textproduct: Boston / Norton

WHAT HAS CHANGED

No changes.

KEY MESSAGES

- Dry and cool weather through at least Tuesday night. Breezy, especially across Cape Cod and the Islands.

- Monitoring the potential for a strong coastal low to form mid to late week, bringing chances for strong winds, rainfall, and coastal flood/erosion concerns heading into the weekend.

DISCUSSION

KEY MESSAGE 1...Dry and cool weather through Tuesday night. Breezy, especially across Cape Cod and the Islands.

Anomalous 1030+ mb high pressure over southeast Ontario Monday, builds eastward into Quebec Tuesday, providing dry and cool weather for Southern New England through at least Tue night. Northeast gradient will maintain breezy conditions, especially across southeast MA, including Cape Cod and Islands. Model soundings here support NE winds 15-20 mph with gusts up to 25 mph at times, less wind away from the coastline.

Abundant sunshine Monday afternoon, except post frontal strato- cu over southeast MA from departing frontal wave, combined with cool northeast flow over the relatively mild ocean. Mid/high clouds will preclude radiational cooling tonight, but a very dry airmass (dew pts in the 40s and 50s) will be sufficient for min temps to fall into 40s, except low to mid 50s along the coast, including metro Boston, Providence and Hartford.

Tuesday, short wave over the OH valley today spreads mid/high clouds into SNE tonight and Tue. However mid level confluent flow over the region will provide sufficient subsidence for dry weather to prevail. These mid/high clouds will yield filtered sunshine Tue. Cooler temps aloft (+7C to +9C) combined with subsidence inversion from 1030+ mb high, will yield another day of cooler than normal highs Tue, with highs only in the low to mid 60s, about 10 degs below normal.

KEY MESSAGE 3...Monitoring the potential for a strong coastal low to form mid to late week, bringing chances for strong winds, rainfall, and coastal flood/erosion concerns heading into the weekend.

Deterministic and ensemble guidance continue to indicate potential for a strong coastal low to form Wednesday into the later half of the week. While there is still a large amount of uncertainty in the guidance, the overarching trend is indicating the low slowly meanders north into the waters south of Nantucket; however, each subsequent model run has trended the track further east. The ECMWF ensemble is showing about a 50/50 member spread on if the low tracks NW of the 40N/70W benchmark while the GFS ensemble is much more spread, stretching from the DelMarVa coast to Georges Bank, and the Canadian ensemble members are majority far east of the benchmark. Regardless, this system is likely to bring strong winds, rough seas, rain, and the potential for erosion/coastal flooding. It is important to note that the exact track and strength of the low will strongly influence the magnitude of gusts, QPF, and how far inland the rain extends.

Highest confidence at this time pertains to winds, especially over the waters. As the low deepens and approaches to the south, the Canadian high over Quebec is also likely to strengthen and expand, leading to a tighter pressure gradient over New England. This will support the development of gusty winds along the south coast, Cape/Islands, and the waters by Wednesday morning... before the arrival of the low. Confidence continues to increase in at least Gale Force winds (34kts/40mph) across the southern waters, and a Gale Watch continues for Tuesday evening through at least Thursday morning. As the low continues to deepen on approach, winds are likely to increase, especially across the outer waters. By Friday night/Saturday morning, the latest ECMWF ensemble shows widespread 20 to 30 percent probabilities of 50 knots (57 mph) across the waters, with a 30 to 40 percent bullseye over Cape Cod Bay. While the winds over land along the south coast, Cape, and Islands throughout the week are more likely to be a touch lighter than the marine winds, they're still expected to be gusty... more on the order of 25-35 mph along the immediate coast.

This prolonged period of gusty northeasterly winds will build rough seas and raise the threat for coastal flooding and erosion, especially for east facing beaches. Latest coastal flooding guidance from the Steven's Institute shows chances for Nantucket and Boston nearing/entering Action Stage. Given the previously discussed uncertainty in track and strength, confidence remains on the lower end for rainfall totals. Ensemble guidance has ticked up slightly in totals compared to previous runs, with the ECMWF ensemble showing 50 to 70 percent probability for 2"+ in 48 hours (20 to 40 percent for 4"+ in 48 hours) over the Cape and Islands, but the GFS ensemble only shows 30 to 50 percent probability (10 percent for 4"+ in 48 hours).

AVIATION /00Z TUESDAY THROUGH SATURDAY/

Forecaster Confidence Levels...

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

00Z TAF Update:

Tonight...High confidence.

Generally VFF. NE winds 10-15 kt with gusts up to 20-25 kt at times across southeast MA terminals.

Tuesday...High confidence.

Low-end VFR around dawn steadily improves thorughout the day. NE winds 8-15 kts with gusts 22-28 kts. Highest gusts expected across the Cape/Island terminals.

KBOS Terminal...high confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Wednesday through Saturday/...

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

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

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

Thursday Night: VFR. Strong winds with areas of gusts up to 40 kt. Chance SHRA.

Friday: Mainly VFR, with local MVFR possible. Strong winds with gusts up to 45 kt. Chance SHRA.

Friday Night: Mainly VFR, with local MVFR possible. Strong winds with gusts up to 50 kt. SHRA likely.

Saturday: Mainly VFR, with local MVFR possible. Strong winds with gusts up to 45 kt. SHRA likely, slight chance TSRA.

MARINE

Forecaster Confidence Levels...

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

Through Tuesday Night...High Confidence.

NE winds 15-20 kt with gusts up to 25 kt at times will continue, as strong 1030+ mb high pressure over southeast Ontario on Monday, advects eastward into Quebec. This northeast fetch will yield rough seas on the ocean waters. Dry weather and good vsby prevail. Potential for gale force gusts appear to hold off until Wed and beyond.

Outlook /Wednesday through Saturday/...

Wednesday: Strong winds with gusts up to 40 kt. Rough seas up to 13 ft.

Wednesday Night: Strong winds with gusts up to 40 kt. Rough seas up to 14 ft.

Thursday: Moderate risk for gale force winds with areas of gusts up to 45 kt. Rough seas up to 15 ft. Slight chance of rain showers.

Thursday Night: gale force winds with gusts up to 45 kt. Rough seas up to 17 ft. Chance of rain showers.

Friday: gale force winds with gusts up to 45 kt. Rough seas up to 19 ft. Rain showers likely, chance of thunderstorms. Local visibility 1 to 3 nm.

Friday Night through Saturday: gale force winds with gusts up to 45 kt. Rough seas up to 20 ft. Rain showers likely, slight chance of thunderstorms.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...High Risk for Rip Currents from 6 AM EDT Tuesday through Tuesday evening for MAZ007-015-016-019>024. RI...High Risk for Rip Currents from 6 AM EDT Tuesday through Tuesday evening for RIZ002-004>008. MARINE...Gale Watch from Tuesday afternoon through Thursday afternoon for ANZ235-237-255-256-282-283. Small Craft Advisory until 3 AM EDT Tuesday for ANZ254>256.