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Excessive Heat Watch Issued for Portions of New England Ahead of Heat Wave

After a beautiful, if not slightly cool Father's Day, the heat will begin to crank up big time in New England as a heat dome sets up to the southwest of New England for much of the upcoming week, a favorable position to send very hot temperatures into New England. Monday will act as a transition day from the comfortable weekend weather to the scorching heat wave.

A warm front will lift through New England during the day, likely bringing an increase in clouds and possibly a shower or two as it does so. Both temperatures and humidity will be on the increase, but overall, seasonable for mid-June. Tuesday will see the heat dome begin to set up for the region.

Tuesday through Thursday will likely be the peak of the heat with many areas seeing their first heat wave of the season during this time frame. An official heat wave in New England is at least three consecutive days with a high temperature of 90° or higher. Interior areas shouldn't have much of a problem reaching well into the 90s during these three days. Excessive heat watches have been issued for the Connecticut River and Merrimack Valleys.

Wednesday and Thursday still look to be the peak of the heat for most with many areas reaching well into the 90s with the typical warm spots of New England pushing toward upper 90s. These warm spots would namely be the Connecticut River Valley, Merrimack Valley and possibly the Champlain Valley. Temperatures have trended up slightly for Wednesday and Thursday.

If clouds remain at bay for the most part, some communities in these areas could reach 100° on Wednesday and/or Thursday. Combine this with dew points well into the 60s, and heat index values could push toward 105°, which is the border between excessive heat warning and heat advisory. Refuge from the heat will be found at the immediate coast, particularly the south facing coasts, where highs may struggle to reach 90° amid a local onshore flow.

This heat dome will bring with it some very impressive temperatures aloft by New England standards. Temperatures at the 850mb level, which is about 5,000 feet above sea level, will surge towards the mid 70s, which indicates an exceptionally warm air mass aloft.

For some sort of idea on this level of warmth at this height, Mt. Washington's summit is 6,288 feet above sea level and their all time record high temperature is 72°. 500mb heights will also be (possibly) increasing to nearly 600 dam (decameters). It's rare for heights to reach this extent in New England (especially in mid-June) and is a strong signal for big time heat. The highest height ever recorded by the Gray, Maine National Weather Service office is 598 dam (July 5, 2018).

500mb height (decameters) for Wednesday (June 19). Values in the 594-597 dam range are expected:

There is the potential for some daily record highs to fall, which is a pretty lofty goal to reach in mid-June as these records are all well into the 90s. Record highs for major cities across New England on June 19 and 20 (Wednesday and Thursday) are pretty much all in the mid or upper 90s.

Whether or not records are reached will likely come down to the extent of cloud cover, which would keep temperatures from reaching their full potential. One record to watch is the all time high record in Caribou, Maine, which is 96°. The current forecast has Caribou reaching 95° on Wednesday.

The main limiting factor, which we've been alluding to all this week, is the potential for clouds and/or unsettled weather to ride around the ridge and into New England. The ridge is looking like it will be centered close enough to New England to keep the area largely dry. With that said, increased clouds will be possible at times with some widely scattered thunderstorms, especially on Thursday. Potential timing of these features could keep some areas from reaching expected forecast highs.

All big heat in New England must end, and the end may begin on Friday as signals show the ridge getting suppressed southward with disturbances dropping into New England along with a cold front. Early indications show that it will not cool significantly behind the front, but will be enough to end the heat wave. This end will also likely be signified by more widespread showers and thunderstorms.

Weather map for Friday (June 21):



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