textproduct: Boston / Norton

WHAT HAS CHANGED

Increased rain chances and winds for the late week/weekend storm as confidence increases. Will need to monitor coastal water levels for the Tuesday morning high tide around Nantucket Sound.

KEY MESSAGES

- Low clouds and rain showers return overnight into Tuesday night with a rumble of thunder possible.

- Isolated shower possible Wednesday, but drier overall. Dry conditions return for Thursday with seasonably cool temperatures.

- Low pressure system expected to bring rain Friday into early Saturday. Gusty northwest winds and below normal temperatures.

DISCUSSION

KEY MESSAGE 1...Low clouds and rain showers return overnight into Tuesday night with a rumble of thunder possible.

Still tracking another band of lower clouds moving south across our region. Eventually, even these clouds will dry up with the daytime heating. The same cannot be said for later tonight. A backdoor front should arrive from the NE and push well into southern New England before a low pressure to our north gets close enough to increase southwest winds and drive this front back to the north some time Tuesday afternoon into Tuesday night.

The main impact will be some lower clouds and perhaps some patchy drizzle late tonight. A passing shortwave over northern New England increases the risk for more significant showers the closer one gets to northern MA. Could even see some rumbles of thunder given some weak elevated instability above am inversion, again with a greater risk towards northern New England.

Persistent east to southeast winds could keep waters level up towards Nantucket Sound. Will need to monitor the Tuesday morning high tide. Impacts look marginal right now, so no headlines at this time.

KEY MESSAGE 2...Isolated shower possible Wednesday, but drier overall. Dry conditions return for Thursday with seasonably cool temperatures.

A trailing cold front shifts across the region Wednesday with surface high pressure building in from the west through the day. Model soundings show increased mixing in the boundary layer supporting drier conditions and also westerly winds with gusts 20-30 mph. Westerly, downslope flow combined with 925mb temperatures +17- 18C will support a warm fall day with high temperatures in the 70s in many areas. There are indications of a stray shower or two as a weak wave moves through, but this will be fairly limited in coverage and at most a light shower/sprinkle due to the drier lower levels.

As high pressure settles in further into Thursday, the airmass dries out even more. Thursday will likely be dry, although cooler than Wednesday. A cooler airmass will also have moved in with 925mb temperatures around 10C cooler than Wednesday. This will yield seasonably cool temperatures mainly in the lower 60s.

KEY MESSAGE 3...Low pressure system expected to bring rain Friday into early Saturday. Gusty northwest winds and below normal temperatures.

Deterministic and ensemble guidance show a stronger/deeper northern stream shortwave trough push across the Great Lakes and the northeast Friday into Saturday. There is synoptic support with this system as an upper level jet positions near southern New England yielding large scale ascent while also advecting in a plume of above normal moisture northward into the region. This will set the stage for a widespread rain. Despite plentiful moisture, instability appears to be limited thus minimizing the potential for heavy rain impacts. Ensemble guidance ranges amounts generally from 0.25 to 0.60" with the ECMWF and Canadian leaning wetter than the GFS solutions. Across those particular models, amounts closer to an inch are possible. These higher amounts are not out of the question if we can tap into better forcing with the warm front. Finer details such as exact timing of arrival and exit of rain are less certain as there are difference across guidance still. Rain more likely arrives in the Friday morning timeframe, continuing overnight. Most guidance shows this system to be fairly progressive meaning it won't be lingering the whole weekend. The bulk of the rain likely exits early Saturday morning. In addition, the initial southerly LLJ with this system nudges in Friday which if mixed sufficiently, will bring breezy to gusty southerly winds (gusts 20-30 mph).

A strong cold front pushes across the region by early Saturday morning putting the northeast within a classic cold air advection pattern with gusty NW winds, cooler temperatures, and isolated showers. A NW 40-50 kt LLJ positions across the region Saturday. The tighter gradient and CAA to help steepen lapse rates will support deeper mixing and thus gusty NW winds. The degree or mixing and strength of the LLJ are still less certain this far out, but ensembles show a signal for gusts 35-45 mph possible, potentially higher across the coastal waters. 925mb temperatures range between - 2C to +2C Saturday yielding likely the coolest temperatures of the season so far. Highs will likely struggle to get out of the 40s across the interior with low 50s in the valleys and coastal plain. RH fields show residual moisture within the cyclonic flow pattern. This may be enough to support some isolated showers. Some of the higher elevation points across the Berkshires may only hit the 40 degree mark on Saturday. It's a bit too early to say the "S word" just yet (snow). However...if the timing aligns, a morning flurry or shower can't be ruled out in those higher terrain spots out west. Expect a cold night Saturday night with high probabilities for the interior to at least drop to freezing. Higher elevation locations may drop into the upper 20s.

AVIATION /18Z MONDAY THROUGH SATURDAY/

Forecaster Confidence Levels...

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

Through 00Z:

Mainly VFR with areas of MVFR cigs. Lingering IFR across the Cape and islands lifts to at least MVFR after 20Z.

Tonight: High confidence in trends, but moderate on timing backdoor cold front.

VFR early tonight for most. A backdoor cold front progresses southward from NH/ME. Timing approximately 02-04z for BED, and around 04-07z for the remaining TAFs. This will bring lowering ceilings again into the MVFR-IFR range, light -RA and winds turning NE around 5-10 kt, strongest near the coast.

Tuesday: Moderate confidence.

MVFR-IFR ceilings, with lighter rains or drizzle. Possible MVFR-VFR optimism by mid to late afternoon. NE/E winds around 5-10 kt, turning SE/S by afternoon and increasing to around 10 kt.

Tuesday Night: Moderate confidence.

MVFR-IFR to start. Some areas improve to VFR late, mainly in the valleys and towards RI and southeast MA.

KBOS Terminal...High confidence in trends. Moderate confidence in timing.

KBDL Terminal...High confidence in trends. Moderate confidence in timing.

Outlook /Wednesday through Saturday/...

Wednesday through Thursday: VFR. Breezy.

Thursday Night: VFR. Chance SHRA.

Friday: Mainly VFR, with local IFR possible. Windy with local gusts up to 30 kt. SHRA.

Friday Night: Mainly VFR, with local MVFR possible. Strong winds with local gusts up to 45 kt. Chance SHRA.

Saturday: VFR. Strong winds with local gusts up to 45 kt.

MARINE

Forecaster Confidence Levels...

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

Through Tuesday Night: High confidence.

The remnants of Hurricane Isaias are currently working their way across the southern waters. Seas 5 to 10 ft and wind gusts 20-30 knots. SCAs are in effect for the southern waters, 15-20 kts with gusts to 25kt possible, especially on the southern outer waters. Seas 6-9 ft on southern waters, 3-6 ft on the eastern waters.

Monday night: High confidence.

NE winds 10-15 kts with gusts 20-25 kts on the eastern waters. Seas 3-5 ft.

Tuesday: High confidence.

SE winds 10-15 kts with increasing gusts to 20 kts. Seas 3-5 ft. RA over the eastern waters.

Outlook /Wednesday through Saturday/...

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

Wednesday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft. Slight chance of rain showers.

Thursday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft.

Thursday Night: Winds less than 25 kt. Areas of seas approaching 5 ft.

Friday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Rain showers.

Friday Night: gale force winds with local gusts up to 45 kt. Rough seas up to 12 ft. Chance of rain showers.

Saturday: gale force winds with gusts up to 45 kt. Rough seas up to 12 ft. Slight chance of rain showers.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 1 AM EDT Tuesday for ANZ235-237. Small Craft Advisory from 8 PM this evening to 2 AM EDT Wednesday for ANZ250. Small Craft Advisory until 8 PM EDT Wednesday for ANZ254. Small Craft Advisory until 8 AM EDT Tuesday for ANZ255-256.