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

- Scattered showers this afternoon and tonight. Brief gusty winds or a rumble of thunder possible. Mainly dry and cooler Saturday.

- A strong cold front brings a chance for scattered showers Sunday followed by much cooler temperatures for the first week of October.

DISCUSSION

KEY MESSAGE 1...Scattered showers this afternoon and tonight. Brief gusty winds or a rumble of thunder possible. Mainly dry and cooler Saturday.

Day Cloud Phase satellite imagery and radar this afternoon show shower development increasing in coverage across portions of southern New England. This activity will continue to expand and push eastward into the early evening along and ahead of a gradually advancing cold front.

Recent SPC mesoanalysis highlights an environment conducive to convective elements with PWATs near 1.6", marginal instability, and steep low level lapse rates. The well mixed boundary layer has contributed to an axis of elevated DCAPE values around 1000 J/kg extending from east CT to a portion of east MA. Given this set-up, expect scattered convective showers this evening. A few showers may be capable of a rumble of thunder, brief downpour, and localized gusty winds. Although not expecting severe criteria.

The front and associated showers will slowly push through tonight. Showers exit most areas by midnight, with the exception of southeast MA and the Cape/Islands where showers may linger through around 5 AM. Skies clear behind the departing showers Saturday morning with post-frontal winds out of the northwest. Colder air advects in post-front for Saturday with 850 mb temperatures +4-6C. This will yield a more fall-like day with temperatures in the mid to upper 60s for most of the southern New England. As surface high pressure shifts east, flow weakens and shift to the east across the eastern coastal areas. Onshore may keep temperatures in those areas locked in the low to mid 60s.

Radiational cooling should optimize cooling Saturday night given the clear skies with lows dropping in the 40s for the interior and low 50s near the coast. The typical radiator spots and higher terrain could drop into the upper 30s.

KEY MESSAGE 2...A strong cold front brings a chance for scattered showers Sunday followed by much cooler temperatures for the first week of October.

Shower chances Sunday have come into better focus with the latest guidance showing a band of anomalously high PWATs advecting poleward along and ahead of a strong cold front Sunday night into Monday morning. While most guidance has shifted towards more widespread showers the fly in the ointment for this forecast is the depth and extent of a layer of dry air (RH <35%). More widespread dry air aloft may serve to help keep the showers more scattered in nature as instead of a broad shield of rainfall that some global guidance is depicting.

Will see strong CAA behind the front later Monday into Tuesday as the parent low deepens over the Canadian Maritimes. Ensemble guidance shows a 30-50% chance of the low deepening >980mb which is an increase from previous runs. A deeper low would bring stronger CAA and thus colder 850 mb temps. At present, guidance shows the 0C isotherm making it as far south as coastal MA and CT by Tuesday morning. These cold mid-level temperatures likely translate to highs in the mid and lower 50s Tuesday and Wednesday. More notably, low temps likely fall into the 30s Monday and Tuesday night. Monday night should see the <40 degree readings limited to northern MA and the higher elevations of eastern CT as the boundary layer stays better mixed. More widespread 30-40 degree readings arrive Tuesday night as the center of continental high pressure crests over the region resulting in calm winds and clear skies. Thus, the best chance of the season's first widespread frost/freeze should arrive Tuesday night.

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 /19Z FRIDAY THROUGH WEDNESDAY/

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 gusts to 20 kts and up to 25 kts for east coast and Cape terminals.

- SHRA approaches TAF sites between 19-03z from west to east with brief period of MVFR visby/cigs within any SHRA. While not zero, the risk for TS is low (< 20%) and predominantly should be SHRA. Winds shift behind the front from SW to W/WNW this evening. Could see a brief gust 20 kts with the frontal passage.

Tonight: High confidence.

VFR for most, with patchy MVFR possible for Cape Cod and Islands overnight. Another "wave" of -SHRA is possible for portions of eastern southern New England tonight. This is more likely to impact PVD and Cape/Island terminals, but could push as north as BOS 00 to approx 04Z. -SHRA move offshore the Cape and Islands by 10z Sat. NW to N winds increase to around 10 kt, especially around/after midnight. Ceilings gradually increase 06-12Z.

Saturday and Saturday Night: High confidence.

VFR. 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. SCT mid-level decks possible. N/NNE winds continue overnight.

KBOS Terminal...High confidence in TAF/trends but moderate on timing. Thinking SHRA begins around 20-22z, with embedded heavier SHRA and MVFR visby between 21-23z. Added in a PROB30 for -SHRA 00-04Z as some additional showers may push near BOS in that timeframe. Winds shift to the NW 22-00Z with the frontal passage this evening.

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

Outlook /Sunday through Wednesday/...

Sunday: VFR.

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

Monday through Tuesday: VFR. Breezy.

Tuesday Night through Wednesday: VFR.

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. Winds shift to the NW/N behind the front this evening into tonight. High pressure briefly returns Saturday with tranquil boating conditions.

Outlook /Sunday through Wednesday/...

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

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 25 kt. Seas up to 5 ft.

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

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

Wednesday: Winds less than 25 kt.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 8 PM EDT this evening for ANZ230>237-251. Small Craft Advisory until 2 AM EDT Saturday for ANZ254>256.