Boston Harbor Marine Forecast (2024)

Synopsis
Showers and thunderstorms continue this evening before tapering off tonight. Hot and humid Monday with a low chance for severe thunderstorms in the late afternoon. Stalled cold front mid week will bring near daily rain chances. Now tropical storm Debby looks to impact our region, bringing the potential for heavy rain and flooding, Friday through Sunday.

Near Term - Until 6am Monday Morning
Tonight:

Line of thunderstorms has progressed east into RI and SE MA. With the loss of diurnal heating and more stable marine air, storms are slowly weakening. With MLCAPE values dropping to 1000 J/kg or less and the boundary layer starting to decouple and stabilize, not expecting any more severe winds at the surface. Lightning activity could last though 8-10pm with the leftover elevated instability. Models have backed off on showers and thunderstorms staying alive over the Cape and Islands late this evening. Overnight lows remain elevated in the upper 60s to low 70s with dewpoints still in the upper 60s as well.

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

Shortwave moves offshore, but little change is the airmass is expected as SW flow continues. Skies tomorrow start out sunny, but mid to high clouds form ahead of another passing shortwave. The main forcing from the shortwave passes to the north of SNE in VT and NH. This should keep the best chance for thunderstorms to the north. However, if any thunderstorms do form in SNE tomorrow, they will be in an environment sufficient to produce severe weather. Mean SBCAPE values range from 1500-2500 J/kg tomorrow afternoon with 30-40 knots of 0-6km bulk shear. Mid-level lapse rates become more favorable tomorrow approaching 6.5C/km. The primary severe threat appears to be wind again with DCAPE values around 700-800 J/kg and steep low level lapse rates approaching 10C/km. There could be some small hail given the marginally steep mid level lapse rates and strong shear, however, freezing levels are still very high at 12-13kft. Heavy rain is possible with any storms that form with PWATs (Precipitable Waters) still near 1.5 inches. The main window for severe thunderstorms tomorrow appears to between 2pm to 10pm, but uncertainty remains high as most of the Hi- res guidance is showing the bulk of the thunderstorm activity staying to the north of SNE.

It will still be hot and humid tomorrow with mostly sunny skies. High temps reach into the upper 80s to low 90s with dewpoints still in the upper 60s to low 70s. Heat indicies will approach the mid 90s.

Monday Night

Cold front drops south form northern New England that may bring more showers and thunderstorms, but confidence is low at this time. Some of the hi-res guidance does show thunderstorm activity starting around midnight on Tuesday, but will likely be dependent on how much instability is leftover. HREF shows 500-1000 J/kg of MUCAPE still available past midnight. With deep layer shear increasing overnight to 40-50 knots, there is a low chance these thunderstorms could become severe, however, as the boundary layer decouples, it will be more difficult for severe wind gusts to reach the surface. Overnight lows stay warm in the upper 60s to low 70s again, but dewpoints begin to fall into the mid 60s.

Long Term - Tuesday Through Sunday
* Stalled front Tuesday/Wednesday will yield unsettled weather mid week, though the potential for severe weather is low

* Best chance for dry weather comes Wednesday night into the first half of Thursday

* Now Tropical Storm Debby may impact the region late next week or next weekend; high uncertainty in timing/impacts

We're expecting an unsettled week ahead with persistent mid level troughing over the northeast and a stalled cold front that will bring near daily rain chances to the region through Thursday. Beyond the first few days of the forecast, our attention is focused on now Tropical Storm Debby, which is slated to make landfall, possibly as a Hurricane in the big bend of Florida, in the next day.

Through Thursday... As mentioned by the previous forecaster, cold front gets hung up near or just south of the south coast Tuesdays and will linger for much of mid-week. While temperatures will be much cooler than we've seen the last few weeks, and actually several degrees below normal, PWATs (Precipitable Waters) will remain high, some 1.5-2 standard deviations of normal, thanks to the tropical moisture plume advective northward from the deep south. Agree that the risk for the most persistent rain/shower activity will be south of the MA Turnpike, and primarily across CT, RI and SE MA, with Wednesday targeted as the wettest day as the front wobbles a bit further north. Previous forecaster also mentioned the still deep warm cloud depths, which combined with high PWATs (Precipitable Waters) will lead to downpours, which will pose a threat for urban and poor drainage flooding. Additionally, the orientation of the frontal boundary, west to east, could result in some training. Continue to feel the marginal risk ERO from WPC covers the risk well. Will note that late Wednesday evening to Thursday morning looks like the best chance for a prolonged dry period, which would be beneficial in allowing the region to dry out slightly before the weekend. Did take this opportunity to lower Probability of Precipitation in the late Wednesday to Thursday timeframe to reflect this possibility.

Tropical Storm Debby... There remains a lot of uncertainty surrounding the late week and early weekend forecast regarding the track of Tropical Storm Debby and what impacts might be seen in southern New England. While there is rather high confidence that Debby gets "hung up" in the Charleston/Savannah area early this week anticipating the low, and moisture, will eventually be steered towards our region. Impacts from Debby may begin as early as Friday and last through Sunday evening. Ensemble probabilities paint a picture of a 10 to perhaps 40% chance of 24 hour Quantitative Precipitation Forecast exceeding 2" and a few pixels of exceeding 4" of Quantitative Precipitation Forecast during the same period. Combining this with deterministic runs, with both the GFS (Global Forecast System) and ECWMF showing wide swaths of 2-5"+ of rain across the region, its quite reasonable to deduce, based on the current forecast track of the storm, that this storm will bring a considerable amount of rain to SNE. In fact, WPC has placed us in an uncommon day FIVE marginal risk for excessive rainfall, which may be elevated in later updates. We will have a better grasp on the expected max precipitation potential and location of heaviest rain over the next several days.

Marine
Forecaster Confidence Levels:

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

Tonight through Monday night...High Confidence.

Persistent southwest flow will result in marginal small craft seas across the southern outer-waters right into Monday night and will allow headlines for that region to continue. In addition, small craft headlines this afternoon for Buzzards Bay,Cape Cod Bay, and Nantucket sound continue though midnight. Good mixing near the land should result in the potential for a period of southwest 30 knot wind gusts and choppy seas. Similar conditions are also possible for Monday afternoon, but likely will remain below small craft criteria.

Showers and thunderstorms over the northern waters this evening will move south into the southern water overnight, eventually moving well out to sea by Monday morning. More showers and thunderstorms possible near the waters Monday night.

Outlook /Tuesday through Friday/... Tuesday: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain showers, isolated thunderstorms.

Tuesday Night: Winds less than 25 kt. Rain showers likely, isolated thunderstorms.

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

Wednesday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Slight chance of rain showers.
Thursday: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain showers.
Thursday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain showers.

Friday: Winds less than 25 kt. Areas of seas approaching 5 ft. Rain showers likely, isolated thunderstorms. Local visibility 1 to 3 nm.

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

Marine
Small Craft Advisory until midnight EDT tonight for ANZ231>234. Small Craft Advisory until 8am EDT Tuesday for ANZ254>256.