Many spots in New England warmed to 90 degrees on Sunday. But at Logan International Airport in Boston, the high temperature of 89 means that we still have not reached 90 in 2015.
This air is from Canada, where it's been very warm, so that explains how we can be warm, too. But also in Canada, on the West Coast, hundreds of forest fires are burning. Upper-level wind has brought that smoke to the New England sky. A smoky sky reflects sunlight back to space - I believe that is one reason many locations did not warm past 90 degrees today.
Another factor is the rainfall earlier this week. Wet ground absorbs some of the energy from the sun that would otherwise go to heating up the air. Locations that missed the rain earlier in the week, like Portland, Maine, and Concord, New Hampshire, did reach 90. But for much of New England, May 26th, remains the hottest day so far in 2015. Portland hit 91 degrees that day.
But it is OK to say it is, or was, hot out.
A batch of slightly cooler air from eastern Canada is sliding into eastern New England overnight. It is a weak back-door cold front.
At the same time in western New England, warmer and more humid air is trying to advance toward the north. This subtle weather boundary will create thunderstorms often on for much of this week.
Upper-level wind will be very light, so any storms that form will move relatively slowly. That means we have potential for flash flooding this week. It will not be widespread. Timing and placing individual thunderstorms will be key to the forecast, a major challenge this week.
Overall, the pattern is a little less hot, and a little more humid, with intermittent sun and storms through Wednesday. High temperatures will be in the 70s at the shore and well into the 80s inland. Nighttime lows will be in the 60s to near 70, with patchy, dense fog, especially near the coast at night.
The next batch of dry air from Canada pushes in from northwest to southeast Wednesday until early Thursday.
But just as quickly, a warm front will come through Friday night, with warm to hot and humid weather next weekend and the threat for renewed thunderstorms.
This may bring the final melt of Boston's last remaining snow pile.