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Posted at 12:48 PM ET, 05/01/2012

Summer-like warmth and humidity return East, but probably not for too long

Round and around we go; cooler, then warmer, then cooler, and so on. Unlike March, when the global circulation almost uninterruptedly delivered warm conditions nearly everywhere in the country for weeks on end, the weather pattern in April gave us a few different looks. Anticipate lots of weather transitions to continue into the first part of May.

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
The map (to the right) of temperature differences from normal averaged over the last 7 days show lots of blue shading in the Northeast.

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. (

Accumulated rainfall (in.) last Sunday. (AHPS/NOAA.)
In fact, as the jet began its slow poleward trek last weekend across the Southern Plains, thunderstorms doused many watersheds across the Missouri Valley with flooding rains on Sunday (see image to the right).

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)
I wouldn’t be surprised to see a swath of 90°F heat extend from eastern Kansas to the mid-Atlantic coast (including Washington, D.C. - have you entered our 90+ day prediction game?) on Friday and/or Saturday. In addition, low-level flow trajectories (black arrows) should bring humid air from the Gulf of Mexico all the way to the East Coast –though the air may dry out some before reaching the I-95 cities as it descends in the lee of the Appalachians.

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.

By  |  12:48 PM ET, 05/01/2012

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