Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 100153 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 853 PM EST Sun Dec 9 2018 .SYNOPSIS... Low pressure will pull away from the Carolina coast tonight. A second area of low pressure will develop offshore of the Carolinas Monday then move out to sea as high pressure builds in from the Ohio Valley. This area of high pressure will retreat to offshore of New England through the end of the week as a large area of low pressure develops over the Tennessee Valley and heads toward the region. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/... Low pressure now resides off the coast of Cape Hatteras with the snow band now weakening and shrinking back off to the south/east across the region. Also seeing reports of rain/sleet/freezing rain mixing in with the snow as warming occurs aloft, and this is also confirmed by dual-pol radar signatures. Thus have begun the process of trimming back the ongoing warnings/advisories to where accumulating snow is still occurring, and will likely be able to trim further over the next 1-3 hours, with all areas ending by midnight. Low pressure will then pull away tonight, with snow coming to an end across all areas by midnight. Low temperatures tonight will remain below normal, generally from the upper teens to upper 20s. Skies should begin to clear to the northwest very late. && .SHORT TERM /6 AM MONDAY MORNING THROUGH TUESDAY NIGHT/... A secondary northern stream shortwave will dive to our south Monday, with surface low pressure developing off the Carolina coast and heading out to sea. No impact is expected locally from this system. High pressure will follow, building in from the Ohio Valley into Tuesday. Multiple shortwaves aloft will begin approaching from the west well out ahead of long wave troughing deepening over the central U.S. by later Tuesday, likely leading to an increase in cloud cover. Below normal temperatures are expected to persist early this week, with highs in the 40s and lows in the 20s (coldest Tuesday night, especially if we have less mid/high level cloud cover). && .LONG TERM /WEDNESDAY THROUGH SUNDAY/... High pressure will become centered overhead and shortwave ridging will move in aloft for Wednesday, resulting in mostly sunny skies and light winds. High temperatures on Wednesday will be in the low- mid 40s. A decaying shortwave will approach the area from the southwest late Wednesday Night into Thursday. The GFS and Euro differ by a fair amount with respect to the track of this feature. The GFS brings it across the Central Mississippi Valley to near Detroit by Thursday morning, while the Euro tracks the feature through the Ohio Valley Wednesday Night and toward the Mason-Dixon Line Thursday morning. The GFS solution would have minimal impacts on the area other than producing clouds. The European solution would also produce cloud cover, but with the disturbance closer by, it could also trigger a few showers across western portions of the forecast area Wednesday Night. Very few ensemble solutions produce any precipitation Wednesday Night, so it appears that the Euro is a bit of an outlier at this point, though temperatures will be cold if precip does occur during this time. A more substantial trough will eject from the Four Corners region onto the Southern Plains during the day Thursday. This trough will become meridionally elongated in nature Thursday Night, and eventually cut itself off from the northern stream flow during the day Friday as it tracks across the lower-mid Mississippi Valley. Although there is some spread in the location of the trough amongst model solutions, they are in good agreement that the trough will be potent, with a well developed area of low pressure at the surface, and a highly anomalous plume of moisture (GFS has precipitable water values between +2 and +3 sigma) streaming northward ahead of the system. The combination of strong forcing for ascent ahead of the trough and the anomalously high moisture levels could lead to a heavy rainfall event across the area Friday into Friday Night. The operational GFS and Euro, as well as the ensemble means from the GEFS and EPS all produce over an inch of precipitation with the system. With a strong area of high pressure departing off of New England at the start of the event, some brief wintry precipitation can`t be ruled out at precipitation`s onset late Thursday Night or early Friday Morning, but it appears that most of the precipitation should fall in the form of rain. Some wintry precipitation also can`t be ruled out Saturday Morning on the back side of the system underneath the upper low in upslope westerly flow. There is also a non- zero chance the storm tries to spawn a secondary low along the coast during this time. && .AVIATION /02Z MONDAY THROUGH FRIDAY/... VFR expected through Tuesday, except in central VA/INVOF CHO where IFR continues this evening in snow. Improvement there after 03z tonight is expected as well. Light north flow is expected the next few days, generally 10 kts or less. Mainly VFR conditions are likely Wednesday and Thursday as a few weak system cross. Condtions may deteriorate to sub-VFR by Friday as the next system approaches. && .MARINE... SCA for middle/lower tidal Potomac and most of the MD portion of the Chesapeake Bay south of the Bay Bridge tonight, scaling back a touch before diminishing entirely by around midday Monday as the gradient/northerly channeling weaken and high pressure builds. Light winds are then expected through Tuesday. Winds are expected to be light on Wednesday and Thursday with surface high pressure located overhead. && .CLIMATE... Rainfall totals continue to creep upward, with Baltimore setting the annual record already. Here are the current rankings for wettest year on record (through December 5th): Washington DC area (DCA) 1. 61.33 inches (1889) 2. 60.83 inches (2003) 3. 60.78 inches (2018) 4. 60.09 inches (1878) Weather records for the Washington DC area have been kept at what is now Ronald Reagan Washington National Airport (DCA) since 1945. Precipitation records observed downtown extend the period of record back to 1871. Baltimore MD area (BWI) 1. 65.67 inches (2018) 2. 62.66 inches (2003) Weather records for the Baltimore MD area have been kept at what is now Baltimore-Washington International Thurgood Marshall Airport (BWI) since 1950. Precipitation records observed downtown extend the period of record back to 1871. Dulles VA area (IAD) 1. 65.67 inches (2003) 2. 61.30 inches (2018) 3. 59.05 inches (1972) Weather records have been kept at what is now Washington Dulles International Airport (IAD) since 1960. NOTE: All climate data are considered preliminary until reviewed by the National Centers for Environmental Information (NCEI). && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...Winter Storm Warning until midnight EST tonight for MDZ017. Winter Weather Advisory until midnight EST tonight for MDZ016- 018. VA...Winter Storm Warning until midnight EST tonight for VAZ025- 036>039-050-055>057-508. Winter Weather Advisory until midnight EST tonight for VAZ051- 052-502. WV...None. MARINE...Small Craft Advisory until 6 AM EST Monday for ANZ536-540. Small Craft Advisory until 1 PM EST Monday for ANZ532>534-537- 541>543. && $$ SYNOPSIS...DHOF NEAR TERM...MM/DHOF SHORT TERM...DHOF LONG TERM...KJP AVIATION...MM/DHOF/KJP MARINE...MM/DHOF/KJP CLIMATE...WFO LWX

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