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Area Forecast Discussion
Issued by NWS Sterling, VA
Issued by NWS Sterling, VA
436 FXUS61 KLWX 201903 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 303 PM EDT Sat Jul 20 2024 .SYNOPSIS... Showers and isolated thunderstorms are expected through this evening as a weak piece of energy traverse a stalled front along the Virginia and North Carolina border. High pressure briefly returns Sunday before a warm front lifts into the region Monday and stalls out heading into midweek. This will result in daily shower and thunderstorm chances along with increased humidity and warming temperatures throughout much of next week. && .NEAR TERM /THROUGH TONIGHT/... The early afternoon surface analysis places a stalled wavy frontal boundary over southern Virginia back into eastern Kentucky. Regional radar imagery shows plenty of activity across the state of Virginia. A bulk of the heftier echoes remain south of the local area, generally from the Northern Neck southward. However, recent scans have shown enhancements along a north- south line extending from near Warrenton southward into Spotsylvania County. This appears to be a local area of convergence owing to low-level easterly flow. Surface-based CAPE values average around 1000 J/kg over this area which should help sustain the activity until convergence weakens. Any rainfall should prove to be beneficial given the increasing drought over the region. Elsewhere, a broad conveyor belt of weak echoes continue to track from southwest to northeast. These are all in response to a slow moving shortwave tracking through central Virginia. However, such activity has not amounted to a whole lot given their innocuous nature. The combination of isentropic lift over the southern Virginia frontal boundary coupled with modest instability should keep showers around into the early evening. However, the expansive cloud deck over the area does cast some doubt on the longevity of any convective elements. These overcast skies have also kept temperatures down with current readings locked in the mid 70s to mid 80s. The slow moving trough begins to exit late this evening into the night. While some showers linger over southern Maryland, most of the area begins to dry out into the night. Forecast low temperatures will mainly be in the 60s, locally hitting the low 70s in D.C., Baltimore, and near the milder waters (i.e., tidal Potomac and Chesapeake Bay). Depending on where rain falls, a night of calm winds and decreasing clouds may support some patchy fog. The best chances would be along and west of U.S. 15. && .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... The stalled frontal boundary starts to make some poleward progress on Sunday. This allows temperatures to crank back up into the upper 80s to low 90s. Adding humidity to the mix yields heat indices in the mid 90s. While seasonable in nature, these certainly fall well short of readings over the past few weeks. Subsidence behind the passing shortwave will also afford more sunshine over the area. Some afternoon/evening shower and thunderstorm chances loom across the Potomac Highlands eastward to the Central Virginia Piedmont. The rest of the forecast area is expected to stay dry underneath mostly sunny skies. Heading into Sunday night, slightly milder temperatures are expected with lows around 2 to 4 degrees above the previous night. A longwave trough currently situated over eastern Iowa/western Illinois begins to make its presence felt to start off the new work week. Although the trough itself begins to open up and somewhat shear out, the residual vorticity centers spread across the Mid-Atlantic region on Monday. Ample lift from the forcing aloft coupled with the stalled frontal zone will make for a wetter day as showers and thunderstorms become likely. The current forecast calls for a broad half inch to inch over the area. Given the ongoing drought issues, these rains should be welcome site. Any remote flooding concerns would be tied to the metro areas given poor drainage. Monday`s temperatures should be a bit lower than the previous day owing to more clouds. Most stay in the mid/upper 80s, with 70s across mountain locales. For the night, shower chances begin to decrease with seasonable temperatures expected. && .LONG TERM /TUESDAY THROUGH SATURDAY/... Key Messages: 1) Daily shower and thunderstorm chances continue throughout the workweek. 2) Locally heavy rainfall possible Tuesday and Wednesday. 3) Temperatures near normal for the extended period. Still looking at a stagnant, but active weather pattern as the region remains sandwiched between the broad ridge over the southwest Atlantic and ridge over the Rockies. In between the upper level trough remains with it`s associated pseudo warm/stationary frontal boundary draped just to the north of the region. With that said, a steady supply of deep moisture will continue to advect in from the Gulf of Mexico on south to southwesterly flow. Showers and thunderstorms will be the continued theme of the forecast especially during the afternoon and evening hours Tuesday through Thursday next week. Several pieces of of mid-level shortwave energy traversing the nearby front will be to blame for the increased shower and thunderstorm activity along with the threat for locally heavy rainfall. 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 for an extended period of time. PW values Wednesday and Thursday will range from roughly 1.50 inches across the northern portions of the area to around 2.00 inches across the south. The high end of this range touches the 90 to 97.5 percentile of the 30 year climatology. On Friday, values decrease slightly to the 1.25 to 1.75 inch range. 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 learning machine probabilities which suggest a non zero chance of severe weather during the middle and latter half of next week. Later this week 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 try to push the front through during this timeframe. As for temperatures, expect values closer to average for late July. 850mb temperatures will trend gradually milder through the period running between +16 to +19 through Tuesday through Thursday timeframe. 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.Overnight lows through the period will fall into the mid to upper 60s and low 70s. && .AVIATION /18Z SATURDAY THROUGH THURSDAY/... Outside of KMRB, all terminals have been teetering between VFR to MVFR ceilings given a scattered to broken 2,500-3,000 foot deck. Have opted to TEMPO this MVFR group given the brief lowering of ceilings will likely be tied to passing showers. With that said, showers are expected to be in and around all terminals except KMRB. The threat for thunder is fairly low so have kept any mention out of the TAFs. Southerly winds weaken tonight with a threat for some patchy fog, especially across KCHO and KMRB where some restrictions are possible. A stalled frontal zone to the south returns northward as a warm front on Sunday. With the disturbance aloft passing off to the east, it should be a dry day underneath mostly sunny skies. Some afternoon/evening showers could push as far east as KCHO. It reamins to be seen if any patchy fog is mustered overnight given the dry soils over the region. A better chance for showers and thunderstorms looms for Monday as a shortwave trough moves in from Illinois. Restrictions are possible at times on Monday with winds shifting to more southeasterly in nature. VFR conditions continue into the middle of next week although the opportunity for sub-VFR remains during the afternoon and early evening hours as a front sits near region. This will lead to increased chances for showers and thunderstorms during the afternoon and evening hours at the terminals although confidence remains low onto which terminals will be impacted. Shower/storm activity wanes in the late evening, though a few showers could linger into the overnight each day. && .MARINE... Some of the convective showers popping up within a broad swath of light activity have begun to raise wind gusts to near 18 knots. With that said, Marine Weather Statements have been hoisted across waters through 6 PM this evening. However, any threat for such levels of gusts should wane with the decrease in showers. Background winds remain on the light side through Sunday as gradients remain weak. By Monday, an approaching shortwave trough will interact with a stalled boundary over the area. This promotes a much better chance for showers and thunderstorms. While not obvious whether any of these will be severe in nature, the threat for Special Marine Warnings is non- zero. Any convective chances slowly diminish into Monday night. South to southwesterly flow will continue to increase midweek ahead of an approaching cold front and upper level trough over the Ohio River Valley. This could result in SCA conditions mainly over the central and lower open waters of the Chesapeake Bay as well as lower tidal Potomac. The highest confidence for a Small Craft Advisory appears be Tuesday night into Wednesday due to southerly channeling. 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 (gusts up to 35 kts) that occur during the afternoon and evening hours. && .TIDES/COASTAL FLOODING... Light southeast to south winds this afternoon and evening will continue to result in a small increase in water levels. Although minor flooding is unlikely, vulnerable shoreline (such as Annapolis) could reach Action Stage on Sunday and Monday mornings. Water levels are likely to remain elevated with south to southwest flow persisting through much of next week. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && $$ SYNOPSIS...EST NEAR TERM...BRO SHORT TERM...BRO LONG TERM...EST AVIATION...BRO/EST MARINE...BRO/EST TIDES/COASTAL FLOODING...EST