textproduct: Boston / Norton

WHAT HAS CHANGED

Confidence continues to increase for the region to see a stretch of warm and humid conditions for late week into the weekend.

KEY MESSAGES

- Weak low pressure brings some clouds and a low risk for a shower early Wednesday across Martha's Vineyard and Nantucket

- Increasing heat and humidity across much of southern New England, along with daily afternoon pop-up thunderstorms from Thursday through this weekend. A widespread washout is not anticipated at this time.

DISCUSSION

KEY MESSAGE 1...Weak low pressure brings some clouds and a low risk for a shower early Wednesday across Martha's Vineyard and Nantucket

Overall a pretty quiet night, weather-wise, is expected for most of the region. Weak high pressure over CT and western MA will result in mostly clear skies with some patchy valley fog perhaps forming. Low temperatures will be seasonable, generally the lower 60s. The only other feature on the weather map is a very weak low pressure several hundred miles of southern New England. This will track northeast, and although it will pass south of Nantucket late tonite/early Wednesday morning, moisture around the low may be enough to produce a period of low clouds as far north as Cape Cod. Low (less than 25%) chance of a light shower is also possible across Martha's Vineyard and Nantucket between roughly 4a-8a.

KEY MESSAGE 2...Increasing heat and humidity across much of southern New England, along with daily afternoon pop-up thunderstorms from Thursday through this weekend. A widespread washout is not anticipated at this time.

There is increasing confidence in a prolonged period of midsummer warmth and humidity, along with daily chances for afternoon pop-up thunderstorms from Thursday through the upcoming weekend. However, a widespread washout is not anticipated. Given the abundant rainfall earlier this week, any storms capable of producing heavy downpours could lead to localized instances of flash flooding. This is reflected in the WPC Marginal Risk (Level 1 of 4) for excessive rainfall across western New England on Thursday and all of southern New England on Friday.

Bermuda high pressure develops this week and remains anchored over the western Atlantic through the remainder of the forecast period. With persistent southwesterly flow, expect above-normal temperatures along with increasing humidity. While temperatures will be above climatological normals, they are not particularly unusual for early August. High temperatures from Thursday through Sunday should climb well into the upper 80s and lower 90s, while dewpoints rise into the lower to middle 70s. This combination will result in heat index values of 95 to 100 degrees. If current trends continue, Heat Advisories may be needed for portions of the region Thursday and Friday, and perhaps Saturday as well. One uncertainty is the degree of afternoon cloud cover and thunderstorm development, which could temper temperatures somewhat. This will be monitored over the coming days.

Regarding thunderstorm chances, the overall synoptic pattern features the Bermuda High over the western Atlantic with a mid-level ridge extending across the Northeast. Embedded shortwave disturbances riding through the flow should provide enough lift to initiate scattered afternoon and evening thunderstorms from Thursday through Sunday. Although deep-layer shear remains weak, instability will be plentiful, with forecast CAPE values generally between 1,500 and 2,000 J/kg. As a result, storms should remain largely pulse-type and disorganized rather than organized severe convection. BUFKIT soundings depict tall, skinny CAPE profiles, suggesting efficient warm-rain processes and the potential for locally heavy rainfall. PWAT values around 1.7 inches further support the threat of torrential downpours. In addition to localized flooding, gusty winds and frequent lightning will be the primary hazards.

That said, not every community will experience thunderstorms each day. Many locations will remain dry, so there is no reason to cancel outdoor plans at this point. The details regarding storm coverage and placement will become clearer once the convection-allowing guidance comes into range over the next couple of days.

AVIATION /00Z WEDNESDAY THROUGH SUNDAY/

Forecaster Confidence Levels...

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

00z TAF Update:

High Confidence.

Weak high pressure overhead tonight will result in generally clear skies and light winds for most of the region. Could see some patchy valley fog, but not expecting any at TAF locations. Slightly different outlook for the Cape and Islands. Weak low pressure will pass south of ACK late tonight, but could be close enough to result in some lower ceilings. Have some MVFR for FMH and HYA, and IFR at ACK. Could even see -SHRA at ACK early in the morning. Any low ceilings should clear out by 14z.

Otherwise Wednesday will feature VFR. Believe a sea breeze will again form. Although an isolated shower can't be ruled out across the western interior. Increasing moisture Wednesday night should result in MVFR/IFR conditions as low clouds, especially across the southern coastal areas. May also see a few -SHRA develop prior to 12z Thursday across CT.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Thursday through Sunday/...

Thursday: Mainly VFR, with local MVFR possible. Breezy. Chance SHRA, chance TSRA.

Thursday Night: VFR.

Friday: VFR. Breezy. Chance SHRA, slight chance TSRA.

Friday Night: VFR. Slight chance SHRA, slight chance TSRA.

Saturday: VFR. Breezy. Chance SHRA, chance TSRA.

Saturday Night through Sunday: VFR. Breezy.

MARINE

Forecaster Confidence Levels...

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

The need for a Small Craft Advisory across the outer southern waters will come to an end by early this evening as seas slowly subside to below 5ft. Otherwise a "Bermuda High" will become firmly entrenched off the eastern sea board resulting in an extended period of SW winds.

Outlook /Thursday through Sunday/...

Thursday: Winds less than 25 kt.

Thursday Night: Winds less than 25 kt. Seas locally approaching 5 ft.

Friday: Winds less than 25 kt.

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

Saturday: Winds less than 25 kt. Seas locally approaching 5 ft.

Saturday Night: Winds less than 25 kt. Slight chance of rain showers.

Sunday: Winds less than 25 kt.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...High Risk for Rip Currents until 9 PM EDT this evening for MAZ020- 023-024. RI...High Risk for Rip Currents until 9 PM EDT this evening for RIZ006>008. MARINE...None.