Marine Weather and Tide Forecast for Nantucket, MA

KEY MESSAGES

- Prolonged, heavy rainfall continues Wednesday leading to potential street and flash flooding, especially across western New England.

- Cloudy weather into Friday with onshore flow and off-and-on light rain showers, to go along with cooler than normal high temperatures.

- Generally dry weather with seasonably warm temps, but turning unsettled late in the weekend and especially into early next week.

DISCUSSION
KEY MESSAGE 1...Prolonged, heavy rainfall continues Wednesday leading to potential street and flash flooding, especially across western New England.

We're approaching the period in which we expect the heaviest rainfall and thus the greatest flood threat in southern New England, generally Wednesday morning through Wednesday evening. In the mid levels, an anomalously strong low center is in the process of closing off over New York. Over the course of the next 12 to 36 hours this neutral to negatively tilted trough will be slow to move east thanks to a blocking high over the north Atlantic, setting us up for a long duration rainfall event. It is a very moist and dynamic setup, a bit unusual for July in the northeast. Strong mid level divergence and upper diffluence will induce deep layer ascent.
At the surface deepening low pressure moves up from the Mid Atlantic causing winds to back to the east and inducing a 30-40kt LLJ. This leads not only to convergence/rising air on the nose of the jet but also strong orographic/upslope flow into the high terrain of southern New England; principally the Berkshires but also the Worcester Hills. This will allow the axis of heavier rain over eastern NY to pivot over southern New England, while showers continue to exhibit training potential, especially over the Berkshires early. Given the very saturated atmosphere (PWATs over 1.7") this is a setup conducive to big rainfall totals. As we've gotten into high-resolution guidance concern has increased for the potential of extreme amounts on the order of 5 to 7 inches where the heaviest bands set up. Unfortunately, exactly where they set up is nearly impossible to nail down short of a nowcast situation.
However, the greatest threat continues to be western and central SNE where the Flood Watch is in effect. A more widespread 2 to 4 inches of rain is more likely for any given area. The most concerning period for western SNE will be this morning into the early afternoon when rainfall rates of 1 to 2 inches per hour are possible.
Eventually that shifts into eastern MA in the later afternoon and evening/early overnight hours. So, the biggest concern for dangerous flash flooding (especially in urban and and poor drainage areas)
will be for the Berkshire slopes/Hartford area during the morning commute. This then shifts toward the PVD-BOS corridor for the evening commute. Even given the extreme amounts, we continue to expect this to mainly be a flash flood/street flooding concern as opposed to river flooding, though we are keeping an eye on the rivers and streams.

By Thursday the best moisture and dynamics have moved off but the vertically stacked low lingers leading to a damp, dreary day with lingering showers off and off throughout the day. As for temperatures, persistent easterly flow and thick cloudcover/rainfall will depress high temperatures into the low to mid 70s both Wednesday and Thursday.

KEY MESSAGE 2...Cloudy weather into Friday with onshore flow and off- and-on light rain showers, to go along with cooler than normal high temperatures.

The stubborn, slow-moving low pressure near southern CT with wound- up, vertically-stacked upper-level closed low atop it begins to open up and fill as we move into Friday, eventually pulling offshore and away by Friday night.

Unfortunately this means a continuation of cloudy, dreary weather with at least a modest onshore NE to N wind through the rest of the workweek. Guarded optimism that later in the day Friday could see some limited breaks in the cloudiness, but it's overall a cloudier- than-not period. Off-and-on periods of showers can be expected as well too, but with the moist layer shown in model forecast RH cross sections being about 5000 ft or less in depth from the surface, these showers should prove light and could resemble more of a drizzle. Lows Thursday night are in the upper 50s to low 60s. Highs on Friday though are more challenging in that if we do get any late- day breaks temps could reach into the low 80s. Better chance of that in western MA/CT, but otherwise have kept temps in the 70s, with even some upper 60s out over the Cape due to more clouds/onshore flow.

KEY MESSAGE 3...Generally dry weather with seasonably warm temps, but turning unsettled late in the weekend and especially into early next week.

Weak high pressure then begins to build in for the weekend, so we should see at least briefly drier weather return, though we could have to watch for lingering low clouds over the Cape and Islands.
Confidence in this part of the forecast is lower-than-average though, as the issue is that most of the global ensembles show another elongated upper trough digging into the OH Valley with southwest flow aloft developing by Saturday. It's a pattern which could spell rain chances coming back into view in the SW flow aloft.
While the non-GFS guidance keeps it a generally dry weekend, the GFS shows scattered showers/storms later Saturday, with more to come as we move into early next week. Thinking it turns more unsettled again early next week, but it is not out of the question there could be showers/storms popping up again by the weekend. Maintained a model- blended approach for this period.

AVIATION /12Z WEDNESDAY THROUGH SUNDAY/
Forecaster Confidence Levels...

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

12z TAF update...

Today and Tonight...Moderate confidence.

Rain has started to overspread the region this morning and will continue to do so today into tonight. IFR/MVFR expected in continued periods of heavy rain. SE to E winds 10-15 kts with gusts to 25 kts possible. Potential for LLWS for eastern terminals overnight. Will have to monitor for isolated instances of embedded TSRA this morning and this afternoon. Confidence in its occurrence varies a bit depending on location, with higher confidence towards BDL this morning. Confidence not high enough to include in most TAFs this afternoon and evening given how little has been produced so far in areas already affected by the rain.

Thursday...Moderate confidence.

IFR/MVFR in scattered showers. NE winds 5-10 kts.

Thursday Night...Moderate confidence.

Widespread IFR and lower end MVFR. Lingering scattered showers with N winds remaining generally below 10 kt.

KBOS Terminal...Moderate confidence. Lower confidence in possible embedded TSRA later this afternoon. Not high enough to include in TAF at this time, but if it does occur, it will likely be between 23z and 02z tonight.

KBDL Terminal...Moderate confidence. Chance for embedded TSRA this morning between 12z-14z as rain moves in.

Outlook /Thursday Night through Sunday/...

Thursday Night: Mainly VFR, with local IFR possible. Chance SHRA, isolated TSRA.

Friday: VFR. Slight chance SHRA, isolated TSRA.

Friday Night through Saturday Night: VFR.

Sunday: Mainly VFR, with local IFR possible. Slight chance SHRA, slight chance TSRA.

MARINE
Forecaster Confidence Levels...

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

Through Thursday night...High confidence.

An anomalous midsummer low pressure system will continue to bring a prolonged period of unsettled weather to the southern New England coastal waters through Thursday. Periods of heavy rain, reduced visibilities, and embedded thunderstorms are expected through much of the period. A Small Craft Advisory remains in effect for all waters through Wednesday, and for the outer coastal waters through Thursday. South to southeast winds increase tonight, with gusts of 25 to 30 kt expected on Wednesday. Seas build to 4 to 6 ft, highest across the outer waters. Wind direction becomes more variable on Thursday as the surface low tracks across the southern New England coastal waters. East to northeast winds ahead of the low will shift to north to northwest in its wake, although the eastern waters may remain northeasterly longer before the system gradually departs Thursday night.

Outlook /Thursday Night through Sunday/...

Thursday Night: Winds less than 25 kt. Seas up to 5 ft. Chance of rain showers, isolated thunderstorms.

Friday: Winds less than 25 kt. Seas up to 5 ft.

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

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

Saturday Night through Sunday: Winds less than 25 kt.

BOX WATCHES/WARNINGS/ADVISORIES
CT...Flood Watch through this evening for CTZ002>004.
MA...Flood Watch through this evening for MAZ002>004-008>012.
RI...Flood Watch through this evening for RIZ001-002.
MARINE...Small Craft Advisory until 8 PM EDT this evening for ANZ230>237-251.
Small Craft Advisory until 8 AM EDT Thursday for ANZ250- 254>256.