Marine Weather and Tide Forecast for Severna Park, MD

NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/
An upper low centered off to our southwest over Kentucky and Ohio continues to slowly drift northeastward toward our area.
Stratiform rain (mostly light to moderate in nature) has continued through much of the day within a zone of large scale ascent ahead of the approaching upper low as low-level moisture continues to be transported north and eastward into our area.
Much of the forecast area has received 1-2 inches of rainfall since last night, with northeast Maryland receiving a bit less.
Rainfall has been enhanced along the east facing slopes of the Blue Ridge and Alleghenies within upslope flow, with a strong southeasterly low-level jet intersecting the terrain. Heavy rainfall has been ongoing through much of the day in those locations, leading to several instances of flooding. The hardest hit location has been southwestern Allegany County, where Georges Creek is at its second highest ever recorded level and still rising (just short of Major Flood stage). A Flash Flood Emergency is in effect for southwestern Allegany County.

While stratiform rain is ongoing across much of the forecast area, the system's mid-level dry slot is starting to work into southwestern portions of the forecast area. As the mid-high level clouds move out, those locations are experiencing some breaks of sun, which has led to subsequent development of instability and then thunderstorms. These thunderstorms are just working into Nelson County, and could impact portions of the Shenandoah Valley and Central Virginia over the next few hours, potentially leading to some isolated instances of flooding. A Flood Watch remains in effect for the bulk of the forecast area through tonight. While some light rain is still ongoing across Western Maryland, the heaviest rain has come to an end as the low- level jet (and the upslope enhancement in rainfall rates it caused) starts to lift to the north out of the area.

Showers will continue on and off through the night, before eventually winding down from southwest to northeast during the second half of the night. Fog may be possible during the second half of the night, especially across southwestern portions of the forecast area where the rain should exit earlier.

SHORT TERM /6 AM WEDNESDAY MORNING THROUGH THURSDAY NIGHT/
The upper level low will begin opening as it approaches Wednesday. While cloud breaks might be limited, especially across the northern half of the area, some low level drying may begin to take place as the steadier showers pivot off to the north. Instability will build in this area to the southwest, and with little inhibition, widespread showers and scattered thunderstorms are expected to develop by afternoon. This activity should gradually spread northeastward, but perhaps with lower intensity as it encounters weakening instability. Shear will be marginal, around 20-30 kt, but this could support a few stronger storms with gusty winds being the primary threat. Storm motions will also be somewhat slow with unidirectional wind profiles. Precipitable water values will be lower, but isolated flooding will be possible with any heavier convection that moves over sensitive areas from previous rainfall. Temperatures should push into the 70s for most areas, warmest in the southwest. Showers will gradually lift to the northeast Wednesday night.

The weakening trough axis will be overhead Thursday. More breaks of sun are likely, which will allow temperatures to rise toward 80. While forcing is somewhat nebulous, dew points will still be well into the 60s, so at least some scattered showers and thunderstorms will be possible during the afternoon. Late Thursday night, eyes will turn to the northwest as some guidance has remnant convection along a warm front moving toward the area. If this does occur, storms may be maintained by steepening lapse rates aloft and elevated instability.



LONG TERM /FRIDAY THROUGH TUESDAY/
An active period continues through the first half of this weekend.
The bigger change on Friday and Saturday will be the notable uptick in heat and humidity levels. A subtropical ridge centered over the Gulf of America is expected to flex northward on Friday which will help raise heights by around 3 to 5 dm. The current forecast package calls for high temperatures in the low/mid 80s, coupled with dew points in the mid 60s to near 70 degrees. This setup would yield a more favorable thermodynamic environment as surface-based CAPE values rise to around 2,000 to 3,000 J/kg. Some of the more robust solutions would carry instability to near 4,000 J/kg, although these appear to be driven by a low-level moisture bias. Given vertical shear in the 30 to 40 knot range, this instability/shear combination is more than sufficient for area-wide severe weather. However, there are question marks in this setup given some potential capping issues as well as contamination from previous upstream convection. Given this system is still 72 hours away, details will likely become revealed in the next day or so as convective-allowing models come into play.

The closed low/upper trough initially over the Upper Midwest will track eastward to the lower Great Lakes by Saturday evening. In response to this trough, heights are forecast to slowly fall offering a grazing blow to the local area. Like the preceding day, conditions remain warm with temperatures in the mid/upper 80s, but with a bit of a decrease in moisture levels. This would be slightly less optimal for convective development. However, forcing is more focused so Saturday could present a threat for additional strong to severe thunderstorms. The cold front attached to this system tracks through the Mid-Atlantic region by Saturday evening. This boundary eventually shifts winds over to westerly into the night and Sunday.

A more tranquil pattern is anticipated for the second half of the weekend and into early portions of next week. High pressure builds over Ontario and extends southward into the Great Lakes and Ohio Valley. This yields a gradual drop in daily temperatures as well as dew points returning to the 50s. The next system to monitor is another very amplified trough that tracks across the central U.S.
during the middle of next week. The associated warm front begins to approach the local area from the south by Tuesday. Consequently, this would also bring the next opportunity for any organized precipitation.

AVIATION /19Z TUESDAY THROUGH SUNDAY/
Low-end MVFR to high-end IFR conditions and showers continue at the terminals this afternoon. All terminals are expected to drop to IFR/LIFR tonight, with rain showers continuing on and off through much of the night. A brief thunderstorm also can't be ruled out this evening. PROB30 groups were maintained to account for that possibility. Winds are gusty out of the east this afternoon, with many sites intermittently gusting to around 20-30 knots. These gusty winds should continue through the first half of the night, before decreasing in magnitude during the second half of the night. Fog may also be possible later tonight, especially at CHO. Showers should move out of the area from southwest to northeast during the second half of the night.

IFR conditions are expected to start the day tomorrow, with ceilings gradually lifting to MVFR and potentially even VFR during the afternoon hours. Conditions should stay dry through the morning, but showers and thunderstorms will form during the afternoon, and some could potentially impact the terminals.
PROB30 groups were added tomorrow afternoon at IAD, DCA, and BWI to account for this potential. Primarily VFR conditions are expected on Thursday, but an afternoon shower or thunderstorm can't be ruled out.

Friday and Saturday each bring the potential of strong to severe convection as a cold front approaches from the west. Restrictions are looking likely at times, particularly during the afternoon to evening hours. The typical hazards of gusty/erratic winds, heavy downpours, and frequent lightning would accompany many of these storms. The cold front pushes through the terminals by Saturday evening which shifts winds from southwesterly to westerly. Gusts up to 20 knots are possible as this occurs. VFR conditions are expected for Sunday with westerly gusts in the afternoon to around 20 knots.

MARINE
High end SCA conditions continue over the waters this afternoon, with gusts out of the east to southeast at around 25-30 knots.
Winds will increase further this evening, with Gale conditions expected over Chesapeake Bay and the Lower Tidal Potomac this evening. Winds should decrease back to SCA levels for the second half of the night. SCA gusts may linger over northern portions of the Bay through tomorrow morning, but elsewhere sub-SCA gusts are expected tomorrow. Light southerly winds are expected over the waters tomorrow night through Thursday.

A strong system will bring a threat for strong to severe thunderstorms on both Friday and Saturday. This may necessitate some Special Marine Warnings for some of these cells. Initial south- southwesterly winds eventually shift to westerly by Saturday evening as the cold front moves through. The current forecast does keep background winds below advisory thresholds. However, the bigger issue would be the thunderstorm-related wind fields.

TIDES/COASTAL FLOODING
As a low pressure system slowly approaches through mid week, continued onshore flow will gradually build anomalies. Sensitive shoreline could approach or reach minor tidal flood levels especially with the Wednesday and Thursday morning high tide cycles.

LWX WATCHES/WARNINGS/ADVISORIES
DC...Flood Watch through late tonight for DCZ001.
MD...Small Craft Advisory until 6 PM EDT this evening for MDZ008.
Small Craft Advisory from 2 AM to noon EDT Wednesday for MDZ008.
Gale Warning from 6 PM this evening to 2 AM EDT Wednesday for MDZ008.
Flood Watch through late tonight for MDZ003-004-501>504-510.
VA...Flood Watch through late tonight for VAZ028-031-050-051- 053>057-501-502-505-506-526-527.
Flood Watch until 2 AM EDT Wednesday for VAZ025>027-029-030- 036>040-503-504-507-508.
WV...Flood Watch through late tonight for WVZ050>053-503-504.
Flood Watch until 2 AM EDT Wednesday for WVZ055-501-502-505- 506.
MARINE...Small Craft Advisory until 6 PM EDT this evening for ANZ530>534-537>543.
Small Craft Advisory from 2 AM to noon EDT Wednesday for ANZ530-531-538-539.
Gale Warning from 6 PM this evening to 2 AM EDT Wednesday for ANZ530>534-537>543.
Small Craft Advisory from 2 AM to 6 AM EDT Wednesday for ANZ532>534-537-540>543.
Small Craft Advisory until 4 AM EDT Wednesday for ANZ535-536.