Area Forecast Discussion
Issued by NWS Sterling, VA

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647 FXUS61 KLWX 170106 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 906 PM EDT Fri Mar 16 2018 .SYNOPSIS... High pressure will build across the region tonight. A weak low pressure will pass just south of the region on Saturday. High pressure will build back into the region for Sunday and Monday. Another complex low pressure system may impact the region Tuesday and Wednesday. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/... Tonight will be the quietest of the three forecast periods. High pressure is nudging into the area from the northwest, and winds have started their downward trend with sunset. While cirrus is moving in from the southwest, it should remain relatively thin until later in the night. With a fresh batch of Canadian air in place, lows will once again be below freezing across the region, but with diminishing winds, wind chills will be less of an issue. Will explore lowering temperatures in some of the more sheltered locations. Dew points remain in the teens, so a dip of temperatures down into that range is possible. Temperatures may climb a bit toward dawn, especially on the ridges, as the atmosphere moistens and clouds increase. && .SHORT TERM /6 AM SATURDAY MORNING THROUGH SUNDAY NIGHT/... For the evening update, have not seen enough in the latest guidance to make wholesale changes for the Saturday event. There are still a range of solutions regarding precipitation placement and type, although the greatest chance for notable accumulations of snow or ice remains somewhere near the US 48 corridor in the Potomac Highlands, where the coldest air overlaps the highest QPF potential. Previous discussion: A fresh dose of Canadian air with some radiational cooling to boot posts a threat to the region Saturday as a weak low pressure crosses just south of our region. Latest guidance has precip moving in after sunrise, but before noon, for portions of the region west of the Blue Ridge, and with clouds moving in, temperatures may remain at or below freezing as precipitation starts. This poses a risk of freezing rain if thermal profiles are conducive, so have issued a winter weather advisory for the zones most at risk. While the main freezing rain threat would likely be in the morning, higher elevations could see the threat persist through the day, and freezing rain is not the only threat. Across the region, the soundings progged for Saturday are quite cold. While surface temps are likely to warm above freezing region-wide as we head through Saturday, with temps so cold aloft, rain may not be the only precipitation type, and soundings in fact suggest sleet and snow may occur across much of the region at times, especially near and north of the I-66/US 50 corridor. Guidance also suggests some sort of fgen band may set up over this region, though there is by no means unanimous agreement with this. Given the great uncertainty, have allowed a small accumulation of snow/sleet in much of the region, and have mixed p-types for much of the day. Its possible that for areas such as the Washington DC metro, the precip could start as rain, change to sleet and snow if intensity gets high enough, then go back to light rain or drizzle as it ends. This could end up reminiscent of the February 17th snow band (also a Saturday) when a burst of heavy wet snow briefly coated many areas roads before tapering off and changing back to rain. However, other models show much less intensity of precip, so its also possible that mainly rain falls and there is little to no accumulation. Given this, will not yet highlight anything in the HWO east of the Blue Ridge. Latest guidance has the system being suppressed mainly southwest of the Baltimore area thanks to stronger confluence aloft north and east of the region, so odds favor precip failing to reach northeast Maryland or even Baltimore proper. Elsewhere, the precip and clouds are likely to keep temps chilly Saturday, with high 30s to low 40s expected for highs. As the system moves away Saturday night, precip should end and clouds should break as high pressure returns. After another cold night, Sunday should see warmer temperatures with more sunshine as high pressure builds overhead. Highs will be mostly in the 50s. Despite the warmer day, another night of radiational cooling should put temps back below freezing in most areas Sunday night. && .LONG TERM /MONDAY THROUGH FRIDAY/... Monday starts dry with high pressure in control. During this time an occluded low pressure system will be moving east over the Tennessee Valley towards the Appalachians with precipitation associated to it starting as early as Monday night. The low pressure will transfer its energy off of the Mid-Atlantic coast Tuesday night into Wednesday and then slowly move NE away from us. Guidance is still in some disagreement varying between snow and rain, but suggesting precipitation to last from Monday night into Wednesday night. The track and speed of this system play an important role on the local impacts we might see. We will continue to monitor the forecast, but confidence is increasing of a coastal storm impacting our area next week. && .AVIATION /01Z SATURDAY THROUGH WEDNESDAY/... VFR conditions are expected tonight as northwest winds continue to slacken and become southwesterly late. Sub-VFR possible at all terminals Saturday (mainly late morning onward) as a system with some rain and perhaps some sleet and snow moves across the terminals. The overall probability of IFR is low, and the amount of impact otherwise could depend on the intensity of the precipitation. Precip may stay just south of MRB and BWI, which would significant impacts at those terminals. However, it would be more likely that these locations see visibility reductions in snow should precipitation make it that far north. VFR should return behind the system later Saturday night and persist through Sunday night. Expecting mainly VFR conditions Monday with high pressure in control. Sub-VFR conditions expected Monday night into Wednesday with coastal storm possibly moving near by during this time. Breezy conditions on Tuesday and Wednesday, some gusts up to 20 kts possible. && .MARINE... Northwest winds have started to subside with sunset, now below 25 kt. Some areas will likely be dropped from the SCA early, while it`s possible some areas on the open bay could hold on to SCA conditions a few hours after midnight. We may be near SCA gusts in the northern bay on Saturday, but confidence was not quite there for an SCA. If precipitation stays south of Baltimore, a few 20 kt gusts are more plausible. Winds should remain relatively light Sunday under high pressure. Wind gusts will be below the Small Craft Advisory criteria on Monday. Winds will increase Monday night and remain above criteria into Wednesday, which means that small craft advisory is likely these days. && .FIRE WEATHER... Winds will continue to relax and humidity will start to increase tonight and especially during the day on Saturday as a system with rain and snow moves into parts of the region. Heavy precip is possible Tuesday and Wednesday which would moisten fuels significantly as well. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...Winter Weather Advisory from 8 AM to 5 PM EDT Saturday for MDZ501-502. VA...Winter Weather Advisory from 8 AM to 5 PM EDT Saturday for VAZ025>031-503-504-507-508. WV...Winter Weather Advisory from 8 AM to 5 PM EDT Saturday for WVZ050-055-501>506. MARINE...Small Craft Advisory until midnight EDT tonight for ANZ530>543. && $$ SYNOPSIS...RCM NEAR TERM...ADS SHORT TERM...ADS/RCM LONG TERM...IMR AVIATION...ADS/IMR/RCM MARINE...ADS/IMR/RCM FIRE WEATHER...LWX is the U.S. government's official web portal to all federal, state and local government web resources and services.