Area Forecast Discussion
Issued by NWS Sterling, VA

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616 FXUS61 KLWX 161837 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 237 PM EDT Sun Jul 16 2017 .SYNOPSIS...
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Weak high pressure will slide offshore tonight. An upper level trough will affect the weather Monday and Tuesday. After another period of tranquil weather during the middle of the week, another cold front may approach the region by the end of the week.
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&& .NEAR TERM /THROUGH TONIGHT/...
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Weak high pressure resides over the Mid Atlantic this afternoon, with an even more subtle cold front along the St. Lawrence Valley and across the Great Lakes. Flow still light, but a cumulus field has once again developed in vicinity of mid-upper level trough axis. While output hasn`t been strong or widespread, there are still signals that there could be a few showers/storms over the Appalachians before sunset. RAP is the most prolific in this regard. That area would have favorable orographic lift, isentropic lift from return flow/850 mb warm advection, lower heights aloft, and a bit of somewhat capped instability...so output seems reasonable. This activity should gradually wane with the loss of daytime heating. Elsewhere, it`s mainly a cloudcover forecast. While instability doesn`t completely erode, and mid-level lapse rates decent (more than 6.5 C/km in the 850-500 mb layer and greater than 7 C/km between 700-500 mb), there`s not much forcing otherwise...and CIN remains in place. Am therefore going with a dry overnight forecast, with scattered to broken clouds. Dewpoints will start to rebound, so low temperatures won`t be as cool as last night. Gridded forecast reflects that.
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&& .SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/...
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A trough axis will inch closer to the forecast area Monday, but doesn`t completely clear area. Instability will be able to build. Questions remain how much and whether it will be able to break the cap. Given lower heights and terrain circulations, storms will best be able to develop across the ridges...and the strongest storms will be capable of taking advantage of the cooler temperatures aloft to produce hail/local downdrafts. Marginal Risk from SPC takes these considerations into account. Shear will be a non-factor. Keeping likely PoPs only across the Potomac Highlands/Northern Shenandoah Valley/Western Maryland... only slight chance across far southern Maryland. The trough axis slowly lifts and fills Tuesday. The same ingredients will be in place, but in a weaker sense. Am keeping primarily diurnal chance PoPs, but with a mid-late morning start given minimal inhibition.
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&& .LONG TERM /WEDNESDAY THROUGH SUNDAY/...
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Bermuda high pressure across the western Atlantic will continue through Thursday into Friday. Near seasonable temperatures and high humidity expected with not much precipitation expected. Isolated thunderstorms are possible through the end of the week mainly associated with terrain circulation and river/bay boundaries. Some guidance hints at northwest flow setting up by the end of the week, which could bring into play any upstream leftover MCS activity, but confidence for this not high. By Friday models suggest a weak cold front will sag down into the area. This front is associated with a surface low passing well north of the region. Some guidance takes this front and drops it just south of our CWA on Friday providing for a brief respite from the high humidity. Other guidance suggests the front lingers over the area on Friday and continues into Saturday. Low confidence in how this will play out. Then sometime on the weekend, the front either washes out or moves north as a warm front. Moisture pooling south of this front allowing for higher humidities, with some guidance suggesting precipitable water values over 2.0 inches next weekend.
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&& .AVIATION /18Z SUNDAY THROUGH FRIDAY/...
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Mainly VFR through the valid TAF period. Diurnal cumulus this afternoon, with bases 040-050, thick enough for ceilings at times. Return of warmer/more humid air aloft may keep clouds in place tonight. Winds will be light but become southeast. Southeast flow will continue tomorrow. Building instability will permit development of showers/thunderstorms by afternoon. The strongest of these storms may produce gusty winds and hail... especially for MRB and perhaps CHO. Areal coverage uncertainties preclude a TAF mention at this time. There will be similar considerations on Tuesday. Wednesday through Friday...expect VFR conditions overall, and generally dry weather. The threat of showers and thunderstorms is generally isolated at best both Wednesday and Thursday, but increases Friday ahead of the cold front.
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&& .MARINE...
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Flow will be become south tonight but remain light (at or below 10 kt) through Tuesday. Overall surface pressure gradient mid-late week is fairly weak. Aside from southerly channeling on the Bay during the afternoon/evening, expecting to remain below SCA levels through the end of the week. The threat of thunderstorms will likely increase Friday.
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&& .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && $$ SYNOPSIS...HTS NEAR TERM...HTS SHORT TERM...HTS LONG TERM...SMZ AVIATION...HTS/SMZ MARINE...HTS/SMZ

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