Marine Weather and Tide Forecast for Indian Head, MD

KEY MESSAGES
- 1) Severe thunderstorms return Sunday.

- 2) A return to warm and humid conditions and the potential for strong to severe thunderstorms Thursday.

DISCUSSION
KEY MESSAGE 1...Severe thunderstorms return Sunday.

The dry conditions are expected to continue through the night tonight. Overnight low temperatures are forecast to be in the 60s for most, with upper 50s in the mountains.

Conditions will turn more active during the day tomorrow. On the synoptic scale, a shortwave disturbance will rotate around the base of the broad upper low located to our north, tracking through the Ohio Valley. As this shortwave approaches, the low- level mass response ahead of it will draw deeper moisture northward into our area within southerly flow. Dewpoints will climb into the upper 60s to around 70 while height falls simultaneously occur aloft. The net result of the increasing low-level moisture and cooling aloft will be increasing instability. This instability will overlap with strengthening wind fields aloft. The combination of increasing instability, shear, and large scale forcing for ascent ahead of the shortwave and an approaching cold front will lead to the development of thunderstorms during the afternoon.

Model guidance is still showing a fair amount of spread with respect to the finer scale details, but the general consensus is for isolated to scattered thunderstorms to form across the area during the early to mid-afternoon hours as large scale lift associated with the approaching shortwave increases. CAMs suggest that the convective mode may remain primarily cellular, which seems plausible given deep layer shear vectors are west- southwesterly, and have a significant component oriented perpendicular to any prefrontal trough that may assist in initiation. With the cellular mode, the areal coverage of storms may end up being a bit lower than preceding days, which featured considerable upscale growth into linear structures. The background environment will be characterized by around 1000-2000 J/kg of MLCAPE and around 40-50 knots of effective bulk shear, so supercells may be possible. DCAPE values around 1000 J/kg should facilitate the development of strong downdrafts. Damaging winds appear to be the primary threat tomorrow, with gusts of 60-70 mph possible in stronger storms.
Very warm surface temperatures and poor mid-level lapse rates should limit the hail threat somewhat, but an isolated instance or two can't be ruled out, given the possibility for supercells.
The tornado threat also appears to be very low, but non-zero, given predominantly straight hodographs with minimal low-level shear. SPC currently has the entire forecast area outlooked in a Slight Risk, with damaging winds being the primary threat. It should be noted that this is a higher end Slight Risk for locations to the east of the Blue Ridge, with the 30 percent wind contour.

In addition to any storms that develop in the open warm sector during the afternoon, additional showers and thunderstorms are expected to form along the system's cold front in the Ohio Valley tomorrow afternoon. These storms will then track southeastward into our forecast area during the evening hours.
The trend over time should be for these storms to weaken as they encounter a stabilizing boundary layer, but a low-end threat for a thunderstorm or two will continue after dark until the system's cold front clears the area later in the night.

Much cooler, drier, and less humid conditions are forecast for Monday. Mostly sunny skies are expected with high temperatures in the upper 70s to around 80, dewpoints in the upper 40s to 50s, and a northwesterly breeze.

KEY MESSAGE 2...A return to warm and humid conditions and the potential for strong to severe thunderstorms Thursday.

A stray shower or thunderstorm could develop later in the day Tuesday through early Wednesday, mainly east of the Blue Ridge, as a weak warm front slides northward from the Carolinas into Virginia.
Otherwise, average temperatures for mid-June expected Tuesday, while we see temperatures climb a couple of degrees warmer on Wednesday behind the warm front.

This warm front will connect to an approaching low pressure system from the west late Wednesday night and early Thursday. The low will push near the region and drag a cold front across the region through the day Thursday into Thursday evening. The cold front could spawn numerous showers and a couple of strong to severe thunderstorms Thursday afternoon and evening.

The front could drag slowly through the region Friday morning; thus, showers and a few thunderstorms could develop and be mainly across our southern zones into the central Chesapeake Bay.

AVIATION /01Z SUNDAY THROUGH THURSDAY/
Southerly winds and VFR conditions are expected tomorrow morning. Southerly winds increase tomorrow afternoon, and may gust to around 20-25 knots. Thunderstorms are expected to develop across the area tomorrow afternoon. Confidence is currently low on timing at individual locations, so PROB30s for thunderstorms have been introduced at DCA, BWI, and IAD for now.
TEMPOs or prevailing groups will likely be needed as we move closer in time and confidence increases with respect to exact timing. The threat for thunderstorms will gradually wind down through the evening. Winds will shift to out of the northwest behind a cold front tomorrow night. Gusts to around 20-25 knots may be possible at times later tomorrow night within northwesterly flow. VFR conditions and lighter northwesterly winds are expected on Monday.

VFR conditions mainly expected Tuesday through Wednesday night. Any heavy showers or a thunderstorm could briefly reduce conditions to MVFR at the DCA, BWI and MTN terminals. Winds light becoming southeast 5 to 10 knots Tuesday. Winds south 5 to 10 knots Wednesday.

MARINE
Direction of the winds will vary by location, but will generally be out of the west to northwest over the Upper Tidal Potomac, and out of the south over much of the Bay. Southerly winds are expected over the waters tonight through the day tomorrow. SCAs are in effect for all waters tomorrow into tomorrow evening within southerly flow. Thunderstorms will likely impact the waters tomorrow afternoon into tomorrow evening, with SMWs likely being needed. Damaging winds will be the primary threat, but a stray waterspout or instance of large hail can't be ruled out. A cold front will move over the waters tomorrow night, shifting winds around to out of the northwest.
Additional SCAs will likely be needed in northwesterly flow tomorrow night. Winds will remain out of the northwest on Monday, but are expected to drop below SCA levels by mid- morning.

No marine hazards expected Tuesday through Wednesday night.
Winds light becoming southeast 5 to 10 knots Tuesday. Winds south 5 to 10 knots Wednesday.

LWX WATCHES/WARNINGS/ADVISORIES
DC...None.
MD...Small Craft Advisory from 11 AM to 11 PM EDT Sunday for MDZ008.
VA...None.
WV...None.
MARINE...Small Craft Advisory from 11 AM to 11 PM EDT Sunday for ANZ530>543.