Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 230801 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 401 AM EDT Fri Jun 23 2017 .SYNOPSIS...
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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.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Lead shortwave trough is currently crossing the area. Most of the forcing appears to be at this level along the leading edge of precipitation, as rain is falling from a mid level cloud. Lightning has not been observed so far, but there is just a little elevated instability, so would not rule out a stray rumble of thunder. Rainfall currently over NC is associated with lower level forcing, though guidance suggests it will be in non-uniform showers crossing during mid morning. There may be a bit of a break during the midday hours due to subsidence behind the trough, although there is enough evident of a few showers to not remove POPs entirely. There may be a little insolation to help push temperatures well into the 80s. However, the subsidence may also lead to a capping inversion and thus make it difficult to initiate convection. Most guidance has little to no convection during the afternoon hours, though there are some hints there could be a lee trough which could trigger a rogue storm. Mid- level energy may also clip the far NW CWA. If something can break the cap, there will be plenty of deep layer shear, so there is a conditional threat for severe weather. Will have to monitor thermodynamic trends through the morning to get a better handle on this potential. Based on this forecast though, flooding looks like a low threat through the day.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH SUNDAY NIGHT/...
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There is fairly good agreement between guidance of tracking the remnant circulation of Cindy across the central portion of the CWA late tonight. That will represent the main chance of moderate to heavy rainfall, though it`s possible some lines of storms could slowly drop south from PA during the evening and prolong the issue over far NW sections of the area. Highest QPF amounts will be to the northwest of the low track. Current consensus still places this within the CWA, though some models keep it to the northwest. The combination of uncertainty with rain amounts today, along with where the axis of heaviest rain tonight will be, gives low confidence on whether there will be flash flood issues. However, the moisture-rich environment and warm rain processes will be in play, so this situation will need to be monitored closely. Current guidance also indicates stable low levels tonight with meager instability, but winds aloft will be very strong (with very high low level shear), so still will not totally rule out an isolated instance of severe weather just southeast of the low. Low will be exiting quickly Saturday morning, taking the heavier rain with it. However, the other player in all this -- the cold front -- does not look like it will completely push to the southeast until later in the afternoon. Have maintained chances for showers and storms east of I-95. Drying will continue to advect in overnight, leading to less muggy lows. A secondary cold front will drop through the area Sunday, although it should remain precipitation-free. Highs will fall back into the lower to mid 80s, with lows Sunday night in the mid 50s to lower 60s.
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&& .LONG TERM /MONDAY THROUGH THURSDAY/...
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The weather will feel more like late May than late June for at least the first half of next week as a trough of upper level low pressure settles over the region and a surface high sets up to the west, funnelling cooler and less humid Canadian air across our region. Temperatures will be several degrees below normal, with highs in the 70s to lower 80s and lows in the 50s to lower 60s. By this time of year, normal highs are in the mid- upper 80s and normal lows are in the mid-upper 60s. A weak cold front crossing the area, reinforcing the cooler and drier air, may spark off a shower or t-storm late Monday into early Tuesday. Otherwise, the period looks generally dry. The trough will swing east of the region late in the week and the surface high will return to a more typical Bermuda-high position, allowing warmer and more humid air to start returning to the region.
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&& .AVIATION /08Z FRIDAY THROUGH TUESDAY/...
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Leading edge of rainfall associated with upper trough is not currently posing any restrictions. There may be a brief period of MVFR cigs and additional showers working north during early to mid morning. However VFR conditions are expected by late morning and will persist through the afternoon, though SW winds may gust to 20 kt. Thunderstorm development this afternoon is conditional this afternoon with a cap in place. Guidance has little to no coverage, so will limit mention. Main restrictions will come tonight as remnants of Cindy cross the area. MVFR to possibly IFR will occur late tonight, but exit quickly Saturday morning. Cold front may not clear the area until afternoon, though chance of additional showers/storms is low. Second cold front will cross on Sunday with little to no impact. VFR expected Monday and Tuesday. An isolated shower or thunderstorm can`t be ruled out late Monday into early Tuesday with a weak cold front, but odds are low.
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&& .MARINE...
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Winds beginning to increase a bit over the open waters early this morning, though will increase in earnest later this morning once showers clear and mixing can increase SCA remains in effect. Any thunderstorms this afternoon should be isolated. May be a lull in the winds tonight, but will increase south of the low track of the remnants of Cindy. Have extended the SCA through Saturday as cold front pushes through the area, although winds may decrease by mid afternoon as gradient weakens. Second cold front will cross on Sunday and winds may marginally reach SCA levels. A weak cold front crossing the area late Monday into early Tuesday could boost winds up to SCA levels. An isolated shower or thunderstorm is also possible with this front.
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&& .HYDROLOGY...
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Highest QPF still looks to remain across the NW parts of the area where 1-2 inches total will be possible. However, there is still considerable uncertainty, especially since preceding events may underproduce, and thunderstorms could produce local variations. With areal averages spread out over the period, MMEFS continues to indicate no potential of mainstem river flooding. The main concern will be flash flooding in any areas which receive repeated and/or prolonged periods of heavy rain. Again, that would be the mountains, primarily tonight into Saturday morning.
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&& .TIDES/COASTAL FLOODING...
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Water levels remain near astronomical normals. While an increase is anticipated through Friday, flow will remain southwest. As such, flooding is not likely. However, Caution/Action stages will be reached today, and any small deviation could bring the sensitive spots to minor.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory from 10 AM this morning to 6 PM EDT Saturday for ANZ530-531-535-536-538>540. Small Craft Advisory until 6 PM EDT Saturday for ANZ532>534- 537-541>543.
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&& $$ SYNOPSIS...RCM NEAR TERM...ADS SHORT TERM...ADS LONG TERM...RCM AVIATION...ADS/RCM MARINE...ADS/RCM HYDROLOGY...ADS TIDES/COASTAL FLOODING...HTS/RCM

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