Marine Weather and Tide Forecast for National Harbor, MD

NEAR TERM /THROUGH TONIGHT/
A nearly stationary boundary remains in place over our region this afternoon. Based on wind observations and temperature gradients, the front is located somewhere around the Potomac River and through the DC metro. Clouds remain thick overhead, but low level cloud cover is slowly scouring out along and south of the DC metro. An interesting weatherish phenomenon is occurring in the northern part of our region this afternoon due to a fire in New Jersey. Smoke from the NJ fire has shifted southward into parts of northern MD, with the highest concentration up in NE MD and the Baltimore metro. Smoke is getting locked in near the surface due to a strong stable layer which is not allowing for the smoke to get mixed out. The concentrations of smoke are high enough that they are causing poor air quality with code red air quality alerts being issued by the EPA and MD MDE.

Models have a mesolow forming along the stalled boundary this afternoon creating showers and thunderstorms that will pass through parts of our region late this afternoon and into this evening. Model soundings are indicate a well saturated boundary layer with PW's approaching 2 inches and CAPE hovering between 1000 j/kg and 2000 j/kg. mbE's are expected to be relatively low which could lead to slower storm motions. The combination of high PW's, over 1000 j/kg CAPE, and a saturated boundary layer coupled with the mesolow will lead to environment favorable for efficient rainfall production.
Rain rates within thunderstorms will likely be the 2 to 4 inch per hour range with instantaneous rain rates approaching 6 inches per hour.High rates combined with urban runoff/ a saturated environment will lead to an increased threat for flash flooding.

Rain amounts will likely be between 0.5 and 1.5 inches with reasonable worse case totals approaching the 2 to 4 inch range. I would not be surprised if somewhere observes localized rain total over 4 inches and as high as 6 inches. High end flash flooding will be possible if the higher end rain totals/ rain rates are realized.
A Flood Watch for flash flooding is in effect for the majority of the region east of the I-81 corridor.

In terms of timing and best threat area this afternoon/evening, the HRRR and 3km NAM have been consistently showing a cluster of thunderstorms impacting the DC metro and parts of the Baltimore metro. Models generally have the storms approaching from the northwest, but there are hints that another round could approaching DC from the south. Based on model trends, the window for thunderstorms in the DC and Baltimore metro would likely be between 5 pm and 11 pm this evening with the mostly likely window being between 6pm to 10 pm.

A second area of concern will be parts of central Virginia and eastward where heavy showers and thunderstorms could impact areas that observed rain on Friday. The threat window for this area would be slightly earlier from 4 pm through 9 pm.

SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/

Not much of a change in the forecast as a pesky stalled front remains nearby. The good news is that Sunday appears drier although cloud cover will remain due in part to onshore easterly flow from strengthening high pressure off the New England coast. This high pressure system looks to force the frontal boundary further south into central and southwest VA Sunday morning into SUnday midday.
Subsidence behind shortwave low pressure from Saturday evening will also promote a decrease in PoPs as well for at least 2/3rds of the forecast region. Even with that said, the risk for showers and thunderstorms will still remain from the far western portions of the Alleghenies south and east into central and eastern VA. This lines up with both 00z CAM and synoptic guidance very well with the front subdued to the south as jet max energy squirts east and high pressure wedges in from the north. Once again, flash flooding will be the primary concern with any showers and thunderstorms that develop given the antecedent conditions. Locally damaging winds cannot be ruled out as well with SPC maintains a Day 2 Marginal Risk for portions of the VA Piedmont mainly along and south of I-64.
Highs Sunday will push into the upper 60s and low 70s with easterly flow on the sable side of the boundary. LOcations along and south of I-64 will sit in the mid to upper 70s to around 80 degrees.

The boundary works slowly back to the north Monday bringing continued chances for showers and thunderstorms. Severe weather does not appear to be an issue as stable marine air will be slow to erode over the region. 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. Overall coverage of showers and thunderstorms will be scattered with a focus mainly south and east of metro areas. Convection chances increase again Tuesday as warmer air works back into the region.

LONG TERM /TUESDAY THROUGH SATURDAY/

The surface boundary over the area finally starts to dissipate and lift north of the area come Tuesday, with a weak upper trough approaching 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.

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.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

A stalled boundary over the central part of our region has led to widespread aviation restrictions. IFR ceiling conditions are being observed north of the front toward the MD/PA border. More MVFR ceilings are being observed toward the DC metro, but the low level cloud deck is starting to scouring out. VFR conditions are building into the DC terminals, but lower ceilings are expected to return later this evening. Showers and thunderstorms are likely to impact all terminals with heavy rain leading to visibility restrictions likely in the IFR range and potentially in the LIFR range at times. The main threat window for DCA/IAD/BWI/MTN will between 5pm and 11pm, but based on guidance the most likely threat window will be 6 to 10 pm. Once the thunderstorms and showers exit our region, IFR ceilings are likely to build over all terminals with MVFR visibilities due to drizzle.

Additional periods of sub-VFR conditions are likely to continue for both Sunday and Monday. Shower and thunderstorm chances will remain during the afternoon and evening hours especially at terminals south and east of KCHO. 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. Showers and thunderstorms are forecast to develop each afternoon/evening, and these could result in brief sub-VFR conditions if they move over a terminal.

MARINE

A stalled front will remain draped across the waters through tis evening. Winds today will switch from the south this morning to the north and east later this afternoon into tonight. Some marginal SCA northerly channeling is possible in the northern and middle part of the bay later this afternoon and evening, but confidence is low.
Shower and thunderstorm chances are expected once again later this afternoon and evening (mainly after 20z/4pm). Some storms may warrant Special Marine Warnings with gusty winds and locally heavy rainfall as the primary concerns. Any convection will wane over the waters after 00z/8pm with passing showers, low clouds, and fog leftover overnight. Less shower and thunderstorm activity can be expected Sunday and Monday. Winds will remain out of the east on both days at less than 15 kts.

Winds are forecast to remain below SCA levels through the middle of the week. Daily afternoon showers and thunderstorms could pose a threat to mariners from gusty winds and lightning strikes.

HYDROLOGY

Model soundings and the SPC mesoanalysis are indicating that PW's will be maximized around or above 2 inches. The boundary layer is forecast to be nearly saturated with CAPE values between 1000 and 2000 j/kg. This environment will be favorable for high rain rates with hourly rain rates approaching the 2 to 4 inches per hour with instantaneous rain rates as high 5 to 7 inches. The combination of high rates along with urban runoff/ saturated ground soils will lead to an increased threat for flash flooding later this afternoon and into this evening. A Flood Watch is in effect from 2 pm to 11pm this evening.

LWX WATCHES/WARNINGS/ADVISORIES
DC...Flood Watch until 11 PM EDT this evening for DCZ001.
MD...Air Quality Alert until midnight EDT tonight for MDZ006-008- 011-014-507-508.
Flood Watch until 11 PM EDT this evening for MDZ003>005-013- 014-016>018-502>506.
VA...Flood Watch until 11 PM EDT this evening for VAZ025>031- 036>040-050-051-053>057-501-502-505>508-526-527.
WV...Flood Watch until 11 PM EDT this evening for WVZ051>053.
MARINE...Air Quality Alert until midnight EDT tonight for ANZ530.