textproduct: Boston / Norton

WHAT HAS CHANGED

A Small Craft Advisory has been issued for gusty prefrontal winds and elevated seas across the southern waters Friday into Friday evening. Also introduced a low chance for an isolated thunderstorm Friday afternoon/evening as a cold front pushes through southern New England. Otherwise, there were no significant changes to the forecast.

KEY MESSAGES

- Late taste of summer Friday with highs around 80, followed by showers and a few thunderstorms Friday night.

- Cooler and drier Saturday, followed by a chilly Saturday night.

- Watching if a wave of low pressure tracks far enough north to bring a period of showers sometime Sun into Sun night.

- Coolest air of the season works into the region Mon night-Wed. Areas of frost expected Tue night/early Wed AM. Dry weather for most if not all of the next work week.

DISCUSSION

KEY MESSAGE 1...Late taste of summer Friday with highs around 80, followed by showers and a few thunderstorms Friday night.

Mid-level flow remains generally zonal through Friday, while high pressure remains anchored off the coast of the Carolinas, returning the low-level flow out of the southwest.

Still a bit cloudy tonight with mild temperatures, as lows only fall into the 60s. Brief clearing develops Friday, allowing temperatures to quickly warm. 925mb temperatures rise to +20C to +23C Friday afternoon, supporting highs in the lower 80s for much of the region. Dewpoints also climb into the 60s, providing a late taste of summer for the second day of October. For perspective, this falls near the climatological last 80 degree day of the year for Boston, which is September 30th. Winds will also be elevated Friday with southwest flow and mixing up to around 925mb, where winds aloft are 20-30 knots. Thus, gusts of 20 to 25 mph are not unreasonable during the afternoon.

High pressure drifts east as a cold front approaches from the west late Friday into Friday evening. Timing-wise, the 12z high-res guidance shows the cold front and associated showers/storms pushing through from northwest to southeast between roughly 3/4 PM and 10/11 PM. Plenty of moisture accompanies the front, with PWATs nearing or exceeding 2 inches, which is well outside of climatology for early October. Given modest forcing along the front and lower values of instability, with only a few hundred J/kg of CAPE, cannot rule out a thunderstorm or two. Low-level lapse rates are fairly steep at 7- 8C/km, though mid-level lapse rates are less impressive around 5C/km. There could be a few stronger gusts with DCAPE values of 600- 800 J/kg, though currently feel sub-severe gusts are more probable than severe gusts. In addition, locally heavy but brief downpours are possible given the anomalously high PWATs.

KEY MESSAGE 2...Cooler and drier Saturday, followed by a chilly Saturday night.

Behind the front, high pressure builds across the eastern Great Lakes Saturday with the aforementioned cold front just offshore. This may lead to a period of clouds and a few showers offshore early Saturday. Otherwise, drier and cooler Saturday with 925mb temperatures falling to around +10C, supporting highs generally in the upper 60s to around 70 degrees. High pressure moves into southern New England Saturday night into early Sunday, setting the stage for good radiational cooling. Lows fall into the 40s for much of the region, with the warmest readings in urban centers such as Boston around 50F. The coldest locations, especially across the higher terrain of western Massachusetts, could fall into the upper 30s.

KEY MESSAGE 3...Watching if a wave of low pressure tracks far enough north to bring a period of showers sometime Sun into Sun night.

There remains considerable disagreement on the northern extent of a precipitation shield associated with a wave of low pressure on Sun- Sun night. Seems like the majority of the 12z deterministic guidance keeps the northern extent of the rain shield south of our region...perhaps briefly flirting with the south coast. However...the ECWMF as well as some individual GEFS/CMC ensemble members are further north and indicate the potential for a period of showers. Greatest risk for a period of showers seems to be south of the MA turnpike...but it is too early to rule out some showers reaching the NH border or on the flip side of the coin remaining completely south of southern New England. Either way...not expecting any significant impacts even if rain does impact part of our region for a time Sun into Sun night. Given the uncertainty...mainly kept pops in the chance levels for now with the highest of those near the south coast. High temps should be in the 60s...but it could end up a bit cooler depending on the northern extent of the rain shield.

KEY MESSAGE 4...Coolest air of the season works into the region Mon night-Wed. Areas of frost expected Tue night/early Wed AM. Dry weather for most if not all of the next work week.

A pleasant/seasonable day of weather is on tap for Mon with highs well into the 60s...perhaps even flirting with 70 in a few spots. However...it will be a bit breezy on Mon as a cold front crosses the region. Low level moisture is lacking...so expecting mainly a dry frontal passage or worst case being just a few brief showers. Either way...the main impacts will be the coolest air of the fall season working into southern New England Mon night through Wed. Highs may struggle to break 60 on Tue along with a breezy northwest wind. High pressure building overhead will allow for the coolest night of the young fall season Tue night. Low temps by daybreak Wed should be down into the 30s for many locations...perhaps flirting with the freezing mark in some of the normally coolest outlying locations. Frost is likely in many spots as well.

The large high pressure system shifts east of the region by Thu. This should allow for a southwest flow of milder/seasonable temperatures to return by Thu afternoon.

Dry weather should pretty much dominate most if not all of the upcoming work week. There is little moisture/forcing to support precipitation.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

Forecaster Confidence Levels...

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

18z TAF Update:

Through 00z Friday: High confidence.

SCT-OVC multi-layered VFR cloud decks. SW winds around 8-12 kt, with occasional gusts to 20 kt for the Cape and Islands.

Tonight: High confidence.

Continued multi-layered VFR cloud decks at SCT-OVC coverage. SWly winds around 5-10 kt, with low-level wind shear starting around 01-03z as jet of 35 kt at 2000 ft traverses Southern New England.

Friday: High confidence in trends but moderate on timing.

VFR continues, but a cold front will be approaching between 18-00z and bring with it a risk for showers, possibly a rumble or two of thunder, but the exact timing is still uncertain. A best estimate would be between 20-23z for BDL and 21-03z for remaining terminals. SW winds around 10 kt, shifting to W/WNW with frontal passage.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Saturday through Tuesday/...

Saturday: VFR.

Saturday Night: Mainly VFR, with areas IFR possible. Patchy BR.

Sunday: VFR. Breezy. Slight chance SHRA.

Sunday Night: Mainly VFR, with local IFR possible. Chance SHRA.

Monday: VFR. Breezy. Slight chance SHRA.

Monday Night: VFR. Windy with local gusts up to 30 kt.

Tuesday: VFR. Breezy.

MARINE

Forecaster Confidence Levels...

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

Thursday afternoon through Sunday morning...High confidence.

Area of high pressure south of New England and off the coast of the Carolinas will shift east as a cold front approaches the southern New England coastal waters Friday evening. Ahead of the front, southwest flow will bring gusts of 20-25 knots across the southern nearshore waters, including Cape Cod Bay, with marginal seas of 5-6 feet across the southern outer waters. A few showers and an isolated thunderstorm are possible with the frontal passage Friday evening. High pressure briefly returns Saturday with tranquil boating conditions.

Outlook /Saturday through Tuesday/...

Saturday: Winds less than 25 kt. Seas up to 5 ft.

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

Sunday: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain showers.

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

Monday: Winds less than 25 kt. Seas up to 5 ft. Slight chance of rain showers.

Monday Night: Low risk for gale force winds with gusts up to 35 kt. Seas up to 5 ft.

Tuesday: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 11 AM to 8 PM EDT Friday for ANZ231>235-237. Small Craft Advisory from 11 AM Friday to 2 AM EDT Saturday for ANZ254>256.