Severe thunderstorm watches expire for Boston, other parts of Mass.

UPDATE | 7:10 p.m. — The severe thunderstorm watch has been extended to 11 p.m. and now includes Boston, Springfield, Worcester, Northampton, Greenfield and Foxborough areas — all of Massachusetts except for Berkshire County. A strong cluster of thunderstorms will move toward Boston in the next couple of hours, possibly bringing damaging wind gusts with occasional heavy downpours. These storms will likely be in the Greater Boston area between 8 and 9 p.m. Lightning will be frequent with the chance for small hail.

UPDATE | 4:45 p.m. — A severe thunderstorm watch remains in effect through 8 p.m. for nearly all of Northern New England (Vermont, New Hampshire and Maine) and much of northern and central Massachusetts, including the Boston suburbs. The city is not included in the watch area. Storms could fire up again, mainly concentrated in areas north of the Mass Pike and west of I-95. Storms that do develop this evening will likely cluster together and move more quickly to the east than earlier storms. Some parts of eastern New England may miss any storm activity.

UPDATE | 4:10 p.m. — Thunderstorm warnings have been dropped for Western Massachusetts after strong storms developed and quickly dissipated.

UPDATE | 3:30 p.m. — Three separate storm cells have strengthened into severe thunderstorms across Western Massachusetts, specifically parts of Hampden, Hampshire and Franklin counties, including the Springfield area and Greenfield. The storms are moving slowly, which raises the risk for flash flooding with heavy downpours in addition to strong wind gusts and potentially small hail. A Severe Thunderstorm Warning has been issued for parts of Western Mass.

UPDATE | 3:15 p.m. — A Severe Thunderstorm Warning has been issued for parts of southwestern Maine, specifically Cumberland County including Portland, Westbrook, Gorham, and Falmouth. Strong wind gusts up to 60 miles per hour and quarter-size hail are possible with this system, which can damage vehicles, roofs, and trees. For your protection, move to an interior room on the lowest floor of your home or building.

UPDATE | 1:10 p.m. — The National Weather Service has issued a Severe Thunderstorm Watch for nearly all of Northern New England (Vermont, New Hampshire and Maine) and much of northern and central Massachusetts, including the Boston suburbs. The city is not included in the watch area. The alert remains in effect until 8 p.m.

Scattered thunderstorms will begin to fire up through the afternoon and track eastward across New England through early evening and past sunset. Storms may cluster together and can be strong enough to bring powerful damaging winds, hail and significant cloud-to-ground lightning.

Wind gusts could reach as high as 70 miles per hour and the strongest of storms may produce quarter-size hail.

PREVIOUS FORECAST: It took about a century, but Boston braved a summer heat milestone: The city topped out at 98 degrees at 4 p.m., Wednesday, breaking the previous daily record of 96 degrees set on June 19, 1923. Temperatures actually climbed 4 degrees in less than an hour to hit the mark, while the heat index, how you feel in the heat, reached a staggering 107 degrees.

Boston was noticeably more hot and humid on what was the second day of this sweltering heat wave. It was the hottest day in Boston since Aug. 9, 2022.

Other records that were broken or tied on Wednesday:

• Caribou, Maine — Tied for all-time high temperature of 96 degrees.

• Manchester, N.H. — Set a new daily record of 98 degrees, beating a record 96-degree temp set in 1941.

• Hartford — Set a new daily record of 97 degrees. Old record was 95 degrees set in 1995.

More records could topple Thursday, which will likely deliver the biggest one-two punch — the most intense heat (high of 98 and heat index of 101) with the added chance for thunderstorms later in the day. I guess this weather pattern wants to kick off astronomical summer with a bang. (The summer solstice arrives today at 4:51 p.m.)

Mostly sunny skies with some haze and clouds will build quickly to bring daily highs into the upper-90s across Greater Boston today, and possibly higher inland. Expect variable clouds to build in throughout the day.

The massive high pressure heat dome that’s been stalled over the Northeast this week will slowly inch south but continue to funnel in a southerly flow to keep the high levels of humidity in place. Dew points will build to the upper 60s and even low 70s for the Boston area — making for another very muggy day.

Heat alerts will remain in place as heat index values will likely hit around 95 to 105 degrees across New England Thursday. Make sure to remain diligent by moderating time spent outside and hydrating frequently. Also an air quality alert has been issued for 11 a.m. to 11 p.m., Thursday for unhealthy ozone levels that could affect sensitive groups, such as children, the elderly and those with lung disease, such as asthma, or heart conditions.

The heat will be most intense in the valleys and cities. As air sinks into valleys, it will compress and generally become warmer than the environment around it. Temperatures usually vary about a degree and a half for every 500-foot change in elevation. This is why the Pioneer Valley in Western Mass., Connecticut River Valley in central Connecticut, Champlain Valley in Vermont, and the Merrimack and Blackstone River valleys are often warmer than surrounding areas.

Cities tend to be warmer due to the urban heat island effect — with more concrete absorbing heat and more vehicles producing heat, cities tend to boost surface temperatures 1 to 2 degrees, especially with already hot air present.

A cold front will sag into the region from the northwest in the afternoon and introduce a forcing mechanism for hot and humid air to rise and condense. The (slightly) cooler and dense air will push pockets of warm air vertically into the atmosphere and lead to developing scattered storms — especially as the hot surface heats and destabilizes throughout the afternoon.

The threat for the strongest thunderstorms is likely in central New England — essentially areas north of the Mass Pike and west of I-95. However, there is a good chance for enough energy to be available for Boston to get clipped by storms later today. The main threat from storms that do emerge will be damaging wind gusts with some hail possible.

Given how hot the surface is, there may be pockets of smaller, but stronger multi-cell clusters of thunderstorms, meaning storms may feed off each other with a tremendous amount of moisture in the air.

Thunderstorms are likely to develop sporadically across New England along the frontal boundary. With plenty of instability in the atmosphere, there may be multiple cells that f near the same areas, especially as the front stalls and hovers over the region later in the afternoon.

• Noon-3 p.m. — Some showers and thunderstorms will begin popping up across parts of Northern New England around midday and drifting from Vermont to Maine through the early afternoon while Southern New England stays dry. A few cells may also develop in Western Mass. and slowly traverse east.

• 3 p.m.-6 p.m. — With peak daytime heating occurring between 1 p.m. and 4 p.m., the atmosphere should be primed and clusters of storms will likely develop once again over Northern New England, but also possibly becoming more widespread throughout western and central Mass.

• 5 p.m.-9 p.m. — Storms could fire up and move east in a hurry. Most will be concentrated in areas north of the Mass Pike and west of I-95, but there is a good chance for potential storms impacting the Greater Boston area by 5 or 6 p.m.

Although storms are more likely as you head north., there can be multiple clusters that develop anywhere in New England, except for the southern coasts of Rhode Island and Connecticut, where weather has been ideal this week thanks to the cooler sea breeze.

This hot and stormy weather will be gone by Friday, with highs slowly rising into only the 80s under partly cloudy skies. Some scattered showers might spring up here and there, but generally it will feel a lot more comfortable outside, so enjoy! More details on the extended weekend forecast to come.

Southern New England: The third day in a row of excessive heat with temperatures once again pushing the mercury above 95 degrees for many areas. It stays very humid out with dew points bouncing between 65 and 70 with sunny skies filling up with clouds as the day progresses. Heat alerts remain in place with heat index values ranging between 97 and 105. Scattered showers and thunderstorms will begin popping up across the region in the afternoon but should stay north of the Mass Pike.

Western Mass.: Heat will spread throughout the valleys and the hill towns once again with temperatures building toward 95 degrees and higher. Plenty of moisture will be in the air and a cold front will help develop scattered storms starting midday. Some storms may have strong downdrafts that can result in heavy rain, gusty winds, and small hail. Heat alerts will remain in place through 7 p.m.

Cape and Islands: The Cape is having the best weather of the year with temperatures once again falling ideally in the upper 70s and low 80s. Winds will be pesky with a breeze to 15 mph. A mostly sunny day will build to a mix of clouds and sun for folks along the peninsula and the islands.

Northern New England: Much of Northern New England will feel the heat once again with temperatures breaking 90, although the timing of the cold front may chip away at afternoon highs for some across Vermont and the most northern tier of the region — still, daily highs should break 90 for most with heat indexes above 95, keeping alerts in place. Partly to mostly cloudy skies will lead to scattered showers and thunderstorms developing as early as noon and may linger into the evening.

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Ken Mahan can be reached at ken.mahan@globe.com. Follow him on Instagram @kenmahantheweatherman.