Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 121439 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 939 AM EST Sun Nov 12 2017 .SYNOPSIS... High pressure will move east off the coast today. Low pressure will then affect the region Sunday night into Monday, followed by high pressure on Tuesday and Wednesday. Another front will cross the region early Thursday with more high pressure for Friday. && .NEAR TERM /THROUGH TONIGHT/...
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Surface high pressure will continue shifting offshore through today, with dry weather and light winds. Plenty of high clouds are streaming overhead this morning, and while these will generally persist through the day, they may tend to thin or scatter out somewhat for a time later this afternoon and into the evening before lower/mid clouds increase, leading to a few breaks of sun. Otherwise, temperatures will be milder than yesterday but still below normal with highs from the mid 40s to around 50F. Tonight is very tricky. We have a stronger shortwave moving into the area, but it is still relatively weak and moisture- starved. The surface low will traverse the Ohio Valley, then dissipate as it crosses West Virginia and energy transfers to a new low pressure developing near the VA/NC border. This sort of energy transfer near us is always tricky, as it can often result in both a persistent wedge of cold air, as well as most of the precip bypassing the region. So, we have the questions of 1) How much of the area will see precip overnight and 2) Will it be cold enough where precip does occur for any of it to be icy or snowy. Atmospheric profiles from various models are very borderline, with many showing warm surface layers, but given the relatively dry low levels and potential for a little radiational cooling in the evening before thicker clouds move in overnight, there remains the possibility of temps getting lower than guidance suggests. However, the other fly is the precip itself happening, and the latest guidance has trended a bit drier, with more separation between the primary and the secondary low pressures and their associated precip. Thus, do not feel any more confident about any wintry precip than I did 24 hours ago, and will maintain existing mention of potential wintry weather as it was inherited. Certainly this is not a problem for the I-95 corridor, with the main concern being north-central and northwestern MD and eastern WV, along with the northern Shenandoah Valley in VA. Most likely, any advisories will be issued either very late today or this evening if their need becomes more clear. Lows tonight will be chilly, but with clouds moving in, feel most of area except region outlined above will stay above freezing, despite some stat guidance to the contrary.
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&& .SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/...
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Most guidance has the secondary low pressure rapidly taking over from the primary low early on Monday, with precip moving out/falling apart over our western zones except for favored upslope areas on the Allegany Front. Showers will likely linger over central MD much of the morning, but should move out by afternoon, with some sun breaking through. With a more Pacific influence coming in behind this system, expect highs to actually end up a smidge warmer than recent days, though this is dependent on the sun breaking through. If for some reason the clouds remain persistent, it could end up cooler than Sunday. Any lingering upslope rain showers on the Allegany Front could become snow showers on Monday night - otherwise, Monday night through Tuesday night should be dry as high pressure moves in. Highs Tuesday should have a better shot at getting into the 50s.
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&& .LONG TERM /WEDNESDAY THROUGH SATURDAY/... A vertically stacked ridge axis will pivot across the eastern U.S. Wednesday resulting in seasonable temperatures and partly to mostly clear conditions. A cold front will cross the region from west to east Wednesday night into Thursday morning. Though at the surface it will be weakening, the mid-to-upper trough will be sharpening some, which should still result in a few scattered showers. Temperatures should be warm enough to support liquid as the precipitation type. More clouds than sun and a few sprinkles or a shower could linger Thursday as the upper trough axis pivots across. 00z GFS/ECMWF are similar with timing/strength of the shortwave, but GFS has a little more low- level moisture resulting in upslope showers Thursday afternoon over the Allegheny Front, with just cold enough air for a few wet snowflakes by nightfall as precipitation ends. A much stronger storm system will affect the area later Friday into the first half of the weekend as a result of a deep trough diving across the Great Lakes. We should be in the warm sector for this with liquid rain showers and breezy conditions the most likely outcome Friday night into Saturday, with upslope snow showers plausible over the highlands as the system departs Saturday night. Long range models/ensembles are in better than average agreement, lending to a relatively high confidence forecast. && .AVIATION /15Z SUNDAY THROUGH THURSDAY/...
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VFR through today. CIGS lower tonight as weak system moves in, with MVFR looking most likely later tonight as a little rain moves through. We could see some MVFR or even high end IFR vis if there is enough moisture from the rain to generate mist late tonight and early Monday. Should return to VFR on Monday afternoon and remain VFR through Tuesday night, though a little patchy fog is not impossible in the morning. Winds generally less than 10 knots. Patchy fog is possible Wednesday morning. Otherwise mainly VFR Wednesday-Thursday with southerly flow of 5 to 10 knots.
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&& .MARINE...
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High pressure today will keep winds light, while weak low pressure Monday looks unlikely to bring winds up to SCA levels, even as it pulls away Monday night. High pressure returns Tuesday and Wednesday. SCA conditions are possible Thursday behind a cold front.
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&& .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && $$ SYNOPSIS...RCM NEAR TERM...MM/RCM SHORT TERM...RCM LONG TERM...DFH AVIATION...MM/RCM/DFH MARINE...MM/RCM/DFH

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