Area Forecast Discussion
Issued by NWS Sterling, VA

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668 FXUS61 KLWX 251334 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 934 AM EDT Fri May 25 2018 .SYNOPSIS...
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Surface high pressure will move offshore this afternoon. Weak disturbances will cross the area over the weekend into early next week. A cold front will push through the area on Tuesday. A low pressure system may approach from the south during the middle of next week.
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&& .NEAR TERM /THROUGH TONIGHT/...
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High pressure will continue to move offshore of the Virginia and North Carolina Coast the remainder of the day. A southerly flow will continue, as well, leading to increasing warmth and moisture. High temperatures will reach the 80s with perhaps a few spots pushing 90. It will also be a little more humid as dew points increase into the 60s. A little better deep layer moisture return along the Appalachians may result in some isolated showers by mid- afternoon, but a cap with limited forcing will likely limit intensity and duration. While any showers over the mountains will likely diminish with sunset, there could be a cluster of decaying showers/thunderstorms which approaches from the Ohio Valley overnight. Even if the showers hold together, the trajectory around the ridge may make them more likely to move toward Pennsylvania than our CWA. Elsewhere, it will remain dry and a bit more muggy, as lows only drop into the 60s to lower 70s.
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&& .SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
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The weekend will likely be unsettled with potential for thunderstorms with heavy rain, but will not be a complete washout. Moisture will increase further, with dew points pushing into the 70s and high temperatures remaining in the 80s. Precipitable water values will also increase to around 1.75". Combined, moderate instability will likely develop by the afternoon hours as a subtle disturbance approaches from the southwest. Given deep layer shear generally less than 25 kt, storms aren`t expected to be especially organized or widespread at any one moment, but many areas will see a storm at some point during the afternoon or evening. The main threat will be locally heavy rain as storm motions may be around 20 mph or less. While storms should generally trend downward after sunset, there will be potential for additional vorticity maxima to cross overnight with the threat for a few showers or perhaps a storm. The set-up doesn`t change much for Sunday, although the ridge may be suppressed a little as a trough moves across the Great Lakes. This system may send a back door cold front to the south, although current projections keep it north of the area. Coverage of thunderstorms will likely become numerous again during peak heating, though some models show an early start due to forcing from the northern system. Locally heavy rain will again be the biggest threat. Given antecedent conditions which are still wet, the could be potential for isolated flooding issues either day. Don`t have a handle at the moment if one day will be favored over the other, nor any particular area, given the expected disorganized nature of the thunderstorms.
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&& .LONG TERM /MONDAY THROUGH THURSDAY/... Southerly flow will continue across our region Monday and into Tuesday with tropical moisture being advected into our region. A weak boundary will be lingering to our north on Monday and move across on Tuesday. The boundary and in addition to diurnal heating will allow for showers and thunderstorms to develop in the afternoon/evening hours both days. A high pressure system will be centered over the Northeast U.S. on Wednesday and move offshore on Thursday. Flow will become easterly on Wednesday and then turning more southerly on Thursday. At the same time a low pressure system will be to our southwest centered over the Gulf Coast states. Some guidance bring this low into the Mid-Atlantic and other keeps it tracking east over the Southeast U.S. This could mean the difference between a dry or wetter second half of the work week as it could bring additional tropical moisture to our region...therefore additional showers and thunderstorms. && .AVIATION /14Z FRIDAY THROUGH TUESDAY/...
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VFR through tonight as high pressure continues to move offshore. Will withhold mentioning any fog for tonight since cloud cover should begin increasing. Southerly flow will remain 10 kt or less. A warm and humid airmass combined with weak disturbances will lead to the potential of showers and thunderstorms Saturday and Sunday, favored during the afternoon and early evening. Impacts will likely be brief, with heavy rain the main threat. Fog could occur overnight in locations that receive rain if skies clear. Warm and moist air Monday into Wednesday will allow for additional showers and thunderstorms mainly in the afternoon/evening hours which brings the possibility of sub-VFR periods.
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&& .MARINE...
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High pressure will move offshore this afternoon, with southerly winds developing. Winds should stay in the 10-15 kt range during the day. Have issued a Small Craft Advisory for tonight for the wider waters of the midday where increasing southwesterly flow aloft will have enough fetch to mix down. Winds may also be marginal on Saturday, but am uncertain enough not to continue the advisory for now. The larger threat may come from thunderstorms during the afternoon and evening. Sub-SCA conditions are expected on Sunday too, though showers and thunderstorms will become likely again during the afternoon and evening. Winds are expected to remain below small craft advisory criteria Monday into Wednesday, therefore not anticipating an advisory for this period.
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&& .TIDES/COASTAL FLOODING... Water levels remain slightly elevated at Washington DC, though trends in the anomalies suggest minor flooding will be a low threat. It will be close for the next several cycles though. No coastal flooding issues are expected elsewhere. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory from 6 PM this evening to 8 AM EDT Saturday for ANZ533-534-543. && $$ SYNOPSIS...ADS/KLW NEAR TERM...ADS/KLW SHORT TERM...ADS/KLW LONG TERM...IMR AVIATION...IMR/KLW MARINE...IMR/KLW TIDES/COASTAL FLOODING...ADS/KLW

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