Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 220759 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 359 AM EDT Mon May 22 2017 .SYNOPSIS...
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A warm front will cross the region this morning. A cold front will move through the area later today. High pressure will briefly return Monday night before low pressure develops over eastern Carolina Tuesday and moves northeast off the DelMarVa Tuesday night. Upper-level low pressure will linger over the area Wednesday through Friday.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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It`s rather difficult to pick out precisely where the warm front resides early this morning. Based on dew points and wind flow, best estimation is somewhere across southside Virginia. However, the isentropic lift and the overruning precipitation it has induced has enveloped central Virginia, with lighter returns across the Baltimore-Washington area. Just about everyone east of I-81 will be seeing precipitation by sunrise. Have likely or categorical PoPs for the morning, with the highest along I-95... coincident with the best lift. Trends this afternoon are more optimistic. There is a common signal that echoes will become more scattered in nature. Further, RAP, NAM, and even GFS suggesting that the cold front will be entering the western portion of the forecast area during the afternoon. Best moisture remains east of this boundary, so won`t have PoPs higher than chance through frontal passage. Will be ending precip chances along with the dry air infiltrating behind the front. Instability will be limited at best due to early precip timing. However, in daytime heating behind the first round of precip, few hundred joules of CAPE will be possible east of the Blue Ridge, and have entertained a chance of thunder in this area.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH WEDNESDAY NIGHT/...
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High pressure will briefly build across the region tonight. However, the frontal zone won`t be all that far offshore, and clearing unlikely for point east of the Blue Ridge. Guidance has, for a couple of cycles now, been inducing cyclogenesis across the southeastern United States late tonight near a 500mb shortwave and the right rear quad of the upper jet. The track of this low will bring rain back to central Virginia and southern Maryland by late tonight. The focus will remain across southern/eastern areas, with a good dose of associated precipitation though Tuesday. One difference this cycle though is the compact nature of the low (tight precip gradient on the northwestern edge) and progressive speed. That means that precip will be exiting by Tuesday evening. Have backed off on Tuesday night precip substantially. Am hesitant to drop PoPs entirely as current guidance suggests, but am at a lower confidence "chance". A closed upper low will be moving south into the Mid-west Wednesday while a sfc low and associated cold front moves eastward across the Ohio Valley. A brief period of subsidence is expected across the Mid- Atlantic Wednesday leading to mostly dry conditions for most of the day however instability will increase across the Allegheny Front and creep northward into central VA Wed afternoon. This may cause a few showers and thunderstorms to form however confidence is low through the afternoon. Mostly dry conditions will come to an end Wed evening as rain progresses from south to north across the Mid-Atlantic region into Thu morning. Elevated instability is expected and thunderstorms are possible Wed night.
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&& .LONG TERM /THURSDAY THROUGH SUNDAY/...
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A cold front will be in vicinity of the Mid-Atlantic region Thursday. Showers and a few thunderstorms should be ongoing Thu morning as low pressure rides along the cold front. Depending on the speed of the front showers and thunderstorms may continue through the day Thursday. Shear profiles show weak shear and therefore thunderstorms are expected to be sub-severe. The caveat is that heavy rain is possible in thunderstorms. Will need to monitor the threat if training storms occur. An upper level trough will move over head Thu-Thu night. The cold front should be east of the Mid-Atlantic region by Fri morning and northwest flow is expected. Disturbances may move across the region under the influence of the upper level trough Friday and a chance for showers exists. High pressure will build into the Mid-Atlantic region Friday- Saturday. Dry weather is expected through Saturday. Low pressure will move into the Ohio Valley Sat night and Sun and showers and thunderstorms are possible during this time.
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&& .AVIATION /08Z MONDAY THROUGH FRIDAY/...
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MVFR flight conditions prevail across most of the terminals except for MRB, which is further away from the forcing. MTN has actually just dropped to IFR. Expect a downward trend through the morning push, with aob IFR common...due more to ceilings than vsbys. After the initial round of showers pass east, ceilings should gradually lift. The afternoon will be at least above IFR. By evening, VFR should prevail. The precise timing of category crossings still a little bit nebulous. Also, there could be a few more showers in the afternoon, and maybe even a thunderstorm DCA/BWI/IAD. Spatial and temporal uncertainties preclude a TAF mention. VFR for the rest of the TAF period. However, more rain, with reduced flight categories will return for the morning push Tuesday. Once again, IFR a good possibility for much of the day, with improvement in the evening. SHRA/TSRA possible Wed night-Thu night as a cold front moves across the terminals. Sub-vfr conditions possible during this time.
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&& .MARINE...
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Winds have dropped to 10-15 kt, and all Small Craft Advisories have been dropped. Mixing should be rather poor today in vicinity of showers. Southerly flow will become west/northwest this evening with a cold frontal passage. The pressure gradient will tighten Tuesday as low pressure moves up the coast. Have raised a Small Craft Advisory for the mid Bay and lower Potomac, where the gradient will be the best. As the situation unfolds, the Advisory may need to be expanded or extended. A cold front will cross the waters Thu-Thu night. Showers and a few thunderstorms are possible Wed night through Thu night. Winds become W-NW Friday.
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&& .TIDES/COASTAL FLOODING...
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Water levels remain about a foot or less above astronomical normals. High tide has passed at Straits Point, and the tide came in under minor threshold at Annapolis. Both of those Advisories have been dropped. At SW DC and Alexandria, guidance indicates that tides will bring a surge along with this high tide. HEC-RAS forecast on track so far, so have no reason to dispute the results. Have a bit of uncertainty is it will indeed be as high as forecast, but at least think the Advisory threshold will be crossed. Therefore, have raised the Advisory for high tide this morning. The next tide cycle will be the lower astronomically, and anomalies may be easing slightly. A cold front tonight will bring northwest winds. Therefore there are no indications that any additional flooding is imminent. However, a coastal low Tuesday provides some uncertainty. Guidance ensembles then diverge substantially through midweek.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...Coastal Flood Advisory until 9 AM EDT this morning for DCZ001. MD...None. VA...Coastal Flood Advisory until 9 AM EDT this morning for VAZ054. WV...None. MARINE...Small Craft Advisory from 2 PM to 6 PM EDT Tuesday for ANZ533- 534-537-543.
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&& $$ SYNOPSIS...HTS NEAR TERM...HTS SHORT TERM...HTS/HSK LONG TERM...HSK AVIATION...HTS/HSK MARINE...HTS/HSK TIDES/COASTAL FLOODING...HTS

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