Nantucket Sound Marine Forecast
|Tonight...Se Winds 5 To 10 Kt. Seas 1 Foot Or Less.|
|Sat...E Winds 5 To 10 Kt. Gusts Up To 20 Kt In The Afternoon. Seas Around 2 Ft.|
|Sat Night...E Winds 10 To 15 Kt With Gusts Up To 25 Kt. Seas 2 To 3 Ft. Patchy Fog. Showers And Tstms Likely. Some Tstms May Produce Gusty Winds And Heavy Rainfall. Vsby 1 Nm Or Less.|
|Sun...S Winds 10 To 15 Kt With Gusts Up To 30 Kt. Seas 3 To 5 Ft. Patchy Fog. Showers And Tstms Likely. Some Tstms May Produce Gusty Winds And Heavy Rainfall. Vsby 1 To 3 Nm.|
|Sun Night...S Winds 10 To 15 Kt With Gusts Up To 25 Kt. Seas 2 To 4 Ft. A Chance Of Showers.|
|Mon...S Winds 10 To 15 Kt With Gusts Up To 25 Kt. Seas 2 To 4 Ft. A Chance Of Showers.|
|Mon Night...S Winds 10 To 15 Kt. Gusts Up To 20 Kt In The Evening. Seas 2 To 3 Ft.|
|Tue...S Winds 10 To 15 Kt With Gusts Up To 20 Kt. Seas 2 To 3 Ft.|
|Tue Night...S Winds Around 10 Kt With Gusts Up To 20 Kt. Seas 2 To 3 Ft.|
|Wed...S Winds 10 To 15 Kt With Gusts Up To 20 Kt. Seas 2 To 3 Ft.|
|Wed Night...S Winds 10 To 15 Kt With Gusts Up To 20 Kt. Seas 2 To 3 Ft. A Chance Of Showers. Winds And Seas Higher In And Near Tstms. 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
645pm EDT Fri July 20 2018
Synopsis: Another dry and cool night is in store as high pressure east of New England slowly slides east. Low pressure along the South Carolina coast will move northward, hugging the coast Saturday and passing to our west Sunday. Its associated warm front will move across southern New England late Saturday night and Sunday, bringing showers and thunderstorms with torrential downpours and gusty winds. Bermuda high pressure then builds west toward southern New England next week bringing tropical humidity and warm temperatures. Scattered showers and thunderstorms are possible each day.
Near Term - Until 6am Saturday Morning
645pm update... Diurnal cumulus is fading over land. South to southeast winds 10 to 15 kt near the shore, light farther inland. Expect one more fair weather night with mostly clear skies and light wind.
Patchy fog is possible overnight from radiational cooling. Dew points are a few degrees higher than past nights, in the mid to upper 50s most places and low 60s near Boston and Providence. Radiational cooling should allow temperatures to fall to near dew point levels. As noted by the earlier shift, this will support patchy fog.
Also as noted earlier, a large fog area is about 2/3 of the way across the Gulf of Maine and slowly moving toward Outer Cape Cod and Nantucket. If it maintains its current movement, it will reach the Outer Cape about 5 AM. The fog/stratus is just north of a small low pressure area that is moving northeast to the south of Nova Scotia. It is possible that the airflow around this low may slow the advance of the fog/stratus
Short Term - 6am Saturday Morning Through Saturday Night
Saturday... Sunshine Saturday morning will begin to fade behind increasing high cloudiness from the southwest and low level cloudiness encroaching from the east by mid-afternoon. Areas of fog are possible along the eastern MA coast by late afternoon. Cannot rule out some drizzle development there toward evening. Winds will be from the east ahead of a strong warm front moving northward along the eastern seaboard. Temperatures will be a little cooler than on Friday, with highs mainly in the mid and upper 70s except some lower 80s in the Connecticut River valley.
Saturday night... * Tropical downpours in showers and thunderstorms, with gusty winds likely late * Low probability of an isolated tornado toward daybreak, mainly in northern CT, RI, and southeast MA
A low pressure system, chock full of tropical moisture, will be working its way northward and hugging the mid-Atlantic coastline Saturday night. It will then head inland, well to our west- southwest toward daybreak Sunday. Its very strong warm front will be moving across south coastal New England late at night.
A very strong low level southerly jet develops...on the order of 55-65 knots at 925-850 mb...after 2am Sunday. With backed easterly flow at the surface, low-level (0-1 km) helicity values are through the roof, with up to 400-600 forecast in western CT and 100-300 in the eastern portions of our area toward daybreak. Experience has shown that where a rapid influx of CAPE intersects with helicity values over 150, and with dewpoints soaring to the lower 70s by daybreak, the potential exists for an early morning tornado. This is especially true at this time of the year. The CAPE is forecast to reach 400-800 J/kg in RI and southeast MA, possibly also northeast CT, by around 09Z, which is 5 AM. Thus, a threat exists for a possible tornado mainly from about 4am into the early daylight hours on Sunday.
With precipitable water values rapidly rising to 2.5 inches, any showers and thunderstorms have the potential to produce 1-2 inches of rain in an hour, not unlike a few days ago. This could produce localized flash flooding, especially in urban areas that are typically known for poor drainage.
Wind gusts to 30-50 mph are possible in the strongest showers or thunderstorms, due to the moderately strong winds above the surface
Long Term - Sunday Through Friday
Big Picture... The upper flow will be driven by high pressure east of Newfoundland and low pressure covering the Eastern USA. Coastal low, surface and aloft, will become trapped under that closed eastern low. This draws the coastal surface low farther west, sweeping across NY and the Eastern Great Lakes. The larger scale gradient favors southerly winds Sunday and Monday, trending from the southwest mid and late week.
The stalled high pressure over the ocean builds west toward New England Monday and Tuesday, then halts and nudges back toward the east Wednesday through Friday. Upper low over British Columbia moves east to central Canada early next week, and eventually approaches New England late week. This will push the northern jet closer to New England by late week.
Model mass and themal fields are in good agreement through Wednesday. Differences emerge mid week mainly with the Canadian shortwave that approaches New England. The GGEM becomes a fast outlier Tuesday while the ECMWF and GFS show better agreement in sending the low into Eastern Canada and hint at a cold front at the end of the week. Confidence is high through the early part of next week, then moderate-high for mid to late week.
Contour heigheights remain above normal through much of next week. Expect temps to be near normal on cloudy days and above normal when there is any sunshine. Clouds and high dew points should keep nighttime temps above normal through the period.
Concerns... Sunday... Coastal low moves northwest across NJ/PA/NY. This lifts a warm front through Southern New England during the morning. Southerly flow over the front will continue to generate lift. The air mass moving up the coast will be very humid. Precipitable water values are forecast to reach 2-2.5 inches, well above normal and capable of producing heavy downpours. Dew points in the air mass are expected to be in the low to mid 70s.
Winds at 850 mb are forecast at 50-60 knots, while winds near the surface and in mediocre lapse rates are around 30 knots and possibly 35 knots at times. Expect gusty south-southeast winds especially along the coast. CAPE reaches 500-1000 joules/km. Helicity values at 0-1Km are forecast to be as high as 300 to 600 as the warm front moves through, although much of this moves through NY and PA. Even so, Southern New England will be in the northeast quadrant of the weather system. This is a low CAPE/high shear environment, which could favor development of rotation in thunderstorms. This will need to be monitored.
Conditions should improve during Sunday afternoon as the warm front moves north and west of our area.
Monday through Friday... Southerly flow continues through most of the week, transporting warm and very humid air into Southern New England. The Atlantic high pressure is expected to drift west and closer to our area Tuesday and Wednesday. This would shift the core of the tropical flow and higher dew point air farther west...this would in turn shift the best chance of showers/tstms into western New England and New York.
By Thursday the northern stream shortwave starts approaching through the Great Lakes. This will push the tropical flow and highest dew points back over Southern New England. Expect showers/thunder to return for these two days.
Cold front associated with the northern shortwave approaches on Friday. GFS and ECMWF suggest the front moves through our area on Friday. This timing may waver over the next few model runs and will need to be monitored.
Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent.
Short Term /through Saturday Night/...High confidence.
High pressure remains in control this evening into Saturday, keeping winds and seas below small craft thresholds.
A Gale Watch has been issued for Narragansett Bay and the waters south of Rhode Island for Saturday night and Sunday. A Small Craft Advisory has been issued for all other waters for Saturday night and Sunday.
A storm system will be moving northward along the mid-Atlantic coast and passing well to the west of the waters by Sunday. A strong warm front will be moving across the waters late Saturday night and Sunday morning. Showers and thunderstorms, with torrential rain and gusty winds are expected late Saturday night into Sunday. Gale force wind gusts are possible, hence the Gale Watch in the waters along the RI coast. Winds are forecast to be near 30 knots over all other waters late Saturday night and Sunday, but it is possible that portions of these areas could need to be upgraded to Gales in subsequent forecasts.
Visibilities will be reduced in possible fog along the MA east coast Saturday afternoon/evening and across all waters Saturday night.
Outlook /Sunday through Wednesday/... Sunday: Low risk for gale force winds with gusts up to 30 kt. Areas of rough seas. Rain showers likely, chance of thunderstorms, patchy fog. Areas of visibility 1 to 3 nm.
Sunday Night through Monday: Winds less than 25 kt. Areas of rough seas. Chance of rain showers, slight chance of thunderstorms.
Monday Night: Winds less than 25 kt. Local rough seas. Slight chance of rain showers, isolated thunderstorms.
Tuesday: Winds less than 25 kt. Seas up to 5 ft. Slight chance of rain showers, isolated thunderstorms.
Tuesday Night through Wednesday: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain showers, isolated thunderstorms
NOAA Boston MA Office: Watches - Warnings - Advisories
CT...None. MA...None. RI...None.
Small Craft Advisory from 2am to 6pm EDT Sunday for ANZ230>234-250-251-254-255. Gale Watch from late Saturday night through Sunday afternoon for ANZ235>237-256