Boston/Norton forecast discussion

What Has Changed

No significant changes.

Discussion

KEY MESSAGE 1...Showers this morning followed by a lull in activity for much of the afternoon aside from a few spot showers. Another round of showers with some embedded thunderstorms possible move through between 5-11 pm. Windier and colder Saturday following a strong cold front.

High pressure continues to drift off to the east early this morning and showers have starting to make their way into parts of western MA and CT. These will continue to track through the region this morning, spreading from west to east. This is associated with a 1004 mb low pressure system currently over the Great Lakes region that will track along the southern Ontario/Quebec and northeast US border as the day goes on. As it does, the low is expected to continue deepening as its associated upper level shortwave trough also deepens, cutting well into the Midwest by this evening. Showers this morning will likely last a few hours at any given location. Winds have already shifted more southerly this morning, which will help kick off the advection of an anomalous moisture plume and warmer air. PWATs up to 1.75" and even approaching 2.0" will creep into southern New England as the low's warm front moves through by the early afternoon. Once this warm front tracks through, the system's warm sector will settle overhead for the afternoon. This will provide a bit of a respite from steady showers ahead of the next expected round of rain, though a few spot showers cannot be completely ruled out. 925 mb temperatures while under this warm sector will increase to the 16-18C range with 850 mb temperatures around 15C as warm air advection moves in full force. Highs will likely reach the upper 60s and low 70s as a result. A southerly LLJ also sets up, with the core of it at 925 mb ranging between 45-50 kt. Amidst some of the model discrepancies, the general agreement is the core of this will be just over the eastern waters in the afternoon, reaching up to 60 kt (though the NAM goes even higher at 65 kt). This same signature is also noted at 850 mb. Model soundings do, however, indicate an inversion at the low to mid levels which would inhibit this jet from fully mixing down. Gusty winds to 30 mph are not out of the question, and some could see 35 mph gusts closer to the eastern coastlines. Cloudy conditions persist through the day as well, given the abundant moisture aloft.

Our concern then turns to the next round of rain towards the evening hours, which could include some embedded thunderstorms. Not much has changed in the CAMs for the 5-11 pm timeframe today, as they've continued to show the development of a "fine line" tracking from west to east across the region. High-res guidance still indicates a few hundred J/kg this afternoon and evening ahead of the low's strong cold front, though 0-6 km effective bulk shear is in the 50- 60 kt range with strong jet dynamics. Given this environment, some embedded thunderstorms could develop as this line of showers moves through southern New England. Even with the forcing from the approaching cold front, the limited instability should keep the severe potential for any embedded thunderstorms that may develop quite low. This high shear/low CAPE environment may still produce some instances of sub-severe gusts to 50 mph though, and much of southern New England is under a Marginal (1/5) risk for severe weather today. Western portions of central CT are in a Slight (2/5) risk for severe weather today, though also for the risk of severe and damaging winds. Latest CSU ML guidance has much of the northeast under a 5-15% chance for severe winds today as well, so it cannot be entirely ruled out. This will continue to be monitored ahead of and during the event.

Some of the high-res guidance also showed a signal for a some post- frontal rain across southeastern New England, though by the time this arrives, the threat for any embedded thunderstorms will have dropped considerably. Event rainfall totals are expected to generally range between 0.3-0.7" across southern New England. Depending on where some embedded thunderstorms/more convective showers set up, some spots could see locally higher totals, especially given the high PWAT environment.

KEY MESSAGE 2...Chilly night Saturday night with below-freezing low temperatures away from the coasts.

As the cold front crosses southern New England through Saturday morning, strong cold air advection is left in its wake. 925 mb temperatures fall below 0C by Saturday evening, and PWATs fall below 0.25" once more as colder, drier air sets in. Lows Friday night fall into the 40s and upper 30s (towards the high elevations). Highs Saturday may struggle to reach the low 50s across the interior and lows Saturday night will likely dip below freezing across the region, with the exceptions being areas closest to the coasts and urban areas. Once the gusty NW winds drop off earlier in the overnight period Saturday, we could see some radiational cooling develop as high pressure builds in post-FROPA, enhancing an already relatively cold night for some locations.

KEY MESSAGE 3...Dry with more seasonable through early next week, though monitoring coastal storminess near or south of the mid- Atlantic waters towards the midweek period.

High pressure continues to build in Sunday and a more seasonably cool airmass moves into the region, and the tighter gradient between the strong exiting low and approaching high pressure slackens. This will allow winds to let up as well, diminishing slightly heading into the second half of the weekend and the start of next week. As high pressure settles overhead as well, temperatures will moderate back to more typical levels for this time of year as highs return to the 50s and 60s into early next week. One thing we will be keeping an eye on, though, is the development of a cutoff low towards the mid-Atlantic/Carolina coastal waters. This feature appears to have detached from the broader upper level trough that was associated with the strong low pressure that is impacting us today. There is some uncertainty with regards to our region seeing any impacts from this cutoff low, especially given the strength of the ridge overhead during this point in the forecast. Even then, at most, it would lead to some scattered showers towards the Cape and Islands at closest and more NE breezes. Much can change between now and this timeframe, but as of now, impacts look minimal to zero.

Aviation /08Z Friday Through Tuesday/

Forecaster Confidence Levels...

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

06Z TAF Update:

No significant changes from the previous update. Forecast remains on track.

Through 12z...High confidence.

VFR, though increasing moisture brings ceilings starting to lower to around MVFR levels for BDL/ORH after 09z; rain showers could develop as soon as 09-11z from west to east, but steadier rains probably holds off until 11-13z from west to east. Light SE winds increase in speed to around 5-10 kt by daybreak.

Friday...Moderate confidence.

Deteriorating conditions to MVFR-IFR levels. Initial warm- frontal rains with 3-6 SM visby SHRA will bring ceilings to mainly MVFR levels starting ~11-13z, but period IFR ceilings possible. There probably will be more intermittent/lighter SHRA/DZ (could even be dry for periods) after 17-19z west to east, but ceilings should then trend IFR as rain lightens. Then a strong cold front brings another period of 2-4 SM SHRA/+SHRA from 23-04z in a line, low chance for TSRA with localized gusty winds in that timeframe but left out for now due to low confidence.

Winds start SE around 10 kt through 16-19z with the warm frontal rains, then shift to S around 10-12 kt with gusts to low-20s kt range, then becoming SW to WSW at similar speeds ahead of the cold front.

Friday Night...Moderate confidence.

Cold front moves offshore early Fri night; showers begin to taper off between 03-06z west to east with ceilings rapidly improving to VFR. Windshift to NW with frontal passage, and becoming increasingly gusty, speeds 13-17 kt with gusts 25-30 kt becoming more common after 06z.

Saturday...Moderate confidence.

Rain clears quickly following the frontal passage, however, some lingering showers could continue in the morning hours towards the Cape and Islands. These should move out by the afternoon. Otherwise, generally VFR. Gusty NW winds up to 30 kt possible, sustained at 10-20 kt.

KBOS Terminal...Moderate confidence in TAF. VFR for tonight with high confidence. SHRA develops around 13z with MVFR visby and MVFR/periods IFR ceilings. More focused period of IFR as initial rain tapers off by midafternoon; line of SHRA/+SHRA with possible rumble of thunder between 00-03z Sat before gusty northwest windshift after 06z Sat.

KBDL Terminal...Moderate confidence in TAF. Mainly VFR tonight, ceilings start to lower to MVFR by 09z Fri. SHRA develops around 11z with MVFR visby and MVFR/periods IFR ceilings. More focused period of IFR as initial rain tapers off by early to midafternoon; line of SHRA/+SHRA with possible rumble of thunder between 22-01z Sat before gusty northwest windshift after 03z Sat.

Outlook /Saturday Night through Tuesday/...

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.

Through Saturday...High confidence.

Intensifying low pressure will continue to track near the northern New England/Quebec border today into this evening. Ahead of this low, strong small craft southerly wind gusts along with choppy seas will develop early today.

Behind this cold front, NW wind gusts of 30-35 knots will develop especially late tonight into Sat morning in the CAA pattern. Latest guidance has still shown more marginal Gale conditions, so a Gale Watch remains over these areas. Despite marginal conditions, cold air advection patterns can often overperform this time of year given relatively warm SST which can enhance mixing. Either way, strong SCA to gale force wind gusts are expected with choppy seas.

Outlook /Saturday Night through Tuesday/...

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 from 8 AM this morning to 6 PM EDT this evening for ANZ230-236. Gale Watch from this evening through Saturday evening for ANZ230>237-250-251-254>256-280>283. Small Craft Advisory until 6 PM EDT this evening for ANZ231>235-237-250-251-254>256.

DISCUSSION...Hrencecin AVIATION...Loconto/Hrencecin MARINE...Frank/Hrencecin