textproduct: Boston / Norton

WHAT HAS CHANGED

The Level 2 of 5 (Slight Risk) for severe weather for Saturday late-afternoon to evening has been adjusted eastward into eastern Massachusetts for localized straight-line wind damage. Additional storms are expected starting early Sunday afternoon as well, but these storms should be in the sub-severe range. Finally, small craft advisories have been posted for the southern waters for Saturday.

KEY MESSAGES

- Warm temperatures this weekend, with scattered thunderstorms (some strong to severe) late in the day into the evening Saturday. Another round of afternoon showers and thunderstorms in greater coverage is expected Sunday, mainly into central/eastern MA and RI.

- Cooler start to the workweek gives way to a warming trend with summer-like warmth mid to late week.

DISCUSSION

KEY MESSAGE 1...Warm temperatures this weekend, with scattered thunderstorms (some strong to severe) late in the day into the evening Saturday. Another round of afternoon showers and thunderstorms in greater coverage is expected Sunday, mainly into central/eastern MA and RI.

For Saturday...we'll still be awaiting the arrival of an initial cold front which will be working its way into northwestern MA later in the day. Until then, very warm to hot temperatures are expected Saturday under full sunshine, reaching into the mid 80s to low 90s with a touch of humidity, but not oppressive or enough to raise heat indices to the point of needing heat headlines. The approach of the cold front will be moving into an environment characterized by moderate instability (CAPEs on the order of 1000-1500 J/kg north and west of I-95) and a well-mixed atmosphere favoring strong downdrafts with progged downdraft CAPE values around 700-900 J/kg. There is also some steeper mid-level lapse rates around 6.5-7 C/km, which linger well into the evening hours. Often with mid-level lapse rates at that level, it can sustain thunderstorms even into the evening hours. Wind fields aloft increase some but are not necessarily eye- popping, with 0-6 km shear values around 30-40 kt. The cold front's late timing and that moisture levels are sub-par are main limiting factors to a more organized severe weather threat. We'll probably be waiting for storms to develop and/or move east into western New England until 5-8 PM, and then slowly evolve into a broken line of storms moving ESE through interior Southern New England to the I-95 corridor between 8 pm to midnight, then moving offshore shortly after midnight for the coasts. A Slight Risk (Level 2 of 5) for severe weather has been expanded eastward from central MA/CT and on into the eastern MA metro areas, and the main severe weather risk is for straight-line wind damage and gusty outflow/"microburst" winds which could down trees and powerlines. Thinking the window for storms to become severe is in the 6-9 PM timeframe, then as conditions stabilize, we should see a continued risk for thunderstorms as they move offshore but could still be capable of heavy downpours, lightning and sub- severe wind gusts. Of the two weekend days, it's late Saturday and into early Saturday night that harbors the greatest potential for storms to produce severe weather. The front moves offshore overnight, and with it brings at least a brief period of drier conditions which continues into Sunday morning.

For Sunday...we'll start off with drier conditions during the morning hours as temperatures start to warm into the lower to mid 80s with some humidity. We then await a stronger mid-upper level disturbance aloft with a cool pocket of air aloft, which will be digging southward during the early afternoon hours into NH. There should be enough instability around to go along with increasing mid/upper level support from the disturbance aloft and weak inhibition to generate showers and thunderstorms in scattered to even numerous coverage by early afternoon (as soon as noon), then building southeast toward the southern coast thru late afternoon. Instability values look less compared to Saturday, which should temper the severe threat, but storms could still be capable of small hail with the cooler air aloft and localized gusty winds. With not much to force storms other than the disturbance aloft, we couldn't say no to a storm occurring virtually anywhere, but the greatest risk looks to be centered from a Fitchburg- Worcester-Willimantic line eastward to the eastern/southern coast, and a lesser risk westward into the CT Valley/Berkshires. In sum, Sunday could offer a greater thunderstorm coverage than Saturday, but the chances any individual storm reaches severe limits is lower.

KEY MESSAGE 2...Cooler start to the workweek gives way to a warming trend with summer-like warmth for the mid to late week.

The workweek starts on a cooler note as northerly flow behind a departing trough keeps cooler temperatures aloft. At the surface, onshore flow keeps temps in the 70s at the coast. Further inland, temps warm into the low 80s away from the marine influence. The column quickly warms Tuesday and stays that way through middle of next week as an expansive upper-level ridge builds over the central CONUS. Aloft, 850 mb temperatures soar to +15-18C. Unsurprisingly, dry conditions are likely through much of next week as surface high pressure builds overhead. Bigger story will be the increasing heat starting Tuesday through much of the upcoming week. Ensemble guidance shows somewhat impressively high probabilities for high temps >90F with values between 50 and 60% in the CT and Merrimack River Valleys Tues-Fri.

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:

Through Tonight: High confidence.

VFR. W/WNW winds around 10 kt, though seabreeze for eastern MA coast should develop by 20z. Winds tonight turn light SW with ENE winds for NE MA/BOS.

Saturday: High confidence.

VFR for most of the time, although the risk for isolated to scattered thunderstorms increases after 21z from ORH westward. Some storms could turn strong with gusty winds leading to local turbulence being the main risk. A greater storm coverage is more likely after 00z eastward. Winds become SW and increase to around 10-12 kt with gusts 20-25 kt.

Saturday Night: Moderate confidence.

VFR outside of scattered thunderstorms which could bring local IFR visby restrictions. Initial widely scattered storms should turn into a broken line of storms moving eastward into the ORH-BED-BOS-PVD corridor between 00-03z, then move offshore. A couple storms could still be strong early with localized gusty winds, but general weakening trends are more likely. Brief clearing between 04-08z north to south as front moves offshore. SW winds around 10 kt then shift to W/WNW 5-10 kt.

Sunday: High confidence in trends but moderate on timing.

VFR initially, though SHRA/TS develops as soon as 16z Sunday in the MA/VT/NH border area and progresses south/southeast. SHRA/TS could be greater in coverage compared to Saturday, with best chances from ORH eastward to the Cape and Islands (lesser chance/coverage west), and a couple could be strong. Thinking decreased coverage after 21z Sunday. Winds become NW around 10-13 kt, with winds coming NE late in the day.

KBOS...High confidence in TAF but moderate on timing of wind directions. VFR through 00z Sunday. Seabreeze has been delayed but thinking it comes in around 20z Friday; by 00/01z Saturday, winds could back to a NE component before turning SW around 14-15z Sat.

KBDL...High confidence in TAF. VFR, with WNW winds around 10 kt becoming SW late today. TS possible after 22z Saturday.

Outlook /Saturday Night through Wednesday/...

Sunday: VFR. Breezy. Chance SHRA, slight chance TSRA.

Sunday Night: VFR. Breezy. Slight chance SHRA.

Monday: VFR. Breezy.

Monday Night through Tuesday Night: VFR.

Wednesday: VFR. Breezy.

MARINE

Forecaster Confidence Levels...

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

Through Sunday Night: High confidence.

Small Craft Advisories have been posted for Saturday afternoon into Saturday night for the southern waters as the combination of SW winds gusts to 25 kt and building seas to around 4-6 ft. A risk for thunderstorms capable of lightning, heavy downpours and localized strong wind gusts too mainly after 7 PM on both southern and eastern waters.

Another round of scattered showers and thunderstorms develops Sunday afternoon. There could be a period Sunday late afternoon and evening where the eastern waters could see northerly to northeast wind gusts to 25 kt, which could prompt the need for small craft advisories on most if not all waters.

Outlook /Saturday Night through Wednesday/...

Sunday: Winds less than 25 kt. Seas up to 5 ft. Chance of rain showers, slight chance of thunderstorms.

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

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

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

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

Tuesday Night through Wednesday: Winds less than 25 kt.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...Air Quality Alert from 11 AM to 11 PM EDT Saturday for MAZ013- 016>024. RI...Air Quality Alert from 11 AM to 11 PM EDT Saturday for RIZ001>008. MARINE...Small Craft Advisory from 2 PM Saturday to 8 AM EDT Sunday for ANZ232>235-237-254>256.