Marine Weather and Tide Forecast for Nantucket, MA

NEAR TERM /THROUGH TONIGHT/
300 AM Update:

Key Messages:

* Increasing cloud cover towards overcast today.

* Mostly dry during the morning, but risk for showers increases late morning to mid afternoon from west to east. Washouts not expected.

* Still rather mild temps in the lower to mid 70s, but cooler along the coast and in western New England.

Details:

Gradual deterioration in conditions expected today with increasing cloudiness and a gradual north/east expansion of the existing rain shield over SW CT. Recent trends have offered a later onset timing of these light rain showers even into western New England, and it may not really develop until late morning/noontime for the vast majority of locations in western MA/CT, and not until mid afternoon for most of central and eastern MA and RI. Overall not looking like a washout today in most areas. The later arrival of rain showers allowing for a little longer period of sun in eastern MA away from the coast should allow temps to rise into the mid 70s, but quite a bit cooler near the coasts in the lower to mid 60s. In CT and western MA, slow rises in temps due to overcast should keep temps in the mid to upper 60s.

SHORT TERM /THURSDAY/
300 AM Update:

Key Messages:

* Periods of showers, even a rumble or two of thunder tonight to early Thurs AM. Best chance interior SNE.

* Rather humid with more clouds than sun on Thurs, with hit- or-miss showers possible at times.

Details:

Tonight:

Approach of deamplifying shortwave trough energy over the OH Valley will allow for an increased SSE low-level jet to develop tonight as a warm front moves thru. This will transport increasing precipitable water values/higher dewpoints along with a burst of weak elevated instability rooted above near-sfc stable layer (Showalter indices around 0 to -1, with CAPE values ranging from 300-700 J/kg using a most-unstable parcel approach). Expect showers to increase in coverage tonight in the interior, then spread ENE into RI and eastern MA toward daybreak. Given the weak elevated instability, included a mention of isolated thunder but severe weather isn't expected.
It would be more of the isolated lightning strike type of mention. Rain amts could be up to a quarter- inch in some spots, but most areas stay below that, with lesser rain totals as one moves north. Should be a rather mild night and a touch humid too, with temps in the mid to upper 50s, maybe staying near 60 out in the CT Valley.

Thursday:

Warm front then moves offshore early-morning on Thurs, putting much of Southern New England in a cloudy warm-sector which is also rather humid by mid-May standards (e.g. dewpoints in the lower 60s). Wouldn't rule out intermittent, slow-moving showers in this warm sector, and although convective temps are pretty low, there isn't really much to hang on to as far as sources of lift go. Given this I couldn't show PoPs any higher than slight/lower Chance. Thinking there will end up being several hours of dry weather, but there will also be some showers to dodge at times. How warm temps may get is contingent on breaks in the cloudiness; I kept temps mainly in the 60s to low 70s under mostly cloudy conditions but potential for quite a bit warmer (well into the 70s) if we do get any sustained breaks in cloud cover.

LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/
Key Messages:

* Unsettled conditions late this week into the start of the weekend, including a low risk for thunderstorms Friday and Saturday

* Temperatures decrease late in the weekend into early next week

Details...

Guidance continues to agree on the overall pattern late this week into the weekend. Drier conditions return Thursday night ahead of our next chance for showers and storms. A low moving across southern Canada will continue to move east and will eject shortwave energy eastward into the northeast US, which may assist in creating a more favorable environment for some storms. The chance that some could turn severe at this time still remains low, and what does develop is more likely to be quite scattered versus being a widespread event.
Ensembles are continuing to hint at some elevated CAPE values Friday into Saturday, with SREF probs for surface CAPE at 500 J/kg or higher ranging from 70 to 90 percent across western and central MA.
Some lower probs Friday exist for MLCAPE values at or above 1000 J/kg across this same area, reaching up to 30 percent. The highest chances for severe weather, though, still remain to our southwest.
This will continue to be monitored as we get closer to the weekend.

A cold front moves through Sunday which is expected to usher in a drier airmass following the exit of the mid-level low. Winds shift more to the NW and ridging begins to push into the region to start next week. 925 mb temperatures cool from around +20C over Friday and Saturday closer to +5C and +10C by Monday, continuing into Tuesday.
Highs Friday are expected to be in the upper 70s and even into the low 80s for most, aside from the Cape and Islands where southerly winds moving over the Atlantic will bring highs down to the low 70s and upper 60s. Saturday and Sunday highs are expected to be similar; primarily in the 70s. Once the cold front moves through, highs to start the week may settle more into the mid to upper 60s.

AVIATION /12Z WEDNESDAY THROUGH SUNDAY/
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.

12z TAF Update:

Today: Moderate confidence.

MVFR stratus has made it into BED/OWD westward, while still VFR for BOS and the Cape airports. Expect deterioration to MVFR for these far eastern airports ~18-22z. Rain showers continue to make very slow northeast progress, best chance at rain during the afternoon from PVD to ORH west. SE winds 5-10 kt.

Tonight: Moderate confidence.

MVFR-IFR, periods of SHRA with embedded TS. Best chance at TS over CT/western and central MA airports but not likely worse than isolated covg. Light SE to E winds.

Thursday: Moderate confidence.

MVFR-IFR likely to continue into Thurs, though SHRA coverage turns more intermittent with several dry periods in between.
SE to S winds under 10 kt.

KBOS TAF...Moderate Confidence in TAF. VFR with periodic MVFR bases thru 18z, then a more widespread MVFR thereafter with IFR more likely after 02z. -SHRA possible by 02z but more likely after 06z Thu. E/SE winds 5-9 kt.

KBDL TAF...Moderate Confidence in TAF. MVFR ceilings but dry until around 17z. IFR ceilings develop late afternoon to early evening in SHRA, outside chance at TS after 02z.

Outlook /Thursday Night through Sunday/...

Thursday Night: Mainly VFR, with areas IFR possible.

Friday: Mainly VFR, with local MVFR possible. Chance SHRA, isolated TSRA.

Friday Night through Saturday: Mainly VFR, with local IFR possible. Chance SHRA, isolated TSRA.

Saturday Night: Mainly VFR, with local IFR possible. Chance SHRA.

Sunday: Mainly VFR, with local MVFR possible. Breezy. Slight chance SHRA.

MARINE
Forecaster Confidence Levels:

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

Today through Thursday...High Confidence.

Winds will remain below SCA criterion through the period.
However waves will be building to around 4-6 ft over the southern offshore waters starting later today and into Thursday. SCAs remain posted due to the building seas. Areas of mist/fog and showers develop tonight and into early Thursday.

Outlook /Thursday Night through Sunday/...

Thursday Night: Winds less than 25 kt. Areas of seas approaching 5 ft.

Friday: Winds less than 25 kt. Seas locally approaching 5 ft.

Friday Night through Saturday: Winds less than 25 kt. Chance of rain showers, isolated thunderstorms.

Saturday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain showers.

Sunday: Winds less than 25 kt. Areas of seas approaching 5 ft.
Slight chance of rain showers.

BOX WATCHES/WARNINGS/ADVISORIES
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 6 PM this evening to 8 PM EDT Thursday for ANZ255-256.