Area Forecast Discussion
Issued by NWS Sterling, VA

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497 FXUS61 KLWX 231900 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 300 PM EDT Fri Jun 23 2017 .SYNOPSIS... Tropical air will envelop the area today. A cold front will cross the area late tonight into Saturday. High pressure will return Sunday and persist through early next week. && .NEAR TERM /THROUGH TONIGHT/...
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In spite of pockets of insolation, atmosphere within the forecast area capped, as evidenced by the lack of agitated cumulus on satpix and just showers on radar. The same cannot be said across western Pennsylvania down the Ohio Valley, in vicinity of the frontal zone. Storms in that region shear-driven, and that will be our concern tonight. Recent runs of HRRR/RAP both depict some enhancement of the showers in Virginia. Given 1500-2000 J/kg SBCAPE, expect some thunder to develop by mid-late afternoon. That area should be scattered in nature and have a diurnal component to it. By 21-22 UTC, the line of thunderstorms, and scattered storms ahead of the line, will be approaching the Potomac Highlands. With loss of daytime heating and residual inhibition, expect a weakening trend in the strength of convection. But, the overall coverage in the Highlands will be increasing. Precipitable water above 2 inches suggests that heavy rain can be expected within any thunderstorm. Latest model guidance (12 UTC cycle) offers a quicker timing. The southwesterly low level jet will be substantive (60-70 kt at 850 mb), so anticipate that some thunder can be expected tonight even without daytime heating. As has been discussed the past couple of days, the question remains whether these storms can root in the boundary layer east of the Appalachians. If they can, it will be busy, with low-topped rotating updrafts. While heavy rain can be expected too, the system overall remains progressive (quicker than previously modeled), which will preclude greater issues. Bottom line... have categorical PoPs crossing forecast area between midnight and 6 am (likely beginning mid-evening), with heavy rain possible. With dewpoints in the lower 70s, low temperatures follow suit.
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&& .SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
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Latest GFS/NAM prog timing to be so quick that all precip will be east of the forecast area by the start of the period. Am a little bit caution of that solution, and have lingered some precip into the Saturday morning hours. Then we`ll need to assess how quickly front leaves area; there are some suggestions in may get caught up across southern Maryland or the Virginia tidewater. Otherwise, high pressure will be building across the area this weekend, with no other weather hazards. Perhaps there will be a reinforcing boundary Sunday, but moisture looks scant for even clouds. Accepted a MOS blend for temps. Sunday will be the cooler day, with lower dewpoints also advecting in Saturday.
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&& .LONG TERM /MONDAY THROUGH FRIDAY/...
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Surface high pressure sets to the west on Monday as a slow moving upper level trough slides east into Tuesday with surface westerly flow. Below normal temperatures, with showers and thunderstorms possible during this period. Conditions should become drier as surface high settles overhead Tuesday night. The high will move offshore on Wednesday and continue moving west into Thursday as flow becomes more southerly. Temperatures will be on the rise as dry conditions continue into Friday, when guidance suggest a boundary will bring showers and thunderstorms over our area.
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&& .AVIATION /19Z FRIDAY THROUGH WEDNESDAY/...
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VFR flight condition across the terminals this afternoon. THere is an outside chance of a shower or tstm, but odds much too low for a TAF inclusion. A cold front, infused with tropical moisture, will cross the terminals tonight, most likely after midnight but before dawn. At this point, kept restrictions at MVFR. There is a decent potential of local/brief IFR, probably due more to reduced vsbys in heavy rain. There may also be localized wind gusts. Where these gusts aren`t realized, there is a 50-70 kt wind max several thousand feet off the deck, so low-level wind shear will be a concern. The front will be east of the terminals by Saturday morning. VFR conditions will prevail for the weekend. Mostly dry/VFR conditions expected Monday into Wednesday as surface high pressure settles in. Some scattered showers and thunderstorms possible Monday afternoon into Tuesday as upper air disturbance moves through, which could cause periods of sub- VFR conditions.
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&& .MARINE...
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Southwest winds 10-15 kt with some gusts to 20 kt. Those winds will be increasing ahead of a cold front tonight. Small Craft Advisories remain in effect. There will be a 50+ kt low level jet several thousand feet off the waters overnight. Do not believe this will mix down in the gradient flow, but local punches may near heavier showers. Marine Warnings may be required tonight. Northwest flow Saturday will contain better mixing. The gradient will decease Saturday night. Have not extended the Advisory further at this time. Mostly dry conditions expected Monday into Wednesday as surface high pressure settles in. Some scattered showers and thunderstorms possible Monday afternoon into Tuesday as upper air disturbance moves through. Winds gusts will be below the small craft advisory criteria.
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&& .HYDROLOGY...
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As mentioned above, precip waters will be exceeding 2 inches, so heave rain expected in storms tonight. However, only 0/1 to 0.3 inches fell this morning, so soils were not saturated. Further, storms look to be progressive. So, while heavy rain expected, believe that the soil will be able to handle it overall, based on flash flood guidance. Thus, will not post any Watches at this time. Local minor flooding is possible, especially in the Potomac Highlands.
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&& .TIDES/COASTAL FLOODING...
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Tides have started to spike, especially in the northern Bay, where departures are now greater than a half foot above normal. The early morning tide cycle will be susceptible for minor flooding. Beyond that, northwest winds should lead to levels returning close to normal.
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&& .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory until 6 PM EDT Saturday for ANZ530>543. && $$ SYNOPSIS...RCM NEAR TERM...HTS SHORT TERM...HTS LONG TERM...IMR AVIATION...HTS/IMR MARINE...HTS/IMR HYDROLOGY...HTS TIDES/COASTAL FLOODING...HTS

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