textproduct: Boston / Norton

WHAT HAS CHANGED

No significant changes. Potential for rain to last a bit longer Monday morning, especially south coast.

KEY MESSAGES

- Cool start Saturday followed by a nice day with a mix of clouds and sun.

- Rain arrives Sunday, likely exiting by Monday morning but there remains uncertainty on how quickly rain clears.

- Quick round of rain around Tuesday/Tuesday night, then dry weather returns Wed-Thu

DISCUSSION

KEY MESSAGE 1...Cool start Saturday followed by a nice day with a mix of clouds and sun.

Off too a cool start, especially where winds have become calm allowing for better radiational cooling; many locations have dropped into the low 40s while the warmer urban centers remain in the mid to upper 50s. It will be a gradual warm-up with sunrise as high pressure eventually becomes centered overhead providing a pleasant Fall day. Mostly sunny skies will be filtered by high cirrus streaming north ahead of an approaching weather system. Expect highs to reach into the mid 60s. Trended lows toward MOS guidance a bit Saturday night as winds will be calm, though high clouds will limit radiational cooling if they're widespread enough.

KEY MESSAGE 2...Rain arrives Sunday, likely exiting by Monday morning but there remains uncertainty on how quickly rain clears.

The main focus for the holiday weekend weather-wise is generally the 24 hour period from 12pm Sunday to 12pm Monday. This, as a surge of moisture makes its way north, the remnants of Hurricane Isaias. After lifting north up the MS Valley it will be pulled east by a northern stream trough and move over Southern New England, strengthening as it exits the Mid-Atlantic coast and passes near the 40/70 benchmark. This plume of anomalous moisture (PWATs approaching 2 inches are 2-3 SD above normal) will interact with good dynamics to squeeze out a good 0.5 (north) to 1 (south) inches of rain in this time period. Some locations could see closer to 1.5" mainly along the south coast where the core of a 40-50kt LLJ will move overhead. Could also see a maxima on the slopes of the Berkshires/Worcester Hills where orographics could help depending on the exact track of the low. Ensemble guidance continues to suggest low probabilities of 1+" rainfall totals outside of the south coast where probabilities are in the 40-50% range. The short duration and manageable totals do not suggest that we'd be concerned with flood threat. The greatest uncertainty continue to be exactly when Sunday afternoon the rain arrives and how quickly it exits Monday. Of late, odds are increasing slightly that showers could linger Monday morning, drying by the afternoon. We'll know more as we get into the hi-resolution guidance over the next 24 hours. Fortunately, the bulk of the aforementioned LLJ should be kept aloft by inverted atmospheric profiles so we are not expecting damaging winds with this system at this time. Small Craft Advisories will likely be needed on the waters for gusts 25-30 kts.

KEY MESSAGE 3...Quick round of rain around Tuesday/Tuesday night, then dry weather returns Wed-Thu

It's but a brief respite from the rain on Monday as the mid level ridging following Sunday night's system will quickly be ushered out as another shortwave swings through on Tuesday with PWATs quickly back up to 1.75". As the surface low passes near or just to our north generating a 35-45 kt 850mb LLJ resulting in breezy and wet weather around Tuesday or Tuesday night. High pressure returns Wednesday into Thursday ushering in a quiet and seasonable stretch.

AVIATION /12Z SATURDAY THROUGH WEDNESDAY/

Forecaster Confidence Levels...

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

12Z update...

Today and Tonight: High confidence.

VFR. NW/NNW winds for much of day at 6-10 kts. Winds shift to the NE, then around to the ESE/SE after 20Z. Winds turn more S/SSW into Sunday after 03Z while winds stay SE for the Cape/Island terminals less than 8 kts.

Sunday: High confidence.

VFR. S/SE winds 5-10 kts with RA arriving from west to east generally between 16Z-20Z. MVFR ceilings develop with the arrival of RA.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Sunday Night through Wednesday/...

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

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

Monday Night: VFR.

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

Tuesday Night: Mainly VFR, with local IFR possible. Breezy. Chance SHRA, slight chance TSRA.

Wednesday: VFR. Breezy. Slight chance SHRA.

MARINE

Forecaster Confidence Levels...

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

Through Sunday: High confidence.

Winds 10-15 kt Saturday, becoming light northeasterly late Saturday into Saturday night as high pressure approaches. Seas 3 ft or less all waters. Sunday winds increase again ahead of approaching low pressure, S/SW 10-15 kts gusting 15-20 kts.

Outlook /Sunday Night through Wednesday/...

Sunday Night: Low risk for Small Craft Advisory winds with local gusts up to 30 kt. Local rough seas. Rain showers, slight chance of thunderstorms. Local visibility 1 to 3 nm.

Columbus Day: Winds less than 25 kt. Rough seas up to 9 ft. Chance of rain showers.

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

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

Tuesday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft. Rain showers likely, slight chance of thunderstorms.

Wednesday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Chance of rain showers.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...None.