Marine Weather Net

Nantucket Sound Marine Forecast


OVERNIGHT

SW
WINDS
10 - 15
KNOTS

FRI

SW
WINDS
15 - 20
KNOTS

FRI NIGHT

W
WINDS
20 - 25
KNOTS

SAT

NW
WINDS
10 - 15
KNOTS

The Marine Weather Forecast In Detail:
ANZ232 Forecast Issued: 1020 PM EST Thu Nov 21 2019

GALE WARNING IN EFFECT FROM 10 AM EST FRIDAY THROUGH FRIDAY EVENING
Overnight...S Winds 5 To 10 Kt, Becoming Sw 10 To 15 Kt With Gusts Up To 25 Kt. Seas 2 To 3 Ft.
Fri...Sw Winds 15 To 20 Kt. Gusts Up To 30 Kt, Increasing To 40 Kt In The Afternoon. Seas 3 To 4 Ft. Showers Likely.
Fri Night...W Winds 20 To 25 Kt, Becoming Nw 15 To 20 Kt After Midnight. Gusts Up To 40 Kt. Seas 3 To 5 Ft.
Sat...Nw Winds 10 To 15 Kt, Becoming W 5 To 10 Kt In The Afternoon. Gusts Up To 25 Kt. Seas 2 To 4 Ft.
Sat Night...Sw Winds 5 To 10 Kt. Seas Around 2 Ft. Rain Likely.
Sun...E Winds 5 To 10 Kt, Becoming N 10 To 15 Kt In The Afternoon. Gusts Up To 25 Kt. Seas 2 To 3 Ft. Rain.
Sun Night...Nw Winds 10 To 15 Kt, Becoming W 15 To 20 Kt After Midnight. Gusts Up To 25 Kt. Seas 2 To 4 Ft.
Mon...W Winds 10 To 15 Kt With Gusts Up To 20 Kt. Seas 2 To 4 Ft.
Mon Night Through Tue Night...S Winds 5 To 10 Kt. Seas 2 To 3 Ft. Seas Are Reported As Significant Wave Height, Which Is The Average Of The Highest Third Of The Waves. Individual Wave Heights May Be More Than Twice The Significant Wave Height.
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Area Forecast Discussion
National Weather Service Boston/Norton MA
1006pm EST Thu Nov 21 2019

Synopsis
High pressure along the USA East Coast will move offshore tonight. Milder air starts moving into Southern New England tonight, bringing clouds and possibly a sprinkle. Gusty southwest winds will keep Friday, with a few showers possible as a cold front moves through the region. Colder air moves in behind the front Friday night. Weak high pressure brings dry weather Saturday. Low pressure moving up from the Gulf of Mexico will bring rain to much of our area late Saturday night and Sunday. Portions of Northwest MA may start as mix of snow/sleet/freezing rain before changing to rain. High pressure brings drier weather Sunday night through Tuesday. A cold front may bring showers Wednesday, followed by dry weather again for Thanksgiving.

Near Term - Until 6am Friday Morning
10pm Update:

Stout radiational cooling with current temps outpacing forecast lows especially towards Cape Cod and southeast MA. Observations at Falmouth and Hyannis were already near or below freezing. Thus made some downward adjustments to hourly temps to get better in- line with current observations, heavily using GFS LAMP temps which showed trends the best. Warm front still resides back into southern NY into northern PA, but prior ideas for gradually warming temps early in the overnight still looks valid. Lows for the evening should be occurring over the next couple hrs with only a couple more degrees left of cooling likely before warming trends commence.

Regarding PoPs...00Z RAOB (Radiosonde Observation or Upper-Air Observation) from Albany showed considerable dry air below 500 mb that will serve to evaporate any light precipitation that could be falling from the area of radar returns moving through the area (and that's also falling through a mid cloud deck). Lowered the chance-ish Probability of Precipitation towards the eastern slopes of the Berkshires to dry until about 10z, which is when RAP- based model soundings show better columnar saturation occurring supporting very light rain/trace Quantitative Precipitation Forecast. Most areas dry however, with lowering/thickening clouds associated with warm advection anticipated.

Final change was to lower wind speeds both on the land and out on the waters. Appears winds have decoupled and gone light in most areas. Gradually increasing p-gradient into the pre- dawn hrs should help to increase wind speeds a bit further.

Otherwise, no other changes needed attm.

Previous Discussion... Visible satellite shows some high clouds from a low pressure system centered over the Great Lakes spreading into Western MA this afternoon. This is the system that will produce a cold front that will swing through SNE tomorrow afternoon. Ahead of the cold front, warm air advection will lead to a non-diurnal temperature trend this evening. The low-level jet increases overnight, with 850mb winds increasing to 40 kts by daybreak. Bufkit soundings suggest some momentum transfer to the surface by early to mid morning, with wind gusts of up to 25 kts. Temperatures would initially drop into the mid 30s interior to low 40s near the coast early evening. But with warm air advection and low level jet strengthening, temperatures would rise a few degrees into the low 40s in the interior to upper 40s near the coast by daybreak on Fri. Clouds will also thicken overnight, becoming mostly cloudy by staying mostly dry by Fri morning.

Short Term - 6am Friday Morning Through Friday Night
Friday:

Most of Fri morning is expected to stay dry. Despite limited insolation from a thick cloud cover, warm air advection will allow most areas outside NW MA and interior higher elevations to warm into the mid 50s. I would not be surprised to see a few locations, especially in RI and SE MA reach the upper 50s, especially if there are peeks of sunshine. It will also be breezy, with SW winds of 10-20 kts with gusts up to 25 kts.

Most model guidance supports the cold front reaching our western counties by around 18z, then racing quickly towards the coast and offshore by around 22-00z. Considering both the limited moisture available and progressive nature of the front, think that most areas will see no more than a tenth of an inch of precip. Moreover, due to the lack of cold air, all the precip is expected to fall as rain. Cannot rule out a few wet flakes mixed in in the higher elevations of the East Slopes of the Berkshires but no accumulations expected. Temperatures fall quickly behind the cold front, with 30s in the interior and 40s by early evening. Most areas will fall below freezing, with mid 20s in the interior and upper 20s/low 30s in Eastern MA and RI. The exceptions are the immediate coast and the Cape, which fall into the mid 30s.

The main headline is, however, the strong NW winds, especially over the waters from Fri afternoon to late Fri night/early Sat morning. The cold air advection will help mix stronger winds to the surface. Expect sustained winds of 20-25 kts with gusts of 35-40 kts. Some interior locations, especially higher elevations of the East Slopes of the Berkshires and Worcester Hills may reach Wind Advisory criteria. But these are expected to be scattered in nature so currently don't have the confidence for issuance. Therefore, have issued Gale Warnings for all waters during this period. As a result of the strong winds, wind chills will be in the teens in the interior and 20s in the coastal areas late Fri night.

Long Term - Saturday Through Thursday
Upper flow is initially split between northern and southern branches. But this evolves by Sunday to a single jet moving through a broad trough centered over North America. Five-wave scale starts with a trough over the Eastern USA and a ridge over the West Coast. The pattern shift west by next week with the five-wave trough over the Rockies and a West Atlantic ridge backing toward the USA East coast.

Heigheights at 500 mb vary during the period, dropping below normal as east storm moves through, then climbing into the 550s Dm in between. Overall expect the deep layer to reflect near normal temperatures, except colder temps Saturday night and Sunday as a coastal storm moves past.

Confidence is low-moderate Saturday night-Sunday with the coastal storm and Wednesday with timing of a midweek cold front. Higher confidence with the dry weather in between.

Daily Concerns... Saturday... Shortwave ridge over the region will provide plenty of subsidence and dry weather. Increasing high level moisture moves into CT and Western MA late in the day, so expect increasing high thin clouds at that time. Mixed layer reaches to about 950 mb, with layer temps supporting surface max temps in the 40s.

Saturday night-Sunday... Weather system coming out of the Gulf will draw moisture north to New England while pulling colder air south from Canada. The combination of these factors brings wet weather to our area Saturday night and Sunday. The question is how much of the northern stream colder air is drawn over Southern New England. There is no sign of a Maritime high or other source of cold air damming, and no sign of other sauces of cold air drainage. The projected track of the storm has the center moving up along the South Coast, farther north than models showed yesterday and suggesting warmer temperatures farther inland. For now the forecast favors mainly rain, except some snow/sleet in NW Mass that changes to rain by midday Sunday.

Monday through Thanksgiving Day... High pressure brings dry weather Monday and Tuesday. Mixed layer reaches to 925 mb, where temps will be 0-2C Monday and 3-5C Tuesday. Mixing of those temps to the surface supports max surface temps in the 40s Monday and lower 50s Tuesday.

A cold front swings through on Wednesday. Current model timing of the cold front is faster than yesterday by 6 to 12 hours. Such run- to-run changes lowers confidence in this timing. Moisture in the column will be limited, so expect scattered showers along and just ahead of the front. Enough of a gradient and cold advection to bring west-northwest gusts to 35 mph Wednesday afternoon and Thursday.

One other note: We are coming into a period of high astronomical tides Sunday and Monday. Depending on the wind direction and timing of highest surge, a worst case scenario would bring some minor coastal flooding with the Sunday high tide on either coastline (5.7 ft MLLW at Providence at 541 AM, 11.2 ft MLLW at Boston at 845 AM, and 3.9 ft MLLW at Nantucket at 946 AM). Ensemble guidance from Stevens Institute shows a potential surge of 1.0 to 1.5 ft. Tides are a little higher Monday morning, but by then we should be dealing with offshore (west) flow which should limit the threat.

Marine
Short Term /through Friday Night/...Moderate confidence.
Tonight: Winds become WSW and increase to 10-15 kts with gusts to 20 kts by daybreak Friday. Seas mainly 2-4 ft, though with leftover 5 footers early. Good visibility.

Friday: WSW winds continue to increase to low-end SCA (Small Craft Advisory) levels (mainly 15-20 kts with gusts to 30 kts) ahead of a cold front expected to cross the waters late Friday. Wind shift to W with sustained winds increasing to 20-25 kt winds, gusts increasing to 35-40 kts...strongest on the outer waters. Seas build to 5-9 ft, highest southern and southeastern waters. Light showers could bring visibilities down briefly to 4-6 miles late Friday afternoon. A Gale Warning in in effect for these winds.

Friday night... Gusty west winds continue with gusts to 35 kt. Winds diminish the latter half of the night. Seas remain at 5 to 10 feet. Gale warning continues.

Outlook /Saturday through Tuesday/... Saturday: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas.

Saturday Night: Winds less than 25 kt. Rain likely.

Sunday: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Rain.

Sunday Night: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Slight chance of rain showers.
Monday: Winds less than 25 kt. Local rough seas.

Monday Night through Tuesday: Winds less than 25 kt.

NOAA Boston MA Office: Watches - Warnings - Advisories
CT...None. MA...None. RI...None.
MARINE...
Gale Warning from 10am Friday to midnight EST Friday night for ANZ231>235-237-251. Gale Warning from noon to 10pm EST Friday for ANZ230-236. Gale Warning from 8am Friday to 4am EST Saturday for ANZ250- 254>256.