Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 230108 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 908 PM EDT Thu Jun 22 2017 .SYNOPSIS... A stalled boundary will return north as a warm front toward the area tonight. A cold front will cross the area late Friday night into Saturday, with tropical moisture streaming into the Mid Atlantic ahead of the cold front. High pressure will return Sunday and persist through early next week. && .NEAR TERM /THROUGH FRIDAY/... A line of showers and thunderstorms will continue to move southeast across central PA and will likely brush northeast MD tonight. Warm and moist conditions continue across the region tonight. Isolated showers and thunderstorms have been popping up across the eastern panhandle of WV. Shear has been weak across this region and activity has been of the garden variety. Previous discussion... A surge of isentropic lift will be approaching the area tonight. While this should be quick moving, precipitable waters will be increasing in excess of 2 inches, so locally heavy rain will be possible. The best opportunity for showers will be in the Potomac Highlands. Instability will be limited, so will be confining that to slight chance at best. && .SHORT TERM /FRIDAY NIGHT THROUGH SATURDAY NIGHT/... The first round of showers will be exiting Maryland in the morning. There is substantial uncertainty what will transpire during the midday-afternoon hours. The latest indications are that mid-levels will be warm enough that the atmosphere will be capped. Have reduced PoPs to indicate a break in the warm sector, but not eliminated the mention entirely due to the uncertainty. There will still be a 850-500 mb jet on the order of 40-50 kt, so if the cap is broken, strong storms will result. Heights lower Friday night as the cold front approaches, with the remnant low of Cindy intertwined. Model guidance remains consistent that another mid level speed max...this time up to 60 kt...will be shooting northeast in advance of the low. While instability will be meager, shear will be plentiful, especially east of the track of the low. At this time, that track seems to be from Staunton/Waynesboro across metro DC/Baltimore. There will be ample convergent forcing for another round of showers and a few thunderstorms. Given precipitable waters above 2 inches (possibly up to 2.5 inches), high confidence that locally heavy rain will result. The question remains if shear can root in the boundary layer. If so, then an overnight severe risk would be realized as well. The front/low will clear the area Saturday...with spread remaining in how quickly that will transpire. Grids depict improvement over the course of the day, but held onto showers southeast of DC during the afternoon. Will have clearing skies by evening. && .LONG TERM /SUNDAY THROUGH THURSDAY/... Below normal temperatures are expected much of next week as a large- scale trough slowly pivots across the northeastern CONUS. Models are in above average agreement across our area during this time, increasing confidence in the forecast (highs generally upper 70s/lower 80s with lows several degrees on either side of 60, about 5-10 degrees below normal). A secondary (moisture-starved) cool front should cross the area late Monday. Again, models/ensembles cluster reasonably well for this time range. An increasing in high and mid-level clouds and a few showers are likely during this time. A gradually warming trend should commence by the end of next week as high pressure moves offshore, resulting in a southwesterly return flow. && .AVIATION /01Z FRIDAY THROUGH TUESDAY/... Overall, VFR conditions are expected at the terminals through Friday. However, A round of showers/thunderstorms will be crossing the area late tonight, some of which may contain moderate to locally heavy rain. The issue remains confidence in impacts with respect to a point forecast. If storms do reach an airfield, then restrictions would result. There will be a lull on Friday midday-afternoon. A few showers/thunderstorms may be around (more conditional). Again, any storm would result in brief restrictions, but the likelihood of that will be lower in the afternoon than early morning. A second round will cross the terminals late Friday night into Saturday morning. Areal coverage should be better. Again, local restrictions likely within storms. Gradual improvement will come during the day on Saturday. Mainly VFR Sun-Mon with WLY flow AOB 10 kts. && .MARINE... Southwest flow will increase tonight, with flow aloft substantive through Friday night. The complication is poor mixing. Am keeping overnight Small Craft for the mid Bay/lower Potomac, and for all waters Friday-Friday night. Given the wind flow aloft, higher winds likely in vicinity of showers/thunderstorms. The best chance for storms over the waters will be late Friday into early Saturday. Mixing will improve during the day on Saturday in the wake of the cold front. The gradient should subside Saturday night. Gusts should remain below Small Craft Advisory levels Sunday into Monday as high pressure builds over the waters. && .HYDROLOGY... Given brief nature of the two round of moderate to heavy rain and scattered showers/storms in between, areal average storm total (tonight through Saturday) rainfall are generally in the 1-2 inch range, Gusts should remain below Small Craft Advisory levels Sunday into Monday as high pressure builds over the waters.possibly less to the southeast. If anywhere exceeds two inches areal average, it would be the Potomac Highlands. Thus MMEFS indicates little to 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 Friday night into Saturday morning, with the preceding events priming/saturating the soils. && .TIDES/COASTAL FLOODING... Water levels remain near astronomical normals. While an increase is anticipated through Friday, flow will remain southwest. As such, flooding is not anticipated at this time. However, Caution/Action stages likely will be reached by Friday. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory from 10 AM Friday to 6 AM EDT Saturday for ANZ530-531-535-536-538>540. Small Craft Advisory from 2 AM Friday to 6 AM EDT Saturday for ANZ532>534-537-541>543. && $$ SYNOPSIS...HTS NEAR TERM...HSK/HTS SHORT TERM...HTS LONG TERM...DFH AVIATION...HSK/HTS/DFH MARINE...HSK/HTS/DFH HYDROLOGY...LWX TIDES/COASTAL FLOODING...LWX is the U.S. government's official web portal to all federal, state and local government web resources and services.