Area Forecast Discussion
Issued by NWS Sterling, VA

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010 FXUS61 KLWX 200129 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 929 PM EDT Mon Aug 19 2019 .SYNOPSIS... Hot and humid conditions will persist through Thursday with daily chances of afternoon and evening thunderstorms. A cold front will cross the area Thursday night with Canadian high pressure building in for the weekend, && .NEAR TERM /THROUGH TUESDAY/... Isolated showers and thunderstorms are still lingering around our area mainly developing due to outflow boundaries and terrain. Expecting thunderstorms activity to continue to diminish in the next few hours. Some fog could again develop over the valleys tonight where skies clear enough. Other than that, warm and muggy tonight with low temperatures in the upper 60s to mid 70s. && .SHORT TERM /TUESDAY NIGHT THROUGH WEDNESDAY NIGHT/... Hot and humid conditions will persist through mid week until cdfnt clears the fcst area late Thu or Thu night. Expect more of the same with showers and thunderstorms each afternoon with Wed being the more active day with a shortwave-trough passing by and more significant height falls. Fcst storm motions are not expected to be particularly very fast, so storms may produce heavy rainfall and isolated instances of flash flooding in addition to damaging winds and hail. Heat indices may approach the heat advisory criteria of 105F east of the Blue Ridge both Tue and Wed. && .LONG TERM /THURSDAY THROUGH MONDAY/... On Thursday, a cold front will approach the area from the Ohio Valley. This front will bring some much needed relief from the heat into the weekend. On Thursday however, with the front coming through in the afternoon, high temperatures probably still reach the upper 80s to low 90s. With dew points in the low 70s too, this will lead to heat indices into the mid to upper 90s, with a few spots approaching 100 in southern MD/central VA. The cold front moving into this environment will certainly result in some thunderstorms across the region, as evident by likely POPs Thursday afternoon. Thunderstorms that do form along the front will have 1000-2000 J/kg of CAPE and around 30kts of deep layer shear to work with. This will mean a threat for severe weather, with damaging winds and large hail being the expected threats. A lot will depend on how quickly the front moves through the area. Obviously, if the front comes through earlier than anticipated, severe threat would be more limited. So, will have to monitor over the coming days for tends in the speed of the front. Models are not very sure at all where the front goes after Thursday. The ECMWF hangs the front up near the forecast area on Friday, with norther parts of the area likely being behind the front, and areas farther south still being in warmer and more humid air. However, the GFS pushes the front out to sea very quickly Thursday night into Friday, resulting in a dry and cooler than average day area-wide. EPS ensembles show some support of both solutions honestly, with the mean hanging the front up in central VA through Saturday. Temperatures across the area will heavily depend on where that front ends up on Friday, with some areas in likely barely in the low 80s, and others closer to the upper 80s farther south. Carrying chance POPs through Friday at least, with best chances farther south. With a stalled front over the area, and a warm and humid airmass to the south, can`t rule out an isolated flooding threat as well, especially if storms train along the frontal boundary. Rain chances decrease into Saturday, but could still see some in southern areas. Even the ECMWF clears the front from the area by Saturday afternoon, so expecting a dry afternoon at the very least. But could not rule out a showers or storms, especially down south, if the front is stubborn to leave. Temperatures on Saturday will be in the low 80s, and with dew points in the low 60s, it will feel much less humid. By Sunday, model guidance all agrees that the front will be long gone. Surface high pressure will be set up to our northeast. Dry conditions expected Sunday, with highs into the mid 80s or so. Again, low dew points will make it feel very comfortable out. && .AVIATION /01Z TUESDAY THROUGH SATURDAY/... Isolated thunderstorms are moving in the vicinity of IAD at this moment but expecting activity to diminish in the next hours to allow for VFR conditions. Some fog may develop later tonight over CHO and MRB, maybe IAD as well. Sct showers/t-storms expected over all terminals Tuesday into Wednesday with boundary over our area. Thunderstorms are expected on Thursday afternoon, in association with a passing cold front. These storms may produce strong winds and some hail. Coverage seems to be pretty high, so all terminals face the risk of restrictions during storms. Other than that, VFR conditions are expected for the rest of the day. More showers and storms possible on Friday, as the cold front could hang up over the region, with the best chance of coverage near CHO. Could see some lower cigs down near the front, as well as showers and storms. && .MARINE... Sct shower/t-storms expected today through Fri. Winds and waves higher near t-storms. SCA conditions possible Wed through Thu night. Generally expecting winds to remain below SCA criteria on Thursday and Friday. Though the central Bay and lower tidal Potomac may get close each afternoon. Additionally, there is a threat for showers and thunderstorms for all waters on Thursday, with potentially severe wind gusts. So Special Marine Warnings may be needed. For Friday, the threat for showers and storms will really be more confined to the central Bay and lower tidal Potomac. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && $$ SYNOPSIS...LFR NEAR TERM...IMR/LFR SHORT TERM...LFR LONG TERM...CJL AVIATION...IMR/LFR/CJL MARINE...LFR/CJL is the U.S. government's official web portal to all federal, state and local government web resources and services.