Marine Weather and Tide Forecast for Nantucket, MA

KEY MESSAGES
- Dry with slowly moderating temperatures through Wednesday

- Storm system to bring a period of strong winds and heavy rain late Thursday into Friday

- Potential for minor coastal flooding along south coastal areas Friday morning

DISCUSSION
KEY MESSAGE 1...Dry with slowly moderating temperatures through Wednesday

Fairly progressive pattern will bring a ridge of high pressure across our region Tuesday, with it moving offshore for Wednesday. As this happens, low level winds will shift from NW to SW and start to bring in moderating temperatures. 925mb temperatures currently around -12C will rise to near -8C tomorrow and then all the way to about +1C for Wednesday. This will support daytime highs here at the ground either side of freezing tomorrow and the upper 30s to mid 40s on Wednesday (which is near normal). Given general high pressure pattern, no precipitation is expected, although periods of mid-level clouds will move overhead.

KEY MESSAGE 2...Storm system to bring a period of strong winds and heavy rain late Thursday into Friday

The end of the week looks to feature the most "notable" weather.
Although there is still considerable variation in the intensity and timing of the storm system, there is enough similarity across the guidance suite to suggest a fairly high probability of seeing wind, rain and mild temperatures. The devil will be in the details, but the current indications are that a robust southerly LLJ (on the order of 65-80kt at 925/850mb) will develop out ahead of an approaching deep upper level trough. The trick in these situations is to figure out how much of that wind aloft will actually mix to the surface. A rough rule of thumb is to take 50% of the 925mb winds to equate to potential wind gusts. However guidance is also showing that temperatures will be warming Thursday night into Friday morning, perhaps well into the mid 50s (especially eastern half of SNE). This would mean a bit more boundary layer mixing, and the ability to bring higher gusts to the surface. BUFKIT soundings from GFS show the potential for over 50kt gusts across SE MA early Friday morning.
NBM5.0 probabilities for 50kt+ are currently running 10-20% across SE MA, Cape and Islands. Thus confidence is still low for those kind of strong winds, but gusts into the lower 40kt seems reasonable at this point, so did nudge gust forecast up some from the baseline NBM. There will also be a surge of moisture, with Precip Water Values up around 1", so that suggests downpours. Most areas should get 3/4-1 inch of rain out of the system. A strong cold front passes through on Friday, with rapidly falling temperatures. Model soundings suggest deep boundary layer mixing post-frontal, with continued breezy/windy conditions late Friday/Friday night. Highest probabilities for 40kt+ wind gusts from the NBM are centered across the Berkshires and Worcester Hills as well as offshore, which is a typical pattern for where the strongest gusts in a CAA pattern occur. Probabilities are only 5-15% at this time, so stay tuned.

KEY MESSAGE 3...Potential for minor coastal flooding along south coastal areas Friday morning

A lot of uncertainty here, but depending on how strong the southerly winds can get early Friday and when that happens, there is a potential for a storm surge to align with high tide in the 6-8am timeframe. Stevens Institute ensemble models are suggesting about a 1.5ft surge along south coastal areas at this point, which given overall tides would keep total water levels just below flood stage. But there is about a 10% chance of seeing close to a 3ft surge. If everything aligns perfectly, we could see some minor coastal flooding.

AVIATION /00Z TUESDAY THROUGH SATURDAY/
Forecaster Confidence Levels...

Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.

00z TAF Update...

High confidence.

VFR through the period. Generally W winds around 10 kts through the night and morning, eventually turn SW Tuesday aft'noon.

KBOS Terminal...High confidence in TAF

KBDL Terminal...High confidence in TAF.

Outlook /Tuesday Night through Saturday/...

Wednesday: VFR. Windy with local gusts up to 30 kt.

Wednesday Night through Thursday: VFR. Breezy.

Thursday Night: MVFR/IFR conditions possible. Strong winds with gusts up to 45 kt. Chance RA.

Friday: Mainly MVFR, with local IFR possible. Strong winds with gusts up to 45 kt. Chance RA.

Friday Night: VFR. Windy with gusts up to 35 kt.

Saturday: VFR. Breezy.

MARINE
Forecaster Confidence Levels...

Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.

Today Through Tuesday Night...High confidence.

Winds are subsiding this afternoon, and gale warnings have been replaced by Small Craft Advisories. As a ridge of high pressure approaches, the winds will continue to diminish, and the expectation is that Small Craft Advisories will not be needed by Tuesday morning, although there may still be some rough seas further offshore.

Outlook /Tuesday Night through Saturday/...

Wednesday: Strong winds with areas of gusts up to 30 kt. Areas of rough seas.

Wednesday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas.

Thursday: Winds less than 25 kt. Areas of seas approaching 5 ft.

Thursday Night: Moderate risk for gale force winds with gusts up to 45 kt. Rough seas up to 14 ft. Chance of rain.

Friday: gale force winds with gusts up to 45 kt. Rough seas up to 15 ft. Rain.

Friday Night: Strong winds with areas of gusts up to 45 kt.
Rough seas up to 13 ft.

Saturday: Winds less than 25 kt. Seas up to 5 ft.

BOX WATCHES/WARNINGS/ADVISORIES
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 7 PM EST this evening for ANZ231>235-237-251.
Small Craft Advisory until 3 AM EST Tuesday for ANZ250-254>256.