textproduct: Boston / Norton

WHAT HAS CHANGED

A Small Craft Advisory remains in effect for the southern waters through this evening for gusty winds and elevated seas. Otherwise, the forecast remains on track.

KEY MESSAGES

- One last warm day on tap today before showers, possibly a few thunderstorms, return this afternoon and evening. Expecting temperatures to drop for Saturday behind the cold front.

- Low potential for a few scattered showers Sunday night south of I- 95. A secondary, sharp temperature drop is then expected for the start of next week.

DISCUSSION

KEY MESSAGE 1...One last warm day on tap today before showers, possibly a few thunderstorms, return this afternoon and evening. Expecting temperatures to drop for Saturday behind the cold front.

Already seeing quite a bit of clearing skies on the satellite this morning; however, dewpoints remain in the 62 to 65 degree range, only 1 or 2 degrees lower than the actual temperatures around southern New England, and combined with mostly calm winds... wouldn't be surprised to see a little bit of patchy, shallow fog to develop in some areas.

Expecting skies to stay mostly clear through the noon hour today, perhaps into the early afternoon, before the cold front starts to push into NW MA. Once the sun comes up, expecting to see the surface temperatures rising rapidly, to near or just above 80F, due to those 925mb temperatures hovering in the +20C to +22C range.

The latest hi-res guidance is in good agreement that the cold front, and the subsequent showers, should start to arrive around 3 to 4 PM and exit offshore around 10 PM to midnight tonight. Given the high dewpoints, PWATs are also expected to surge to near 2.0" (although the NAM is much more robust, in the 2.0"-2.25" range along the immediate frontal boundary), and the low-level lapse rates remain on the steeper side, near 7C/km. This leads to the potential for some localized heavy downpours as the line of showers passes. While instability is on the lower end with only a few hundred J/kg of CAPE, it doesn't take much this time of year to spark a few rumbles of thunder.

Behind the cold front, a rather strong continental high pressure system will build over the Great Lakes and southern Ontario, sinking those +4C to +6C 850mb temperatures down into New England and sharply dropping surface temperatures through the weekend. Saturday's highs are likely to be on the order of 10-15 degrees less than today's with overnight temps dropping into the low 40s (low 50s on the Cape and Islands).

KEY MESSAGE 2...Low potential for a few scattered showers Sunday night south of I-95. A secondary, sharp temperature drop is then expected for the start of next week.

Still quite a bit of uncertainty on if showers will push far enough north into southern New England Sunday evening. The latest ECMWF and Canadian runs indicate some scattered showers brushing across the Cape/Islands and immediate south coast while the GFS shows widespread showers reaching all the way into southern NH. Opted to keep low-end PoPs south of I-95 for now.

Regardless of the showers, a strong high pressure system is expected to build in right on the heels of a deep Canadian low, bringing a secondary shot of even colder air to New England for the start of next work week. Latest guidance indicates the 530mb line sinking possibly as far south as the Hartford-Providence- Plymouth corridor, meaning 850mb temperatures could drop to near or just below 0C by Tuesday morning. This would support surface highs in mid 50s with nighttime lows possibly falling into the 30s through Wednesday.

The high then pushes east by Thursday, marking the return of predominantly southwest flow and seasonable temperature to southern New England for the end of the week.

AVIATION /12Z FRIDAY THROUGH TUESDAY/

Forecaster Confidence Levels...

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

12z TAF Update:

Today: High confidence in TAF/trends but moderate on timing.

Mainly VFR, with SW winds around 8-12 kt with occasional sub-20 kt gusts through at least 20z. Thereafter cold frontal light -SHRA approaches TAFs during the approx 19-03z from west to east with PROB30 for MVFR visby SHRA. While not zero, the risk for TS is minimal and predominantly should be SHRA. Post- frontal windshift from SW to W/WNW and decreasing to 5-10 kt.

Tonight: High confidence.

Cold frontal SHRA moves offshore the Cape and Islands by 03z Sat, with decreasing VFR cloudiness. NW to N winds increase to around 10 kt, especially around/after midnight.

Saturday and Saturday Night: High confidence.

VFR through the period. N winds around 10 kt to start Saturday, though decrease to around 5-10 kt by afternoon, with ENE/E winds for the eastern MA coast. These wind conditions should stay steady through the overnight hours as well.

KBOS Terminal...High confidence in TAF/trends but moderate on timing. Thinking SHRA begins around 20z, with embedded heavier SHRA and MVFR visby between 21-23z.

KBDL Terminal...High confidence in TAF. Thinking SHRA begins around 19z, with embedded heavier SHRA and MVFR visby between 20-22z.

Outlook /Sunday through Tuesday/...

Sunday: VFR.

Sunday Night: Mainly VFR, with local MVFR possible. Slight chance SHRA.

Monday through Tuesday: VFR. Breezy.

MARINE

Forecaster Confidence Levels...

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

Through Sunday...High confidence.

A cold front approaches the southern New England coastal waters this evening. Ahead of the front, southwest flow will bring gusts of 20-25 knots across the southern nearshore waters, including Cape Cod Bay, with marginal seas of 5-6 feet across the southern outer waters. A few showers and an isolated thunderstorm are possible with the frontal passage this evening. High pressure briefly returns Saturday with tranquil boating conditions.

Outlook /Sunday through Tuesday/...

Sunday: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain showers.

Sunday Night: Winds less than 25 kt. Seas up to 5 ft. Chance of rain showers, slight chance of thunderstorms.

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

Monday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas.

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

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 11 AM this morning to 8 PM EDT this evening for ANZ231>235-237. Small Craft Advisory from 11 AM this morning to 2 AM EDT Saturday for ANZ254>256.