textproduct: Boston / Norton

WHAT HAS CHANGED

Global and ensemble guidance as trended a bit farther NW with the track of an offshore storm later this week. While there will need to be more multi-run consistency to call this a true trend, this adjustment shows that the eventual path of this system is still far from set in stone at this time. Otherwise, a Gale Watch has been upgraded to a Gale Warning for the outer waters Tuesday into Thursday. Closer to shore, a Gale Watch has been issued for the same timeframe.

KEY MESSAGES

- Not as cold tonight with clear skies and light to calm winds. Dry for most through Monday Night.

- Increasing chances for rainfall across coastal areas for this upcoming work week as a offshore Storm gradually approaches from the south. The system likely brings significant marine impacts in the form of strong winds, rough seas, and beach erosion through much of the upcoming week.

DISCUSSION

KEY MESSAGE 1...Not as cold tonight with clear skies and light to calm winds. Dry for most through Monday Night.

Center of a high pressure reaches the Maritimes this evening, but is still strong enough to dominate our weather through Monday. Then southern new England is stuck between a high pressure over the North Atlantic and another over Ohio. This slight weakness in the pressure pattern opens the door for a low pressure off the Mid Atlantic coast to nudge slightly farther north late Monday into Monday night.

For most of our region, dry weather will continue through Monday night with a slight warming trends. The exception will be towards the outer Cape and Nantucket, where some showers are possible Monday evening and night. Still, there will be plenty of dry hours there as well.

KEY MESSAGE 2...Increasing chances for rainfall across coastal areas for this upcoming work week as a offshore Storm gradually approaches from the south. The system likely brings significant marine impacts in the form of strong winds, rough seas, and beach erosion through much of the upcoming week.

Confidence is somewhat lower than normal for the eventual track and intensity of this week's nor'easter. While the majority of guidance tracks the low center well to the S of the 40/70 benchmark, there has been a trend NW amongst AI, high res, and an increasing number of global guidance. This subtle NW jog is likely driven by the interaction between the cutoff low driving the nor'easter and an additional shortwave trough moving S from Canada by later Wednesday. The main change has been to amplify the shortwave from a flat/positive tilt to more of a neutral tilt. This positioning allows for a stronger more amplified system and thus a track further N. In terms of QPF, the northward trend has brought a notable uptick in the probability of >0.5" of rainfall along and S of the I-95 corridor. As of the 12z suite, most models show a 40-60% chance of rain totals in this range. There are still some models, namely the NAM and AI guidance that show a track even further north and the chance for a soaking rain event for much of the region. This is certainly a low probability event but nonetheless a possibility at this time.

The higher confidence aspect of the forecast continues to be significant marine impacts driven by increasing NE winds. Low pressure to the S will move against high pressure over southern Quebec later Monday. The resulting pressure gradient will bring an increasing likelihood of Gale force winds well to the north of the actual surface low. Winds build up towards Monday night as the low pressure deepens and drifts to the north. Ensembles have shown near 80-90 percent probabilities of sustained winds reaching gale-force over the southern waters despite a low track to the S of the benchmark. Thus, we opted to upgrade Gale Watches to Gale Warnings for the outer waters and have introduced Gale Watches for the near coastal Waters. Finally, we will have to monitor the chance for significant beach erosion as well with a prolonged easterly fetch of winds and waves through the week.

Later this week, the aforementioned trough likely pushes the coastal low farther offshore putting an end to any lingering impacts. 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 after originating from the Canadian Prairies, which would lean towards drier conditions for the whole region once again.

AVIATION /18Z SUNDAY THROUGH FRIDAY/

Forecaster Confidence Levels...

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

Through Tonight...High confidence.

VFR. Calm/light winds continuing. Stronger E winds towards Nantucket, with G20kt possible by daybreak.

Monday and Monday Night...High confidence.

VFR, with the exception of the Cape and Islands towards the late afternoon and evening hours, when MVFR ceilings may develop. Chances for -SHRA increase there as well and should remain generally over the 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.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Tuesday through Friday/...

Tuesday: VFR. Windy with gusts up to 30 kt.

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 local gusts up to 30 kt. Slight chance SHRA.

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

Thursday Night through Friday: VFR. Windy with areas of 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 Monday Night...High confidence.

Relatively light winds and seas expected through most of tonight. A low pressure well south of the waters will begin to generate stronger winds and rougher seas late tonight into Monday across the outer coastal waters. Small Craft Advisory conditions are expected across the southern coastal waters, but will keep the focus on the period of stronger winds mid week.

Outlook /Tuesday through Friday/...

Tuesday: Strong winds with gusts up to 40 kt. Rough seas up to 14 ft. Slight chance of rain showers.

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

Wednesday: Moderate risk for gale force winds with gusts up to 40 kt. Rough seas up to 15 ft. Chance of rain showers.

Wednesday Night: Strong 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 35 kt. Rough seas up to 13 ft. Chance of rain showers.

Thursday Night: Strong winds with gusts up to 35 kt. Rough seas up to 12 ft.

Friday: Strong winds with gusts up to 35 kt. Rough seas up to 13 ft. Slight chance of rain showers.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Gale Watch from Monday evening through Wednesday evening for ANZ231>235-237. Gale Warning from 8 PM Monday to 8 PM EDT Wednesday for ANZ254>256-281>283.