Area Forecast Discussion
Issued by NWS Sterling, VA

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468 FXUS61 KLWX 171957 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 357 PM EDT Fri May 17 2024 .SYNOPSIS...
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Low pressure will track to our south tonight, before progressing offshore this weekend. High pressure will build into the area early next week. A cold front will approach from the west during the middle of the week.
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&& .NEAR TERM /THROUGH TONIGHT/...
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As of 3 PM, visible satellite imagery shows cloudy skies across the entire forecast area. Showers continue to spread eastward from the Ohio Valley, and have reached western and central VA, the WV Panhandle, and western MD. These showers are expected to gradually weaken as they spread eastward through the remainder of the afternoon, eventually dissipating before reaching I-95. Additional showers and thunderstorms are expected to fire further west across the WV Panhandle as weak DCVA driven large scale ascent spreads eastward from the Ohio Valley. These showers and storms will form in an environment that is nearly saturated through the entire vertical profile. Model soundings show around 500 J/kg and around 1.5 inches of precipitable water. While the values of instability and moisture present aren`t off the charts, they will be sufficient to produce heavy rainfall. With the saturated vertical profiles in place and weak flow (less than 20 knots) through the lower depth of the troposphere), storms should be relatively slow moving (with weak flow limiting advection driven movement, and saturated profiles limiting propagation driven movement due to cold pool generation), and some training of cells may be possible at times. In terms of convective evolution, CAMs generally show a cluster of storms forming later this afternoon in the Potomac Highlands of the WV Panhandle. These storms then slowly drift eastward into the Shenandoah Valley later this evening. This activity is then followed by additional, more strongly forced (in the left exit region of a jet streak moving to our south) storms moving in from the southwest later this evening. Multiple rounds of storms may be possible across the Potomac Highlands, Shenandoah Valley, and central Virginia late this afternoon through the overnight hours. As a result, this raises concerns for instances of flooding. Many of the 12z CAMs show widespread rainfall totals in excess of an inch across this area, with isolated pockets of 2-5 inches. As a result, Flood Watches have been issued through 4 AM for the Potomac Highlands, Shenandoah Valley and central Virginia. Further east, showers will gradually move in later this evening through the first half of the overnight. A steadier, soaking rain will ensue through the second half of the night, but no issues with flooding are anticipated. Low temperatures overnight are expected to be in the upper 50s to near 60.
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&& .SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
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Weak upper troughing will progress overhead tomorrow, causing cloudy and showery conditions to linger through the day. With the thick cloud cover in place and easterly winds off the ocean, conditions will remain cool, with temperatures only reaching into the low 60s. Upper troughing will start to progress off to our south and east on Sunday. This should lead to a reduction in shower activity, with some breaks of sun developing by afternoon, especially to the west of the Blue Ridge. Low cloud cover and periods of drizzle may try to linger through the morning to the east of the Blue Ridge within easterly low-level flow, but some breaks in the clouds cover still appear likely there during the afternoon. Highs on Sunday are expected to reach into the 70s (warmer further west, cooler further east).
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&& .LONG TERM /MONDAY THROUGH FRIDAY/...
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High pressure and dry conditions prevail for the start of the extended. Winds will be generally light, starting off out of the east early Monday, then becoming southerly return flow behind the center of high pressure as it moves east. This, combined with ample sunshine, will allow temperatures to rise into the upper 70s on Monday, and possibly into the 80s by Tuesday. A potent low pressure system moves from the Northern Plains into the Great Lakes on Wednesday, swinging a strong cold front through our area Wednesday into Thursday. The area will be well primed with temperatures in the 80s and ensembles indicating upwards of 1500 J/kg of CAPE. Shear is less certain, the more favorable shear arriving Wednesday night into Thursday. Much could stand to change this far out, but we will continue to monitor the severe threat for this system.
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&& .AVIATION /18Z FRIDAY THROUGH WEDNESDAY/...
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Ceilings are currently high end MVFR to low end VFR this afternoon, and should eventually reach low end VFR in most locations before dropping back down to MVFR and then IFR later tonight. Showers have overspread western VA, WV, and western MD, but are gradually dissipating as they spread further east. Additional showers and thunderstorms are expected to form this afternoon into this evening, mainly to the west of the Blue Ridge. Thunderstorms may be possible late this afternoon into this evening at CHO and MRB. Further east, instability is lacking. Showers will eventually spread further east to all terminals this evening, but thunderstorms are expected to remain confined to near CHO and MRB. Rain will be steadiest during the second half of the night. Showers and IFR ceilings will linger across much of the area tomorrow into tomorrow night. Chances for showers will decrease on Sunday, but IFR to MVFR ceilings will continue. Winds will be out of the northeast today, southeast tonight, and then east this weekend. VFR conditions are expected Monday and Tuesday with high pressure and dry conditions.
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&& .MARINE...
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Sub-SCA northeasterly winds are expected through the remainder of the day. Winds turn southeasterly tonight, and then easterly this weekend. Winds may reach SCA levels in easterly flow tomorrow. Sub-SCA criteria winds are expected Monday and Tuesday with light easterly winds to start on Monday turning out of the south. Winds increasing out of the south on Tuesday could result in some southerly channeling, but not currently expected.
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&& .TIDES/COASTAL FLOODING... Persistent onshore flow will continue through early next week. This will result in elevated tide levels. Minor flooding is expected in more sensitive locations over the next several tidal cycles. && .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...Coastal Flood Advisory until 8 AM EDT Saturday for DCZ001. MD...Coastal Flood Advisory until 7 AM EDT Saturday for MDZ014. Coastal Flood Advisory until 2 AM EDT Saturday for MDZ017-018. VA...Flood Watch through late tonight for VAZ025>027-029-030- 036>040-050-051-503-504-507-508. WV...Flood Watch through late tonight for WVZ055-501-502-505-506. MARINE...None.
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&& $$ SYNOPSIS...KJP NEAR TERM...KJP SHORT TERM...KJP LONG TERM...CAS AVIATION...KJP/CAS MARINE...KJP/CAS TIDES/COASTAL FLOODING...KJP