Marine Weather and Tide Forecast for Washington, DC

KEY MESSAGES
- 1) Severe weather and flash flooding are possible through tonight.

- 2) Thunderstorm chances return Wednesday night into Thursday.

DISCUSSION
KEY MESSAGE 1...Severe weather and flash flooding are possible through tonight.

Very active weather is expected over the next 12 to 24 hours, with the threat for flash flooding, damaging wind gusts, and a few tornadoes.

As of 10 AM, regional radar shows two well-defined MCVs. The first is centered over southwestern PA, with ongoing showers to the south and east across much of our forecast area. This activity will continue to spread eastward over the next few hours. A second MCV is located across northeastern Indiana and northwestern Ohio. A few thunderstorms are already ongoing ahead of this disturbance across Indiana and Ohio, and additional storms are expected to pop up ahead of it this afternoon, and then move in from the west this evening into the early overnight hours. Questions still remain regarding how much development of storms will occur in-between these two disturbances. As of now, there is isolated activity developing across central Ohio in the clearing between the two systems. Recent HRRR runs try to have this activity dissipate before making it into our forecast area, but that seems questionable given that there's full sun in place ahead of it, which should allow it to move into an increasingly unstable environment. If this activity were to hold together, it would likely make it to the Allegheny Front around 4 PM and then track eastward across the area through the evening. The storms associated with the MCV in IN/OH currently should reach the Alleghenies by around 8 PM, and then eventually clear the bay by around 2 or 3 AM.

Trends will need to be very closely monitored given the very high PW airmass and potential for repeat/training thunderstorms.
Should heavier thunderstorms train over any one area, 3 to 6 inches of rain would be possible on a localized basis. This could cause scattered to numerous instances of flash flooding, with considerable flash flooding possible in urban/poor drainage areas or areas that have seen heavy rain recently. If and when it becomes clearer where the greatest coverage of heavy rain will set up, additional Flood Watches may be issued.

Regarding the severe weather threat, strong low-level winds will boost SRH in a high moisture content environment. Wet downbursts and brief spin ups would be possible in this environment, with the tornado threat maximized along the slowly lifting warm frontal zone - especially if any cell or cluster interacts with/tracks along it. Shear is favorable for right- moving (i.e. NW to SE) supercells if enough instability is realized, and in those cases storms would be more apt to ingest streamwise vorticity along the warm front (or residual outflow).

The wave this evening, though timed a bit after peak heating, may pose the relative highest risk of damaging wind gusts due to increased low-level flow orthogonal to what will probably a linear type feature.

Convection (and corresponding severe/flood threats) may wane only gradually tonight with continued low-level moist advection beneath height falls aloft. This causes instability to remain or even increase through the night.

KEY MESSAGE 2...Thunderstorm chances return Wednesday night into Thursday.

As a cold front slowly moves across our area late Wednesday and Thursday, shower and thunderstorm chances return across most of our forecast region. Friday and Saturday, models continue to depict a stalled front lying in our south, bringing sustained chances for summer-time showers and thunderstorms. Currently, it's unclear if the position of synoptic features and instability will be sufficient for hazardous weather. For now, this will be a feature to monitor as the synoptic setup becomes more defined. Temperatures may fluctuate a little depending on what side of the front we're on, but likely fall some by next weekend, assuming the front pushes to the southeast.

AVIATION /14Z SUNDAY THROUGH THURSDAY/
Active weather with multiple rounds of storms is expected through tonight. The TAFs attempt to hone in on the highest threat windows for TS with PROB30 groups at the moment, but confidence is admittedly low with respect to timing, given the potential for multiple rounds of storms. It should be noted that, given the airmass in place, a shower or thunderstorm could pop up at or near a TAF site at basically any time. Persistent, backbuilding convection is also possible, which could prolong restrictions if/when TS move near or over a TAF site.

South winds through tonight become northwest Monday. Gusts of 15 to 20 knots are possible this afternoon. A few storms could contain gusts of 30-50 kts on an isolated basis through evening.

VFR is expected Monday into Tuesday as high pressure builds, though a pop up shower or storm is possible Monday until a cold front crosses by evening.

Potential showers and thunderstorms re-enter the forecast late Wednesday.

MARINE
Light southeast winds early this morning become south and increase by late morning. Gusts of 15 to 25 knots are expected, and Small Craft Advisories are in effect for all waters through early this evening. Gusts likely taper somewhat over the middle and upper tidal Potomac River, but persist elsewhere through much of the night.

Thunderstorms become increasingly likely through the day, and a few may produce wind gusts of 35 to 50 knots. The storm threat could linger through the night, with even some chances Monday prior to the passage of a cold front that will turn winds to the northwest. Dry weather and light winds are expected Tuesday as high pressure builds.

Winds turn southerly on Wednesday, and channeling may bring marginal SCA conditions along the bay during the evening/overnight. A cold front may bring some thunderstorms on Thursday.

TIDES/COASTAL FLOODING
Southerly winds will increase water levels through tonight.
Vulnerable shoreline such as the Annapolis waterfront could approach minor flood stage during high tide through the night.

LWX WATCHES/WARNINGS/ADVISORIES
DC...None.
MD...Small Craft Advisory until 6 AM EDT Monday for MDZ008.
Flood Watch until noon EDT today for MDZ501-502.
Flood Watch through this evening for MDZ509-510.
VA...None.
WV...Flood Watch until noon EDT today for WVZ050-055-501>506.
MARINE...Small Craft Advisory until 6 AM EDT Monday for ANZ530>534- 537>543.
Small Craft Advisory until 8 PM EDT this evening for ANZ535- 536.