Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 221415 CCA AFDLWX Area Forecast Discussion...CORRECTED National Weather Service Baltimore MD/Washington DC 1015 AM EDT Thu Jun 22 2017 .SYNOPSIS... High pressure will remain west of the area today. 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 /UNTIL 6 PM THIS EVENING/... Early morning showers have beein dissipating, as RAP/HRRR output has been suggesting. Will still have a cluster of low-mid level clouds crossing the forecast area, in addition to the veil of cirrus. Have added more clouds to the database today, which should yield less optimistic skycover forecasts. A bit of warm advection will result in temperatures rising a few degrees over yesterday, so expect lower 90s to be more common. Those aforementioned mid and high clouds could be a limiting factor though. Theta-e advection will also begin this afternoon. Assuming warming/destabilization does occur, around 1000 J/kg of MLCAPE and 1500-2000 J/kg of MUCAPE will be possible, especially over the mountains. Terrain circulations and a lee trough will help initiate showers and thunderstorms this afternoon, perhaps assisted by a little increase in upper level winds. Hi-res models have various solutions on how far east and south the thunderstorms will make it. Have slowed development slightly, but ultimately top out late day PoPs at the same level as my predecessor. Deep layer shear is expected to remain less than 30 kt, so storms should be mostly pulse variety unless a cluster forms along outflow. && .SHORT TERM /6 PM THIS EVENING THROUGH SATURDAY NIGHT/... Overall setup for the influx of tropical moisture remains similar from tonight through early Saturday. There will be two opportunities for a slug of moderate to heavy rainfall (though likely not long duration), with potential for convection in between on Friday afternoon. Lead shortwave/LLJ/isentropic lift will arrive first tonight. Have trended POPs toward the speedier timing of the NAM/WRFs, as these types of events often arrive and depart faster than progged. LLJ will be transporting PWATs of around 2 inches, which will then remain in place. However, overall duration of this rain is on the order of 3-4 hours, so even with efficient rain rates, am thinking flooding will be a low concern with this round other than ponding. This round of rain should be departed by mid morning Friday. Thermodynamic environment remains uncertain heading into Friday afternoon as clouds will be abundant and atmosphere will stabilize some after morning rain. However in the moisture rich environment, some instability is likely to develop. Deep layer shear will also be increasing as upper trough digs to our west. Latest guidance suggests the best chance of storms will be across northern/western portions of the area where convergence may be a little better along with weak mid level impulses. Will have to monitor this time period for severe weather as well as heavy rain rates and flash flooding, though wind fields should be sufficient to keep storms moving. Final opportunity for heavy rain will arrive with the remnants of Cindy and a southward moving cold front late Friday night into Saturday morning. Once again, the forcing will be fast moving, so prolonged heavy rain is unlikely. However, rates may be especially intense given the continued high PWAT and deep warm cloud layers. Given ground may be more saturated in some areas by this point, will have to monitor potential for flash flooding. There is some spread on how quickly the cold front sweeps east on Saturday. There is at least some chance thunderstorms could redevelop east of I-95 during the afternoon. However, dry conditions are expected by Saturday night, along with lower temperatures and humidity. && .LONG TERM /SUNDAY THROUGH WEDNESDAY/... Upper level troughing looks to dominate the region as we head into next week, centered near the Great Lakes. This will position a high pressure in a semi-permanent fashion over the central US. In turn, this will result in a somewhat unusual northwesterly flow at low levels through at least the first half of next week. This will result in slowly cooling temperatures as weak reinforcing shots of cooler air push southeastward into the region, likely accompanied by little precip. Bottom line is that it won`t feel very much like late June. The trough and associated cooler air may move away late in the week as surface high pressure shifts eastward, eventually assuming a more Bermuda-high like position by next weekend. && .AVIATION /14Z THURSDAY THROUGH MONDAY/... Scattered showers and storms may form near the mountains this afternoon. MRB has the highest chance of being affected, though wouldn`t totally rule out something reaching the metros. The first half of the evening will be VFR. Main concern the next couple of days will be periods of heavy rain as tropical moisture flows into the area. One will be late tonight into Friday morning. The second will be late Friday night into Saturday morning. MVFR or lower cigs and vsby will be possible with each. Thunderstorms may form in between these two slugs on Friday afternoon, though coverage and intensity are uncertain. Gradient winds could also gust 20-25 kt Friday afternoon. VFR likely Sunday and Monday. && .MARINE... There`s a very low chance a shower or storm could reach northern parts of the Bay and upper tidal Potomac late this afternoon. Otherwise southwest winds at 10-15 kt will prevail. Wind fields start increasing late tonight ahead of deepening storm system and remnants of Cindy. SCA conditions will become likely by Friday, though a period of moderate to heavy rain could temper winds a bit in the morning. Winds are expected to remain elevated through Saturday as the remnants and a cold front cross the area. High pressure will most likely result in sub-SCA winds Sunday. A weak reinforcing cold front may bring gusts to SCA levels Monday. If so, they will likely subside again Tuesday. && .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, possibly less to the southeast. 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. && .TIDES/COASTAL FLOODING... Water levels a few inches above astronomical normals this morning. These will slowly increase over the next couple of days. There is the potential for minor flooding by Friday at sensitive sites, in the gradient flow ahead of a cold front which would be merged with moisture from the remnants of Cindy. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory from 10 AM to 6 PM EDT Friday for ANZ530- 531-535-536-538>540. Small Craft Advisory from 2 AM to 6 PM EDT Friday for ANZ532>534-537-541>543. && $$ SYNOPSIS...RCM NEAR TERM...HTS/ADS SHORT TERM...ADS LONG TERM...RCM AVIATION...ADS/RCM MARINE...HTS/ADS/RCM HYDROLOGY...ADS TIDES/COASTAL FLOODING...HTS/RCM is the U.S. government's official web portal to all federal, state and local government web resources and services.