Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 201859 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 259 PM EDT FRI MAY 20 2016 .SYNOPSIS... High pressure will retreat to the northeast this evening. Low pressure will track east across North Carolina tonight before turning up the Mid-Atlantic coast Saturday. Upper level low pressure will persist along the Mid-Atlantic coast Saturday night through Monday night. High pressure returns Tuesday. && .NEAR TERM /THROUGH TONIGHT/... A ridge of high pressure remains from the eastern Great Lakes to Cape Hatteras this afternoon. As a result...the bulk of the moisture has been delayed. Cirrus now streaming across the area, and there have been a few cumulus clouds that have been able to develop. Denser moisture will overspread the area tonight as low pressure tracks from the southeastern states toward the Appalachians. Based on meso-model trends, this translates to increasing clouds for the evening hours. Am shifting the onset of precipitation to late evening in the far southwestern section of the forecast area, and during the overnight hours for the bulk of the area. We still will have a ribbon of strong theta-e advection advancing eastward, so I`m fairly confident that rain will begin before dawn for most of the area. && .SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/... A sharpening trough axis in the Great Lakes/Ohio Valley will assist in transferring energy to a coastal low off the Carolinas early Saturday. This coastal low will then head northeast up the coast during the day. The area will have ample moisture in place, with a conveyor coming from the Atlantic ahead of the low. Therefore, the morning to midday hours likely to see the heaviest rain, as the low level jet interacts with an airmass with precipitable water above an inch. Due to the synoptic pattern, there will be a wedge of cool air in place; am therefore going with a forecast of steady rain. Amounts of an inch to an inch and a half should be quite common in central Virginia and southern Maryland by midday Saturday. Could see some upslope enhancement along/east the spine of the Blue Ridge, especially in central Virginia. The lowest flood guidance values extend west of there, so after collaboration a slight risk of excessive rainfall was painted by WPC in these areas. As the low tracks north of the area by mid-late afternoon, we will experience a greater influence from the trough axis. There will be multiple shortwaves embedded within the flow that will cross the area. This will last for the rest of the weekend. Therefore, needs to continue at least a chance of showers (with mostly cloudy to cloudy skies). Timing these vorts will ultimately determine when the best chance of rain will be. While skill not present to do that specifically, it would seem as though Saturday evening and Sunday afternoon would have the best chance at additional showers. This pattern will keep the daytime hours quite cool. It may be a struggle to see 60 degrees Saturday. Since Sunday looking to be cloudier/rainier, high temperatures for Sunday have been lowered. Not much change for low temperatures, which should be kept near or above 50 degrees due to high dewpoints. && .LONG TERM /MONDAY THROUGH FRIDAY/... An upper level low near the Delmarva Peninsula will gradually pull away from the coast and move toward the northeast Monday and Monday night. There is a chance for a couple of showers or a thunderstorm Monday with a slight chance expected in the eastern half of our region Monday night. Temperatures will be slightly below normal. High pressure will build into the region Tuesday through Wednesday. Dry conditions and warmer temperatures expected each period. By Thursday, a trough of low pressure will develop and bring the next chance of showers and thunderstorms. Temperatures will be above normal. A cold front should slowly work its way southward across the region Friday. The threat for showers and thunderstorms will exist along and ahead of the front with temperatures remaining above normal and increasing humidity. && .AVIATION /19Z FRIDAY THROUGH WEDNESDAY/... VFR conditions this afternoon, and it should continue as such into this evening. Rain spreads from southwest to northeast tonight. It likely won`t affect the terminals until the overnight hours. Since winds ahead of the rain should be easterly, anticipate a rather sudden drop from VFR to IFR. Am not certain when this will happen, but it should be complete by mid-morning Saturday. If may very well be ongoing during the morning push. IFR conditions may continue for a good portion of Saturday. The best to hope for will be MVFR. The pattern will remain wet through the rest of the weekend, but not a washout. In general, majority of the time should be MVFR. Periods of IFR certainly possible, especially during the overnight and morning hours. Precipitation will become more showery, which does mean local/brief restrictions possible. Mvfr conditions possible with showers and thunderstorms around Monday. Winds northwest around 5 knots Monday. Vfr conditions possible at MRB, CHO, and IAD terminals Monday night. Mvfr conditions possible elsewhere to the east. Winds becoming light and variable Monday night. && .MARINE... Light and variable winds ongoing, and that should be the case for much of the evening. low pressure will be approaching tonight, and passing east of the waters tomorrow. Will have an increased flow ahead of this low. Small Craft Advisories have already been in place for the lower waters overnight and all waters Saturday. Momentum transfer looks reasonable during the evening hours on the Bay/mouth of the Potomac; have extended Advisory for then. Shortwave energy will be swinging across the waters Saturday night through Sunday, but will ample cloudcover, mixing will be poor. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory from 8 AM Saturday to midnight EDT Saturday night for ANZ530>533-538>541. Small Craft Advisory from 4 AM Saturday to midnight EDT Saturday night for ANZ534-537-542-543. Small Craft Advisory from 8 AM to 7 PM EDT Saturday for ANZ535- 536. && $$ SYNOPSIS...HTS/RCM NEAR TERM...HTS SHORT TERM...HTS LONG TERM...KLW AVIATION...HTS/KLW MARINE...HTS is the U.S. government's official web portal to all federal, state and local government web resources and services.