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Area Forecast Discussion
Issued by NWS Sterling, VA
Issued by NWS Sterling, VA
510 FXUS61 KLWX 191800 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 200 PM EDT Fri Jul 19 2024 .SYNOPSIS... Dry conditions and lower humidity are expected today as high pressure builds into the region. Shower and thunderstorm chances return Saturday as a weak piece of energy traverses a stalled front draped along the Virginia and North Carolina border. High pressure briefly returns Sunday before a warm front lifts into the region Monday and Tuesday. This will result in daily shower and thunderstorm chances along with increased humidity throughout much of next week. && .NEAR TERM /THROUGH TONIGHT/... Northeast to southeast flow and a mix of cumulus and cirrus are resulting in slightly below normal temperatures and lower humidity across the Mid-Atlantic. A front remains stalled over the Carolinas while high pressure builds from the Ohio River Valley toward New England. Any shower activity looks to be very brief, isolated, and limited to near the higher terrain given weak forcing and dry air. Clouds (especially cirrus) look to linger tonight as a jet streak pivots across and weak low pressure develops to the south. Humidity ticks up slightly with low temperatures near to slightly below normal for mid July. && .SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/... On Saturday, the stalled boundary to our south is expected to meander northward as a shortwave passes overhead. Precip chances will increase from southwest to northeast through the day, bringing some much needed rainfall to the area. 12Z guidance continues to trend down with overall QPF amounts. Cloud cover will keep high temperatures below normal, topping out in the low to mid 80s for most of the area. Rain chances drop off overnight as the shortwave exits and forcing is lost. Low temperatures will be a few degrees cooler than Friday, in the low to mid 60s for most. The front likely remains stalled to our south into Sunday, limiting PoPs to our southern zones. Expect coverage to be reduced compared to Saturday with subsidence on the back end of the exiting shortwave. Reduced cloud cover and southerly flow brings highs back to the upper 80s to low 90s. Clouds build in overnight resulting in milder lows, hovering in the 70s along and east of the I-95 corridor, with mid-upper 60s expected elsewhere. && .LONG TERM /MONDAY THROUGH FRIDAY/... Key Messages: 1) Confidence is high for daily shower and thunderstorm chances throughout the workweek. Localized flood issues are possible especially in areas that see repetitive rounds of rain. 2) Seasonable temperatures and increasing humidity during the extended period. No big changes in the synoptic pattern remain amongst the latest deterministic and ensemble model runs. Still looking at a blocked pattern with upper-level ridging in the western U.S and a slight retrograde westward with the ridge over the Bermuda/central Atlantic. In between lies the upper-level trough and associated stalled frontal boundary which will slowly lift north as warm front Monday before oscillating near the region for the middle part of the workweek. As a result, showers and thunderstorms look to become a mainstay of the forecast, in particular during the afternoon and evening hours. Humidity values will also increase as a result of steady south to southwesterly flow off the Gulf of Mexico aiding in the potential for locally heavy rainfall and drought relief for some mid next week. Any rainfall that we do see is likely to be mostly beneficial, especially along and west of US-15 where severe/extreme drought conditions are being observed. Locally heavy rainfall in the I-95 urban corridor could result in some flooding issues if storms persist over an area. As for severe weather, the threat remains low given the abundant cloud cover from the stalled front nearby. This is highlighted by both the CSU and NSSL machine learning probabilities which suggest a non- zero chance of severe weather during the middle and latter half of next week. By Thursday, the upper-level trough and associated cold front approach the area from the Ohio River Valley and Great Lakes region. Some uncertainty remains in the exact timing and strength of this system. For now, will leave chance PoPs in the forecast through Thursday and perhaps even Friday as models still hold the front up from clearing the area. As for temperatures, expect values closer to seasonable norms for late July. Highs will range from the upper 70s and low 80s over the mountains to mid and upper 80s and low 90s east of the Alleghenies each afternoon. This aligns with the 6-10 Day and 8- 14 Day Temperature Outlooks from the Climate Prediction Center. Overnight lows through the period will fall into the mid to upper 60s and low 70s. && .AVIATION /18Z FRIDAY THROUGH TUESDAY/... VFR conditions are forecast to prevail through the weekend, though some patches of MVFR CIGs are possible mainly near CHO during the early morning hours as they are closer to a front meandering to the south. Weak low pressure along this front may deliver occasional shower activity to the terminals along with brief restrictions Saturday (especially near CHO). Generally weak instability lowers confidence in thunder potential for most. Winds will become SE to S through Saturday, then NW Saturday night through Sunday at 5-10 kts. VFR conditions continue into next week, although the opportunity for sub-VFR remains during the afternoon and evening hours as a warm front lifts into the region. This will lead to increased chances for showers and thunderstorms at the terminals, although confidence remains low on which terminals will be impacted. The highest coverage for thunderstorms appears to be Monday through Wednesday. Winds will will turn southwesterly Monday and Tuesday generally AOB 10 knots. && .MARINE... SE to S winds are forecast through Saturday. Showers and a few thunderstorms are possible Saturday as weak low pressure moves along a front moving up from the south. Winds become NW Sunday, and look to remain below SCA levels through the weekend. Southerly winds kick back into gear over the waters Monday and Tuesday. Sub-SCA level conditions are expected although some channeling could occur especially over the open waters. Stronger southerly channeling is expected Tuesday night into Wednesday, especially over the open and middle waters of the Chesapeake Bay/lower tidal Potomac River. SCAs may be needed during this time. Outside of channeling, expect daily shower and thunderstorm chances each afternoon and evening next week as a stalled front sits nearby. An SMW cannot be ruled out for any potential strong thunderstorms that look to pop up during the afternoon and evening hours. && .TIDES/COASTAL FLOODING... Light easterly to southerly flow through Saturday will lead to an uptick in water levels. Although minor flooding is unlikely, vulnerable shoreline (such as Annapolis) could come close early Saturday morning. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && $$ SYNOPSIS...EST NEAR TERM...DHOF SHORT TERM...DHOF/CAS LONG TERM...EST AVIATION...DHOF/EST MARINE...DHOF/EST TIDES/COASTAL FLOODING...DHOF