textproduct: Boston / Norton

WHAT HAS CHANGED

Small craft advisories have been issued for many of our coastal waters tonight into Thursday.

KEY MESSAGES

- Temps continue to moderate Thu with highs reaching into the lower-middle 70s in many locations. Mainly dry weather continues with a few brief spot showers possible with a cold front.

- Dry weather prevails Friday through early Sunday, with another frosty start possible Saturday. Rain is increasingly likely late Sunday into Columbus Day, though uncertainty remains with onset timing and rainfall amounts.

DISCUSSION

KEY MESSAGE 1...Temps continue to moderate Thu with highs reaching into the lower-middle 70s in many locations. Mainly dry weather continues with a few brief spot showers possible with a cold front.

High pressure will continue to shift east of the region tonight as a cold front approaches from the west on Thursday. A modest southwest LLJ ahead of this cold front should keep most locations from decoupling tonight. This will result in a much milder night than what we saw last night. Overnight low temps should only drop into the middle 40s to the lower 50s in most locations with middle 50s in the Urban Heat Island of Boston.

A moisture starved cold front and associated pre-frontal trough will be approaching the area from the west near daybreak Thu. This will result in a period of mid-level cloudiness and a few brief passing showers possible...mainly Thu morning. However...any showers will be very short-lived in a given location and pretty much looking at a dry day. Otherwise...partial sunshine with 925T around +14C should result in high temps mainly in the lower to middle 70s in many locations. There is a low risk for a few additional spot showers ahead of the cold front too...but again the majority of the day is dry. A bit of a breeze out of the west southwest too...but overall a very nice day.

Winds diminish Thu night with skies becoming clear. This will allow for a good night of radiational cooling with low temps mainly in the 40s...but some upper 30s possible in the normally coolest outlying locations.

KEY MESSAGE 2...Dry weather prevails Friday through early Sunday, with another frosty start possible Saturday. Rain is increasingly likely late Sunday into Columbus Day, though uncertainty remains with onset timing and rainfall amounts.

The extended forecast remains on track with no major shifts. Strong 1030+ mb high pressure builds over the Northeast Friday and Saturday, supporting sunny and dry conditions. Highs Friday are seasonably cool in the post-frontal airmass, mainly in the lower to middle 60s. WAA develops aloft Saturday as 925 mb temperatures increase from around +8C Friday to +9C to +12C Saturday, supporting highs in the middle 60s to lower 70s. Another frosty start is possible Saturday, especially in the typically colder locations where light/calm winds favor effective radiational cooling. The NBM doesn't capture this particularly well, so blended in MOS guidance, resulting in lows in the mid 30s across the colder locations while the urban core remains in the mid to upper 40s.

Confidence is high in dry weather Friday through early Sunday given strong ensemble agreement on high pressure dominating the Northeast. Confidence decreases thereafter with respect to timing and rainfall amounts, though it is looking increasingly likely that rain arrives sometime late Sunday and continues into Columbus Day. High pressure shifts east Sunday as remnant moisture from Isaias begins to approach southern New England. Isaias, currently over the southwest Gulf, is expected to make landfall Friday night on the Gulf Coast. Guidance still shows a fairly wide window for the onset of rain, ranging from late Sunday morning to after sunset, but confidence is moderate to high that rain occurs Sunday night into Monday.

There are signals that this could be a soaking rain, though confidence in amounts remains lower. PWATs increase to 2-3 standard deviations above climatology, while DESI probabilities for 0.50+ inches of rain from 8 PM Sunday to 8 PM Monday range from around 45 percent across northern Massachusetts to 60-70 percent along coastal Rhode Island and Massachusetts. Probabilities for 2+ inches remain low, around 10-15 percent and mainly confined to Block Island, Martha's Vineyard and Nantucket.

Forecast confidence becomes quite low by Tuesday, though the pattern favors continued unsettled weather as a cold front potentially approaches from the west. Otherwise, no significant changes to temperatures, with highs generally in the 60s to around 70 Sunday through Tuesday and overnight lows in the upper 40s and 50s.

AVIATION /18Z WEDNESDAY THROUGH MONDAY/

Forecaster Confidence Levels...

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

18z TAF Update...High confidence.

VFR conditions continue through Thu night. S winds of 5-15 knots tonight with some 20+ knot gusts at times. A few brief passing showers possible mainly Thu morning...but this would be short-lived. WSW wind gusts of 15 to 25 knots expected Thu...but these should diminish and become W by Thu night.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Friday through Monday/...

Friday: VFR. Breezy.

Friday Night through Saturday Night: VFR.

Sunday: Mainly VFR, with areas MVFR possible. Breezy. Chance SHRA.

Sunday Night: Mainly MVFR, with areas VFR possible. Breezy. SHRA.

Columbus Day: Mainly VFR, with local IFR possible. Breezy. Chance SHRA.

MARINE

Forecaster Confidence Levels...

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

Tonight through Thursday night...High confidence.

A SW low level jet ahead of an approaching cold front will result in SW wind gusts of 20-25 knots developing tonight and persisting into Thu afternoon. Small craft advisories have been posted for most of our waters. Wind gusts should drop below small craft thresholds later Thu and especially Thu night.

Outlook /Friday through Monday/...

Friday through Saturday Night: Winds less than 25 kt.

Sunday: Winds less than 25 kt. Chance of rain showers.

Sunday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Rain showers likely, slight chance of thunderstorms.

Columbus Day: Winds less than 25 kt. Areas of rough seas. Chance of rain showers, slight chance of thunderstorms.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 9 PM this evening to 3 PM EDT Thursday for ANZ231>235-237-254>256. Small Craft Advisory from 6 PM this evening to 11 AM EDT Thursday for ANZ250-251.