near 30 degrees. && .SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/... The coastal low will strengthening Wednesday morning off the coast with much of the area continue to see snow falling. Progression of this low to the east northeast is a bit slower than previous guidance had been indicating. This is depicted with the lastest ECMWF run and to a lesser extent the GFS/NAM. Implications of this would be a longer duration event extending in to the evening hours on Wednesday before precipitation pulls off to our east. Will continue with the Winter Storm Watch for much of the area with this second wave of low pressure, allowing the day shift to assess the extent/duration of the precipitation for the latter half of Wednesday. Behind the exiting system, high pressure of Canadian origin will build in to the area from the northwest, delivering drier conditions and continued below normal temperatures through weeks end. && .LONG TERM /FRIDAY THROUGH MONDAY/... High pressure will remain to the west on Friday. An upper level trough may swing across the area. If it does, some mountain snow showers and lower elevation rain showers or sprinkles could occur. Temperatures will remain 10-15 degrees below normal. The high will quickly cross the area Saturday and the next low pressure system will approach from the central Plains. The 00Z GFS/GEFS solutions took a substantial dive to the south, leaving the local area dry. However, other guidance still has the low tracking across the Carolinas. With this track occurring early in the day on Sunday, will be leaving a chance of rain/snow in the forecast. A narrow upper level ridge will cross the area during the beginning of next week. The 00Z GFS/GEFS are on the fast side of the envelope in racing the next system eastward on Monday. Forecast will show a drier solution. Temperatures are likely to remain a little below normal. && .AVIATION /08Z TUESDAY THROUGH SATURDAY/... Deteriorating conditions expected this morning across all terminals as rain moves in from the south as low pressure moves eastward across the Carolinas. MVFR/IFR VIS and CIGs are forecast through Wednesday night as subsequent areas of low pressure develop off the coast and bring wintry precipitation to the terminals. First low pressure today will help filter in cold air from the north this morning, turning rain showers in to a wintry mix from north to south. A lull is expected late this afternoon and evening before the second area of low pressure impacts the terminals. Snow is expected overnight and during the day on Wednesday, bringing continued periods of MVFR/IFR conditions. Winds will generally be out of the north northeast through Wednesday night at 10 to 15 knots, gusting in excess of 20 knots during the day on Wednesday. Precipitation pulls east of the terminals Wednesday evening, with improving flying conditions from west to east thereafter. VFR conditions expected on Thursday as high pressure builds in to the region, while northwest breezes will gust upwards of 25 knots. VFR conditions are likely Friday and Saturday. A disturbance could bring a shower on Friday, but any precipitation would have minimal impact. && .MARINE... SCA conditions across all waters through the day on Wednesday as two areas of low pressure impact the region with a mix of wintry precipitation. A Gale Warning is in effect through this afternoon for the middle portion of the Chesapeake as low pressure strengthens off the coast. Small Craft Advisories are likely Wednesday night and in to the day on Thursday as the waters remained wedged between exiting low pressure to the east and building high pressure to the northwest. Northwest flow will continue Friday, with Small Craft Advisory conditions remaining possible. High pressure will slide eastward across the area on Saturday, which should result in lighter winds. && .TIDES/COASTAL FLOODING... Two areas of low pressure will pass off the coast today and Wednesday, resulting in prolonged onshore flow. Water levels will be rising during this time. Have trended forecast on the high side of the guidance, which may not be capturing the surge / back-up of water in the bay correctly. Minor flooding will be possible tonight through Wednesday night, before the low exits and water levels start decreasing. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...Winter Storm Watch from late tonight through Wednesday evening for DCZ001. MD...Winter Weather Advisory until 6 PM EDT this evening for MDZ004>006-503-505-507-508. Winter Storm Watch from late tonight through Wednesday evening for MDZ003>006-011-013-014-503>508. Winter Storm Warning until 8 PM EDT this evening for MDZ003. Winter Storm Warning until 8 PM EDT this evening for MDZ501- 502. Winter Storm Watch from this evening through Wednesday afternoon for MDZ501-502. Winter Storm Watch from Wednesday morning through Wednesday evening for MDZ016>018. VA...Winter Weather Advisory until 6 PM EDT this evening for VAZ025>027-029>031-503>505-507-508. Winter Storm Watch from late tonight through Wednesday evening for VAZ025>031-040-052>054-501-503>508. Winter Storm Warning until 8 PM EDT this evening for VAZ028. WV...Winter Weather Advisory until 6 PM EDT this evening for WVZ053- 505-506. Winter Storm Watch from late tonight through Wednesday evening for WVZ051>053. Winter Storm Warning until 8 PM EDT this evening for WVZ051- 052. Winter Storm Warning until 8 PM EDT this evening for WVZ050- 055-501>504. Winter Storm Watch from this evening through Wednesday afternoon for WVZ050-055-501>506. MARINE...Small Craft Advisory until 6 PM EDT Wednesday for ANZ531>533- 537-539>542. Small Craft Advisory until 6 PM EDT Wednesday for ANZ530-535- 536-538. Small Craft Advisory until 6 AM EDT early this morning for ANZ534-543. Small Craft Advisory from 6 PM this evening to 6 PM EDT Wednesday for ANZ534-543. Gale Warning until 6 PM EDT this evening for ANZ534-543. && $$ SYNOPSIS...BKF NEAR TERM...BKF SHORT TERM...BKF LONG TERM...ADS AVIATION...ADS/BKF MARINE...ADS/BKF TIDES/COASTAL FLOODING...ADS is the U.S. government's official web portal to all federal, state and local government web resources and services.