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ANZ530 Chesapeake Bay North Of Pooles Island- 707 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...
Today..S winds 5 kt. Waves flat. Areas of dense fog this morning with vsby 1 nm or less.
Tonight..SE winds 5 to 10 kt. Waves 1 ft. A chance of showers.
Sun..S winds 15 to 20 kt with gusts to 25 kt. Waves 2 ft. Rain. Vsby 1 nm or less.
Sun night..W winds 15 to 20 kt...becoming nw 20 to 25 kt after midnight. Gusts up to 35 kt. Waves 2 ft. Rain.
Mon..NW winds 20 to 25 kt with gusts to 35 kt. Waves 2 ft.
Mon night..W winds 15 to 20 kt with gusts to 25 kt... Diminishing to 5 to 10 kt after midnight. Waves 1 to 2 ft.
Tue..SW winds 5 to 10 kt. Waves 1 ft.
Wed..NW winds 5 to 10 kt. Waves 1 ft.

ANZ500 707 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 KCTP 091201 AFDCTP

Area Forecast Discussion National Weather Service State College PA 701 AM EST Sat Dec 9 2023

SYNOPSIS
Dry weather with increasing clouds will highlight today's weather with afternoon high temperature departures ranging from around 5 degrees above normal across the Lower Susquehanna Valley to nearly 20 degrees above normal over the Western Mountains of the state.

A plume of deep tropical moisture and surge of warmer air will precede and accompany the passage of a relatively strong cold front Sunday afternoon and night.

Widespread heavy rain of 1 to 2 inches will fall Sunday into Sunday night with locally higher amounts over and to the east of the Susquehanna Valley.

The rain will be followed by a several hour period or more of snow that will bring light accumulations to many valley locations Monday morning with a few to several inches possible across the higher terrain of Northern and Western PA.

Gusty winds up to around 40 mph will occur on Monday as the colder air deepens across the region, and ushers in drier conditions for Monday night and Tuesday.

Beyond Monday, the pattern signal for the rest of next week favors seasonal temperatures and little to no precipitation.

NEAR TERM /UNTIL 6 PM THIS EVENING/
Strengthening/deepening warm advection was spreading layer cloud cover across the Laurel Highlands and NW Mtns of PA early today. this combined with a 5-8 KT south/serly breeze was helping to keep temps quite mild and in the mid 40s to around 50F with mid to upper 50s further west and poised to push into the Western counties of our CWA later this morning and this afternoon.

Persistent clear to partly cloudy skies and nearly calm air over the Central Ridge and Valley Region and much of the Susq Valley supported the coolest overnight temps in the upper 20s to mid 30s with patchy light fog. Long southerly fetch of the 10 to 15 KT flow in the lowest 50 mb of the atmos across the northern half of the Chesapeake Bay tapped ample low level moisture to steadily expand (NWWD) a low stratus deck and locally sub 1SM vsbys in fog along and to the east of a line from KTHV to KMDT and KHZL.

These low clouds will erode slowly late this morning as the aforementioned thickening clouds across the west advect overhead. This will keep temps coolest across the east as the sfc based inversion will be tough to mix out completely before 18Z, while the big head start in temps over the Western Mtns will lead to the highest temps +15 to 20 deg F temp departures there.

Latest high res components of the NBM indicate rather fast NNE moving and brief, isolated to scattered showers across the Western Mtns this afternoon and evening with a few hundredths of an inch of rainfall at most. Elsewhere across the Central Mtns and Susq Valley it should stay dry and mild with high temps in the upper 40s to low 50s.

SHORT TERM /6 PM THIS EVENING THROUGH MONDAY/
*Flood watch in effect for east-central PA from Sunday afternoon through Monday afternoon

Rain chances increase Precip probs tick higher Saturday afternoon into Saturday evening over the western Alleghenies as strong cold front reaches the Upper Ohio Valley by 00Z Sun.
There is good agreement in the front moving slowly across CPA on Sunday as the associated longwave mid/upper trough becomes negatively tilted.

There is increasing confidence in the potential for heavy rain ahead and along the cold front given strong upper jet and GOMEX moisture feed surging pwats to 1+ inches or +2SD above the mean.

There will be a broad axis of at least a few hundred J/KG of mixed layer CAPE sliding east across the CWA late tonight (west)
through mid afternoon Sunday (East) with 925 mb based lifted indices dipping to about -1C overnight, and the elevated instability increasing Sunday morning across the Lower Susq Valley. So although there won't be any real severe weather threat, rainfall rates will increase significantly to over 0.3 inch per hour as this unstable layer drifts across Central PA and the Susq Valley.

Periods of enhanced Meso-B forcing beneath the thermally direct branch of several jet segments riding NNE through the larger scale 140-150 KT jet core will help to generate some heavier rainfall rates of ~0.5"/hr and 1"+ per 3 hours, with good model agreement for event total rainfall of 1.00-2.50" across the area.

The heaviest rain (2+ inches) is expected over the eastern portion of the CWA where isolated minor (urban/poor drainage and river) flooding is most likely. We plan to issue a flood watch adjacent to BGM/PHI to highlight the flood risk heading into the weekend.

Colder air will pour into the area behind the front Sunday night behind increasingly gusty NW flow. This will result in a rain to snow transition from west to east across the Alleghenies between 00-12Z Monday. There is still some uncertainty in how this baroclinic zone translates to snowfall amounts as quick cooling catches up to backside of the precip axis. Marginal sfc to BL temps also adds to low confidence especially from the central ridges to the eastern/Susquehanna Valley. Elevation will be a factor with latest forecast showing at least few inches of snow accum across the highest terrain from the Laurel Highlands into the north central mtns and adjacent Endless mtns/western Poconos. Min temps in the 35-40F range will significantly reduce the probability of accumulating snow in the lower to middle Susq Valley.

Peak wind gusts Sunday night into Monday (especially later Monday morning and Monday afternoon as the colder air dramatically deepens and taps stronger winds aloft) are forecast in the 35 to 45 mph range (just under Wind Advisory Criteria). As temperatures drop below freezing, there could be a freeze-up risk of water/slush with icy spots possible Monday morning.

Pattern eventually transitions full-send to seasonably cold NW flow with snow showers likely on Monday before fading into Monday night.

LONG TERM /MONDAY NIGHT THROUGH FRIDAY/
After the deepening surface low lifts northeastward Monday night, high pressure centered over the southeastern US will bring tranquil weather to the Commonwealth for the rest of the week. Prevailing westerly winds will oscillate some through the week as a boundary stalls just north of the area. When the wind exhibits a more northerly component, a few light rain/snow showers could affect the far northern tier of PA. Outside of that very low chance of precipitation, the rest of the area should stay dry with plenty of sunshine.

The sunshine will be a bit deceiving, though. Temperatures will be near average with highs in the middle 30s in the north to middle 40s across the southeast and overnight lows generally be in the 20s. That being said, a breezy westerly wind each day will shave 5 to 10 degrees off of the temperature making it feel more like 10 to 20 degrees in the morning and 25 to 35 degrees in the afternoon.

AVIATION /12Z SATURDAY THROUGH WEDNESDAY/
12z update... Lower stratus and fog will be the main issue in parts of the Susquehanna Valley this morning.

Then, later tonight, lower ceilings, rain, and low-level wind shear are expected overspread the terminal sites from west to east.

For the Alleghenies (KBFD and KJST), we have confidence (80%)
that unrestricted conditions will prevail into this evening.
There is a low chance (about 20%) that MVFR ceilings could briefly sneak into KBFD prior to about 15z.

Later tonight, there is fairly high confidence (60-80%) that fuel alternate-IFR restrictions will develop in steadier rain. A 35-45 kt SW low-level jet is also likely (50-70%) to produce LLWS conditions.

For the ridge and valley of central PA (KAOO, KUNV, and KIPT), confidence is high that unrestricted conditions will prevail well into this evening (about 80%). There is a low chance (10-20%) that lower ceilings could briefly (fuel alternate-IFR)
sneak into KAOO, KUNV, and KIPT this morning, as they approach from the east. Although we're not necessarily expecting this, we will monitor closely.

Later tonight, there is fairly high confidence (60-80%) that fuel alternate-IFR restrictions will develop in steadier rain. A 35-45 kt SW low-level jet is also likely (50-70%) to produce LLWS conditions. Given a well formed low-level inversion and the expectation of light easterly surface flow at KIPT, conditions could be more highly sheared at this site.

For the Lower Susquehanna Valley (KMDT and KLNS), IFR-LIFR restrictions are likely (50-70%) until late morning (15-16z).
The early afternoon period represents some uncertainly as to how quickly things will improve. Right now, we have VFR re- establishing itself by around 18z, but confidence is only moderate here (40-60%). It is conceivable that lower ceilings could persist longer.

During the pre-dawn hours Sunday, there is fairly high confidence (50-70%) that MVFR-fuel alternate restrictions will develop as rain gradually moves in. A 35-45 kt SW low-level jet is also likely (50-70%) to produce LLWS conditions. Given a well formed low-level inversion and the expectation of light E-SE surface flow at KMDT and KLNS, conditions could be strongly sheared at these sites.

Higher confidence (80+%) of lower (IFR) conditions is there beyond 12z Sunday.

Outlook...

Sun...CFROPA. Widespread reductions in rain/snow. Windy.

Mon...Rain/snow showers poss N/W. Breezy west wind.

Tue-Wed...Mainly dry with VFR conditions.

CLIMATE
It has been 316 days since the last measurable snowfall (>=0.1") at Harrisburg. This ranks as the 3rd longest run on record.

1. 330 days ending 12/13/2020 2. 328 days ending 01/21/2007 3. 316 days ending 12/07/2023

CTP WATCHES/WARNINGS/ADVISORIES
Flood Watch from Sunday afternoon through Monday afternoon for PAZ042-051>053-057>059-065-066.

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Port Deposit
Click for Map
Sat -- 03:01 AM EST     0.60 feet Low Tide
Sat -- 03:38 AM EST     Moonrise
Sat -- 07:02 AM EST     1.61 feet High Tide
Sat -- 07:13 AM EST     Sunrise
Sat -- 02:19 PM EST     Moonset
Sat -- 02:22 PM EST     -0.28 feet Low Tide
Sat -- 04:40 PM EST     Sunset
Sat -- 08:15 PM EST     2.35 feet High Tide
Tide / Current data from XTide NOT FOR NAVIGATION

Port Deposit, Susquehanna River, Maryland, Tide feet

Havre de Grace
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Sat -- 01:31 AM EST     0.17 feet Low Tide
Sat -- 03:38 AM EST     Moonrise
Sat -- 07:00 AM EST     1.43 feet High Tide
Sat -- 07:12 AM EST     Sunrise
Sat -- 12:51 PM EST     0.10 feet Low Tide
Sat -- 02:19 PM EST     Moonset
Sat -- 04:40 PM EST     Sunset
Sat -- 07:06 PM EST     2.15 feet High Tide
Tide / Current data from XTide NOT FOR NAVIGATION

Havre de Grace, Susquehanna River, Maryland, Tide feet

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