Marine Weather and Tide Forecast for Indian Head, MD

NEAR TERM /UNTIL 6 AM SATURDAY MORNING/
Current radar imagery shows numerous showers and a few embedded thunderstorms ongoing, primarily to the west of the Blue Ridge.
This activity is occurring within a zone of weak synoptic scale ascent downstream of a low-amplitude mid-upper level trough over the Ohio Valley. Those showers and storms will continue to spread northeastward through the rest of the night. Most locations will experience on and off showers through much of the night, with a few embedded lightning strikes possible. A few brief heavier downpours may also be possible, given a very deep layer with near saturation, weak instability, and PWAT values nearing 1.5 inches evident on the 00z IAD sounding. Overall, a general quarter to half inch of rain is expected across much of the area, which should be beneficial given ongoing drought conditions. Temperatures will remain mild overnight with lows in 60s (mid- upper 50s mountains).

SHORT TERM /6 AM SATURDAY MORNING THROUGH SUNDAY NIGHT/
The surface low will be moving into the St. Lawrence Valley by Saturday morning. A cold front will trail southwestward along the Appalachian Mountains. Aloft, a compact shortwave/closed low will be moving through the Great Lakes, while a more subtle shortwave accompanies the surface front. Scattered showers will likely be occurring along and ahead of the front as it moves eastward during the morning. The extent of these showers and low cloud cover could play a role in destabilization ahead of the cold front. However, most guidance does have some instability available along and east of I-95 during the afternoon. Wind fields aren't terribly strong, but around 30 kt of deep shear may aid in storm organization for anything that develops. The main threat with any stronger storms will be gusty winds, with CAMs focusing on southern Maryland for the greatest potential of these stronger storms. Highs should make it back into the 70s for most, with 80s most likely in central Virginia and southern Maryland where some thinning of clouds may occur.

Showers and storms will depart to the southeast during the evening. The most potent part of the northern trough will remain north of the area, so any light lingering upslope precipitation in the Alleghenies should wind down before colder temperatures arrive. Lows will be in the 40s, with 30s in the higher elevations.

The other aspect of this cold front will be the gusty westerly winds in its wake. Despite the setting sun, winds may increase into Saturday evening due to cold advection and strong pressure rises. Most gusts will be 20-35 mph but higher on the ridge tops. While winds may decrease the second half of the night, especially in sheltered areas, expect them to increase again after sunrise Sunday. While cooler, highs in the mid 60s to lower 70s on Sunday will be near to a few degrees below normal.

Winds rapidly diminish Sunday evening as high pressure builds in from the northwest, and skies should be clear. This will provide favorable radiational cooling conditions with temperatures falling into the upper 30s to mid 40s (warmer in urban and bayshore areas). Dew points have been trended downward however, which could present an opportunity for frost, in particular across the Shenandoah valley, but potentially in colder spots across northern Maryland as well. Growing season for the far western zones starts May 1.

LONG TERM /MONDAY THROUGH FRIDAY/
Surface high atop the area Monday quickly slides offshore of the Carolinas Monday night as a strong upper trough approaches from the west. The main upper trough is forecast to slide across the Great Lakes into Ontario, though its trailing cold front moves south into the OH Valley Tuesday, then across the Mid-Atlantic Wednesday. There could be a chance for strong to severe thunderstorms as the front moves through the Potomac Highlands on Tuesday, then possibly along and south of US-50 on Wednesday. Temperatures each day are forecast to be well above normal, in the mid to upper 80s on Tuesday and upper 70s to mid 80s on Wednesday.

Taking a closer look, Tuesday looks to start dry as sharp mid-level ridging and residual surface air cap convection during the first half of the day. As the mid-level ridge moves east some slight height falls Tuesday evening accompanied by modest instability ahead of the front could support some strong to severe thunderstorms in the Potomac Highlands. Shear looks to be marginal at 30-35 knots in mostly zonal flow aloft, but enough to support some organized convection. East of the Alleghenies storms could struggle with the loss of daytime heating and increasing stability/capping.

The front moves south into our area Tuesday night into Wednesday morning where it flattens out and slows down around US-50. However, the exact location of the frontal surface will be key in whether severe storms are possible Wednesday afternoon/evening. Abundant cloud cover in stable northwest winds to the north of the boundary likely prevent any storms from developing. To the south of the boundary, ample instability and moisture with slightly better deep- layer shear profiles (35-40 kt) could support more robust convection. Based on the current 12Z guidance this puts portions of Central VA and far southern MD with the better chances for some strong to severe storms. Something to keep watching in the days ahead.

A reinforcing trough pushes the nearly stalled cold front south of our area Wednesday night. Still, shower and thunderstorm chances will remain in the forecast through the end of the week due to another area of low pressure approaching from the Mid-South on Thursday. Its associated cold front moves across the Mid-Atlantic on Friday. Highs remain in the 70s to low 80s each day.

AVIATION /01Z SATURDAY THROUGH WEDNESDAY/
Cloud bases continue to lower as a system approaches from the west. A more substantial increase in shower coverage is expected later this evening, which will likely bring some periods of MVFR conditions to the area TAF sites. While a few thunderstorms may affect the metro areas this evening, still think the chance is too low to include in the TAFs. There may be a relative lull in shower chances late tonight into Saturday morning.

Ceiling forecasts are rather uncertain tonight, perhaps contingent on the placement of the advancing surface warm front.
In fact, there could be a tight gradient between areas that are VFR compared to IFR. For now, limited IFR conditions to MTN, with MVFR at MRB/IAD/DCA/BWI. Lower ceilings will gradually lift by midday or early afternoon Saturday. After further evaluation, LLWS in previous TAFs appeared unlikely, so removed it from the TAFs.

A cold front will work across the area Saturday. Potentially light showers during the morning may take an increasingly convective nature by midday, especially moving southeast from BWI/DCA. With coverage of thunder uncertain, have included PROB30 for thunder for the metro TAFs and heavier convective showers at MRB/CHO. Activity should be southeast of the TAF sites by 22 or 23Z.

In the wake of the front, winds shift to the west and northwest.
A surge of gusts 25-35 kt may occur during the evening, with renewal of 20-30 kt gusts after sunrise Sunday. VFR conditions prevail however, and winds will rapidly diminish Sunday evening.

VFR conditions continue through the start of next week. Showers and thunderstorms return to the forecast for Tuesday evening, though coverage looks to be low at the terminals at this time. Still, something to watch as a storm or two could be strong (again chances are low at this time). Southerly winds will increase Tuesday into Tuesday night and possibly gust to around 20 knots.

MARINE
South to southeasterly winds continue to produce spotty advisory level conditions this evening. The intermittent and spotty nature of the gusts could continue through the night.
Because of this potential, Small Craft Advisories are in place across all waters through early evening, continuing everywhere outside the upper tidal Potomac into tonight.

Showers and isolated thunderstorms move through tonight. As a cold front approaches Saturday afternoon, some stronger thunderstorms may develop which require Special Marine Warnings for gusty winds. Behind the cold front, an increasingly gusty west to northwest wind overspreads the waters. This will bring gusts to around 25 to 30 knots, which may include occasional gale force winds at times. The most likely time for this will be in the post frontal surge, but some potential may linger along the bay until Sunday morning. Due to uncertainties in mixing depth, have only issued Small Craft Advisories at this time, but upgrades may eventually be warranted. Small Craft Advisories will likely be needed into Sunday afternoon. Winds drop off pretty quickly Sunday evening given the influence from Canadian high pressure.

Marine conditions look to be favorable Monday as high pressure over the area moves offshore of the Carolinas. As that happens southerly winds increase Tuesday morning into Tuesday night, with SCA conditions possible over most of the waters.

TIDES/COASTAL FLOODING
Tidal anomalies currently range from 0.25 to 0.50 feet this afternoon. There will be some uptick in water levels ahead of a cold front that tracks through Saturday afternoon. The current forecast brings Annapolis and Baltimore into Action stage, but all others remain below. Behind the front, a west to northwesterly wind will quickly lower anomalies across the area.
No tidal flooding is expected on Sunday into Monday.

LWX WATCHES/WARNINGS/ADVISORIES
DC...None.
MD...Small Craft Advisory until 6 AM EDT Sunday for MDZ008.
VA...None.
WV...None.
MARINE...Small Craft Advisory until 6 AM EDT Sunday for ANZ530>534- 536>543.
Small Craft Advisory from 11 AM Saturday to 6 AM EDT Sunday for ANZ535.