Extended period of 90+ heat Sat thru at least Tue along with
increasing humidity Tue and Wed. ECMWF Ensemble Situational
Awareness Table indicating 1000 mb temps Sat into Wed are in the 97
percentile to Max range, with Max indicating the ensemble is
forecasting an event that would fall outside the 1979-2009
climatology for this time of year. Thus, strong signal for
oppressive heat Sat thru Tue, possibly into Wed. Given this we will
incorporate some of the warmer guidance to derive max temps.
90+ highs likely Mon, Tue and possibly extending into Wed depending
on the timing of an approaching backdoor front. In addition, cloud
cover Wed could preclude highs from reaching 90+. Deterministic and
ensembles both support Monday as the hottest day, with 850
temps around +19C and +25C at 925 mb! These temps aloft will
translate to highs in the low to mid 90s, even a low prob of a
few upper 90s possible across the Merrimack River Valley. Dew
pts are in the low to mid 60s, which may not be high enough to
boost heat indices into the heat advisory category of 100-104.
Nevertheless, Monday could be day 3 of 90+ heat, hence
oppressive. Regarding winds, appears sufficient WSW gradient
Monday to preclude seabreeze development in eastern MA. Thus,
Boston should see another day of 90+ heat. South coast, SSW
winds off SSTs in the mid to upper 50s will result in not as hot
conditions across south coastal MA/RI.
Tuesday, still hot with anomalous warm airmass remaining over the
region combined with WSW pressure gradient precluding seabreeze
development along the eastern MA coast. Along the south coast, SW
winds will continue to keep the south coast not quite as hot as the
interior. Mainly dry weather Monday, may give way to risk of late
day scattered storms Tue, as deep layer moisture begins to circulate
around the ridge into New England. This moisture may also yield
increasing cloud cover and cap highs to 85-90. Although this
increasing moisture will result in dew pts climbing to near 70 or
so, thus yielding oppressive heat and humidity for day 4.
Highs Wednesday will hinge on timing of backdoor front. If backdoor
front is delayed until later in the day, WNW winds ahead of boundary
would increase probs of one more day of 90+ heat. Ensembles have
trended toward a slower frontal passage, so another day of 90+ heat
is possible. However, another limiting factor will be a very moist
airmass with potential increased cloud cover and also PWATs
approaching 200% of normal per ECMWF ensembles. This abundant deep
layer moisture combined with mid level short wave trough and
attending backdoor front, increasing chance for scattered showers
and thunderstorms. Still could potentially see day 5 of oppressive
heat and humidity, with highs 85-90 and dew pts 65-70.
Thursday and Friday...
Post frontal with dry weather and a significant cool down per
ensembles, with 850 temps lowering to +8C to +10C! A spring-like
airmass translating to highs only in the 70s and lows in the 50s,
along with dew pts in the 40s! Quite comfortable for early to mid
June!
