textproduct: Boston / Norton

WHAT HAS CHANGED

Gale Watch issued for Tuesday afternoon through Wednesday early evening.

KEY MESSAGES

- Seasonable summer weather today with temperatures in the 70s to mid 80s, with seabreezes near the coasts.

- Active weather Tuesday and Wednesday. Potential for hazardous marine conditions, high surf, soaking downpours and a limited risk for severe weather.

- Drying out late-week with a return to more seasonable temperatures and lower humidity levels into next weekend.

DISCUSSION

KEY MESSAGE 1...Seasonable summer weather today with temperatures in the 70s to mid 80s, with seabreezes near the coasts.

Another great day of pleasant summer weather to open the workweek. High pressure near vicinity of Cleveland OH builds south, and brings a modest WSW to SW wind later in the day. Local seabreezes still look to develop, but full sun with highs in the upper 70s to mid 80s. A touch cooler along the coasts with the seabreezes. Clear skies continue tonight but we'll be starting to increase moist advection. There could be some patchy mist or low clouds in interior portions of Southern New England but probably not much impact.

KEY MESSAGE 2...Active weather Tuesday and Wednesday. Potential for hazardous marine conditions, high surf, soaking downpours and a limited risk for severe weather.

As the aforementioned high pressure cell moves offshore, in its wake is an approaching frontal system over the Great Lakes region with rather strong mid-upper level winds accompanying it. Besides also leading to a tightening southerly pressure gradient, a southerly 925-850 mb jet upwards of 60 kt will permit strong moist advection and high IVT as PWATs rise to 2.25" along the system's cold front/frontal occlusion Tue night/Wed. Several potential issues to Southern New England Tuesday into Wednesday. Ranked in order of highest to lowest confidence, these range from hazardous marine conditions/high surf, another round of soaking downpours, and a more limited risk for strong/severe thunderstorms potentially into the overnight hours.

See the Marine portion for more details. Gale Watches have now been hoisted on most waters starting Tuesday afternoon through Wednesday early evening; multi-model-ensemble 34+ kt wind probs remain in the 50-75% range in the Gale Watch area, highest the further east/offshore one goes. The issue is on mixing in the southwesterly warm advection pattern, which those coarser-res ensembles can struggle to resolve, but potential exists for parts of or all of the Watch to see gusts into gale force range. Although they're mostly wind waves in a southerly fetch, waves build to upwards of 12 ft and while not a beach day by any stretch of the imagination, the potential exists for high surf and later shifts will need to consider high surf or rip current headlines. Less common to see gale watches in the summer months, so hopefully getting the Watch out now conveys to mariners that conditions could turn hazardous especially Tuesday night and into Wednesday when factoring in the winds and building rough seas.

Showers and thunderstorms begin to increase Tuesday afternoon, and especially late in the day and into the Wednesday morning as a combo of rising dewpoints and increasing upper support from being placed in the poleward exit region of a 70-kt 500 mb jetstreak provides additional upper support. Although severe weather potential looks quite a bit greater well to our west, increasing MUCAPEs and K- indices into the Tue overnight support a risk for embedded stronger storms. Given the lower-level winds improve overnight, some risk for straight line wind damage and/or a brief spin-up are possible, but would hinge on mustering enough 0-3 km CAPE. Wednesday might harbor a better chance at stronger daytime storms as the front will have moved offshore allowing for greater destabilization, and we'd be placed under the upper trough with still favorable mid/upper windfields.

In terms of rainfall, it looks like areas along the southern CT, south coast and the Cape and Islands stand to see the most rain, at the expense of interior Southern New England. Ensemble 24-hr QPF probs paint moderately high (40-70%) probs of rain totals AOA 1 inch along the southern/southeast coast, and lower (< 40%) to the north and west. These areas have been getting quite a bit of rain lately, especially out over Cape Cod. But high FFG owing to the sandy soil could limit flooding. If we do get persistent downpours in some of our RI/southeast MA larger cities along a stalling frontal zone, then the risk would increase but this is uncertain.

KEY MESSAGE 3...Drying out late-week with a return to more seasonable temperatures and lower humidity levels into next weekend.

Cold front stalls well offshore by Thurs, with an expansive high pressure across the Great Lakes ridging into the area late this week into the weekend. Temperature profiles are pretty cool for late July, at around +6 to +10C, but hard to get too cool with the sun angle being as high as it is. Thus expect seasonable temperatures, low humidity and dry weather late this week right through most of the weekend. Really not until Sunday when we get into a southerly return flow and with that comes increasing humidity levels.

AVIATION /06Z MONDAY THROUGH FRIDAY/

Forecaster Confidence Levels...

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

06z TAF Update:

Through 12z Monday: High confidence.

Mainly VFR. Patchy valley fog is still possible (recently developed at OWD), but coverage should be spotty and away from the TAFs. Light winds.

Today: High confidence.

VFR. Winds become SW around 5-10 kt by afternoon, although southeasterly seabreezes at BOS and PVD now appear more likely between 16-18z.

Tonight: Moderate confidence.

Mainly VFR; chances for fog increase more during the early morning in interior Southern New England and mentioned MVFR mist as a starting point. Winds become light SW.

Tuesday: Moderate confidence.

General deterioration to MVFR-IFR ceilings by afternoon, with risk for showers and possible storms after 18z. Better potential for storms Tue night. Winds become SE to S and increase to 10-15 kt with gusts 20-25 kt.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Tuesday Night through Friday/...

Tuesday Night: MVFR/IFR conditions possible. Windy with areas of gusts up to 30 kt. SHRA, chance TSRA.

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

Wednesday Night: VFR. Windy with local gusts up to 30 kt. Chance SHRA.

Thursday through Friday: VFR.

MARINE

Forecaster Confidence Levels...

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

Tranquil boating conditions are expected today and into tonight. Winds turn southeast near the coasts around 10 kt this afternoon; otherwise winds to turn SW around 10 kt, to around 10-20 kt overnight. Seas around 3 ft or less, but build to around 3-4 ft tonight.

Gale Watches have been posted for most of the eastern and southern waters for Tue afternoon through Wed evening. Strengthening southerly winds Tuesday but more into the SCA range during the daytime hours. Mariners will need to be prepared for onset of gale-force gusts for Tue night into the first part of Wed. Seas will build and become increasingly rough, especially by Tue night, and could reach upwards of 12 ft on the southern outer offshore waters. Risk for thunderstorms, too. The combo of all of these means dangerous marine conditions are possible.

Outlook /Tuesday Night through Friday/...

Tuesday Night: Low risk for gale force winds with gusts up to 35 kt. Areas of rough seas. Rain showers, chance of thunderstorms.

Wednesday: Moderate risk for gale force winds with gusts up to 40 kt. Rough seas up to 11 ft. Chance of rain showers, chance of thunderstorms.

Wednesday Night: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 11 ft. Chance of rain showers.

Thursday: Winds less than 25 kt. Areas of rough seas. Chance of rain showers.

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

Friday: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain showers.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Gale Watch from Tuesday afternoon through Wednesday evening for ANZ231>235-250-254>256-280>283.