Marine Weather and Tide Forecast for Alexandria, VA

NEAR TERM /THROUGH TONIGHT/
Current water vapor imagery shows a well defined upper low centered over Mississippi, with its attendant circulation encompassing much of the eastern US. At the surface, an accompanying surface low is located over Mississippi as well, with high pressure in place offshore. Deep moisture is being drawn north to northeastward out of the Eastern Atlantic ahead of the upper low. Warm/moist advection ahead of the upper low is maintaining precipitation across the Carolinas into Southern Virginia. Showers are just starting to reach portions of the Central Shenandoah Valley and Central Virginia, and will continue to slowly progress northeastward late this afternoon into this evening. Showers should begin in the DC Metro late this afternoon/early this evening, and in the Baltimore Metro late this evening.

Most of the rain overnight should stay light, but a few embedded thunderstorms can't be ruled out. Any of these thunderstorms could be capable of producing a quick half inch to an inch of rain in an hour, but coverage of storms is expected to be low. A Flood Watch goes into effect for locations in the vicinity of the Blue Ridge at 2 AM, but most flooding impacts are expected to hold off until Tuesday into Tuesday night. Temperatures will remain mild overnight, with lows generally in the 60s (upper 50s mountains).

SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/
The aforementioned upper low will continue to drift northeastward toward the area tomorrow, and will eventually become centered overhead during the day on Wednesday. A steady rain will continue through much of the morning tomorrow within the zone of large scale ascent downstream of the approaching upper low. As we work into the afternoon hours, a mid-level dry slot may start to work in from the southwest, which may allow for a few breaks of sun and some limited surface heating/development of surface based instability. If this occurs, thunderstorms may start to develop within the dry slot and lift northward into the area during the late afternoon/evening hours. These thunderstorms may be capable of producing heavier rainfall rates, and may potentially lead to some instances of flooding. The earlier issued Flood Watch was expanded to include all of the WV Panhandle, most of Northern Virginia, and Allegany County in Maryland. A general 2-4 inches of rain is expected by late Tuesday night across the watch area, with localized higher totals along the eastern facing slopes of the Blue Ridge and Alleghenies, where upslope southeasterly flow should enhance totals. Further north and east, rainfall totals are expected to be slightly lower, but an instance or two of flooding can't be completely ruled out. The steadier rain should come to an end from southwest to northeast during the second half of the night, with the Flood Watch ending as well late Tuesday night.

The upper low will drift overhead on Wednesday. Some more breaks in the cloud cover are expected to form compared to Tuesday, which should lead to a bit stronger surface heating and a bit more instability. Showers and thunderstorms will form in response to that surface heating, and most locations will experience on and off showers and storms through the afternoon and evening hours. Although there doesn't appear to be a boundary to focus any thunderstorm activity, storm motions will be relatively slow beneath the upper low, and storms will be capable of producing heavy rainfall. Following heavy rains on Monday and Tuesday, some more localized instances of flooding may be possible. High temperatures on Wednesday should generally be in the 70s.

LONG TERM /THURSDAY THROUGH MONDAY/
A continued high amplitude pattern remains in place across the nation. The early week trough is likely to be exiting off the Atlantic coast on Thursday morning. As this occurs, a strong subtropical ridge over the Gulf of America will begin to nudge northward. The period of height rises should be rather short lived as the upstream upper low crosses the Upper Midwest late Thursday into Friday morning. Although the core of lower heights with this system remain confined to latitudes north of the Mason-Dixon Line, a grazing blow from this trough is expected.

The potent nature of this trough is reflected in the 40 to 50 knot westerly winds streaming overhead at 500-mb. By Friday, continued warm/moist advection ushers temperatures well into the 80s, accompanied by dew points reaching the upper 60s to low 70s. This will contribute to decent instability profiles with surface-based CAPEs around 1,500 to 2,000 J/kg. However, some guidance indicate some inhibition the atmosphere may be contending with. The combination of deep-layer shear and ample instability will support a severe thunderstorm risk over the area on Friday into Friday night.
If capping inversions and earlier anvil cirrus do not get in the way, a decent severe episode may unfold.

The broad upper low to the north is forecast to track across the lower Great Lakes by Saturday. This is accompanied by a series of additional shortwaves which race across the Ohio Valley into the Mid- Atlantic region. Thermodynamically speaking, temperatures and dew points drop off by around 3 to 5 degrees on Saturday. Overall instability profiles may not be quite as potent as the previous day.
However, forcing from the trough amidst strong atmospheric flow may afford an additional threat for strong to severe thunderstorms.
Winds begin to turn westerly behind a cold front by Saturday evening. A post-frontal air mass fills out the region into the second half of the weekend. This favors a drier finish on Sunday with highs in the upper 70s to low 80s. The westerly downslope flow also begins to decrease humidity levels heading into the new work week.

By Monday, high pressure builds to the north of the region before another frontal system approaches from the middle of the country.
Based on the global ensembles, the local area will be in between systems as a trough sits offshore of the Eastern Seaboard and another amplifies across the Rockies. Some shower chances should eventually emerge toward the middle of next week.

AVIATION /18Z MONDAY THROUGH SATURDAY/
Conditions are VFR at all terminals except CHO at the moment, but ceilings are expected to rapidly drop this evening as showers move in, with conditions becoming IFR overnight.
Prevailing IFR and rainy conditions will continue through the day Tuesday into Tuesday night, with a thunderstorm also possible, especially during the afternoon and evening. We may see some improvement back to MVFR or even VFR for a time Wednesday, but drops back to IFR ceilings are expected again Wednesday night. An afternoon thunderstorm may be possible once again on Wednesday.

Winds will be light out of the south to southeast, before picking up a bit out of the southeast tomorrow (gusts to around 20 knots during the afternoon). Lighter easterly winds are expected on Wednesday.

On Thursday, mainly VFR conditions are expected although an afternoon/evening risk of thunderstorms could lead to a few restrictions. The better chance for convection lies on Friday and Saturday ahead of a potent frontal system. Strong to severe thunderstorms are possible each day which will support some convection-related restrictions at times. Winds during the period will initially be southerly before shifting to southwesterly by Friday. A cold front brings westerlies back to the area on Saturday with gusts up to 15 to 20 knots.

MARINE
A few low-end SCA gusts may be possible within channeled southerly flow along the main channel of the bay this afternoon.
Elsewhere, sub-SCA level southeasterly winds are expected. The southeasterly winds will start to pick up late tonight into tomorrow, when SCAs are in effect for all waters. A few models indicate that winds may even near low-end Gale levels tomorrow evening. Sub-SCA level easterly winds are expected on Wednesday.
A stray thunderstorm can't be ruled out over the waters this evening through tomorrow evening.

Expect sub-advisory caliber winds on Thursday and Friday with winds shifting from southerly to southwesterly during this period. There will be a risk of thunderstorms each day, particularly on Friday as stronger storms may impact the waters. Special Marine Warnings may be necessary for the more robust convection.

TIDES/COASTAL FLOODING
As a low pressure system slowly approaches through mid week, continued onshore flow will gradually build anomalies. Sensitive shoreline could approach minor tidal flood levels between Tuesday and early Thursday.

LWX WATCHES/WARNINGS/ADVISORIES
DC...None.
MD...Small Craft Advisory from 10 AM to 6 PM EDT Tuesday for MDZ008.
Flood Watch from Tuesday morning through late Tuesday night for MDZ501-502-510.
VA...Flood Watch from Tuesday morning through late Tuesday night for VAZ028-031-050-051-053>056-501-502-505-506-526-527.
Flood Watch from 8 PM EDT this evening through late Tuesday night for VAZ025>027-029-030-036>040-503-504-507-508.
WV...Flood Watch from Tuesday morning through late Tuesday night for WVZ050>053-503-504.
Flood Watch from 8 PM EDT this evening through late Tuesday night for WVZ055-501-502-505-506.
MARINE...Small Craft Advisory from 10 AM to 6 PM EDT Tuesday for ANZ530>532-535-536-538>542.
Small Craft Advisory from 6 AM to 6 PM EDT Tuesday for ANZ533- 534-537-543.