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ANZ535 Tidal Potomac From Key Bridge To Indian Head- 934 Am Est Sat Dec 9 2023
.small craft advisory in effect from Sunday morning through Sunday afternoon...
.gale watch in effect from Sunday evening through Monday afternoon...
Rest of today..S winds 5 kt. Waves less than 1 ft.
Tonight..S winds 5 to 10 kt. Waves 1 ft. A chance of showers.
Sun..S winds 10 to 15 kt. Gusts up to 20 kt... Increasing to 25 kt in the afternoon. Waves 1 ft. Rain. Vsby 1 nm or less.
Sun night..NW winds 10 to 15 kt. Gusts up to 25 kt...increasing to 35 kt after midnight. Waves 1 to 2 ft. Rain.
Mon..NW winds 15 to 20 kt with gusts to 35 kt. Waves 2 ft.
Mon night..W winds 10 to 15 kt. Gusts up to 20 kt in the evening. Waves 1 ft.
Tue..SW winds 5 to 10 kt. Waves 1 ft.
Wed..NW winds 5 to 10 kt. Waves 1 ft.

ANZ500 933 Am Est Sat Dec 9 2023
Synopsis for the tidal potomac and md portion of the chesapeake bay..
broad high pressure will move offshore through tonight. A strong frontal system will impact the region Sunday into Sunday night. High pressure will return early next week. A small craft advisory will likely be needed Monday night.

FXUS61 KLWX 091448 AFDLWX

Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 948 AM EST Sat Dec 9 2023

SYNOPSIS
High pressure will progress offshore to our south and east through tonight. A potent weather system will impact the area on Sunday, with a strong cold front moving through Sunday night.
Low pressure will track off to our north and east on Monday, with high pressure building back in for the middle of next week.

NEAR TERM /THROUGH TONIGHT/
Areas of dense fog remain over far northeast MD this morning. Light east to southeast winds and a strong nocturnal inversion (evident on the 12Z RAOB from IAD) has caused areas of dense fog to remain. This will continue through the morning before gradually burning off. Low clouds may hang around into this afternoon for these areas. Otherwise, no major changes to the today/tonight period and previous discussion follows...

For the rest of today, high pressure will continue to move out into the Atlantic while a potent upper-level trough and its associated cold front move through the Great Lakes, central CONUS, and southern Plains. A southerly flow will allow for a milder afternoon (despite a chilly start from radiational cooling) with highs well into the 50s and 60s for most areas.
There will be more high and mid-level clouds as the southerly flow transports more moisture into the area (note that low clouds may hang around into this afternoon across northeastern MD). A few showers are possible, mainly in the Allegheny and Potomac Highlands/central Shenandoah Valley late this morning and afternoon as a weak disturbance well ahead of the main system moves through the area. Any precipitation will be light.

The upper-level trough will continue to dig as it extends over the Great Lakes into the Gulf Coast states tonight. The southerly flow will strengthen, and this will cause increasing clouds as deeper moisture from the Gulf of Mexico and Atlantic Ocean begin to move into the area. Although most of the dynamics will remain to our west, the strengthening low-level jet along with warm advection will cause some showers to move into the area, mainly overnight. The best chance for showers will be west of Interstate 95. A stationary boundary near the Potomac River will slowly lift north as a warm front late. Low clouds and areas of dense fog are possible to the north of the boundary later tonight.

SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/
Have extended the flood watch a tier west this morning based on RFC headwater guidance for flooding coupled with areas of greatest confidence for more than 2.5 inches of rain. Otherwise, the forecast remains on track. Will have to monitor trends in system evolution wrt snowfall on the backside of the system.
Previous discussion follows...

The potent upper-level trough with northern and southern stream energy will continue to slowly pass through the Great Lakes, Tennessee Valley, and Gulf Coast states Sunday while surface high pressure remains well offshore over the Atlantic. The cold front associated with the trough axis will slowly approach from the west during this time. Most guidance indicates that the northern and southern stream systems will not completely phase, and the system should be more progressive (as more energy dives into the Pacific Northwest). However, it is still a potent system and this can be seen by the strong meridional component to the upper-level trough axis. Therefore, warm advection and moisture advection will be strong ahead of the cold front.
Also, the thermal gradient will be quite strong with subtropical air being pulled into the area ahead of the cold front and a Canadian airmass on the other side of the boundary. A strong low-level jet will also aid in moisture advection as well.
Latest NAEFS continues to indicate pwats to be about 3-4 standard deviations above climo which significant.

Therefore, widespread rain is expected across the area Sunday into Sunday evening and it will be breezy. There is some elevated instability progged in much of the latest guidance, suggesting and embedded thunderstorm is possible with the convection later Sunday morning into Sunday evening. A line of heavier showers and perhaps a thunderstorm will also accompany the frontal passage Sunday afternoon/evening. Given the strong winds aloft, this has the potential to mix down gusty to perhaps even locally damaging wind gusts. One mitigating factor will be the lack of instability that is progged to be rooted within the boundary layer. Therefore, certainty for damaging wind gusts is low at this time (with the best chance for strong to damaging wind gusts east of the blue Ridge Mountains). A soaking rain is expected given the dynamics and moisture associated with this system. Rainfall amounts around 1-3" are most likely, and much of that may fall in a 3 to 6 hour period Sunday afternoon and evening. A Flood Watch is in effect for the Washington and Baltimore Metropolitan areas into portions of northern VA and northeastern MD where the heaviest rainfall is most likely to overlap with relatively lower FFG values due to urban areas.

Colder air will move in behind the front, first later Sunday evening in the Allegheny Highlands and then Sunday night from northwest to southeast across the rest of the area. With the upper-level trough axis lagging behind the frontal passage by a few hours or so, this suggests that there will be some anafrontal characteristics to the boundary. Therefore, moderate precipitation is expected for a few hours or so behind the cold front before drier air eventually works its way in overnight.
There will be enough cold air for rain to change to snow in some areas. As of now, it appears that the best chance for accumulation will be along the ridges of the Blue Ridge, Potomac Highlands, and Allegheny Highlands where cold air will move in sooner. For locations along and west of the Allegheny Front, an upslope component to the low-level flow will cause additional snow showers to last longer behind the cold front, enhancing potential for accumulating snow. Winter weather headlines may be needed for ridges of the Blue Ridge, Potomac Highlands and Allegheny Highlands. There is a nonzero chance for accumulating snow to make it into the valleys and even toward the Washington and Baltimore metro areas. If this precipitation rates on the cold side of the boundary can be heavy enough well into the overnight this would take place. However, the most likely scenario is for the boundary layer to remain above freezing while precipitation rates decrease as low-level drier air moves in toward morning. Will continue to monitor.

The cold front will move off to the east Monday and a northwest flow will usher chilly conditions. There will be a snow showers to start along/west of the Allegheny Front (flurries may spill east) before drier air works its way into the area during the afternoon.

The main story for Monday will be the blustery and gusty northwest winds due to low pressure rapidly intensifying to our northeast (system phases during this time). Frequent gusts around 30 to 45 mph are possible for most areas. Even stronger winds are possible over the ridges and an a Wind Advisory may be warranted. High pressure will build toward the area Monday night. Winds will gradually diminish, but it will be cold with lows in the 20s for most areas (teens in the mountains and portions of the Shenandoah Valley).

LONG TERM /TUESDAY THROUGH FRIDAY/
Surface high pressure builds into the region Tuesday and will be the feature dominating the weather for our area through the remainder of the week, leading to dry conditions. A dry reinforcing cold front may pass through Wednesday/Thursday. The next shot for precipitation doesn't look to approach the area until the weekend, if not early next week.

The extended period starts with slightly below normal temperatures with highs in the mid to upper 40s for most of the area on Tuesday, but gradually moderate to near normal by midweek and continue warming through the end of the week.

AVIATION /15Z SATURDAY THROUGH WEDNESDAY/
Low clouds and dense fog are expected around KMTN and KBWI through the morning before gradually burning off.

VFR conditions are expected for the rest of the area through this evening. However, low clouds may be stubborn to lift around KBWI and KMTN into this afternoon. A southerly flow will usher in more moisture and the flow will strengthen Sunday. Widespread rain is expected with subVFR (IFR at times) cigs/vsbys Sunday.
A strong low-level jet and a warm front lifting through the terminals tonight suggests that low-level wind shear may be an issue around the boundary overnight into early Sunday.

A line of heavier showers and perhaps a thunderstorm will accompany the frontal passage late Sunday afternoon into Sunday evening. Strong winds are possible with this line of heavier activity, especially across the eastern terminals. However, overall confidence remains low due to limited instability within the boundary layer. Rain may end as a few snow showers Sunday night, but chances for accumulating snow are low as temperatures are most likely to remain above freezing.

Gusty northwest winds are expected Monday with drier conditions.
Gusts around 25 to 35 knots are most likely. Winds will gradually diminish Monday night.

VFR conditions are expected Tuesday and Wednesday. Flow out of the S/SW on Tuesday may gust in the afternoon to around 15-20 kt. A dry reinforcing cold front may pass through on Wednesday, in which case winds shift out of the W/NW with similar gusts to Tuesday.

MARINE
Areas of dense fog are likely across the northern Bay and nearby inlets through the morning.

A southerly flow will strengthen tonight through Sunday behind a warm front. An SCA is in effect for portions of the open waters late tonight, and for all the waters Sunday. A line of heavier showers and perhaps a thunderstorm will pass through the waters Sunday evening. This line may produce strong winds.

Gusty northwest winds are expected behind the cold front Sunday night through Monday. A Gale Watch is in effect for the waters.
Winds will gradually diminish Monday night, but an SCA will likely be needed.

Winds over the waters Tuesday are expected to be out of the S/SW and may approach just below SCA criteria in the afternoon and into the overnight. Elevated winds continue into Wednesday as winds shift out of the W/NW with a passing dry cold front.

TIDES/COASTAL FLOODING
Water levels will hover around action stage today with light southerly flow over the waters. Southerly winds increase Sunday ahead of an approaching system and increase tidal anomalies, increasing the chances for minor tidal flooding.

A strong offshore flow will return Sunday night into Monday.

LWX WATCHES/WARNINGS/ADVISORIES
DC...Flood Watch from Sunday afternoon through late Sunday night for DCZ001.
MD...Flood Watch from Sunday afternoon through late Sunday night for MDZ005-006-008-011-013-014-016-503>508.
Dense Fog Advisory until 10 AM EST this morning for MDZ004>006- 008-011-014-503>508.
VA...Flood Watch from Sunday afternoon through late Sunday night for VAZ053>055-502-506-526-527.
WV...None.
MARINE...Small Craft Advisory from 6 AM to 6 PM EST Sunday for ANZ530>532-535-536-538>540-542.
Gale Watch from Sunday evening through Monday afternoon for ANZ530>543.
Small Craft Advisory from 1 AM to 6 PM EST Sunday for ANZ533- 534-537-541-543.

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