For the past several days, late July more closely resembled late September across the eastern U.S.. But as early August begins, it will feel like August as the extreme jet stream pattern that directed polar air into the mid-latitudes relaxes.
The animation below shows a radical transformation in the jet stream pattern over the next several days. The huge dip in the jet stream over the East surely and steadily flattens.
As a result, rather than northerly flow from polar regions over the eastern U.S., the air streams more from west to east – which is a lot more characteristic for mid-summer. Notice how the deep blues and greens – representing an unusually intense pool of cold air at high altitudes – vanish over the East, signaling a return to a near normal temperature regime.
It was this cold pool that resulted in temperatures averaging 10 to 20 degrees below normal since Monday over the eastern U.S.. Look at all the record lows and near-record lows from Wednesday morning:
Record lows were set or tied Wednesday in Atlanta, Baltimore (BWI Airport), Washington, D.C. (Dulles Airport), Birmingham, Tallahassee, Nashville and Pittsburgh among dozens of other locations.
Wednesday’s temperature difference from average is shown in the map below:
Lest anyone be misled that this recent cool spell signifies the Earth is going into an ice age and that we’re having a year without a summer, notice how warm it was in north central Canada on the warm side of the see-saw-like jet stream pattern.
The repeated spells of much cooler than normal air in the eastern U.S. this July reflects a continuation of state of affairs since the start of the calendar year.
Cold air outbreaks have afflicted (or in the case of this summer, delighted) the eastern North America repeatedly making it the coldest region of the world, relative to average.
Even as the cool pattern eases over the next several days and temperatures trend towards normal, long-range forecasts offer no signs of extreme heat. Rather, they favor below normal temperatures in the central U.S. which could eventually bleed once again towards the East Coast.
The question of whether this pattern will carry into winter keeps coming up. The answer is we simply don’t know (although there are some tentative indicators that point that way).