Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 211420 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 1020 AM EDT SAT MAY 21 2016 .SYNOPSIS... Low pressure will track up the Mid-Atlantic coast today. Upper level low pressure will persist along the Mid-Atlantic coast tonight through Monday night. High pressure returns Tuesday and persists through mid-week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Shield of stratiform moderate to locally heavy rain has been progressive, and is currently over the Bay...primarily affecting the Delmarva/southeastern tidewater region. Elsewhere, clouds, areas of light rain, and perhaps drizzle will be affecting area through the midday hours. Low pressure developing atop Cape Hatteras will be moving up the coast this afternoon. Easterly flow ahead of this low will be drawing more moisture into the area. That will produce enough upglide to initiate a new round of showers for northeast Maryland. Therefore PoPs will be higher there than other areas along the I-95 corridor. Secondly, height falls will be advancing into the mountains as shortwave energy carves out a trough axis. Instability in the mountains meager, but there will be a couple hundred joules of CAPE, especially south of W99. Have likely PoPs returning to the mountains this afternoon, along with a slight chance of thunder. Am not making any temperature changes at this time. Given the saturated low levels and easterly flow, it will be a struggle to see 60 degrees, although central Virginia may manage it.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH MONDAY NIGHT/...
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The best way to describe the short term period is "unsettled"... although gradually less so with time through Monday night. As energy rotates into the upper trough tonight, a closed low will eventually develop, reaching the Outer Banks by Monday, then perhaps drifting offshore by Monday night. With the subtlety in disturbances within this pattern, timing and placing the highest rain chances will be difficult. Have favored north/west tonight where greatest height falls and moisture will be. On Sunday, have brought a period of likely POPs across the area as trough axis/upper low moves overhead. Can`t rule out some thunder, although looks like a very minimal chance...best in the far west. Have calculated a "storm total" through Sunday, which ranges from less than one inch NE of Baltimore, with 1.5 to 2 inches from DC south/west. Of course, locally higher is possible. Expect shower chances to diminish Sunday night, only to renew Monday, with highest chances in the east closer to the upper low. There`s a little better chance of a thunderstorm or two on Monday as it looks like there will be a better chance of seeing some breaks of sun. The drying trend should begin in earnest Monday night. Sunday`s temperatures will largely depend on where it rains the longest, and models differ strongly regarding this. Splitting the difference maintains continuity of lower to mid 60s. Right now Monday is looking warmer...in fact guidance is has widespread 70s. Given unsettled pattern, played conservatively closer to 70. Overnight lows remain fairly constant in the 50s.
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&& .LONG TERM /TUESDAY THROUGH FRIDAY/...
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Upper level low pressure will be departing eastward on Tuesday...with convergence and warming temperatures aloft. Temps will rise and clouds will break, but still a slight risk of a shower is showing up in guidance. A ridge axis will approach from the west Wednesday, continuing the confluence and the warming. This ridge axis will weaken Thursday as southwesterly flow develops aloft, but heights will continue to build resulting in more warming. Instability may result in convective showers or thunderstorms developing over the mountains and possibly moving east into the metro. Another weak ridge axis will cross the area Friday with heights remaining high, though a trough will be approaching from the west. More showers and storms may form over the mountains and advect into the metro.
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&& .AVIATION /14Z SATURDAY THROUGH WEDNESDAY/...
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Shield of rain advancing northeast across the area, and for the most part have cleared the terminals. Restrictions are taking some time as atmosphere needs to saturate. MVFR more common at this point, although CHO/MRB have achieved IFR. Further restrictions still possible today since low levels saturated. The forecast for tonight through Sunday night is rather uncertain with upper level low pressure moving through the region. It will be unsettled to say the least with MVFR looking likely for much of the time with occasional showers. Don`t have confidence to time in any IFR. Monday should see improved conditions but a lingering chance of showers, especially afternoon. VFR Tuesday and Wednesday with high pressure regaining control.
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&& .MARINE... Expect SE to E winds to increase this morning as low pressure develops off the coast. The strongest winds may actually occur through midday until clouds and rain can sufficiently limit mixing. However, no changes to the SCA at this time. Wind should diminish tonight, but will have to monitor for northerly channeling. N to NW flow will continue through Monday around 5-10 kt. Light and variable by Monday night. && .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory until midnight EDT tonight for ANZ530>534- 537>543. Small Craft Advisory until 7 PM EDT this evening for ANZ535- 536.
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&& $$ SYNOPSIS...RCM NEAR TERM...HTS SHORT TERM...ADS LONG TERM...RCM AVIATION...HTS/ADS/RCM MARINE...ADS/RCM

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