textproduct: Boston / Norton

WHAT HAS CHANGED

No significant changes to the forecast. See Marine section for notes on coastal water levels at Nantucket.

KEY MESSAGES

- Low clouds continue to move in this morning, lasting into Tuesday night. Rain showers possible heading into Tuesday night, as well as early this morning.

- Isolated shower possible Wednesday, but drier overall. Dry conditions return Thursday with seasonably cool temperatures.

- Low pressure system expected to bring rain Friday into early Saturday. Gusty NW winds and below normal temperatures expected to be left in its wake.

DISCUSSION

KEY MESSAGE 1...Low clouds continue to move in this morning, lasting into Tuesday night. Rain showers possible heading into Tuesday night, as well as early this morning.

Low clouds have continued to descend across southern New England from the north as a backdoor front pushes in from the north. Some patchy drizzle remains possible this morning, particularly closer to the MA/NH/VT border. Latest HREF guidance indicates the possibility for two rounds of showers across the region today: one round later this morning, then another this evening heading into early tonight. Latest guidance has shown that these may extend a bit more south into SE MA, including the Cape and Islands this evening. Low pressure associated with a shortwave moving across northern New England is the primary driver for the later showers, which could lead to them becoming more robust towards northern MA.

Notably, lows Tuesday night will be a bit warmer following this disturbance as the flow shifts more SW and 925 mb temperatures climb to nearly 18C overnight. Lows will likely only be in the 50s to low 60s ahead of quite a warm day for Wednesday.

KEY MESSAGE 2...Isolated shower possible Wednesday, but drier overall. Dry conditions return Thursday with seasonably cool temperatures.

The low centered over northern New England continues tracking off to the northeast and surface high pressure with upper level ridging begins to build in over SNE behind it. With the continued westerly winds, downslope flow with the aforementioned elevated low level temperatures (~18C at 925 mb) and good mixing with the boundary layer will all combine to support a much warmer fall day. Highs will likely climb into the 70s, with some areas even approaching 80. Based on the latest ensemble probabilities, the highest temperatures are most likely towards the CT Valley and in much of the coastal plain of SE MA, where highs are expected to reach well into the upper 70s. With the westerly downsloping flow, gusts around 25 mph are not out of the question, especially for the higher elevations Wednesday afternoon.

Isolated showers remain a possibility Wednesday given the trailing cold front associated with the disturbance to our north and PWATs remaining quite elevated. Latest CAMs have a signal for some of these showers towards the afternoon towards eastern MA and RI. Guidance generally has PWAT values between 1.0-1.25" Wednesday, drying out considerably heading into Wednesday night and Thursday with high pressure and a drier airmass over eastern Ontario continuing to build east. 925 mb temperatures Wednesday night drop drastically compared to the afternoon, falling to below 10C and generally remaining there through Thursday. This will be supportive of lows dipping back into the 40s and 50s with daytime highs Thursday in the 60s. With high pressure in full force Thursday, expect drier and more seasonably cool conditions.

KEY MESSAGE 3...Low pressure system expected to bring rain Friday into early Saturday. Gusty NW winds and below normal temperatures expected to be left in its wake.

A deeper, more potent shortwave is expected to deepen well into the Great Lakes region heading into Friday morning and continue tracking east into the northeastern US Friday into Saturday. Latest guidance indicates a LLJ from the SW may set up over the region in conjunction with an anomalous moisture plume of PWATs up to around 1.5" Friday, which given this environment is quite favorable for widespread rain. However, instability is quite lacking across the guidance, so heavier downpours aren't as favored. Ensemble totals for this event range between 0.30" to around 0.75", with the Canadian and the ECMWF ensembles favoring the wetter solutions for southern New England. Given the time out from this system, more refined timing details are a bit uncertain at this time, but the general idea across the guidance is that rain moves in sometime Friday morning and moves out Saturday morning.

Following this rainfall, a strong cold front is expected to push through the region Saturday morning, kicking off strong cold air advection underneath more cyclonic flow. Latest guidance even has the 540dam line extending south into southern New England as 925 mb temperatures sink below 0C Saturday. PWATs fall below 0.25" as this dry, cold air sets in. Lows Friday night fall into the 40s and upper 30s, with the higher elevations likely falling below to at freezing. Highs Saturday may struggle to reach the low 50s across the interior and lows Saturday night will likely dip below freezing across the region, with the exceptions being areas closest to the coasts and urban areas. In the midst of this colder air, strong NW winds prevail, with gusts to 45 mph becoming a possibility given the steeper lapse rates and tight gradient between the exiting strong low and high pressure situated well to our SW.

AVIATION /12Z TUESDAY THROUGH SATURDAY/

Forecaster Confidence Levels...

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

Today...Moderate confidence.

MVFR-IFR ceilings, with -SHRA or -DZ. Likely a round this morning, then another this evening. Used PROB30s, VCSH due to the lower confidence in showers and coverage at BOS/BED/ORH. Potential for patchy LIFR, though somewhat low confidence. NE winds around 5-10 kts, turning S/SE by afternoon and increasing to around 8-12 kts.

Tuesday Night...Moderate confidence.

MVFR-IFR for most. Areas in western MA and CT improving toward VFR after 10z. Breezy S/SW winds up to 15 kt particularly towards the Cape and Islands. Elsewhere, ranging between 5-12 kt. LLWS could be a concern overnight.

Wednesday...Moderate confidence.

Improvement to VFR across the region through the morning hours, especially after 12z; lower cigs may linger a bit over the Cape and Islands. Breezy W winds with gusts to 25 mph possible for the higher elevations and towards the Islands.

KBOS Terminal...High confidence in trends. Moderate confidence in timing.

KBDL Terminal...High confidence in trends. Moderate confidence in timing.

Outlook /Wednesday Night through Saturday/...

Wednesday Night through Thursday: VFR. Breezy.

Thursday Night: Mainly VFR, with local MVFR possible. Chance SHRA.

Friday: Mainly VFR, with local IFR possible. Windy with local gusts up to 30 kt. SHRA, slight chance TSRA.

Friday Night: Mainly VFR, with local IFR possible. Strong winds with local gusts up to 40 kt. Chance SHRA.

Saturday: VFR. Strong winds with local gusts up to 40 kt.

MARINE

Forecaster Confidence Levels...

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

Through Wednesday...High confidence.

E to SE winds 10-25 kt, gusting to 20 to 25 kt. Gusts increase heading into Tuesday night and Small Craft Advisories remain in effect through Wednesday evening. Seas increase Tuesday night into Wednesday, generally ranging between 5-7 ft for the outer waters and up to 8 ft for the ocean waters. RA possible mainly Tuesday night across the eastern waters. Seas and winds drop off Wednesday night, falling below SCA criteria.

Regarding the water levels at Nantucket: latest guidance and observations indicate that they should remain just below Minor flood criteria for this morning's high tide. At most, some splashover can be expected, but not much more than that.

Outlook /Wednesday Night through Saturday/...

Wednesday Night through Thursday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft.

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

Friday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Rain showers, slight chance of thunderstorms.

Friday Night: Moderate risk for gale force winds with gusts up to 40 kt. Rough seas up to 14 ft. Chance of rain showers.

Saturday: gale force winds with gusts up to 40 kt. Rough seas up to 15 ft. Chance of rain showers.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 2 AM EDT Wednesday for ANZ250. Small Craft Advisory until 8 PM EDT Wednesday for ANZ254. Small Craft Advisory until 8 AM EDT this morning for ANZ255- 256. Small Craft Advisory from 1 AM to 8 PM EDT Wednesday for ANZ255-256.