textproduct: Boston / Norton

WHAT HAS CHANGED

Confidence has increased in the arrival of remnant tropical moisture Sunday afternoon/evening, with the heaviest rainfall favoring the south coast and improving conditions expected later Monday.

KEY MESSAGES

- Mostly clear to partly cloudy through Friday night, with westerly afternoon breezes in the 20 to 30 mph range. Frost possible Friday night.

- Sunny and dry for Saturday with light winds.

- Rain is increasingly likely Sunday afternoon into Columbus Day as remnant tropical moisture moves into southern New England. A soaking rain is possible, especially near the south coast, before conditions improve later Monday. Additional showers Tuesday, then cooler Wednesday.

DISCUSSION

KEY MESSAGE 1...Mostly clear to partly cloudy through Friday night, with westerly afternoon breezes in the 20 to 30 mph range. Frost possible Friday night.

Main synoptic-scale features of interest through Friday night is a broad cyclonic mid-level flow aloft across the US/Canada border, with an expansive 1024-1028 mb surface high pressure from the Gt Lakes region to the northern mid-Atlantic, which moves into Southern New England by late Friday night into Saturday.

There is an embedded mid-level shortwave disturbance moving through the broader cyclonic flow, which could spark isolated pop-up showers especially north of the Mass Pike through this afternoon, but the vast majority of the time through today is dry.

The mid/upper level trough will maintain a cooling boundary layer with strong diurnal mixing for the rest of today and again on Friday. Mixing depth progs show the top of the PBL as high as 800 mb, with westerly jet of 3O to 35 kt progged on the NAM/GFS. This will support westerly breezes and falling dewpoint temperatures, both this afternoon and again by Friday mid-morning until sundown. Westerly gusts through sundown today in the 20-25 mph range with gusts up to 30 mph, and should be a similar if not up to 5 mph higher on Friday. Highs mainly in the mid 60s for Friday with mostly sunny skies.

The high pressure over the mid-Atlantic states begins to shift eastward into Southern New England Friday night, with decreasing winds and increasingly efficient radiational cooling. The timing of wind cessation - which hinges on how quickly will the high move in - will be important on low temps and if frost can develop, but have shown lows Friday night in the mid/upper 30s in the typically cooler outlying locations and the low to mid 40s for the urban centers and the coastlines. That could warrant frost headlines for parts of the area pending how quickly winds ease up.

KEY MESSAGE 2...Sunny and dry for Saturday with light winds.

Saturday is the pick of the weekend, as high pressure crests over Southern New England, then moves offshore into Saturday evening. After a bit of a chilly start, highs should rebound back into the low to mid 60s under full sun. With light winds, it's a great day for any outdoor plans. Warm advection takes place Saturday evening and with a modest increase in SE winds, lows should be quite a bit milder than Friday night, in the mid 40s to low 50s.

KEY MESSAGE 3...Rain is increasingly likely Sunday afternoon into Columbus Day as remnant tropical moisture moves into southern New England. A soaking rain is possible, especially near the south coast, before conditions improve later Monday. Additional showers Tuesday, then cooler Wednesday.

Guidance continues to come into better agreement regarding the arrival of remnant moisture from Isaias, with rain increasingly likely Sunday afternoon/evening and continuing into Columbus Day. Important to emphasize that southern New England will not be directly impacted by a tropical system, but rather moisture associated with what was once a tropical system. While confidence in the timing has increased, uncertainty remains regarding how far north the precipitation shield extends. The heaviest rainfall continues to favor areas closer to the south coast. Fortunately, this looks to be a fairly progressive system, with conditions improving during the second half of Monday.

There remains potential for a soaking rain, especially across southern portions of the region. DESI probabilities for 0.50+ inches of rainfall, from 8pm Sunday to 8pm Monday, remain around 45 percent across northern Massachusetts, increasing to 60-75 percent along and south of Hartford, Providence and Taunton. Cannot rule out locally higher amounts, with probabilities for 2+ inches around 10-20 percent from Montauk Point through coastal Rhode Island, including Block Island, into southeast Massachusetts and the Islands. Given the progressive nature of this system, the overall flooding threat appears low, though cannot completely dismiss localized poor drainage flooding, especially where fallen leaves may clog storm drains. While the severe weather threat remains uncertain, the tropical origins of this moisture warrant continued monitoring for any low-end severe potential.

A brief lull in precipitation is anticipated late Monday into early Tuesday before another round of showers accompanies a cold front Tuesday. Temperatures remain mild by October standards ahead of the front, with highs in the upper 60s to lower 70s Sunday through Tuesday. Dewpoints increase into the middle to upper 50s Sunday, then into the 60s Monday and Tuesday. Behind the front, a cooler and drier airmass arrives Wednesday, with highs returning to the upper 50s to lower 60s and dewpoints falling into the 40s to lower 50s.

AVIATION /00Z FRIDAY THROUGH TUESDAY/

Forecaster Confidence Levels...

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

00z TAF Update...High confidence.

VFR conditions continue through Friday. Initially gusty W/NW winds relax overnight to between 5-10 mph at most terminals. VFR Friday with W wind 10-15 kts gusting to 20 to 30 kts through the afternoon. Could see a SCT layer develop at 045, but otherwise Friday is a VFR day.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Friday Night through Tuesday/...

Saturday through Sunday: VFR.

Sunday Night: MVFR/IFR conditions possible. SHRA likely.

Columbus Day: Mainly VFR, with local IFR possible. Slight chance SHRA.

Monday Night: VFR.

Tuesday: Mainly VFR, with local MVFR possible. Breezy. Slight chance SHRA.

MARINE

Forecaster Confidence Levels...

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

Through Friday Night: High confidence.

Small craft advisories remain in effect through 7 PM this evening, as westerly winds have been gusting in the 25 to 30 kt range on most waters and should remain so through sundown. Although winds should ease some into tonight to around 10-15 kt, we westerly gusts resume in the SCA range (25-30 kt) starting Friday mid to late morning through the afternoon with strong mixing expected over the waters. We'll likely need to re-issue SCAs for most waters once the existing ones expire. Seas around 4 ft or less for all waters, with good visibility expected.

Outlook /Friday Night through Tuesday/...

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

Saturday Night through Sunday: Winds less than 25 kt.

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

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

Monday Night: Winds less than 25 kt. Areas of rough seas. Slight chance of rain showers.

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

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 8 PM EDT this evening for ANZ230>237-250-251-254>256.