If we look back at April, the brief mid-month flirtation with 90°F heat in parts of New England was replaced about a week ago by relatively cool temperatures over much of the East (and even a rare late April snowstorm for higher elevations of Appalachia).

Temperature departures from normal, averaged over the last week

But, as one might expect based on recent trends, the same areas that were cooler than average over the last week (shaded in blue) are starting to be visited by an almost summer-like, air mass to persist for a few days.

The change comes as one branch of the jet stream lifts northward from its current location over the middle part of the country to latitudes near the Canadian border by the weekend. Transitions like this (shown below in the maps of the jet stream) are often accompanied by stormy weather this time of year, as warm and humid air near the ground surges northward and eastward underneath the jet. This pattern shift is no exception.

High-altitude winds today (left) and Saturday (right) from the GFS model. Black arrows denote jet streams. (TwisterData.com)

Similar bands of big storms will likely develop during the next few days over the nation’s heartland as the warmer air heads northeastward. Experts at the Storm Prediction Center (SPC) have outlined a heightened risk for severe storms with heavy rains in this part of the country through midweek. Just last night, tornado watches blanketed parts of the Oklahoma and Kansas.

By Friday, the summer-like air mass that was just yesterday confined to the Southern Tier of the U.S. will stretch across the eastern half of the country. As shown in the picture below, temperature anomalies in its core will likely exceed +15°F.

Temperature departures from normal expected this Friday, from GFS ensemble mean. Black arrows are surface winds. (Penn State)

Make no mistake … this is a far cry from the overnight lows in the 30s and 40s that have been common in the mid-Atlantic and Northeast lately (not to mention the 17°F at Saranac Lake, NY yesterday morning). Still, though, the ongoing unsteadiness in the global circulation will likely limit the warm-up to just a couple of days. In fact, there are already signs that by late in the weekend or early next week much cooler-than-normal air will return to many locations east of the Mississippi.

Round and around we go.