Marine Weather and Tide Forecast for National Harbor, MD

NEAR TERM /UNTIL 6 PM THIS EVENING/
Daily shower and thunderstorm chances look to continue today although coverage looks to be far less than recent days and mainly confined to portions of the Allegheny Highlands as well as central VA. The pesky stalled front continues to sit in the vicinity of Potomac River. Several ripples of upper level energy continue to traverse this boundary with one exiting early this morning and another ejecting out of western WV and southeast Ohio. This is noted per water vapor imagery this morning along with radar/satellite which shows a subtle swirl down around the Huntington/Parkersburg, WV area. Widespread shower and thunderstorm activity continues in those aforementioned locations early this morning with a decrease in activity across our region as subsidence settles in behind the departing ripple of shortwave energy/jet max. Meanwhile, weak high pressure off the Canadian Maritimes will wedge southward today helping push the front further south into central Virginia and westward into the Allegheny Mountains. Areas south of the boundary will have the greatest concern for widespread shower and thunderstorm activity today along with instances of flash flooding. Locations further north still have a risk at a few showers and thunderstorms although coverage will be far less pronounced especially along and north of I-66/US-50. Low and mid level clouds will continue to prevail leading to mostly cloudy to overcast skies as easterly onshore flow increases. Some wildfire smoke may also be noted below the inversion this morning and as things mix out this afternoon due in part to ongoing wildfires in southern NJ. This may hamper air quality in both the Baltimore/Washington DC metro areas throughout the day where concentrations look to be the highest.

Highs today will be much cooler compared to recent days with many locations struggling to get out of the upper 60s and low to mid 70s, especially north of the boundary where a maritime airmass will take shape. Locations south of the boundary (i.e the southern Shenandoah Valley and VA Piedmont will see highs in the mid to upper 70s to around 80s degrees. These locations have a better bet of seeing filtered breaks of sunshine which will help the atmosphere reload for convective development during the afternoon and evening hours. Current 00z CAM guidance suggest storms along and south of line from Grant Co. WV down toward Staunton/Richmond, VA. Once again looking at multicellular clusters developing between 2-8pm along the boundary before waning after midnight.
Storms will fuel off of MUCAPE values of 500-100 j/kg with 0-6 km shear values under 35 kts. Rain rates of 1-2"/hr remain possible with PWATS still hovering between 1.5 to 2 inches.
Flood Watches may need to be considered for portions of central VA and the Allegheny Highlands later this morning to encompass this threat. Decided to forgo for now given some uncertainty in the placement of the boundary and overall convective coverage.

Showers and thunderstorms will diminish tonight with areas of fog and low clouds persisting. Lows tonight will fall back into the low to mid 60s. A few upper 50s are possible in the river valleys and areas along the PA/MD line as slightly drier air works in.

SHORT TERM /6 PM THIS EVENING THROUGH TUESDAY NIGHT/
High pressure over eastern Canada and northern New England will shunt a stationary boundary south of the region. Onshore maritime flow will continue as a result with multiple pieces of shortwave energy traversing the boundary to the south. The stalled front will remain close enough for increased chances of shower activity especially south of the Baltimore/DC metro areas. Highest shower chances will be during the early morning and late afternoon/evening hours. Once again isolated instances of flash flooding are possible given the antecedent conditions over the last few days. Warm stratiform rain processes will be more of the theme compared to convective due in part to a slightly more stable airmass overhead. Highs Monday will remain in the upper 60s and low to mid 70s with the influence of high pressure along the New England coast still wedging south and the front slowly push back north from central and southern VA.

The stalled surface boundary that will have lingered over the area for several days will finally start to dissipate and lift north of the area as a warm front Tuesday into Wednesday. Meanwhile, a weak upper trough will approach from the Ohio Valley. Showers and thunderstorms are expected to form during the afternoon hours within a very humid airmass (surface dewpoints around 70). High temperatures on Tuesday are expected to be reach into the upper 70s and lower 80s for most.

LONG TERM /WEDNESDAY THROUGH SATURDAY/
High pressure strengthens offshore moving into the middle of the week, leading to south to southwesterly low-level flow and increasing heat and humidity locally. Temperatures should reach into the 90s on both Wednesday and Thursday. When combined with dewpoints in the lower 70s, heat indices may approach 100 on Thursday.
Thunderstorms appear possible both days, but the greater coverage should be on Thursday as an upper trough tracks off to our north across the Great Lakes, with its associated cold front approaching from the west. Current model guidance suggests that ample instability and shear may be present on Thursday, so some storms may be on the stronger side. Upper troughing is expected to dig in from the Great Lakes on Friday. Cooler and drier conditions are expected locally in the wake of the system's cold front heading into next weekend.

AVIATION /08Z SUNDAY THROUGH THURSDAY/
Low clouds, fog, and drizzle continue across the terminals this morning as a stalled front sits nearby. The corridor terminals as of 08z continues to bounce between IFR to LIFR with MVFR TO IFR conditions across the Shenandoah Valley and Alleghenies. IFR to pockets of LIFR cigs and vsbys can be expected through at least 15z amongst the terminals with subtle improvement into the mid-afternoon hours. Onshore flow and a moist marine airmass look to linger throughout the day and into the first part of next week. BKN-OVC MVFR TO IFR cigs are expected this afternoon and evening with the front wavering nearby. Another round of showers and thunderstorms will also follow with the highest concentration of activity south of a line form KEKN to KCHO/KSHD and KRIC.
Storms look to bubble in these aforementioned locations between 19-00z/3-8pm. Areas further north into the corridor terminals will contend with a passing shower or thunderstorm after 20z/4pm. Locally heavy rainfall will be the biggest threat with storms today which will reduce vsbys. Gusty winds also cannot be ruled out along with frequent lightning. Any convection will slowly wane after 00z-04/8pm-12am with passing showers and drizzle/fog leftover during the overnight hours.

IFR/sub-IFR cigs/vsbys are expected across all terminals tonight into Monday morning as onshore flow remains. Winds today will remain light out of the north/east at 5-10kts with occasional guts up to 15 kts along the stalled front to the south and near the waters. Additional periods of sub-VFR conditions are likely to continue for Monday and Tuesday.
Shower and thunderstorm chances will remain during the afternoon and evening hours especially at terminals south and east of KCHO Monday before becoming more scattered areas wide as the front lifts northward Tuesday. Areas further north will remain in a marine airmass with persistent low clouds and areas of fog/low clouds during the overnight/early morning periods.

Sub-VFR conditions may be possible Tuesday morning, but improvement back to prevailing VFR conditions is expected Tuesday afternoon through Wednesday. Temporary drops to sub-VFR conditions may be possible in thunderstorms both Tuesday and Wednesday afternoon.

MARINE

A stalled front will sit south of the waters today through Monday.
Onshore easterly flow will persists with occasional gusts up close to SCA levels this morning and into the afternoon/evening hours.
This is especially true over middle and lower portions of the Chesapeake Bay compared to the sheltered harbors/inlets. Low clouds and some marine fog is possible this morning and again tonight given the moist airmass in place. Less shower and thunderstorm activity is also expected today through Monday given the proximity of the boundary south of the region. Even with that said, an isolated shower or thunderstorm cannot be ruled out during the afternoon and evening hours. Sub-SCA level winds less than 15 kts are expected through Monday.

Sub-SCA level southerly winds are expected over the waters on both Tuesday and Wednesday.SCA conditions look to return Thursday into Friday as a strong cold front crosses the waters.

LWX WATCHES/WARNINGS/ADVISORIES
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...None.