“Unusually comfortable” air mass breaks records across eastern half of U.S.

Dozens of records have fallen this week across the U.S. thanks to the infusion of cold air, from the Northern Plains to the Southeast.

Summer respite is being felt in the Southeast. Atlanta, Ga. dropped to 61 degrees on this morning, and Pensacola, Fla. saw a chilly morning low of 65 – both of these were records. Jackson, Ky. also beat its former record low for the date by 5 degrees — bottoming out at 54 this morning.  Mobile, Ala. fell to 64 breaking its record of 65 from 1886.

Unofficial map of locations that set or came close to record low temperatures this morning (CoolWx.com)

Unofficial map of locations that set or came close to record low temperatures this morning (CoolWx.com)

Several records lows were also logged this morning in Mississippi:

Farther  north, Washington Dulles Airport tied its record low of 55 on Thursday morning.

On Wednesday morning, Joplin Mo.tied its all-time July record when it bottomed out at 50 F (9 degrees below its daily record low). Springfield and West Plains, Mo. also tied their record lows for the date.  To the north, International Falls set a record low of 39 degrees.

As we noted in a report Wednesday, Marquette, Michigan’s high only managed 53 degrees Tuesday – its chilliest high temperature on record for the date by an 11-degree margin.

In many places, it’s not so much the reprieve from the heat, but the low humidity that has everyone heading outdoors. In Chicago, the National Weather Service is reading positively giddy this morning as they explain how “unusually comfortable” these low dew points are:

Not only were temperatures fall-like Wednesday, but humidity values, the likes of which we rarely see in July, greeted us courtesy of summer brand of arctic air that moved into the area. Dewpoints yesterday dropped to an incredible 42F in Chicago and spent 6 hours at or below 44F, a feat that occurs less than 1% of the time in July!!

Chicago dew point climatology for the month of July. (NWS Chicago)

Chicago dew point climatology for the month of July. (NWS Chicago)

The Chicago office explains the polar origins of this cool, dry air mass:

Using computer forecast models, NOAA’s Air Resources Laboratory can calculate the approximate trajectory of our incredibly dry low level air mass from yesterday afternoon. Notice that the origins of yesterday’s air mass are from just north of the arctic circle, which helps explain the remarkably low humidity levels for this time of year!

Model simulations trace Chicago's air back to the Arctic Circle. (NOAA)

Model simulations trace Chicago’s air back to the Arctic Circle. (NOAA)

On the opposite end of the extreme temperature see-saw out West, record warm temperatures fell in Washington state. Seattle set a record high temperature of 89 F on Wednesday while Pasco, Wash. soared to a record-besting 109 degrees. Numerous other Washington cities either tied or broke their records, including Pendleton, Walla Walla, Yakima, and Hanford.

Also on Capital Weather Gang

Washington Dulles Airport ties record low temperature at hottest time of year