textproduct: Boston / Norton

WHAT HAS CHANGED

Global and ensemble guidance shifted back towards the east with the track of an offshore low later this week. Given the wobble back, confidence remains lower than normal in the exact track for this system at this point in the forecast. The remaining Gale Watch over the southern coastal waters, along with Cape Cod Bay, was upgraded to a Gale Warning given increased confidence in gale- force winds impacting those areas.

KEY MESSAGES

- Dry through most of the day today with rain chances increasing overnight towards the outer Cape and Nantucket.

- Chances for rainfall towards coastal areas for much of the workweek due to an offshore nor'easter well to the south. Main impacts from this system will be marine-based: rough seas, beach erosion, and strong winds.

DISCUSSION

KEY MESSAGE 1...Dry through most of the day today with rain chances increasing overnight towards the outer Cape and Nantucket.

Broad high pressure extending from the Maritimes to the Ohio Valley across the northeast remains overhead today, though with the two centers in the Maritimes and over the Ohio Valley, we find ourselves between them in a weaker spot. This weakened area of high pressure becomes more noticeable as the day goes on with increasing cloud cover from the SE to the NW and especially into tonight with rain chances increasing over the outer Cape and Nantucket. These are associated with low pressure continuing to deepen near the mid- Atlantic/Carolina coast. A slight warming trend continues as highs approach the low 60s and upper 50s.

As low pressure well to our south continues to strengthen, the pressure gradient between it and the high pressure over southern New England will start to tighten, increasing NE winds heading into tonight. This will be especially noticeable over the Cape and Islands, where gusts to 30 mph are a possibility by daybreak Tuesday.

KEY MESSAGE 2...Chances for rainfall towards coastal areas for much of the workweek due to an offshore nor'easter well to the south. Main impacts from this system will be marine-based: rough seas, beach erosion, and strong winds.

Confidence for the track and intensity for the nor'easter well to our south remains somewhat lower than normal at this point in the forecast. Latest guidance (with the exception of the AIGEFS) appears to have backtracked a bit on the more NW shift and instead takes the system slightly north and east instead. What hasn't changed much, though, is that among the ensembles, their members have continued to keep the low's center location south of the 40N/70W benchmark once it does track out to sea. Aside from this, the general trend this week is a drier, somewhat warmer pattern developing as a broad ridge of high pressure remains over much of the northeastern US and into Ontario. Temperatures continue to moderate through the week with highs generally in the 50s and 60s, with cool nights in the 30s and 40s. The exception remains parts of SE MA where rain chances tick up once again towards Thursday.

The latest high-res guidance has shown some of the showers associated with the northern precipitation shield of this nor'easter making their way up to southeastern parts of New England, mainly the outer Cape and Islands. Compared to previous runs, the latest global deterministic guidance has seemed to back off on any rain making it to even Nantucket, for tonight. Given the return to more of a north and east track amongst a lot of the guidance later this week, higher QPF has shifted back more towards just the Cape and Islands. The NAM remains a bit of an exception, as it is a bit more bullish with this system and indicates a signal for greater rainfall totals around 1.5" towards the outer Cape. It does, though, also take the north and east jaunt for this system later this week. Ensemble probabilities remain between 20 and 50 percent for totals reaching or exceeding 0.5" by Thursday evening, which remain focused over the Cape and Islands. For measurable precipitation, probabilities generally up to 30 percent exist over much of the coastal plain in SE MA. The GEFS is the exception as probabilities reach up to 70 percent, though there is a very tight gradient between that and the 50 percent mark in SE MA for this same timeframe. Dry air aloft along with a very tight moisture gradient that looks to be focused over SE MA would limit the spread of any rainfall across anywhere north or west of the I-95 corridor. Again, the highest probabilities remain focused on Cape Cod and the Islands for the second half of the week. Despite the uncertainty with precipitation associated with this system over southern New England, marine impacts continue to look more significant as it remains relatively stationary over the same area, churning over the same waters for what appears to be most of this week. This will lead to seas building up to 15 ft offshore of Nantucket.

Briefly mentioned in the previous Key Message, the pressure gradient between this nor'easter and the high pressure over the northeast US will tighten greatly, increasing NE winds over the waters and towards the Cape and Islands, RI, and the coastal plain in SE MA. These start to really build up tonight and Tuesday as the low pressure deepens and drifts very slightly north and eventually east (based on most of the latest guidance). Not much has changed amongst the guidance regarding the possibility of gale-force winds, sustained and gusts, despite track changes. If anything, confidence has only increased, which is why we now have Gale Warnings in effect for the southern coastal and outer waters starting tonight and lasting through at least Tuesday. Ensembles have continued to indicate favorable probabilities of sustained winds reaching gale- force over the southern waters, with the GEFS and Canadian ensemble up to 50% (and even up to 90% in the Canadian guidance over the ocean waters) just south of the Islands. In terms of gale-force gusts, probabilities have remained in the high (80-100%) range in the ECMWF ensemble and up to 80-90% in the GEFS. Persistent easterly fetch associated with these winds, along with building rough seas, will need to be monitored as well, as beach erosion for east- and northeast-facing beaches may become an issue.

Towards the end of the week, ensembles generally agree in indicating more positively-tilted troughing from Canada making a pass towards New England, which would aid in steering this low pressure out to sea. This would allow for seas to start diminishing and the pressure gradient responsible for the expected strong winds to relax. Significant ridging aloft over much of the CONUS looks to persist and even intensify, then becomes more tilted towards the Great Lakes region towards the weekend. This would be associated with a strong Canadian high pressure that looks to strengthen over the northeast, originating from the Canadian Prairies. This return of high pressure would lean towards drier conditions for the whole region once again.

AVIATION /12Z MONDAY THROUGH FRIDAY/

Forecaster Confidence Levels...

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

Today and Tonight...High confidence.

VFR, with the exception of the Cape and Islands towards Monday night, when MVFR ceilings may develop. Chances for -SHRA increase there as well and should remain generally overthe Cape and Islands, along with an increased chance for gusts to 25 kt. Winds from the NE and light to calm further inland, 5-10 kt sustained closer to the coasts. N to NE winds increase Monday night, with gusts 15-30 kt towards daybreak Tuesday.

Tuesday...Moderate confidence.

VFR. Rain chances toward the Cape and Islands have diminished by this point with the latest guidance, though cloudy skies remain over much of eastern MA and RI. NE winds sustained between 10-20 kt. Gusts to 30-35 kt possible for the Cape and Islands; up to 25 kt for the rest of eastern MA and RI. Gusts to 20 kt may extend inland towards ORH and even BDL.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Tuesday Night through Friday/...

Tuesday Night: VFR. Windy with gusts up to 35 kt.

Wednesday: Mainly VFR, with local MVFR possible. Windy with gusts up to 35 kt.

Wednesday Night: VFR. Windy with gusts up to 30 kt. Slight chance SHRA.

Thursday: VFR. Windy with areas of gusts up to 30 kt.

Thursday Night through Friday: VFR. Windy with local gusts up to 30 kt.

MARINE

Forecaster Confidence Levels...

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

Through Tuesday...High confidence.

Low pressure well south of the waters juxtaposed with the high pressure towards our region will develop a tight pressure gradient, generating stronger winds. This low pressure will also aid in building rougher seas today into Tuesday, reaching up to 12 ft in the southern ocean zones by Tuesday morning. Gale-force winds are expected to develop heading into Tuesday across the southern waters as well, with seas and winds reaching Small Craft Advisory criteria in the northern waters by later tonight. The Gale Watch that was previously over the southern coastal waters, along with Cape Cod Bay, Nantucket Sound, and Buzzard's Bay, has been upgraded to a Gale Warning given the increased confidence in gale conditions developing there. Seas in these zones may also build up to 8 ft.

Outlook /Tuesday Night through Friday/...

Tuesday Night: Moderate risk for gale force winds with gusts up to 40 kt. Rough seas up to 14 ft.

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

Thursday: Low risk for gale force winds with gusts up to 40 kt. Rough seas up to 12 ft. Chance of rain showers.

Thursday Night: Strong winds with gusts up to 35 kt. Rough seas up to 10 ft.

Friday: Strong winds with gusts up to 30 kt. Rough seas up to 10 ft.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Gale Warning from 8 PM this evening to 8 PM EDT Wednesday for ANZ231>235-237-254>256-281>283. Small Craft Advisory from 8 PM this evening to 8 PM EDT Wednesday for ANZ250-251.