textproduct: Boston / Norton

WHAT HAS CHANGED

No significant changes.

KEY MESSAGES

- Quiet through the rest of today, cooler tonight, with clouds returning before sunrise.

- Briefly dry early to mid morning, then beneficial rain overspreads southern New England and continues through the evening and overnight hours. Flooding concerns remain low.

- High pressure Tuesday gives way to a coastal system for the second half of the week

DISCUSSION

KEY MESSAGE 1...Quiet through the rest of today, cooler tonight, with clouds returning before sunrise.

A 1020-1025 mb high pressure system responsible for today's gorgeous weather drifts offshore tonight ahead of our next system for Sunday. Before that arrives, continued fall-like weather is expected this afternoon and overnight. Winds decouple overnight, becoming fairly light to calm in some areas. In addition, feel the cloud cover will be slower to move in. While clouds push in from the west overnight, thinking these will initially be mainly high clouds. Thus, should still see a period of good radiational cooling during the first part of the night, while eastern areas of southern New England may see a majority of the night before clouds fully arrive.

As is typical with this setup, the NBM appears too mild, so have blended in the cooler MOS/MET/MAV guidance. This really highlights the colder pockets along the I-495 corridor, the hollows of northern Massachusetts, and the Vineyard. All of these areas fall well into the lower 40s tonight, though it is not out of the realm of possibility to see an upper 30 sprinkled in there. Otherwise, urban centers such as Boston, Providence, and Hartford remain in the upper 40s to lower 50s. Worcester stays relatively mild as well, given the city's elevation and location near the top of the nocturnal inversion.

KEY MESSAGE 2...Briefly dry early to mid morning, then beneficial rain overspreads southern New England and continues through the evening and overnight hours. Flooding concerns remain low.

As advertised, conditions Sunday aren't ideal for outdoor activities, with a raw, cool, and rainy day expected. Highs are mainly in the lower to middle 60s across the coastal plain, with middle to upper 50s across the higher elevations of southern New England.

A subtle mid-level northern stream trough arrives from the west early Sunday morning, accompanied by a modest 850 mb jet, though the jet weakens in magnitude as it shifts east. This should provide enough forcing to capitalize on PWATs ranging from 1.4 to 1.9 inches, or roughly 150-200 percent of normal. Despite the anomalous moisture, we aren't concerned about impacts similar to those experienced with last Sunday's heavy/torrential rainfall and flooding. Last Sunday's event was considerably more convective in nature, while Sunday's rainfall should be predominantly stratiform. This should favor lower rainfall rates and greatly reduce the potential for localized torrential downpours that contributed to flooding last weekend.

Timing this out, widespread rain develops during the mid to late morning and persists well into the evening. There is pretty good agreement among the 19/12z suite of high-res guidance. By 18z/2 PM, rain should be widespread across much of the region. The exception will be the outer Cape and Nantucket, which may remain dry for an additional hour or two. There is some, albeit limited, instability near the south coast, where we cannot rule out a rumble of thunder and briefly heavier rainfall. One other area to monitor is near the Mass Pike and the transition from the coastal plain into the higher terrain of central Massachusetts. High-res guidance shows a convergence boundary developing in this vicinity, which may lead to a corridor of enhanced rainfall.

Overall, rainfall amounts of 0.5-1.0 inch are expected, though some variability from town to town is likely, especially near any areas of enhanced rainfall. The HREF maximum does show localized amounts approaching 2 inches, representing the upper end of the guidance envelope, but confidence in amounts this high is quite low. Given the generally lower rainfall rates, the risk for flooding remains low. Some minor ponding is possible where leaves or other debris clog storm drains, but otherwise this should be a beneficial rainfall for the region.

With plenty of cloud cover overnight and a breezy northerly wind, there are no concerns for a particularly chilly night. It will remain raw and wet, with lows in the upper 40s to lower 50s. Rain shifts offshore with the front early Monday morning, coming to an end by or before daybreak.

KEY MESSAGE 3...High pressure Tuesday gives way to a coastal system for the second half of the week.

Ensemble guidance remains in good agreement that an anomolously strong high pressure system will develop over Quebec Tuesday and linger into Wed/Thurs. This system likely keeps temperatures across New England cooler than normal, in the low to mid 60s. Both deterministic and ensemble guidance continue to indicate potential for a coastal low to form sometime Thursday into Friday. While there is still a large amount of uncertainty in the forecast this time, the anomolously strong high to our north and a low offshore will certainly lead to a tight gradient and strong winds throughout the week, especially over the south coast, Cape/Islands, and waters. Depending on how close the low tracks, there is also potential for a few rounds of rain.

AVIATION /20Z SATURDAY THROUGH THURSDAY/

Forecaster Confidence Levels...

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

18Z TAF Update:

Through Tonight...High confidence.

VFR through tonight. Light NE winds becoming E/ESE this afternoon. Winds shift to the S this evening at less than 10 kt, with some areas becoming calm overnight.

Sunday...High confidence in trends, moderate confidence in timing.

VFR to start, deteriorating to MVFR and then IFR from west to east during the afternoon as rain overspreads the region. LLWS could become a concern after 18z as the system moves through. Surface winds generally SE at 5-10 kt. Chances for TS are low to near zero.

Sunday Night...High confidence in trends, moderate confidence in timing.

IFR to LIFR with areas of 2-4SM visibility in continued RA. Winds shift to the N at 8-12 kt.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Monday through Thursday/...

Monday: Mainly VFR, with local MVFR possible. Breezy.

Monday Night through Tuesday: VFR. Breezy.

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

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

Wednesday Night: VFR. Strong winds with local gusts up to 40 kt. Slight chance SHRA.

Thursday: Mainly VFR, with local IFR possible. Strong winds with local gusts up to 40 kt. Slight chance SHRA.

MARINE

Forecaster Confidence Levels...

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

Through Sunday Night...High Confidence.

An area of high pressure moves east of the coastal waters overnight and toward the Canadian Maritimes by Sunday night. With the departure of the high, a low pressure system riding along a frontal boundary ushers in unsettled weather by late Sunday morning, with rain becoming widespread Sunday afternoon. There is a low chance for a few rumbles of thunder, primarily across the southern waters. Rain ends early Monday morning, though a few showers could linger across the southern outer waters as the front sags to the south. Seas throughout this period remain between 2-4 ft, with the greatest wave heights across the outer waters. Winds are northeast through this evening, then become southeast on Sunday. As the front pushes south, winds shift to the north/northeast. Gusts begin to increase with cold air advection Monday morning, which may require Small Craft headlines.

Outlook /Monday through Thursday/...

Monday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of seas approaching 5 ft. Chance of rain showers.

Monday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas.

Tuesday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 10 ft. Slight chance of rain showers.

Tuesday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 11 ft. Slight chance of rain showers.

Wednesday: Strong winds with gusts up to 40 kt. Rough seas up to 13 ft. Slight chance of rain showers.

Wednesday Night: Strong winds with gusts up to 40 kt. Rough seas up to 16 ft. Slight chance of rain showers.

Thursday: Moderate risk for gale force winds with gusts up to 45 kt. Rough seas up to 12 ft. Chance of rain showers.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...High Risk for Rip Currents until 8 PM EDT this evening for MAZ007- 015-016-019-022-024. RI...None. MARINE...None.