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Area Forecast Discussion
Issued by NWS Sterling, VA
Issued by NWS Sterling, VA
838 FXUS61 KLWX 130725 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 325 AM EDT Tue Aug 13 2024 .SYNOPSIS... Tranquil weather conditions are expected through Thursday as surface high pressure resides over the Northeast and Mid- Atlantic states. Precipitation chances return by Friday and into the weekend as a slow moving area of low pressure and series of fronts cross the region. This system eventually exits the coast by early next week. && .NEAR TERM /THROUGH TONIGHT/... A frontal zone initially stretched out over the Carolinas has drifted further south into the southeastern U.S. North of this boundary, high pressure has taken charge as evidenced by light winds across the local area. GOES-16 nighttime satellite imagery depicts an axis of enhanced mid/high clouds pushing eastward out of West Virginia. Regional radar imagery does show some weak returns as light showers attempt to fall out of this deck. The lack of rainfall being observed at upstream personal weather stations suggests much of this is struggling to reach the ground. The layer of clouds overhead has kept temperatures a bit milder than last night. As of 07Z/3 AM, most locations are reporting temperatures in the 60s with a few spotty upper 50s in the cool spots. The usual warm spots are inside D.C. and Baltimore where low/mid 70s are being observed. Heading toward daybreak, some patchy fog cannot be ruled out over the Potomac Highlands if evaporative cooling from upstream echoes can help raise local humidity values. Any such instance of fog should quickly burn off as boundary layer mixing ensues. Northwesterly flow continues in the column thanks to the deep- layer cyclonic pattern. Within this flow regime, multiple disturbances are embedded which should keep a steady stream of clouds over the area. While most high-resolution models are dry, an isolated pop-up shower could fire up over the Blue Ridge Mountains this afternoon. Otherwise, expect a dry day over the region with high temperatures in the upper 70s to low 80s. Northwesterly flow also keeps dew points down which results in much less humidity than mid-August typically offers. High pressure remains a fixture over the area into the night. The resultant light winds and mostly clear skies should make for a decent night of radiational cooling. With that said, the current forecast calls for mid/upper 50s from I-81 westward. Those to the east are likely to see low/mid 60s readings. && .SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/... Behind the conveyor belt of weak to modest impulses passing off to the east, expect decent subsidence over the Mid-Atlantic region on Wednesday. This will result in plenty of sunshine along with a further warm up. Model soundings show deep mixed layers punching into the 800-775 mb layer. In this setup, the nature of the drier air mixing down can often lead to slightly warmer temperatures than models suggest. As such, while mid/upper 80s are currently being advertised, would not be shocked to see a few 90 degree readings on Wednesday afternoon. Another tranquil night lies ahead with seasonable temperatures. Skies stay mainly clear to start off Thursday morning. However, expect increasing clouds through the day ahead of the next weather maker. Depending on the duration of unabated sunshine, many spots could hit the 90 degree mark on Thursday. While a few warm advection showers could reach the Allegheny Front Thursday night, expect a mainly dry period. This shift to southerly flow does slowly increase humidity levels heading toward the end of the work week. Most spots will see overnight lows in the 60s. && .LONG TERM /FRIDAY THROUGH MONDAY/... On Friday, an upper trough will be exiting to the east while brief weak upper ridging builds in over our area. Another trough digs into the Ohio Valley/Mid-Atlantic Friday and into the weekend. The arrival of this second trough is currently progged to coincide with the exit of surface high pressure to the east and a low pressure system moving into the Great Lakes/SE Canada. The last few runs of guidance have trended a bit faster with the arrival of warm frontal precip, and come into more agreement on timing as well. However, there is still significant spread among ensembles, with precip reaching the Alleghenies as early as late Thursday night or as late as Saturday afternoon. That said, the current most likely scenario appears to be precip moving into the Alleghenies by Friday afternoon and expanding to the Blue Ridge and northern Maryland through the evening. Coverage looks to be more scattered with 30% PoPs elsewhere. By Saturday, we`ll have better lift from the approaching trough and ample moisture in the warm sector, so higher coverage of showers and thunderstorms is likely. The cold front is progged to swing through Saturday into Sunday, resulting in more widespread shower activity. Surface high pressure is expected to build in behind the cold front on Monday, but some showers could continue as a trailing shortwave acts on lingering moisture. Temperatures Friday and Saturday will depend on the timing of the warm front, but are currently expected to be in the mid-upper 80s for most of the area. Only a few degrees of cooling is anticipated behind the cold front Sunday into Monday. Lows will be in the 60s and 70s outside of the mountains for the duration of the extended. && .AVIATION /06Z TUESDAY THROUGH SATURDAY/... VFR conditions can be expected at all terminals through Thursday night. Any shower chances likely hold off until late Thursday and be initially confined to the Alleghenies. With high pressure in charge over the region, northwesterly flow persists. Some afternoon gusts could push into the 10 to 15 knot range each day before subsiding after dark. Southerly return flow ensues by Thursday evening ahead of the next system. VFR conditions are expected for most of Friday and Saturday, but showers and thunderstorms could bring brief restrictions to terminals, especially in the afternoon and evening hours. Winds increase out of the south both days, and gusts to around 15 kts are possible in the afternoon. && .MARINE... With high pressure sprawled over the region, expect mainly north to northwesterly winds over the waterways through Thursday afternoon. This is accompanied by a dry forecast. Given deep mixing in the northwesterly flow regime, some afternoon gusts could push into the 10 to 15 knot range. A shift to southerlies is expected by Thursday evening into the night. Some channeling effects could lead to a ramp up to around 15 knots over the southern waters. Winds increase out of the south Friday and Saturday and could approach SCA criteria, especially with the aid of southerly channeling in the afternoons. The open waters in the central and southern Bay are the areas most likely to reach SCA criteria. && .TIDES/COASTAL FLOODING... Tidal anomalies remain low under light northwest flow. However, a few sensitive sites may approach or reach Action stage during the next few late night/early morning high tide cycles, such as Havre de Grace and Annapolis, and possibly DC/SW Waterfront and Alexandria. Winds turn southerly late this week behind a warm front which will result in increasing tidal anomalies. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && $$ SYNOPSIS...BRO NEAR TERM...BRO SHORT TERM...BRO LONG TERM...CAS AVIATION...BRO/CAS MARINE...BRO/CAS TIDES/COASTAL FLOODING...CAS