textproduct: Boston / Norton

WHAT HAS CHANGED

No significant changes.

KEY MESSAGES

- Isolated shower possible today, but drier overall. Dry Thursday with seasonably cool temperatures.

- Low pressure system expected to bring rain Friday into early Saturday. Gusty NW winds and below normal temperatures expected to be left in its wake.

DISCUSSION

KEY MESSAGE 1...Isolated shower possible today, but drier overall. Dry Thursday with seasonably cool temperatures.

Low pressure centered over central NY will continue to track east into northern New England through the day today. With this passing low, its trailing cold front will cross over southern New England and high pressure will build in slowly behind it. Skies are expected to steadily clear through the day today as a result. An isolated shower cannot be ruled out this afternoon given the passage of this disturbance, and latest CAMs did hint at a low chance for an isolated shower or two to develop.

Latest model soundings continue to indicate decent mixing during the day today, which will lend itself to gusty W winds up to 20-30 mph, though locally higher gusts in the higher terrain cannot be ruled out. With these aforementioned gusty westerly winds, downsloping flow is anticipated today which will combine with warmer temperatures aloft to support a much warmer fall day. 925 mb temperatures are expected to range between 16-18C, with the southern coastal areas possibly reaching 20C aloft. Though temperatures are not expected to hit 80F, highs in the 70s today will overspread southern New England, and some spots in the southern CT River Valley and SE MA could get very close to 80F. Despite these warmer temperatures, dew points will remain comfortably in the 50s and 60s.

Heading into tonight and Thursday, high pressure strongly takes hold of the region along with a significantly drier airmass. PWATs fall to as low as 0.2" and winds slacken considerably, becoming light to calm. The warmer airmass overhead today shifts off to the east and is replaced by a notably cooler one with 925 mb temperatures falling below 10C. This will be supportive of cooler lows tonight and a cooler day Thursday: lows will fall into the 40s and low 50s, and highs Thursday will top out in the low 60s.

KEY MESSAGE 2...Low pressure system expected to bring rain Friday into early Saturday. Gusty NW winds and below normal temperatures expected to be left in its wake.

A strong northern stream shortwave trough is expected to shift across the Great Lakes and into the northeast Friday into Saturday. The overall consensus across the deterministic and ensemble guidance is that the surface low sets up across western NY and southeastern Ontario by Friday morning, with rain moving into southern New England at this same time. An upper level jet sets up near southern New England (at 250 mb over the region, this jet may reach around 120-150 kt based on latest guidance), providing support for broad synoptic ascent. With this setup, an anomalous moisture plume will be advected in from the SW. NAEFS guidance has PWAT values approaching 3 standard deviations above normal over the region by Friday morning, and IVT values reaching the maximum of the CFSR climatology by early Friday afternoon. Notably, too, temperatures aloft with this plume are quite warm, with 925 mb temperatures briefly returning to around 16-18C and 850 mb temperatures to around 10C. Ensemble guidance has a bit more agreement this morning regarding rainfall totals, generally ranging between 0.4-0.7". Across all ensembles, some higher probabilities of totals reaching or exceeding 1.0" exist over parts of central and western MA, especially towards the Berkshires. Mean totals also trend slightly higher there across guidance, generally getting to just under an inch. Deterministic guidance indicates some signal for lines of more convective showers, but details regarding how much may fall within those and where they set up are still uncertain at this time.

Aside from the upper level jet, a southerly LLJ is slated to set up over the region Friday, with values at 925 mb ranging between 40-50 kt at its maximum. If mixed well, we could see some breezy or gusty SW winds. Gusts could range between 25-35 mph, with higher gusts possible for the Cape and Islands, especially as the core of this jet sets up over these areas.

Following this rainfall, a strong cold front is expected to push through the region Saturday morning, kicking off strong cold air advection underneath more cyclonic flow. Latest guidance indicates 925 mb temperatures sinking below 0C Saturday as this pattern kicks off. PWATs fall below 0.25" once more as this dry, cold air sets in. Lows Friday night fall into the 40s and upper 30s, with the higher elevations likely falling below to at freezing. Highs Saturday may struggle to reach the low 50s across the interior and lows Saturday night will likely dip below freezing across the region based on the latest ensemble probabilities, with the exceptions being areas closest to the coasts and urban areas. Though, those areas will likely be sitting just around/above freezing overnight. In the midst of this colder air, strong NW winds prevail, gusting well into the 30 mph range with some gusts to 40 mph not to be ruled out.

High pressure builds in Sunday and a more seasonably cool airmass moves into the region, and the tighter gradient between the strong exiting low and approaching high pressure slackens. This will allow winds to let up as well, diminishing slightly heading into the second half of the weekend.

AVIATION /18Z WEDNESDAY THROUGH MONDAY/

Forecaster Confidence Levels...

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

12Z TAF Update:

Today: High confidence.

Warm-frontal IFR stratus and fog at BED should take longest (by 15z) to lift to VFR, otherwise VFR should govern. Widely-SCT light SHRA possible 17-23z from west to east but coverage too limited for a TAF mention. Winds drive most of the change groups, with SW winds 5 kt or less north of the Mass Pike, increasing to around 10-13 kt with gusts to 22 kt south. Windshift to W between 15-17z around 10 kt, then becoming NW around 10 kt between 22-00z Thurs.

Tonight: High confidence.

VFR. NW winds increase to around 10-12 kt, with gusts 20-25 kt for most developing 02-04; upwards of 25-28 kt gusts for the eastern MA airports and the Cape.

Thursday...High confidence.

VFR. Winds diminish greatly with high pressure settling overhead, becoming light and variable to calm.

KBOS Terminal...High confidence in TAF. BOS is VFR, although OWD still has LIFR mist/fog. Expect that to erode at OWD by 14z. Windshift to W around 10 kt by 16z, then turning NW around 10 kt by 00z, increasing to 12-14 kt / gusts to 25 kt overnight. Slow cessation into Thurs AM.

KBDL Terminal...High confidence in TAF. Windshift to W around 10 kt by 15z, then turning NW around 10 kt by 22-23z, with gusts to 20 kt developing overnight.

Outlook /Thursday Night through Monday/...

Friday: MVFR/IFR conditions possible. Windy with areas of gusts up to 30 kt. SHRA, patchy BR.

Friday Night: Mainly VFR, with local IFR possible. Windy with gusts up to 35 kt. SHRA, patchy BR.

Saturday: VFR. Windy with gusts up to 35 kt.

Saturday Night: VFR. Windy with areas of gusts up to 30 kt.

Sunday: VFR. Breezy.

Sunday Night: VFR.

Monday: VFR. Breezy.

MARINE

Forecaster Confidence Levels...

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

Small craft advisories were extended and/or hoisted for tonight for the eastern waters into 18z Thursday as a period of enhanced NW gusts of 25-30 kt are expected to develop early tonight. Seas around 3-5 ft on most waters through Thursday, with dry weather and good visibility expected. Gusts should subside below SCA criterion by Thursday night.

A frontal system brings returning SW winds into the SCA range on Friday (25-30 kt), which will also bring periods of light to moderate rain reducing visibililty through Friday early evening. A short period of small craft advisories will likely be needed once the existing SCAs expiring Thursday afternoon expire.

Looking ahead into Friday night and into Saturday...a period of strong northwest winds are forecast. We've issued a Gale Watch on most waters for Friday night into Saturday, with seas becoming increasingly rough at up to 12 ft. Dry weather is expected by Saturday.

Outlook /Thursday Night through Monday/...

Friday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Rain showers, patchy fog.

Friday Night: Strong winds with gusts up to 35 kt. Rough seas up to 12 ft. Rain showers likely.

Saturday: Strong winds with gusts up to 40 kt. Rough seas up to 12 ft. Chance of rain showers.

Saturday Night: Strong winds with gusts up to 35 kt. Rough seas up to 11 ft.

Sunday: Winds less than 25 kt. Local rough seas.

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

Monday: Winds less than 25 kt.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 8 PM this evening to 2 PM EDT Thursday for ANZ230>234-250-251. Gale Watch from Friday evening through Saturday evening for ANZ230>237-250-251-254>256-280>283. Small Craft Advisory until 2 PM EDT Thursday for ANZ254-255. Small Craft Advisory until 8 PM EDT this evening for ANZ256.