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FXUS61 KLWX 040126 AFDLWX

Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 926 PM EDT Tue Oct 3 2023

SYNOPSIS
High pressure will continue to bring dry conditions and warm temperatures throughout much of the eastern U.S through Thursday. A strong cold front will cross the region late Friday into Saturday bringing renewed chances for showers and thunderstorms. Crisp and cool temperatures will follow the front with high pressure returning Sunday into early next week.

NEAR TERM /THROUGH WEDNESDAY/
Skies have cleared this evening, with mostly calm winds across the area. Patchy river/valley fog is likely once again tonight.
Lows drop into the 50s for most locations with low to mid 60s in the metros and directly along the tidal Potomac/Chesapeake Bay.

SHORT TERM /WEDNESDAY NIGHT THROUGH THURSDAY NIGHT/
Surface high pressure and upper level ridging aloft will remain overhead through Wednesday before shifting offshore Thursday as the upper level trough over the mid-Mississippi River Valley pushes eastward toward the region. Dry conditions will remain throughout the midweek period although clouds will steadily increase Thursday as moisture builds on south to southeasterly return flow. Warm temperatures will continue as well with the latest NAEFS/EPS output for 850 mb temperatures running between +14 and +18 degrees C Wednesday afternoon. High temperatures in many locations as result will surge into the low to mid 80s with even a few upper 80s possible across the metros/southern portions of the forecast area.
By Thursday, 850 mb temperatures cool ever so slightly toward the +13 to +16 degree C range yielding highs in the mid to upper 70s and low 80s under increased cloud cover.

Outside of an isolated shower or thunderstorm over the western mountains Thursday afternoon and evening, a majority of the midweek period stays precipitation free. It's not until late Thursday night into Friday when shower chances increase from west to east across the area. The bulk of any shower or thunderstorm activity will hold off until Friday as the cold front pushes through.

LONG TERM /FRIDAY THROUGH TUESDAY/
High pressure will begin to break down late in the week as an upper trough continues to progress further east from the Ohio Valley, particularly Friday into Saturday. By Friday, increasing clouds and some low-level moisture will creep into the Mid-Atlantic. Some showers may be possible, especially later in the day but coverage and intensity should be fairly limited. Highs will top out in the mid to upper 70s for most of the lower elevations, with 60s for the mountains. Overnight lows will dip down into the 40s across the Allegheny Front with a gradual increase the further east you go towards the waters where low 60s will be common.

A cold front likely crosses the region sometime late Friday into early Saturday. Ensemble guidance continues to fluctuate with respect to timing and intensity of the cold front passage across the Mid-Atlantic. Showers will be common through the period with maybe even a few rumbles of thunder possible. Some individual ensemble members suggest SBCAPE values creeping into the 600 to 800 j/kg range and that coupled with nearly 60 knots of bulk shear would be enough to cause some potential hazardous weather with respect to a QLCS setup. Luckily for now, this solution's probabilities appear fairly low at this point but it will be worth monitoring over the next several days.

Behind the front, the main concerns will be the winds and the shift in temperatures. For the winds, gusty northwest winds will build in across the area post-frontally sometime Friday into Saturday. Some medium-range guidance has near Wind Advisory criteria, especially for some of the higher elevations where the gradient may be a bit tighter. These increased winds should begin to dissipate Monday into Tuesday. For temperatures for the weekend, conditions may still be relatively mild for this time of year on Saturday but as the northwesterly flow kicks in there will be many areas that could see some of their coldest temperatures in 6 months. Areas along the Allegheny Front may see temperatures getting closer to freezing. By Sunday morning, if any lingering low-level moisture is coupled with the near freezing temperatures then some rain-snow mix may be possible at the highest elevation but overall accumulation looks to be little to none. Conditions improve into the beginning of next work week as high pressure builds in from the Deep South.

AVIATION /01Z WEDNESDAY THROUGH SUNDAY/
Patchy fog is possible late tonight at MRB and CHO, some of which could be dense and reduce visibility to IFR conditions.
Given the dry air in place, do not expected widespread fog, and any fog that does develop could be transient with impacts lasting for short periods of time. Any fog that develops will burn off between 12Z-13Z Wednesday morning.

VFR conditions look to continue through Wednesday with high pressure nearby. High pressure slides offshore Thursday allowing for south to southeasterly onshore flow to ensue. This will result in increased cloud cover Thursday afternoon into Thursday night as the approaching upper level trough/cold front advance toward the region. Sub-VFR conditions could return with low clouds at terminals east of the Blue Ridge as early as Thursday night.

Sub-VFR conditions are possible on Friday. A cold front likely brings showers Friday night into Saturday. Sub-VFR conditions continue Saturday with maybe a rumble of thunder possible along with gusty winds behind the frontal passage late Saturday and into early Sunday.

MARINE
Sub-SCA level winds will remain over the waters through Thursday. Winds will generally remain light and variable today/Wednesday before switching to the southeast Thursday at less than 10 kts.

Winds will increase Friday ahead of a cold front passage Friday into Saturday. SCA conditions continue throughout the day and into Saturday. There is an outside threat for gale-force winds for the waters, especially further south. No marine headlines are currently in effect but some may be needed towards the end of the week and into next weekend.

TIDES/COASTAL FLOODING
Anomalies continue to hold steady between 0.75-1.2 feet this afternoon. Caution stages have been observed at vulnerable locations such as DC Waterfront and Annapolis during the recent high tide cycle. Minor flooding remains possible at sensitive locations this evening although confidence remains low to if the threshold will be met. This is largely due in part to light and variable winds over the region with high pressure nearby. The highest confidence for minor flooding appears to be at Straits Point per the latest 12z guidance with slightly lower confidence at both Annapolis and DC Waterfront. For now, have held off of any Coastal Flood Advisories as we await to see how the anomalies bounce back after the concurrent high tide cycle.

Water levels may decline somewhat in the middle of the week as light/variable winds allow some water to flow out of the bay, and astronomical tides decrease. However, increased SE flow late in the week could renew a risk for spotty minor flooding.

LWX WATCHES/WARNINGS/ADVISORIES
DC...Coastal Flood Advisory until 2 AM EDT Wednesday for DCZ001.
MD...Coastal Flood Advisory until midnight EDT tonight for MDZ014.
VA...None.
WV...None.
MARINE...None.

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