textproduct: Boston / Norton

WHAT HAS CHANGED

Gale Watches have been replaced with Small Craft Advisories for the applicable waters, excluding the outermost zones where Small Craft Advisories are not issued. Post-frontal NNW winds are expected to gust 25-35 kt Saturday, with occasional gusts approaching gale force possible, especially across the southern and southeastern outer waters Saturday night into early Sunday.

KEY MESSAGES

- Morning showers exit, followed by a brief lull this afternoon before a cold front brings additional showers and thunderstorms this evening. A few strong to isolated severe thunderstorms are possible.

- A few showers may linger along the southern coast early Saturday. Otherwise, drier and noticeably colder conditions are expected Saturday.

- Dry with a modest warmup through mid next week, but monitoring coastal storminess near or south of the mid-Atlantic waters which could bring periods of light rain, enhanced northeast winds and building waves/possible erosion to southeast New England.

DISCUSSION

KEY MESSAGE 1...Morning showers exit, followed by a brief lull this afternoon before a cold front brings additional showers and thunderstorms this evening. A few strong to isolated severe thunderstorms are possible.

As anticipated, gloomy conditions to wrap up the workweek. Earlier this morning, a warm front lifted through southern New England, bringing the first round of showers. The region is now positioned within the warm sector. Dew points are increasing into the lower 60s along the coast, while interior locations remain in the 50s. However, by midafternoon, anticipate temperatures and dew points continuing to climb into the 60s. Increasing dew points could lead to pockets of instability. High-resolution guidance suggests limited surface-based CAPE; however, as we've previously mentioned, the jet dynamics associated with the approaching cold front are impressive.

A line of showers and embedded thunderstorms is developing across western New York and extending southwest into Ohio. This activity is associated with our cold front, which is marching eastward, aided by a fairly robust mid-level trough. Ahead of the trough, SSW flow allows for WAA, supporting mild afternoon temperatures despite the clouds and rain. Highs should reach the middle 60s across northern areas, while upper 60s to near 70 degrees are possible farther south.

Today is not anticipated to be a high-end severe weather day by any stretch of the imagination; however, it is a day we cannot completely ignore. Instability remains limited, generally 100-300 J/kg of MUCAPE, though strong bulk shear may compensate for the limited instability and support a few stronger storms. Over the last 24 hours, the greatest shear has shifted a bit farther south, favoring areas around NYC and points southwest. Nevertheless, a couple of stronger storms remain possible. Feel as though these storms could produce sub-severe wind gusts of 40 to 50 mph, with a much lower chance of severe gusts exceeding 60 mph. CAMs continue to indicate that the greatest potential for thunderstorms across southern New England will be between 5 PM and 11 PM as the cold front progresses from west to east.

Additional rainfall amounts of 0.25 to 0.50 inches are expected.

KEY MESSAGE 2...A few showers may linger along the southern coast early Saturday. Otherwise, drier and noticeably colder conditions are expected Saturday.

Heading into the overnight hours, the area of low pressure associated with the cold front tracks northeast through the Saint Lawrence Valley as broad high pressure expands across the Great Plains. The surface low deepens to below 990 mb, with some guidance suggesting it could reach 985 mb by Saturday afternoon. A tightening pressure gradient supports gusty NNW winds at the surface, while northerly flow strengthens aloft. Meanwhile, 850 mb temperatures fall to between -6C and -4C, a rather significant drop considering values were between +15C and +17C Friday afternoon.

While the majority of precipitation should exit with the cold front, some guidance suggests the potential for an anafrontal wave to develop along the departing boundary early Saturday. This could allow showers to linger across the southern coast, Cape Cod, and the Islands into Saturday morning. Highs Saturday afternoon are not anticipated to be much warmer than the lower 50s across the coastal plain and middle to upper 40s farther inland. However, with gusty winds, apparent temperatures are expected to be 5 to 10 degrees colder than the actual air temperatures.

Very cold conditions are expected Saturday night into Sunday morning as strong high pressure builds into the region. With the combination of very cold temperatures aloft, diminishing winds, and favorable radiational cooling, temperatures are expected to fall below freezing across much of southern New England, leading to widespread frost/freeze conditions. However, since we are outside the climatological growing season, our frost/freeze program ended on October 11, and no Frost or Freeze headlines will be issued. Nevertheless, if you purchased any sensitive tropical plants or palms from the garden center this summer, now would be a good time to bring them indoors!

KEY MESSAGE 3...Dry with a modest warmup through mid next week, but monitoring coastal storminess near or south of the mid-Atlantic waters which could bring periods of light rain, enhanced northeast winds and building waves/possible erosion to southeast New England.

Expansive area of high pressure will maintain itself over most or much of Southern New England through midweek and potentially longer as it becomes reinforced by another strong high pressure coming out of the northern Canadian Prairie region by midweek. This will at least drive dry weather and a gradual warming trend away from the southeast coast with pretty strong diurnal temperature ranges (mild days, cool nights). These conditions could linger into much of the upcoming workweek, away from the southeast coast.

However we continue to monitor coastal storminess depicted across the NWP and ensemble suite near the mid-Atlantic waters Sunday. There seem to be an increasing number of model guidance which brings a portion of this feature's moisture northward or northeastward into a weakness in the above surface ridge at some point Monday into Wednesday, mainly impacting the coastal plain and southeast New England. Exactly when and/or for how long it rains is still quite a bit unclear as the rain will be moving into a very dry ridge and should create a sharp NW to SE gradient in rain, but it doesn't look to be a soaking rain in looking at ensemble QPF probabilities. The bigger potential issue is mainly for the southern waters/coastlines with respect to increasingly gusty ENE winds and building rough seas which could lead to beach erosion for east- or northeast-facing beaches. With a persistent easterly fetch over several days, it's a conduit for building waves and latest significant wave heights are in the 12-15 ft range south and east of Nantucket and Martha's Vineyard. ECMWF probabilities of gale force gusts have increased into the moderate to high (50-70%) range for Monday into Tuesday, while the GFS is more tempered at around 30-50%, with lower probabilities for the northeast waters. It may also turn breezy over the land as well with gusts up to 30 mph at times. We'll re-evaluate the risk for coastal flooding given several tidal cycles, but astro tides are decreasing through midweek, which should mitigate significant issues. So as it looks right now, there appears to be a growing risk for at least impacts to mariners, including potential gale force gusts that could warrant headlines in the coming days as well as rough seas. Some risk for rains would exist as well for southeast New England, but at the moment rainfall doesn't look particularly significant or substantial at this point.

AVIATION /19Z FRIDAY THROUGH WEDNESDAY/

Forecaster Confidence Levels...

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

18z TAF Update...

This afternoon into this evening...Moderate confidence.

IFR to localized LIFR conditions dominated the region at mid afternoon. There were still a few showers across RI/southeast MA...but otherwise most of the region was in a lull with just areas of drizzle and fog. Main concern is between 22z and 02z as a cluster showers and potentially embedded t-storms sweeps across the region. Greatest risk for thunder will be across CT/RI/SE MA...but that is possible across the rest of the region as well. Brief localized strong wind gusts/heavy rainfall may accompany this cluster of showers even if there is not any lightning.

Otherwise...LLWS will continue to be an issue at times into this evening.

Overnight into Saturday...High confidence.

Conditions should improve to VFR across most areas northwest of I-95 later this evening. Scattered showers will linger well into the overnight hours and even into part of Sat across parts of RI/SE MA...especially across the Cape/Islands allowing lower ceilings lingering a bit in this region. Meanwhile...the rest of the region will see VFR conditions as drier air works into the region. NW winds gusts will increase to between 20 and 30 knots during the day Sat.

KBOS Terminal...Moderate confidence in TAF. Main concern is between 23z and 02z when a cluster of showers with perhaps embedded thunder work across the region. Even if there is not any thunder...brief strong wind gusts are possible with the frontal passage.

KBDL Terminal...Moderate confidence in TAF. Main concern is between 22z and 01z when a cluster of showers with perhaps embedded thunder work across the region. Even if there is not any thunder...brief strong wind gusts are possible with the frontal passage.

Outlook /Sunday through Wednesday/...

Saturday Night through Sunday: VFR. Breezy.

Sunday Night: VFR.

Monday: VFR. Breezy.

Monday Night: VFR. Breezy. Slight chance SHRA.

Tuesday: VFR. Breezy.

MARINE

Forecaster Confidence Levels...

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

Outlook /Sunday through Wednesday/...

Saturday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 8 ft.

Sunday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of seas approaching 5 ft.

Sunday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt.

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

Monday Night: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Chance of rain showers.

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

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 8 PM EDT Saturday for ANZ230-236. Small Craft Advisory until 2 AM EDT Sunday for ANZ231-251. Small Craft Advisory until 8 AM EDT Sunday for ANZ232>235-237- 250. Small Craft Advisory until 11 AM EDT Sunday for ANZ254>256. Gale Watch from Saturday evening through Sunday morning for ANZ281>283.