Area Forecast Discussion
Issued by NWS Sterling, VA

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918 FXUS61 KLWX 290936 AAA AFDLWX Area Forecast Discussion...UPDATED National Weather Service Baltimore MD/Washington DC 536 AM EDT Thu Sep 29 2016 .SYNOPSIS... An upper level low will settle over Kentucky today into Friday before lifting back northward across the Great Lakes over the weekend. Persistent onshore flow and periods of heavy rain will occur across the Mid-Atlantic through Friday. The low is forecast to pass to our north as it tracks east across the northern Mid-Atlantic late in the weekend and into early next week, with high pressure building into the area. && .NEAR TERM /THROUGH TONIGHT/...
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Satellite imagery indicates southward-moving upper-level low is currently nearing its forecasted southern most latitude (which is somewhere over Kentucky) it nestles itself between high pressure over the Atlantic and ridging over the central CONUS. Becoming quasi-stationary through tonight as it stalls out in a blocking pattern. This will set up a continued fetch of warm/moist air (from several tropical source regions) as it converges over the area through the period. Current radar trends show bulk of moderate/heavy rainfall is currently located across the eastern half of our area...with more scattered showers across the west. HRRR-TLE shows high probability of at least 0.5-1 inch/hr rain rates in the strongest cores as this activity continues to develop and move across the eastern half of the CWA through the morning. By afternoon...backing flow at 850 hPa strengthens to between 30-50 kts through the day leading to a significant upslope component. This should allow for showers and scattered thunderstorms...heavy at re-develop over the higher terrain. Elsewhere...guidance is a little less bullish after the initial activity during the morning...keeping activity slightly more scattered in nature. Despite this...convective elements coupled with efficient rain processes and anomalously high PWATs suggest flooding concerns could arise in locations that experience training activity. SPC currently has SW portion of CWA in a marginal risk. While ML CAPE remains relatively poor (At best 1000 J/KG), effective bulk shear values AOA 40 kts suggest transient supercell structures are possible across the marginal risk area...with isolated hail/wind threats. Furthermore, nearby frontal boundary and strongly backed low-level flow suggest an isolated tornado is also possible across the far southwest.
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&& .SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/... By Friday...ridging across the central CONUS begins to weaken. This will allow the upper-level low to gradually retrograde and lift back to the north as it begins to become vertically stacked and fills/weakens. Rain rates are expected to gradually become less intense as activity becomes more scattered through the day. For now...did not adjust the expiration time of the Flash Flood Watch...though that will need to be revisited later today. Current thinking is that flooding issues would be more localized Friday. Gradual drying trend continues into vertically-stacked upper-level low continues to weaken and drift northward. While not a washout by any means...scattered shower activity will remain possible. && .LONG TERM /SUNDAY THROUGH WEDNESDAY/... Influence of upper-level low finally begins to diminish by late in the weekend into early next week as it passes by to our north and moves out to sea...while high pressure builds into the area in its wake. Most of Sunday appears dry...aside from some possible lingering early morning showers. Remaining dry with temperatures near climatology as high pressure remains over the area through early next week. && .AVIATION /10Z THURSDAY THROUGH MONDAY/... Upper-level low will continue to impact the area today through Friday...bringing periods of IFR/LIFR to all the terminals. Heaviest rain expected today into Friday. Isolated thunderstorms are also possible today. Low ceilings are possible through Friday night with persists onshore flow. Gradual improvement Saturday...with a return to VFR by Sunday. && .MARINE...
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Gale Warning continues until 14z today for the main channel of the Chesapeake Bay and eastern inlets (with SCA elsewhere). SCA continues everywhere after 14z BUFKIT soundings and high-resolution guidance show winds remaining just below Gale. However...observations/guidance will continue to be monitored as current guidance might be underestimating strength of low-level wind field. SCA currently expires at 12z Friday...though that may need to be extended into Friday afternoon...but confidence is too low at this time. Gradient should begin to relax by later Friday evening...with SCA winds becoming less likely Saturday into the Weekend.
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&& .TIDES/COASTAL FLOODING... Tidal departures increased with the incoming high tide overnight, assisted by gale force east winds. Anomalies running around 1.5 feet, although its a little less near Solomons and the mouth of the Potomac River. Feel that there is enough coming up the Bay to add southern Baltimore to the Coastal Flood Advisory. Since an easterly component to the wind will be in place through at least early Friday, water will only be reinforced. Pondered going with a Watch for the lower reaches (St Mary`s Co primarily... due to the susceptibility of St George Island) but ultimately opted not to since flow should be pushing water away from that part of the Potomac. However confidence is sufficient to extend Advisories through the Friday morning tide cycle. That`s not saying that moderate flooding is out of the question at some point between now and the weekend; rather certainty is too low to issue a Watch at this time. We will continue to monitor. Water levels will continue to be an issue through at least Saturday. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...Flash Flood Watch through Friday morning for DCZ001. Coastal Flood Advisory until noon EDT Friday for DCZ001. MD...Flash Flood Watch through Friday morning for MDZ003>006-011- 013-014-016>018-501>508. Coastal Flood Advisory until 10 AM EDT Friday for MDZ011-016. Coastal Flood Advisory until 8 AM EDT Friday for MDZ014-017- 018. VA...Flash Flood Watch through Friday morning for VAZ025>031- 036>040-050>057-501-502-505>508. Coastal Flood Advisory until 10 AM EDT Friday for VAZ057. Coastal Flood Advisory until noon EDT Friday for VAZ054. WV...Flash Flood Watch through Friday morning for WVZ050>053-055- 501>504. MARINE...Small Craft Advisory from 10 AM this morning to 8 AM EDT Friday for ANZ530>534-539>541-543. Gale Warning until 10 AM EDT this morning for ANZ530>534- 539>541-543. Small Craft Advisory until 8 AM EDT Friday for ANZ535>538-542. && $$ SYNOPSIS...MSE NEAR TERM...MSE SHORT TERM...MSE LONG TERM...MSE AVIATION...MSE MARINE...MSE TIDES/COASTAL FLOODING...HTS is the U.S. government's official web portal to all federal, state and local government web resources and services.