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ANZ536 Tidal Potomac From Indian Head To Cobb Island- 1233 Am Est Sun Dec 10 2023
.small craft advisory in effect from 6 am est this morning through this afternoon...
.gale warning in effect from this evening through Monday afternoon...
Overnight..S winds 10 kt. Waves 1 ft. A chance of rain.
Sun..S winds 5 to 10 kt. Waves 1 ft. Rain. Vsby 1 to 3 nm.
Sun night..NW winds 15 to 20 kt. Gusts up to 25 kt...increasing to 30 kt after midnight. Waves 2 ft. Rain. Vsby 1 nm or less.
Mon..NW winds 15 to 20 kt with gusts to 30 kt. Waves 2 ft.
Mon night..NW winds 5 to 10 kt. Waves 1 ft.
Tue..SW winds 5 to 10 kt. Waves 1 ft.
Tue night..SW winds 5 to 10 kt. Waves 1 ft.
Wed..NW winds 5 to 10 kt. Waves 1 ft.
Thu..W winds 5 to 10 kt. Waves 1 ft.

ANZ500 1233 Am Est Sun Dec 10 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 waters Sunday into Monday. Periods of gales are likely during this time. High pressure will return Tuesday through Thursday next week. A small craft advisory will likely be needed Monday night.

FXUS61 KLWX 100228 AAA AFDLWX

Area Forecast Discussion...UPDATED National Weather Service Baltimore MD/Washington DC 928 PM EST Sat Dec 9 2023

SYNOPSIS
A strong cold front and potent area of low pressure will impact the area Sunday into Monday morning bringing heavy rain, gusty winds, and mountain snow. The potent weather system pulls to the north and east Monday with high pressure building in for the middle of the workweek.

NEAR TERM /THROUGH SUNDAY/

Latest satellite imagery shows low clouds advancing quickly northward this evening, faster than indicated by some of the guidance. Temperatures have likely reached their minimum tonight and will likely remain steady or rise overnight. Overall, the forecast hasn't changed much other than the probability of exceeeding 2 inches of rain has come down since yesterday and is now focused more to the south reducing the risk of flooding.
Severe wx threat looks almost negligible, although an isold elevated t-storm can't be completely ruled out. The strongest winds look to be in the post-frontal environment in the 06Z-18Z Monday time frame.

Previous afd...

The deep upper level trough and associated strong cold frontal boundary will continue to trek eastward toward western MD, eastern WV, and western portions of VA heading into sunrise Sunday morning.
Rainfall probabilities will increase during this time especially west of I-95 where the strengthening low level jet and warm air advection will promote shower development. Fog should be limited with the warm/stationary boundary lifting north of the region. Low and mid level clouds will persist with increasing southerly winds at 5 to 15 mph gusting up to 20 mph Sunday morning. Gusts of 25 to 35 mph remain possible along the highest ridges of the Blue Ridge, Catoctins, and Allegheny/Potomac Highlands.

SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/
A potent upper-level trough with northern and southern stream energy will continue to push east from the Great Lakes and Tennessee River Valley Sunday. At the surface, a strong cold front will slowly approach from the west Sunday morning into Sunday afternoon before working east of the area early Monday morning. 12z guidance continues to illustrate a fairly progressive system with limited phasing between the northern and southern stream. Even with that said, a strong meridional component remains to the upper level trough axis with ample warm/moist advection ahead of the cold frontal boundary. 12z upper level guidance from the CAMS and global solutions even show a negative tilt to the trough as the low level jet strengthen overhead.

The strong thermal gradient combined with ample moisture advection will lead to widespread heavy rain and gusty winds across the region. Thunderstorms are also a possibility as the region resides in the warm sector with some elevated instability (i.e MLCAPE values less than 150 j/kg) 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 latest 12z CAM guidance. Given the strong winds aloft (0-6km shear 60-80kts), this has the potential to mix down leading to a locally damaging wind threat. One mitigating factor for the thunderstorm threat 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 across south-central MD and the central VA Piedmont).

Any thunderstorm activity that we do see will enhance the heavy rainfall threat across the region given the anomalously high PWATS and given dynamic. Rainfall amounts will range between 1-2 inches west of I-81 with 2-2.5 inches farther east. Much of this rain may fall within a 3 to 6 hour period Sunday afternoon into late Sunday evening. This will lead to instances of urban flooding where relatively low FFG guidance overlaps. For that reason, a Flood Watch remains in effect for the Baltimore/Washington DC metros as well as portions of northern VA.

Now for winter - this is a complex anafrontal system with multiple waves of low pressure progged to move along the front.
The colder air will first move into the Allegheny front Sunday evening, and spread southeast through the area overnight Sunday.
Given the anafrontal nature of the cold front, moderate precipitation is likely to persist behind the front before cooler & drier air works into the region. Guidance continues to bring in more cold air which does result in more snow across the region. The best chance for accumulating snow is in areas of higher terrain (generally above 1750'). Have issued a Winter Weather Advisory for portions of the Allegheny Front given the strong signal for advisory level snow on the backside of the system coupled with the potential for snow squalls and showers lingering into Monday morning. Model soundings have a favorable overlap in moisture and lift through the DGZ through mid-morning Monday. Now the second area of winter headline is the northern Virginia Blue Ridge zone. Have issued a Winter Storm Watch for this area given the overwhelming signal for several inches of snow. There is uncertainty with respect to how quickly the dry air moves in and snow ends, but given some of the more robust scenarios, felt a watch was prudent for this area. There is a non-zero chance for accumulating snow further east, even near the DC and Baltimore metros. If the 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 40 mph are possible for most areas. 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 SATURDAY/
As the early week system lifts up toward Greenland, high pressure settles over the Eastern Seaboard on Tuesday. This should remain the general theme through most of next week which keeps conditions mainly dry, accompanied by seasonable temperatures.

The upper pattern is quasi-zonal through mid-week with any meaningful height falls confined to the more northern latitudes. As the longwave trough exits into the western Atlantic, heights build from Thursday into early portions of next weekend. A fairly potent upstream upper low is noted by a number of deterministic models, although spread is fairly high given it is a 7 day forecast.
Whatever materializes out of this feature should be the next weather maker in the forecast.

Forecast high temperatures gradually rise through Friday, generally getting to just above mid-December climatology. This carries afternoon temperatures into the upper 40s to lowFor that reason, 50s, locally about 10 to 15 degrees cooler across mountain locales. High pressure should support plenty of sunshine through the rest of the work week. Clouds increase by Saturday which is accompanied by an uptick in rain chances.

AVIATION /02Z SUNDAY THROUGH THURSDAY/
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.
For that reason, 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 through Thursday given high pressure will largely be in charge. Initial southerly winds should give way to northwesterlies by late Tuesday into Thursday morning in response to a weak cold front. Westerlies take shape thereafter with continued dry weather.

MARINE
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 Warning is now in effect for the waters. The strongest winds are expected between 06Z and 18Z Monday. Winds will gradually diminish Monday night, but an SCA will likely be needed.

As a weak cold front tracks through the waters, there may be a brief uptick in winds up to 20 knots late Tuesday into Wednesday morning. After a drop off in wind fields, north-northwesterlies pick up in strength late Wednesday, particularly across the lower waters. Marine winds turn more westerly by Thursday.

HYDROLOGY
Widespread rainfall is expected Sunday into early Monday morning as a strong cold front and multiple waves of low pressure pass through the region. A strengthening low level jet combined with increased warm air advection/isentropic upglide will yield efficient heavy rainfall over a 6 to 12 hour period extending from Sunday afternoon into Sunday night. Rain should be stratiform initially leading to slightly lower rates per hour before turning convective with a strengthening low level jet ahead of the front Sunday afternoon/evening. A few thunderstorms may also enhance totals east of the Blue Ridge with SBCAPE/MLCAPE values running less than 150 j/kg and 0-6km shear hovering at 40-60 kts.

PWATS ahead of the boundary per the 12 HRRR will surge into the 1.2 to 1.6" range along and east of the Blue Ridge. This aligns with the NAEFS/EPS guidance which suggest PWATS running 3 to 4 standard deviations above climo which is fairly significant for this time of year.For that reason,

Storm total rainfall will range between 1.75 to 2.25 inches along and east of the Alleghenies. The probability of exceeeding 2" has come down since yesterday and is now more focused to the south away from the big metro areas. Fairfax, Prince William and Stafford Counties seem to stand the greatest risk of seeing any flooding.

A Flood Watch remains in effect for both the Washington DC and Baltimore metros as well as much of northern/central MD and urban areas of northern VA. The watch extends from Sunday afternoon into late Sunday evening to encompass the heaviest rainfall. Urban flooding due to runoff looks to be the main concern here although smaller streams and creeks could see rises due to the amount of rain coming in a short period. Antecedent drought concerns should help limit any widespread flooding across the region.

TIDES/COASTAL FLOODING
Caution stages look to continue this evening with light south to southeasterly flow over the waters. Increasing tidal anomalies and chances for minor flooding remain Sunday into Sunday evening under increased southerly flow. Strong offshore flow returns Sunday night into Monday in the wake of a strong cold frontal boundary.

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.
Winter Weather Advisory from 7 PM Sunday to 10 AM EST Monday for MDZ001.
VA...Flood Watch from Sunday afternoon through late Sunday night for VAZ053>055-502-506-526-527.
Winter Storm Watch from Sunday evening through Monday morning for VAZ507.
WV...Winter Weather Advisory from 7 PM Sunday to 10 AM EST Monday for WVZ501-505.
MARINE...Small Craft Advisory from 6 AM to 6 PM EST Sunday for ANZ530>532-535-536-538>540-542.
Gale Warning from 6 PM Sunday to 1 PM EST Monday for ANZ530>543.
Small Craft Advisory from 1 AM to 6 PM EST Sunday for ANZ533- 534-537-541-543.

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