Nantucket Sound Marine Forecast
|This Afternoon...N Winds 15 To 20 Kt, Diminishing To 10 To 15 Kt. Seas 2 To 3 Ft.|
|Tonight...Ne Winds 5 To 10 Kt. Gusts Up To 20 Kt In The Evening. Seas 2 To 3 Ft.|
|Mon...E Winds 5 To 10 Kt. Seas Around 2 Ft.|
|Mon Night...E Winds Around 10 Kt. Seas Around 2 Ft.|
|Tue And Tue Night...Se Winds Around 10 Kt. Seas Around 2 Ft.|
|Wed Through Thu...Se Winds 10 To 15 Kt With Gusts Up To 20 Kt. Seas 2 To 4 Ft.|
|Thu Night...Se Winds 10 To 15 Kt With Gusts Up To 25 Kt. Seas 2 To 4 Ft. A Chance Of Showers. 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.|
| Area Forecast Discussion|
National Weather Service Boston/Norton MA
657am EDT Sunday September 19 2021
Large high pressure over the region today. This will bring dry weather and increasing sunshine, as well as low humidity along with mild days and cool nigheights through mid week. An approaching cold front will be accompanied by showers and possible thunderstorms sometime Thursday into Friday, followed by dry and seasonably mild conditions Saturday.
Near Term - Until 6pm This Evening
A cold front off Nantucket will move farther offshore today. Clouds lingered at sunrise from Worcester and Windham Counties eastward. But there were some breaks in the clouds over SE Mass. North winds are bringing drier air into the region. Dew points were in the upper 40s in NW Mass and 50s over much of the rest of the three states...some low 60s lingered along the South Coast.
The main uncertainty is with the exact timing for the clearing of the clouds. But the actual clearing trend is still expected.
No significant changes to the forecast at this time.
* Gorgeous stretch of dry weather begins later this morning, featuring mild days and cool nights
High pressure and post frontal airmass beginning to overspread the region this morning from NW to SE. Definitely a noticeable change in airmass with PWATs (Precipitable Waters) falling to about 50% of normal! At the surface, this translates to dew pts falling in the 50s today, even a few upper 40s possible in the high terrain of western MA. Quite a difference from the 60s yesterday.
Core of cooler post frontal airmass will eventually be across eastern MA, where 925 mb temps are about +12C today, then warming to +14C to +16C over western MA/CT. This will result in a noticeable temp range across the region this afternoon, with highs in the upper 60s to lower 70s across eastern MA, then warming to the mid-upper 70s across CT into western-central MA and RI.
Gusty NNE winds along the eastern MA coast this morning behind the front, will slacken and give way to afternoon seabreezes. These NNE winds will initially yield scattered to broken strato-cu across eastern MA this morning, but eventually very dry air aloft mixes to the surface and gives way to abundant sunshine all areas later this morning. Away from the coast, light winds will prevail thru the day. Just a gem of a day with abundant sunshine, mild temps, low humidity and light winds this afternoon, especially away from eastern MA coastline.
For those heading to the beach today, leftover southeast swells from Odette will result in one more day of rough surf and increased risk of strong rip currents. Thus, rip current statement remains posted. Main concern is at east facing ocean beaches of Cape Ann, Cape Cod and Nantucket, especially this time of year as beaches no longer have lifeguards.
Short Term - 6pm This Evening Through 6pm Monday
Tonight... The combination of a very dry airmass in place (dew pts in the 40s and 50s) and 1025+ mb high pressure overhead, providing light winds & clear conditions, will result in a very cool night. Low temps were derived from the cooler MOS guidance datasets, which supports lows in the 40s across many of the suburbs, 50s in the urban areas. These cool temps will serve as a reminder that the first day of Autumn is just days away, Wednesday 9/22 at 3:21 pm EDT. Normal low for this time of year is 53 to 58.
Monday... Another absolute gem of a day, with abundant sunshine, light winds and low humidity with dew pts in the 40s and 50s! Subtropical ridge builds to 593 dam heights, centered over southern New England. Still a coolish airmass overhead with 1030 mb maritime high extending into southern New England. Subsidence inversion may preclude mixing to 850 mb, therefore with 925 mb temps around +13C to +14C, expecting highs in the low to mid 70s, cooler along and near the coast with seabreezes.
Long Term - Monday Night Through Saturday
Big Picture... High amplitude pattern through the week will favor a slow change in weather. High pressure surface and aloft over the Eastern Seaboard Monday night shifts to the West Atlantic Tuesday. Great Plains shortwave digs and deepens over the Midwest Tuesday, forming a closed upper low Wednesday which lifts north through the Great Lakes Thursday-Friday.
Upper flow shows well-above normal contours, which would support summertime temps. But the departing surface high will leave southeast surface winds in its wake which should modify temps closer to normal levels. Contours fall as the upper low passes to the northwest, but remain near season normals. So expect near normal temperatures, cooling a little late in the week.
Mass fields are similar through Wednesday, then diverge. The GFS quickly becomes 24-hours ahead of the other models with the position of the closed low. But all models carry the system through the same geography, so the concern on its effects is more with timing. Confidence is high through Wednesday, then moderate through Saturday.
Details... Monday night to Wednesday... High pressure slowly moves east offshore, but maintains dry weather in Srn New England during the period. Mixing reaches between 850-mb and 900-mb. Dew points in the upper 40s and low 50s Monday night with light wind, so Monday night min temps will be in the upper 40s to mid 50s. Dews increase a little Tuesday, so Tuesday night mins will be in the 50s. Temps at the top of the mixed layer will be 5-8C Tuesday, supporting max temps upper 60s to mid 70s. These temps nudge up a little Wednesday, supporting max temps in the low-mid 70s.
Thursday through Saturday... Low pressure lifts through Ontario Thursday, sweeping the associated surface cold front east. GFS moves the cold front through during Thursday night, while the ECMWF (European Centre for Medium-Range Weather Forecasts) moves it through late Friday or Friday night. Tend to favor the slower ECMWF timing. PW values (Precipitable Water values) start climbing Wednesday, reaching highest values around 2 inches during Thursday and early Friday. This suggests the potential for local downpours. Convective parameters are so-so Thursday. Will forecast low-end chance probability of precipitation for showers in the CT Valley starting Wednesday night. The chance of showers spreads east across the region Thursday. Showers continuing through Thursday night in the west and through Friday in the east. probability of precipitation will include a chance of thunder.
Generally dry Saturday, but upper trough sweeps east from the Great Lakes and brings a -26C cold pool. Much of this moves northeast into Canada. But cooler air does move over New England, and this cold advection may destabilize the airmass and bring a few showers, especially to Western Mass.
Forecaster Confidence Levels.
Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent.
Today...High forecast confidence.
High pressure builds into New England, with NNW winds eventually shifting to NNE late this morning. Gusts build up to 20-25 kt, then diminish this afternoon. Seas a combination of NNE wind waves and SE swells from Odette. Dry weather.
Tonight and Monday...High forecast confidence.
Large 1025+ mb high pressure builds over the maritimes, yielding fine boating weather with light winds, dry weather and good vsby.
Outlook /Monday Night through Thursday/... Monday Night through Tuesday Night: Winds less than 25 kt.
Wednesday: Winds less than 25 kt. Seas locally approaching 5 ft.
Wednesday Night: Winds less than 25 kt. Seas up to 5 ft. Slight chance of rain showers.
Thursday: Winds less than 25 kt. Seas up to 5 ft. Slight chance of rain showers, slight chance of thunderstorms.
NOAA Boston MA Office: Watches - Warnings - Advisories
CT...None. MA...High Rip Current Risk through this evening for MAZ007-019-022- 024. RI...None.
Small Craft Advisory until 8pm EDT this evening for ANZ250- 254. Small Craft Advisory until 4pm EDT this afternoon for ANZ251. Small Craft Advisory until 2pm EDT this afternoon for ANZ255- 256.