Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 160750 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 350 AM EDT Wed Aug 16 2017 .SYNOPSIS...
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An area of high pressure will build into the region through tonight. A warm front will move northward Thursday into Thursday night followed by a cold front that should pass through the region Friday night into early Saturday. Weak high pressure will build into the area during the rest of the weekend.
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&& .NEAR TERM /THROUGH TODAY/...
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A combination of light winds, lingering low level moisture, and wet soils has led to the formation of dense fog and low stratus late last night and early this morning. Therefore a Dense Fog Advisory is in effect for much of the region through 9 AM for visibilities down to near zero in some locations. Ridge of surface high pressure moving eastward through the Great Lakes early this morning will then crest near the region this afternoon. This will provide for mainly dry conditions and partly- mostly sunny skies once the fog and low clouds break, which may take until late morning (11 AM or so) in some locations. There has been a couple of isolated showers this morning as well towards the higher elevations, but the vast majority of the area is not expected to see any measurable rainfall. Temperatures will be slow to rise with clouds/fog early, but should still reach the upper 80s to locally near 90F by late in the afternoon.
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&& .SHORT TERM /TONIGHT THROUGH FRIDAY NIGHT/...
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For tonight, surface high will be slow to exit so another night of light winds and mostly clear skies will be in store. We`ll be one day removed from significant rainfall however, so while areas of fog and stratus are likely again, coverage/density should be less than this morning. Lows from the mid 60s to low 70s. High pressure shifts offshore for Thursday allowing for the return of southerly flow. As an approaching low pressure system lifts through the Great Lakes, a warm front will lift northward and into the region by Thursday afternoon. This and a developing area of broad low level convergence will serve as a focus for shower/thunderstorm development on Thursday. MLCAPE south/west of the warm front is currently progged to be in the 1000-2000 J/KG range, which is quite sufficient for thunderstorm development. Further examination of the modeled forecast soundings would indicate potential for both an isolated severe and isolated flood threat, with the presence of a low level boundary, directional shear along the boundary, 0-6KM shear of around 30 knots, and precipitable water values 2-2.25 inches. Initial focus likely to be west of I-95, with propagation eastward during the evening/overnight hours. Highs Thursday should reach into the mid/upper 80s, with lows Thursday night in the low to mid 70s. The area of low pressure will push into southern Canada on Friday, with its trailing cold front entering the Mid-Atlantic states during the day. This will serve as another focus for scattered showers/thunderstorms during the day Friday. A potential severe threat will exist again on Friday, as temperatures and low level moisture surge out ahead of the front, leading to the development of 1500-2500 J/KG of MLCAPE, with 30-35 knots of shear moving overhead. Highs upper 80s to around 90F. Convection will wane Friday night.
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&& .LONG TERM /SATURDAY THROUGH TUESDAY/...
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Weak high pressure will build toward the area for Saturday and Sunday. Most areas will remain dry with less humidity. However...a pressure trough will likely be hung up near the Chesapeake Bay into southern Maryland. Humidity may remain across these areas along with an isolated shower/t-storm...but much of the time will be dry. High pressure will move offshore for Monday and Tuesday. Hotter and more humid conditions are expected.
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&& .AVIATION /07Z WEDNESDAY THROUGH SUNDAY/...
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Widespread reductions in both ceilings and visibilities expected this morning in fog and low stratus, with LIFR/IFR forecast at all TAF sites. DCA has lowest risk of dense surface fog. Conditions will gradually improve during the morning hours, with VFR returning at most locations by 15z. Potential for additional fog/stratus exists tonight, although coverage/density should be less than this morning. Next weather concerns will be the possibility of scattered showers/thunderstorms both Thursday and Friday, where brief reductions of both ceiling/visibility are possible. High pressure should bring VFR conditions most of the time for Saturday and Sunday.
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&& .MARINE...
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Sub-SCA conditions expected through Thursday. South winds will then be on the increase Thursday night and Friday ahead of a frontal system, and Small Craft Advisory conditions are possible. Isolated to scattered showers and thunderstorms are also possible both Thursday and Friday, mainly in the afternoon/evening, and Special Marine Warnings may be necessary. A pressure trough will remain near the waters during the weekend. Wind speeds should be below SCA criteria for the most part since the gradient will be week. A return flow from the south will develop early next week as high pressure moves offshore.
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&& .TIDES/COASTAL FLOODING...
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A light northerly flow will allow for anomalies to drop early this morning. However...southerly winds will increase for Thursday through Friday night ahead of a cold front. Minor tidal flooding is possible near high tide Thursday night into Friday morning and again Friday night into Saturday morning for sensitive areas.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...Dense Fog Advisory until 9 AM EDT this morning for DCZ001. MD...Dense Fog Advisory until 9 AM EDT this morning for MDZ003>006- 011-013-014-016-503>508. VA...Dense Fog Advisory until 9 AM EDT this morning for VAZ025>031- 037>040-050>057-501-502-505-506. WV...Dense Fog Advisory until 9 AM EDT this morning for WVZ051>053. MARINE...None.
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&& $$ SYNOPSIS...BJL NEAR TERM...MM SHORT TERM...MM LONG TERM...BJL AVIATION...BJL/MM MARINE...BJL/MM TIDES/COASTAL FLOODING...BJL

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