textproduct: Boston / Norton

WHAT HAS CHANGED

Updated for the aviation section.

KEY MESSAGES

- Showers through the evening, perhaps a rumble of thunder.

- Mild and breezy day Wednesday. Low chance for an isolated shower or sprinkle in the afternoon. Dry conditions Thursday with seasonably cool temperatures.

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

DISCUSSION

KEY MESSAGE 1...Showers through the evening, perhaps a rumble of thunder.

A fairly dynamic system, the bulk of which is passing by well to our north, will be the forcing mechanism to bring the chance of showers, and perhaps a brief rumble of thunder, to portions of the area late this afternoon through late evening. Hi-res CAMs are not capturing very well the cluster of stronger convection across parts of Ontario into central New York, so confidence in the evolution and timing of any showers that make it into southern New England is on the lower side. The convection appears to be primarily elevated, and mid- level lapse rates across our area are progged to be marginal to maintain some of the activity. With no specific model doing well with the current activity, I stuck with the blended guidance approach, showing 30-40% PoPs in a general WNW to ESE orientation from western MA down to SE MA and the Cape and points north of that. Rainfall amounts should be generally 1/4" or less, though up to 1/2" is possible, especially across far northern areas closer to the VT/NH borders. Otherwise lots of low clouds, and some fog along the coast courtesy of a moist low level southeast flow. As the evening and overnight goes on, the winds will transition to southwest and start to bring some drying to the low levels. So after any showers move through, we should start to see partial clearing prior to sunrise. On the flip side, those clouds will keep low temperatures from dropping too much. In fact, some areas might see a slow rise in temperatures.

KEY MESSAGE 2...Mild and breezy day Wednesday. Low chance for an isolated shower or sprinkle in the afternoon. Dry conditions Thursday with seasonably cool temperatures.

A trailing cold front shifts across the region Wednesday with surface high pressure slowly building in from the west through the day. This will gradually clear out the lower clouds through the morning. Model soundings show sufficiently mixed supporting drier conditions and breezy westerly winds. Gusts likely range between 20- 30 mph with a locally higher gust in the higher terrain of the Berkshires. Westerly, downslope flow combined with 925mb temperatures +16-18C will support a warm fall day with highs in the 70s in many areas. Isolate shower are possible as a weak wave moves through in the afternoon, but this will be fairly limited in coverage and more likely limited to a 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 have moved in with 925mb temperatures around 10C cooler than Wednesday. This will yield seasonably cool temperatures mainly in the low 60s and lighter northerly winds.

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

Deterministic and ensemble guidance show a deep northern stream shortwave trough push across the Great Lakes and the northeast Friday into Saturday. An upper level jet setting up near southern New England will provide support for broad synoptic ascent and advect a plume of anomalous moisture into the region. This will set the stage for a widespread rain. Ensemble guidance ranges amounts generally from 0.30 to 0.80". The ECMWF and Canadian still lean wetter than the GFS despite its subtle upward trend in amounts. The wetter solutions show potential for banded or localized amounts of 1.0-1.4", likely a result of the additional frontogenetical forcing combined with marginal instability. Location of where the bands of heavier amounts set up is still uncertain. Rain most likely arrives in the Friday morning timeframe, continuing through the evening. Most guidance shows this system to be fairly progressive meaning it shifts out fairly quickly. The bulk of the rain likely exits late Friday night with the pre-frontal trough. There is also an axis of instability associated with this trough which may provide enough forcing/support for a line of convective showers and perhaps even a thunderstorm in the mix.

In addition, the initial southerly LLJ with this system nudges in Friday which if mixed sufficiently, will bring breezy to gusty SW winds. Gusts 20-35 mph are possible with potential for gusts up to 40 for the Cape and Islands and also Block Island.

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 30-40 kt 850mb jet pushes into the region Saturday. CAA will provide the steeper lapse rates to 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 in most places. Some of the higher elevation points across the Berkshires may struggle to hit the 40 degree mark! RH fields show residual moisture within the cyclonic flow pattern. This may be enough to support some isolated showers. It's a bit too early to say the "S word" just yet(snow) with confidence. However, if the timing aligns, a morning flurry can't be ruled out in those highest terrain in the northern Berkshires. No accumulation expected. Expect a cold night Saturday night with high probabilities for non-coastal areas to be at or below the freezing mark. Higher elevation locations may drop into the mid-upper 20s.

AVIATION /19Z TUESDAY THROUGH SUNDAY/

Forecaster Confidence Levels...

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

18z Update

Overall moderate confidence.

Lots of IFR across the region, primarily due to low ceilings. Though a few sites are also showing some lower visibilities due to BR/FG. Fairly confident that the low clouds will be sticking around for most of the night. They will begin to lift and break up as low level winds start to transition to the SW. The wind change will happen by 06z, with VFR conditions expected to become common after 09z. Much lower confidence on the timing of showers, or perhaps even a brief - TSRA. Expect the best chances for showers to be across MA, northern RI and across the Cape and points northward. Probabilities for thunder are fairly low, but not 0. Generally less than 30%. Did keep a PROB30 for ORH, but kept the mention out for other locations. Figured that it was best to see how the convection across central New York evolves as it moves this way. Any threat for showers will end by 03z out across western areas and 07z across the east.

For Wednesday, expecting VFR conditions areawide. Winds will transition to W and NW by mid morning. Looks like the potential for some gusts 20-25kt developing by mid-day and into the afternoon. Can't rule out a g30kt for ORH. VFR conditions continue Wednesday night and into Thursday.

KBOS Terminal...Moderate confidence. Timing of showers through this evening and ultimate improvement to MVFR and VFR conditions late tonight have up to a couple of hours of error.

KBDL Terminal...Moderate confidence. Expect showers to remain north of the airport this evening so did not include in the TAF, however there is a 10-20% chance for -SHRA between 21-01z.

Outlook /Wednesday Night through Sunday/...

Thursday: VFR. Breezy.

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

Friday: MVFR/IFR conditions possible. Windy with gusts up to 35 kt. SHRA, slight chance TSRA.

Friday Night: Mainly VFR, with local IFR possible. Windy with gusts up to 35 kt. Chance SHRA.

Saturday: VFR. Strong winds with local gusts up to 40 kt.

Saturday Night: VFR. Windy with local gusts up to 35 kt.

Sunday: VFR. Breezy.

MARINE

Forecaster Confidence Levels...

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

Through Thursday...High confidence.

Winds turn SW early this evening as low pressure passes by to the north. Expect to see winds increasing to 20-25kt away from the coasts. Winds will turn more west/northwest for Wednesday, but still continue in the 20-25kt range through mid-day. Winds start to diminish later in the afternoon into evening. This will help to push waves up to around 5-6ft, especially further offshore. Thus Small Craft Advisories have been continued. Looking toward Thursday, winds and seas will diminish as a ridge of high pressure moves across the region.

Outlook /Wednesday Night through Sunday/...

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. Chance of rain showers.

Friday: Strong winds with gusts up to 35 kt. Areas of rough seas. Rain showers, slight chance of thunderstorms.

Friday Night: gale force winds with gusts up to 40 kt. Rough seas up to 13 ft. Rain showers likely.

Saturday: gale force winds with gusts up to 40 kt. Rough seas up to 13 ft. Chance of rain showers.

Saturday Night: Strong winds with gusts up to 35 kt. Rough seas up to 12 ft.

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

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from midnight tonight to noon EDT Wednesday for ANZ231>235-237. Small Craft Advisory until 2 AM EDT Wednesday for ANZ250. Small Craft Advisory until 8 PM EDT Wednesday for ANZ254. Small Craft Advisory from midnight tonight to 8 PM EDT Wednesday for ANZ255-256.