Boston Harbor Marine Forecast (2024)

Synopsis
Scattered showers are expected to develop this afternoon from east to west across Cape Cod and the Islands and the South Coast as low pressure approaches from the ocean. Showers will overspread the rest of the region tonight into Monday along with isolated thunderstorms as the low pressure moves into New England. The rest of the week from Tuesday into next weekend will feature an unsettled pattern with daily chances of showers and thunderstorms and oppressive humidity.

Near Term - Through Tonight
945am Update: No significant changes.

Just some minor changes to account for cloud cover through afternoon otherwise the forecast is on track.

Satellite loop shows clouds over ocean advancing NW toward Cape Cod and Islands this morning. Thicker cloud cover was pivoting south of islands but will eventually reach much of south coast by early afternoon, and rest of SNE later in the day. Also starting to see some showers well offshore on radar which will begin to reach Cape Cod and Islands after 2-3 PM, and probably into Providence and Boston 6-8 PM.

High-res guidance continues to indicate that we'll see showers and embedded thunderstorms tonight as coastal low approaches. Forecast soundings show steep lapse rates due to colder air aloft associated with 500 mb low, and some instability aloft. Increasing PWATs (Precipitable Waters) also mean we'll see localized downpours as well but trying to pinpoint more favored locations is going to be difficult. We're starting to see a trend in guidance that portions of CT, RI, and SE MA will be more likely to see these downpours. Pattern doesn't show a significant flood threat so we do not anticipate the need for any Flood Watches, but certainly the usual minor urban flooding could be an issue for some of the more susceptible locations around greater Providence, Fall River, and New Bedford.

One thing to keep in mind - these types of setup with a closed low moving back toward coast is something that models can struggle with, even within 24 hours, leading to lower than usual confidence in how things will play out. This includes location and amount of rainfall, thunderstorm potential, and wind gusts near south coast and over coastal waters. One thing we are more confident in is the potential for strong rip currents along eastern MA ocean beaches Monday and we are considering a High Surf Advisory with afternoon forecast.

Short Term - Monday
Monday... Do want to note that any delay in the arrival of the surface/mid level low will shift the impacts of the "near term" forecast a few hours later into this "short term" period. Thus, there is a scenario in which the heaviest rain is delayed until after 12Z Monday.

Surface low pressure will track along the eastern MA coastline on Monday which will allow showers to continue through the day. With that said, do anticipate the intensity of showers will wane through the day with guidance showing a significant finger of dry air, with PWATs (Precipitable Waters) at or just below 1", wrapping around the center of the low after 12/15Z Monday. With the low pushing north in Maine by Monday PM, thinking that our best shot at substantial additional precipitation will come before the lunch hour. General thought is that we could see up to another half inch of rain during the daylight hours of Monday, with greater Quantitative Precipitation Forecast being confined to any lingering convective downpours as bands of showers sweep to our northeast.

The departing low is really just the catalyst towards setting up an unsettled week. By 00Z Tuesday, surface winds will have shifted solidly to the southwest, which will begin a long period of moisture advection into the region. Seasonable, but muggy conditions should be expected the second half of the day as PWATs (Precipitable Waters) climb back towards 1.75".

Given residual wave action, do anticipate an elevated risk for rip currents for our eastern facing beaches on Monday. Later shifts will likely issue some sort of headline to account for this risk.

Long Term - Monday Night Through Saturday
Key points... * Daily chances of showers and t-storms Tuesday into next weekend * Oppressive humidity develops by Tuesday and continues into next weekend * Heat indices may approach the mid-upper 90s by the end of the week

Monday night into Tuesday... The upper low will be lifting into northern New England Monday night before moving into Canada on Tue. Best forcing will follow the upper low to the north so expect decreasing areal coverage of showers Monday night, but scattered showers and t-storms expected to redevelop during Tuesday as diurnal instability increases with heating and increasing moisture. Highs will reach the 80s Tuesday with oppressive humidity developing as dewpoints climb into the lower 70s.

Wednesday through Saturday... Broad trough sets up across the Gt Lakes to NE with a series of upper level waves moving through. This pattern will support continued unsettled weather with chances of showers/t-storms each day as diurnal instability develops in moisture rich environment. Interior locations will likely be favored over the coast for best chance of showers and storms. But timing of convection is very much uncertain and there will be dry periods with subsidence behind departing shortwaves. It appears Wednesday may be the wettest day as PWATs (Precipitable Waters) spike over 2 inches ahead of the approaching trough, then another favored wet period may be sometime Fri or Sat as another trough approaches with an accompanying increase in PWATs.

Oppressive humidity will be persistent through the period with daytime dewpoints expected to reach low-mid 70s. High temps may average a bit below normal Wed, but possibly reach 90 degrees in the interior Thu to Sat bringing heat indices to mid-upper 90s.

Marine
Forecaster Confidence Levels:

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

Overall high confidence.

Low pressure retrogrades from east to west across the southern waters this evening before moving up the eastern waters on Monday. Gusts are likely to exceed 25kt overnight tonight and into Monday across all of the outer waters with seas building to 5-7ft. Thus, a small craft advisory has been issued for the outer waters, Nantucket sound, and the northeastern inner waters for various period through 00Z Tuesday. Heavy rain and thunderstorms are likely after 00Z tonight through ~16Z Monday.

Will note there is a risk for high surf/rip current headlines on Monday given residual swell/wave action.

Outlook /Monday Night through Thursday/... Monday: Winds less than 25 kt. Local rough seas. Chance of rain showers, slight chance of thunderstorms.

Monday Night through Tuesday: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain showers, isolated thunderstorms.

Tuesday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Slight chance of rain showers.
Wednesday: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain showers, isolated thunderstorms.

Wednesday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain showers, isolated thunderstorms.

Thursday: Winds less than 25 kt. Seas locally approaching 5 ft. Slight chance of rain showers, isolated thunderstorms.

NOAA Boston MA Office: Watches - Warnings - Advisories
CT...None. MA...None. RI...None.

Marine
Small Craft Advisory from 11pm this evening to 10am EDT Monday for ANZ232. Small Craft Advisory from 8pm this evening to 8pm EDT Monday for ANZ250-254>256. Small Craft Advisory from 2am to 8pm EDT Monday for ANZ251.