Marine Weather and Tide Forecast for Ashton-Sandy Spring, MD

KEY MESSAGES
- 1) Increased rain chances, cool temperatures, and abundant cloud cover continue through at least Thursday.

- 2) Dry conditions return later this week and into the weekend.

DISCUSSION
KEY MESSAGE 1...Increased rain chances, cool temperatures, and abundant cloud cover continue through at least Thursday.

Still monitoring what is left of a decaying MCV work across the area this evening. A line of moderate to heavy showers continues to exit east of the Chesapeake Bay over the next hour or so with just a few showers and t-storms leftover the VA/WV Allegheny Highlands where better instability resides. Expect any remnant t-storm activity to settle down through 8pm with a lull into the front half of the evening before more scattered to numerous pockets of showers/rain develop overnight. Low pressure remains back toward eastern Ohio and western PA with a stalled boundary sitting just south of the region.
Meanwhile, 1040 mb high pressure over eastern Quebec will continue to strengthen heading into Tuesday and Wednesday. This will lead to increased onshore flow and a persistent CAD wedge pattern heading into midweek.

Large scale forcing for ascent will gradually increase from west to east tonight into Tuesday as a shortwave trough and embedded upper low approach from the Ohio Valley. As large scale forcing for ascent increases, the coverage of showers will increase substantially through tonight, and likely persist at times through Wednesday morning. The precipitation may even resemble more of a continuous stratiform rain shield as opposed to convective showers given the stability overhead due in part to the CAD. Once again, the coverage and timing of the heaviest rain will be hard to pinpoint given the placement of the shortwave trough as it pivots through.

A few model solutions continue to produce several inches of rainfall over localized areas (along/west of the Alleghenies) as we become positioned in the right entrance region of a strong jet streak developing off to our north and east, but there is lots of model to model inconsistency as to if that will occur and where. As of now, the most likely scenario appears to be for most of the region to receive 1-2 inches of rainfall spread out over a prolonged period time tonight through Wednesday morning. If this were to occur, the rainfall would be beneficial in nature given the ongoing drought conditions. However, if the solutions that produce isolated totals of 3-6 inches were to play out, there could be localized issues with flooding. Even if the solution with higher end totals plays out, instantaneous rainfall rates won't be overly high given the lack of instability, so it would be more of a prolonged soaking rainfall leading to areal flooding, as opposed to thunderstorms producing flash flooding. As of now, no Flood Watches are planned, but we'll continue to monitor.

High confidence remains in regards to low clouds, breezy northeast winds, periods of rain, and cool temperatures continuing through Wednesday. Highs Tuesday will likely hold in the 60s Tuesday with 50s for highs by Wednesday as the CAD peaks. Thursday looks to be the day of transition with high pressure nudging further south allowing slightly drier air to push into the region. Shower chances will likely continue, but should not be as widespread. Highs Thursday will push back into the 60s.

KEY MESSAGE 2...Dry conditions return later this week and into the weekend.

Surface high pressure builds northwest of the forecast area beginning on Friday as a surface low pressure tracks northward off the East Coast. Primarily dry conditions are expected, although a nearby low pressure system brings increased precipitation chances to the eastern portions of the forecast area.

Uncertainty remains on the track and timing of the low pressure system, but recent guidance has it tracking north towards the Northeast away from the Mid-Atlantic, possibly making a cyclonic loop near Long Island. The most likely impact from the nearby system will be increased winds due to the tight pressure gradient overhead. North/northwest winds blow around 15-20 mph Friday and Saturday before decreasing Sunday and into the workweek.

In addition to mainly dry conditions and gusty winds, temperatures gradually warm each day. On Friday, highs will be in the 60s for most (50s mtns) with overnight lows dipping into the 40s and 50s. By the end of the weekend, high temperatures will be in the 70s for most (60s mtns) with most staying in the 50s overnight.

AVIATION /00Z TUESDAY THROUGH SATURDAY/
Ceilings will continue to drop this evening as onshore flow increases with the strengthened high wedging in from the north, stalled front nearby, and area of low pressure approaching from the Ohio River Valley. As of 00z/8pm it's a smattering of MVFR to IFR cigs with LIFR cigs back towards the mountains. FOr now, we remain in a lull with lingering showers over northeast MD and the southern Shenandaoh Valley. Expect this lull through 2-4z/10pm-midnight with shower coverage likely increasing overnight into Tuesday morning. IFR conditions will be the theme of the valid TAF period and really through the middle of the workweek as the onshore flow increases. Periods of light to moderate showers will continue as well likely lingering through at least Thursday morning as a series of low pressure systems push through. Winds will remain out of the northeast through Wednesday, and should gust to around 15-25 knots on Tuesday and Wednesday. Gusts will begin to increase at the corridor terminals prior to 9z/5am Tuesday morning as the gradient tightens.

Northeast winds on Thursday shift to north/northwesterly on Friday, gusting 15 to 20 knots each afternoon. On Friday, metro terminals gust 20 to 25 knots. VFR conditions are expected both days under dry conditions.

MARINE
Expect a lull in the shower activity over the waters the next few hours before an increase heading into early Tuesday morning.
Small Craft Advisories are now in effect with east to northeast winds increasing heading into midweek. Gale Watches are in effect for the waters around southern MD Wednesday into Thursday. We'll see if we need to expand or upgrade this watch to a warning in the subsequent shift given better agreement amongst the guidance potentially kicking in gale conditions as early as Tuesday night over the aforementioned areas. Periods of rain are also expected, which will reduce visibility.
Persistent onshore flow may result in waves as high as 3 to 6 feet in the wider waters of the Chesapeake Bay off southern Maryland by mid week.

Northeast winds on Thursday blow 15 to 25 knots over the waters with gusts up to 35 knots possible. A Gale Watch is in effect until Thursday afternoon for the middle portions of the Chesapeake Bay off southern Maryland. On Friday, north/northwest winds blow 20 to 30 knots over the waters with gusts up to 35 knots possible. Gale conditions are possible over the southern portions of the waters with additional watches possible.

TIDES/COASTAL FLOODING
Tides are expected to snap back to the north, with anomalies rapidly rising again later tonight through the day Tuesday.
Elevated tide levels are then expected through much of the week as a strong area of high pressure builds to our north, leading to persistent onshore flow. The wind direction will be closer to northeasterly rather than easterly, so the tidal levels are expected to increase with southward extent in the Chesapeake Bay. Many locations will likely reach Minor Flood Stage on Tuesday and Wednesday, and Annapolis could come close to flooding tonight as well. Moderate flooding can't be ruled out at some locations, especially at Annapolis by mid week. A coastal low is expected to develop offshore later this week, potentially prolonging coastal flood concerns.

LWX WATCHES/WARNINGS/ADVISORIES
DC...None.
MD...Small Craft Advisory until 6 AM EDT Wednesday for MDZ008.
VA...None.
WV...None.
MARINE...Small Craft Advisory until 6 AM EDT Wednesday for ANZ530>543.
Gale Watch from Wednesday morning through late Wednesday night for ANZ533-537-541-542.
Gale Watch from Wednesday morning through Thursday afternoon for ANZ534-543.