Area Forecast Discussion
Issued by NWS Sterling, VA

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265 FXUS61 KLWX 150858 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 358 AM EST Wed Nov 15 2017 .SYNOPSIS...
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High pressure will depart to the northeast today. A cold front will cross the region early Thursday. High pressure will traverse the area Friday, then a strong cold front will cross the region late Saturday. High pressure will return by early next week.
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&& .NEAR TERM /THROUGH TONIGHT/...
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Most of the stratus has (finally) broken up, with just a small ribbon slowly dwindling over the northern Shenandoah Valley as of early this morning. Meanwhile, more stratus is developing over the eastern shore of Maryland and advecting westward. Raw model guidance doesn`t bring appreciable overcast into the I-95 corridor until this afternoon, but given the current position and motion of this cloud deck as well as RH profiles in BUFKIT soundings, have outpaced the models a bit and have clouds developing from east to west shortly after daybreak, reaching the Blue Ridge by early afternoon. Mid and high level clouds will also begin increasing from west to east ahead of an approaching cold front/upper trough through the day. High temperatures will likely be observed by midday as clouds increase, with temperatures holding steady or slowly falling this afternoon, especially beneath developing stratus. By this evening, low-level moisture will be deep enough to support occasional drizzle and at least spotty light rain along what looks to be an inverted trough-like surface feature across northern Virginia east into central and eastern Maryland including the DC/Baltimore areas. Exact timing of drizzle and rain depends on what model you look at. Some guidance (i.e. NAM) starts around sunset (5 PM) and other models (i.e. GFS) do not start until mid-evening (9 PM). Current forecast splits the difference. The aforementioned cold front will bring showers into western parts of the CWA this evening, but as they move east they should begin to die out due to a lack of substantial upper forcing. After midnight, showers to the west and light rain/drizzle to the east should simultaneously dissipate as the front moves to the east and drier air moves in.
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&& .SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
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The surface cold front will have departed to the east of the area by daybreak Thursday, but the upper trough axis will lag behind. As a result, expected scattered to broken cumulus to develop. Strong cold/dry advection on NW flow will lead to breezy conditions, with wind gusts to around 30 mph likely. Upslope showers along the western ridges of the Allegheny Front will be slow to dissipate, and should mix with snow by mid-morning Thursday, then change to all snow Thursday afternoon as cold air rushes in. Model soundings show low-level moisture in tact through about midnight Thursday night, so have extended at least low end PoPs to account for this. Precipitation looks to be relatively light, so accumulations should be minimal. Winds will be slow to subside as high pressure continues to surge into the area Thursday night. Cooler than normal temperatures are expected Friday as the high moves overhead, and the forecast leans on Superblend for highs which splits the difference between the warmer statistical guidance and cooler raw model 2-meter temperatures. The high quickly moves offshore by Friday evening with clouds quickly filling in from the west ahead of the next approaching storm system. Given strong warm/moist advection and isentropic lift, nudged PoPs upward more in line with the SREF by daybreak Saturday.
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&& .LONG TERM /SATURDAY THROUGH TUESDAY/...
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Changeable weather can be expected this weekend as a strong cold front crosses the region. Low pressure will be deepening as it lifts northeast from the Great Lakes on Saturday. Gusty southwesterly winds will develop during the day, which should help propel temperatures into the 50s, if not 60s in some locations. At the moment, the best chance for warm advection showers will be the northwestern half of the CWA. Models are beginning to converge on a Saturday evening frontal passage. There`s a possibility showers along the front may break apart somewhat crossing the mountains. Post-frontal showers will also be possible as the sharp mid level trough will lag the surface front. Any precipitation should clear the area by Sunday morning except along the Allegheny Front, where snow showers will be occurring by that time. There is still some uncertainty in the wind forecast (both ahead of and behind the front) lying in the amount of mixing, but expect breezy to windy conditions which will persist into Sunday. In addition to the wind, strong cold advection Sunday will lead to temperatures in the 40s much of the day. A secondary trough axis may swing across the area, which could enhance upslope snow showers and allow some precipitation to spill east of the mountains -- especially eastern WV and northern MD. Surface wet bulb temperatures will be low enough that snowflakes are possible in these areas as well. Northwest flow at the surface and aloft will continue into Sunday night, although moisture begins to decrease, so upslope snow showers should be diminishing. 00Z operational GFS and ECMWF are in better agreement on high pressure building in from the southwest on Monday and sliding off the Carolina coast on Tuesday. However, much greater spread is noted in the ensembles. Overall the beginning of the week should be dry though, with temperatures below normal (but modifying by Tuesday).
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&& .AVIATION /09Z WEDNESDAY THROUGH SUNDAY/...
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VFR through early this morning with light and variable winds. Some brief patchy fog is possible in outlying areas but is expected to be too brief and sparse for mention at any TAF site. MVFR stratus develops/advects in from east to west starting this morning, reaching the Blue Ridge by early afternoon. Drizzle develops this evening (shortly after sunset). IFR possible for a time this evening, mainly between 7 PM and midnight. Conditions should improve markedly after midnight as drier air moves in behind a cold front. NAM is more pessimistic but has had a better handle on low level moisture/clouds in this setup, so gave it a little more weight for this forecast. As the front departs to the east Thursday, expected gusty NW winds 15-20 kts with gusts 25 to near 30 kts. Winds subside by midday Friday as high pressure moves across. Showers will become increasingly likely Saturday afternoon and evening as a strong cold front crosses the area. Duration of impacts is uncertain at this time, although a period of sub-VFR conditions will be possible. Strong winds will also be possible Saturday through Sunday (southwest shifting to northwest with the front).
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&& .MARINE...
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A Small Craft Advisory is in effect for the middle Chesapeake Bay and lower tidal Potomac River through this morning due to a modest pressure gradient and northerly channeling. It`s a marginal setup with even the normally more aggressive HRRR on the low end of the SCA spectrum, but believe there will be enough coverage of 20 knot gusts to warrant the headline this morning. Light winds tonight will veer to the NW and become gusty Thursday with solid SCA gusts of 25- 30 kts expected. Gale force gusts are possible Thursday night (925 mb winds 35-40+ kts in most guidance in cold NW flow over relatively warmer water). Winds abate by midday Friday as high pressure moves across, then increase again late Friday night ahead of the next cold front. Southwesterly winds will continue to increase on Saturday. It`s not out of the question winds could approach gale force. The cold front will likely cross Saturday evening with a wind shift to the northwest. Gales will also be possible behind the front and could last into Sunday. Winds will decrease some by Sunday night, although Small Craft Advisory conditions will likely continue.
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&& .TIDES/COASTAL FLOODING...
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Tidal anomalies of around 1 foot above normal are lurking in the southern waters of the Chesapeake Bay. A brief period of light onshore flow is expected this evening into early Thursday which should allow water levels to rise, and could lead to near minor flooding at Strait`s Point and Annapolis this evening.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory from 10 AM Thursday to 6 AM EST Friday for ANZ530>543. Small Craft Advisory until noon EST today for ANZ532>534-537- 540-541-543.
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&& $$ SYNOPSIS...DHOF NEAR TERM...DHOF SHORT TERM...DHOF LONG TERM...ADS AVIATION...ADS/DHOF MARINE...ADS/DHOF TIDES/COASTAL FLOODING...DHOF

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