![National Oceanic and Atmospheric Administration](/build/images/header/noaa.d87e0251.png)
![National Weather Service](/build/images/header/nws.4e6585d8.png)
![United States Department of Commerce](/build/images/header/doc.b38ba91a.png)
Area Forecast Discussion
Issued by NWS Sterling, VA
Issued by NWS Sterling, VA
172 FXUS61 KLWX 160759 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 359 AM EDT Fri Aug 16 2024 .SYNOPSIS... Today starts with dry weather, then shower and thunderstorm chances return late this evening and overnight as a warm frontal boundary lifts in from the Ohio River Valley. Additional showers and thunderstorms are likely this weekend with a cold front slowly crossing the region. Shower chances will linger into Monday before cool Canadian high pressure builds in by the middle of next week. && .NEAR TERM /THROUGH TONIGHT/... An upper trough fully exits to the east today while the western half of a weak upper ridge moves over the area. A trough digging into the Ohio Valley/Mid-Atlantic will be our weather-maker for the weekend. As surface high pressure slides offshore and the trough and associated low pressure system approach, PoPs will very gradually increase across the area from west to east through Saturday morning. Areas along and east of the Blue Ridge likely remain dry during daylight hours, with areas west seeing isolated to scattered showers and thunderstorms late in the morning through the late afternoon. A lull in shower activity is expected around sunset and through the early evening before a southerly LLJ develops after midnight and allows showers and thunderstorms to increase in coverage and expand east of the Blue Ridge. With areas west of the BR potentially getting several rounds of rain, WPC has this area highlighted with a Marginal risk (1/5) for Excessive Rainfall. Temperature wise, highs today will be in the upper 80s for most, and reaching to near 90 along the I-95 corridor. Lows tonight will be more mild than previous thanks to the increase in cloud cover and strengthening southerly flow, hovering in the upper 60s to low 70s. && .SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/... Saturday will be interesting with the area solidly in the warm sector by the afternoon. Recent high res guidance has continued to slow the system, and actually nearly stalls the cold front over our area for the weekend. With a decaying MCS east of the Alleghenies Friday night/Saturday morning, we`re presented with a problem that our area often is: the previous day`s convection and any lingering cloud debris in the morning/afternoon will be key for convection. The environment appears more favorable with around 1500-2000 J/kg of CAPE during peak heating and 30-40 kts, all with the cold front parking along the I-81 corridor. SPC maintains a Marginal risk (1/5) for along and west of the I-95 corridor Saturday. We will continue to monitor, but currently looks to be a typical "conditional severe" set-up for us. The story will be similar Sunday with very little movement eastward of the cold front meaning the environment won`t change much. If anything, it appears we have more favorable shear as the upper trough swings into the Mid-Atlantic. SPC continues the Marginal into Sunday for I-95 and east. Along with the severe threat, another concern is potential flooding. WPC currently has most of the area outlined with a Marginal risk (1/5) for Excessive Rainfall Saturday, and then adds a Slight (2/5) for the Baltimore metro and northeast. Northeast MD recently got a good amount of rain (before and during Debby), so the signal for repeated rounds of rainfall in this area with the cold front stalled to the west bares monitoring. && .LONG TERM /MONDAY THROUGH THURSDAY/... Models continue to advertise a highly amplified hemispheric wave pattern with a deep upper trough across the eastern U.S., while a broad ridge remains over the Four Corners and Southern Plains. The fact this longwave trough sits over the Eastern Seaboard is what has shielded the coastline from direct impacts from Hurricane Ernesto. The latest National Hurricane Center forecast track keeps this tropical system well offshore (east of 70W). Looking more locally, despite the surface cold front moving off the coast early Monday, the trailing upper trough will keep convective chances around. This comes with a mixed bag of clouds and sun. Forecast high temperatures stay fairly close to mid-August climatology, generally topping out in the mid 80s. For cooler conditions, the mountains will see readings in the low 60s to mid 70s. Most opportunities for showers and thunderstorms should wane after nightfall given the loss of heating. Nighttime conditions turn progressively cooler owing to a strengthening of the northwesterly flow. Low temperatures should range from the upper 50s to mid 60s east of the Blue Ridge, to mainly 50s off to the west. For Tuesday through Thursday, Canadian high pressure brings a pattern shift to the eastern U.S. With surface pressures expected to be around 1024-1026 mb, this is easily 1.5 sigma above average. As expected with such an anomalous anticyclone, temperatures will return to below normal with significantly reduced humidity. On Tuesday and Wednesday, many locations will barely crack the 80 degree mark. This comes with dew points that stay in the low/mid 50s. Strong subsidence within the column favors mainly sunny skies each day. Heading into the night, there should certainly be a bit of a chill in the air, particularly well west of I-95. From the Blue Ridge westward, a number of areas drop into the low 50s with some mid/upper 40s along the Alleghenies. For the I-95 corridor, upper 50s to low 60s are more likely. Heading toward the latter part of the work week, a gradual warming trend is expected. Although global ensemble box-and-whisker plots indicate some temperature spread, the general trends support this forecast. The dry pattern persists through Thursday, perhaps even into Friday if the dome of high pressure lingers another day. && .AVIATION /07Z FRIDAY THROUGH TUESDAY/... VFR conditions during the day today with winds increasing out of the south. Rain chances increase west to east today, but precip is unlikely to reach any of the 6 main terminals until Friday evening or later. In the event showers and thunderstorms do reach any of the terminals, expect brief restrictions. Winds could gust 15-20 kts Friday afternoon, as well as overnight with the development of southerly LLJ. Showers and thunderstorms are expected Saturday behind a warm front and ahead of a slow moving cold front, particularly in the afternoon and evening. Some storms could be strong to severe. Similar to Friday, these could bring restrictions to terminals, and gusts to 15-20 kts are possible outside of thunderstorms before winds diminish overnight. Sunday will be similar with no sensible change in air mass. With the upper trough remaining overhead, the continued threat for showers and thunderstorms may yield some restrictions on Monday afternoon/evening. As the trough exits, winds shift from westerly to northwesterly late Monday. Northwesterly breezes could gust up to 15 to 20 knots on Tuesday, particularly during the afternoon hours. Expect VFR conditions on Tuesday with mainly sunny skies. && .MARINE... Winds increase out of the south on Friday and Small Craft Advisories are in effect Friday night. SCA conditions likely continue through Saturday afternoon and evening, and possibly into Sunday as we remain in persistent southerly flow. Additionally, showers and thunderstorms possible Friday and through the weekend and could bring stronger gusts to the waters. Stronger storms could require Special Marine Warnings on Saturday and Sunday. Given the lingering threat for showers and thunderstorms, conditions may be hazardous at times over the waters on Monday, particularly during the afternoon to evening hours. Most activity decays into the night with winds increasing out of the northwest. A breezy northwesterly wind on Tuesday may require Small Craft Advisories over all waters. However, conditions will be dry, accompanied by plenty of sunshine. && .TIDES/COASTAL FLOODING... Nearly calm winds early this morning has led to a spike in tidal anomalies. Most locations feature such anomalies between 0.4 to 0.6 feet. With southerly warm advection ensuing through the weekend, expect multiple opportunities for coastal flooding during high tide, particularly at more sensitive sites (i.e., Havre de Grace, Annapolis, and D.C. Southwest Waterfront). Based on the latest forecast, the most likely times for Minor flooding would be the early morning high tides (higher of the two astronomical high tides) this weekend. Thereafter, some reduction in water levels is expected by late Sunday into Monday as winds shift to westerly. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...Small Craft Advisory from 6 PM this evening to 6 AM EDT Saturday for MDZ008. VA...None. WV...None. MARINE...Small Craft Advisory from 6 PM this evening to 6 AM EDT Saturday for ANZ530>534-537>543. && $$ SYNOPSIS...DHOF/EST/CAS NEAR TERM...CAS SHORT TERM...CAS LONG TERM...BRO AVIATION...BRO/CAS MARINE...BRO/CAS TIDES/COASTAL FLOODING...BRO