Marine Weather and Tide Forecast for Nantucket, MA

KEY MESSAGES

- Isolated shower possible this afternoon, but otherwise mainly dry and warm. Dry Thursday with seasonably cool temperatures.

- Fast-moving system to bring periods of rain Friday, followed by drier and much colder conditions Saturday.

- Quiet and seasonable weather Sunday through Tuesday.

DISCUSSION
KEY MESSAGE 1...Isolated shower possible this afternoon, but otherwise mainly dry and warm. Dry Thursday with seasonably cool temperatures.

For the rest of the afternoon, expect dry conditions overall with the exception of an isolated shower through the evening. Tonight will be dry with mainly clear skies with temperatures falling into the 40s. Cooler air advects in from the north overnight while surface high pressure shifts eastward into the region. The tightened pressure gradient along with steeper lapse rates should support some mixing to bring down gusts around 20 kts, perhaps a little stronger for the Cape where the gradient is tighter.

Surface high pressure settles over southern New England on Thursday.
As it builds in, the pressure gradient will relax, allowing winds to decrease through the morning. Large scale subsidence will maintain dry conditions alongside light northerly flow. A cooler airmass will be in place characterized by 925mb temperatures +6- 8(approx 10C cooler than Wed). This will bring a seasonably cool day with high temperatures in the upper 50s and low 60s.

KEY MESSAGE 2...Fast-moving system to bring periods of rain Friday, followed by drier and much colder conditions Saturday.

Forecast trends continue to indicate a rather soggy end to the week.
The good news is that this is a fairly progressive system, with rain clearing out in time for the weekend. High pressure drifts east and offshore overnight into early Friday. Meanwhile, to our west, a low pressure system develops across the eastern Great Lakes and deepens as it travels northeast through the Saint Lawrence Valley. The low is expected to deepen by roughly 10 to 15 mb within 24 hours, reaching around 995 mb by late Friday evening. Out ahead of the low, a warm front brings an initial band of rain to southern New England during the early morning hours Friday, beginning during the predawn hours and spreading from west to east. There is the potential for a lull in activity between late morning and early afternoon. While in the warm sector, anticipate mild, muggy, and breezy conditions with south-southwesterly winds. A second round of showers, with the potential for thunder, is expected late Friday afternoon through the evening with the passage of a strong cold front. Drier and cooler conditions follow beginning Saturday morning.

In terms of precipitation, PWATs are quite elevated for mid-October, with values ranging between 1.5 and 1.8 inches, roughly 2 to 3 standard deviations above normal. Locally heavy downpours are possible, though the flooding threat is minimal considering the progressive nature of the system. As mentioned, warm frontal showers are expected during the predawn hours Friday. There will be less lift during the daytime; however, spotty showers may continue due to ongoing WAA. As the surface low tracks closer to Quebec City, a cold front brings strong forcing during the late afternoon and evening, with a final round of showers and embedded thunderstorms. While we do not anticipate widespread severe weather, there is a fair amount of bulk shear, ranging from 50 to 80 knots, and MUCAPE values around 500 J/kg. It could be one of those days where the higher shear compensates for the relatively limited instability. This is something we will continue to monitor. As for rainfall amounts, there is still a bit of spread among the guidance, though consensus is leaning toward 0.3 to 0.8 inches. That being said, locally higher amounts are possible, with the greatest potential for more than an inch of rainfall across northwestern Massachusetts, where DESI probabilities are 15 to 25 percent.

As mentioned, it will be breezy on Friday, with a 30 to 40 knot low- level jet. However, the low-level thermal inversion will play a significant role in determining how much of that wind can mix down to the surface. Given a neutral inversion, did not want to mix the full strength of the low-level jet to the ground. Therefore, gusts of 25 to 35 mph on Friday seem the most reasonable. Behind the front, strong CAA aloft sends 850 mb temperatures falling from +17C on Friday to -5C on Saturday. With steep low-level lapse rates, there will be efficient mixing through the lower levels. Gusts Saturday afternoon may approach 30 to 40 mph. Mixing may be further enhanced over the warmer ocean waters; thus, Gale Watches have been issued for all waters from Friday evening through Saturday evening.
While temperatures will be mild on Friday, the bigger story will be the cold on Saturday. As mentioned, temperatures aloft will be in free fall. Highs will struggle to reach the upper 40s to lower 50s.
However, when factoring in the wind, it will likely feel as though temperatures are in the upper 30s to middle 40s Saturday afternoon.
Saturday night will be quite cold, with many areas falling into the lower 30s and upper 20s. Thus, frost is possible, though lingering breezy conditions may limit its development. Nevertheless, our frost/freeze program has ended for the growing season as of October 11, and no frost or freeze headlines will be issued.

KEY MESSAGE 3...Quiet and seasonable weather Sunday through Tuesday.

A broad area of high pressure builds into the region Sunday through Tuesday, leading to a period of quiet and dry weather. The stronger winds from Saturday diminish on Sunday as the pressure gradient relaxes with the return of high pressure. During this time, temperatures begin to moderate and return to more seasonable levels, with highs in the upper 50s to lower 60s and overnight lows ranging from the upper 30s to middle 40s.



AVIATION /19Z WEDNESDAY THROUGH MONDAY/
Forecaster Confidence Levels...

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

18Z TAF Update:

Today: High confidence.

VFR with SCT to BKN decks between 035-150. Isolated light SHRA/sprinkle possible through 23z as showers moves west to east.
Coverage is very limited with little hitting the ground so threw in as a VCSH for the east terminals. Winds WNW this afternoon at 8-15 kts. Gusts 20-25 kts. Gusts diminish after 21Z. Although a few lighter gusts up to 20 kts may linger for the Cape terminals. Winds shift to the NW this evening.

Tonight: High confidence.

VFR. Gusts 20-25 kt for most developing 02-04Z; 20-25 kt gusts, highest for the Cape and Island terminals.

Thursday...High confidence.

VFR. Winds diminish 12-15Z for most terminals with high pressure settling overhead. Gusts around 18kts may linger through 16/17Z for HYA/ACK. Winds NNW shifting to the N at 5-10 kts. Winds may shift around to the NE-E in the afternoon/early eve for the coastal terminals.

KBOS Terminal...High confidence in TAF.
Chance of an isolated shower or sprinkle mainly between 18-21Z.
It's likely that little hits the ground.

KBDL Terminal...High confidence in TAF.

Outlook /Thursday Night through Monday/...

Friday: MVFR/IFR conditions possible. Windy with areas of gusts up to 30 kt. SHRA, patchy BR.

Friday Night: Mainly VFR, with local IFR possible. Windy with gusts up to 35 kt. SHRA, patchy BR.

Saturday: VFR. Windy with gusts up to 35 kt.

Saturday Night through Sunday: VFR. Breezy.

Sunday Night: VFR.

Monday: VFR. Breezy.

MARINE
Forecaster Confidence Levels...

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

Small craft advisories were extended and/or hoisted for tonight for the eastern waters into 18z Thursday as a period of enhanced NW gusts of 25-30 kt are expected to develop early tonight. Seas around 3-5 ft on most waters through Thursday, with dry weather and good visibility expected. Gusts should subside below SCA criterion by Thursday night.

A frontal system brings returning SW winds into the SCA range on Friday (25-30 kt), which will also bring periods of light to moderate rain reducing visibility through Friday early evening.
A short period of small craft advisories will likely be needed once the existing SCAs expiring Thursday afternoon expire.

Looking ahead into Friday night and into Saturday...a period of strong northwest winds are forecast. We've issued a Gale Watch on most waters for Friday night into Saturday, with seas becoming increasingly rough at up to 12 ft. Dry weather is expected by Saturday.

Outlook /Thursday Night through Monday/...

Friday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Rain showers, patchy fog.

Friday Night: Strong winds with gusts up to 35 kt. Rough seas up to 12 ft. Rain showers likely.

Saturday: Strong winds with gusts up to 35 kt. Rough seas up to 12 ft. Chance of rain showers.

Saturday Night: Strong winds with local gusts up to 30 kt.
Rough seas up to 11 ft.

Sunday: Winds less than 25 kt. Local rough seas.

Sunday Night: Winds less than 25 kt. Seas locally approaching 5 ft.

Monday: Winds less than 25 kt.

BOX WATCHES/WARNINGS/ADVISORIES
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 8 PM this evening to 2 PM EDT Thursday for ANZ230>234-250-251.
Gale Watch from Friday evening through Saturday evening for ANZ230>237-250-251-254>256-280>283.
Small Craft Advisory until 2 PM EDT Thursday for ANZ254-255.
Small Craft Advisory until 8 PM EDT this evening for ANZ256.