Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 201401 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 901 AM EST Sat Jan 20 2018 .SYNOPSIS... High pressure to the south will dominate the weather through the weekend. A cold front will approach the region Monday before passing through Monday night into Tuesday. High pressure will return for Wednesday through Thursday of next week before moving offshore Friday. && .NEAR TERM /THROUGH SUNDAY/...
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High pressure will remain over the southeastern United States through the day today leading to dry conditions and light southwest surface flow. Aloft west/northwest winds will lead to downsloping flow which will provide for warming conditions and mostly sunny skies through the day, aside from some high cirrus passing overhead and some cumulus along the westerly facing slopes. Highs should reach the 50s area-wide to potentially 60F in some locations. Tonight, with the high sliding a bit further east, a bit more low level warm/moist advection looks likely to take place, which appears to result in weak frontogenesis over the region. This could result in a bit of a wedge forming over the region late tonight and Sunday, which would cause a milder night but a cooler, cloudier Sunday. However, the NAM is one of the models showing this, and it has goofed these scenarios before by moistening it too fast. This is problematic in particular given how dry the air mass remains at present. Thus, have half stepped between the NAM and GFS for cloud cover and highs on Sunday, bringing them down from today`s forecast but not as cool as the NAM would suggest. If the NAM is right, it could be stuck in the 40s most of the day Sunday, while the GFS would suggest another very mild day like we are expecting today. Further west, along the Allegheny Front, the warm/moist advection looks more certain to significantly moisten the atmosphere, so have some chance pops along the ridgeline late tonight and Sunday, but no further east. With warmer temps, expect precip to be in the form of rain, but it might be close to freezing late tonight.
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&& .SHORT TERM /SUNDAY NIGHT THROUGH MONDAY NIGHT/...
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Whatever wedge manages to form on Sunday will likely linger Sunday night before trying to scour out on Monday as southerly winds increase ahead of the approaching cold front (currently located over the Rockies). However, once again, the NAM tries to hold it in place, suggesting a much cooler day. Again have half-stepped this, given we are expecting little if any precip to reinforce the cold wedge, though a little light rain and drizzle will still be possible along the Allegheny Front and Mason-Dixon line. The uncertainty with highs on Monday is the largest of the forecast period, with the NAM suggesting high 40s in much of the area while the GFS brings temps into the 60s. Did go warmer than Sunday given the increasing southerly flow expected. Monday night the cold front moves in with rain showers. Forcing is decent and there could be some elevated instability, but surface inversion may preclude thunder if the NAM is right. For now have not included any thunder. Temps will likely only drop a little from Monday`s highs, whatever they may be, and if the cool NAM is ultimately right, temps may in fact rise Monday night. Regardless, southerly winds should increase, and the jet just above the surface looks quite strong, so if there is any convection late, some strong gusts should try to mix down if there is little to no inversion.
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&& .LONG TERM /TUESDAY THROUGH FRIDAY/...
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50+ knot low level jet and several hundred joules of CAPE ahead of a cold front/negatively tilted upper trough yield some concern of gusty surface winds Tuesday morning. Question will be if surface/boundary-layer air is still relatively cool as surface high pressure wedge slowly retreats to the north and east. Regardless, anomalously high moisture combined with strong frontogenetic/jet forcing will result in a soaking period of rain showers during the morning, quickly coming to an end from west to east as drier air filters in. High pressure building in behind the front will bring a return to more seasonable temperatures (highs in the 40s, lows in the 20s) for the second half of next week. Otherwise, mainly dry conditions are expected. One exception is a secondary upper trough that pivots through early Thursday which could increase clouds and result in a few upslope snow showers.
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&& .AVIATION /14Z SATURDAY THROUGH WEDNESDAY/... VFR should prevail through today at all terminals. A backdoor cold front may then drop south across the region tonight, with an increasing risk of MVFR or IFR cigs later tonight into Sunday. Not certain on this, however, as the atmosphere is currently quite dry, so a lot of moistening has to take place for these low clouds to form. If they do form, they may linger through Monday, before an approaching cold front also causes reduced visibility via rain showers Monday night. IFR seems likely Tuesday morning with showers ahead of a front. Low-level wind shear and perhaps gusty surface winds would be a concern well given 50+ kt low-level jet forecasted by most models. VFR should return for the second half of Tuesday. Winds will shift from S to NW after frontal passage. VFR Wednesday. && .MARINE... High pressure will provide light winds through Monday. Increasing southerly winds ahead of an approaching cold front will likely lead to SCA gusts Monday night, especially after midnight and especially over the middle portion of the bay. Strong gusty winds out of the south area possible especially in heavier showers Tuesday morning. Winds will become NW and remain gusty later Tuesday through Wednesday behind the front w/ SCA likely. && .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...DHOF AVIATION...MM/RCM/DHOF MARINE...MM/RCM/DHOF

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