July 15-17 are – on average – the hottest days of the year in Washington, D.C.  But thanks to the polar vortex (or – to satisfy the purists – a remnant spoke of the tropospheric circumpolar vortex), we welcome below average temperatures and delightful humidity levels.

Capital Weather Gang  intern Jordan Tessler computes that today is the day of the year with the highest occurrences of highs at or above 90 in D.C.  Since 1872, D.C. has reached at least 90 76 times or 53.5 percent of the time. In 10 of the 12 previous July 16 days in D.C., it’s reached 90 or better.

With today’s highs in the low-to-mid-80s, we’re defying the odds and bucking the trend.

Highs and lows in Washington, D.C. on July 16 back to 2002. (NWS)
Highs and lows in Washington, D.C. on July 16 back to 2002. (NWS)

The average high and low in Washington July 16-18 are 89 and 71.  The forecast highs and lows?

Today: 84 and 69
Thursday: 85 and 63
Friday: 86 and 65

If Thursday morning’s low drops to 63 (as forecast), it will be the coolest low for the date since 1954 when it dipped to 62.

On balance over the next three days, temperatures will average about 5 degrees cooler than normal.  This cool anomaly isn’t anything extraordinary or record-breaking but it’s well-timed and a roughly a 1 in 10-year occurrence over a 3-day stretch.

The core of chilly air mass has focused in the Midwest, where the high in Marquette yesterday was just 53 degrees, shattering the record for coolest high temperature for the date by 11 degrees (previous record was 64, records date back to 1875).  For some perspective, 53 is the average high in Marquette on October 15.  Other cities that set record low highs Tuesday include Rhinelander (66), Appleton (62), and Oshkosh (69).

Temperature difference from average forecast for 2 p.m. Wednesday (WeatherBell.com)

Rhinelander also set a record low of 42 this morning.  Lows this morning in the Upper Midwest and Great Lakes region were as cool as the mid-to-upper 30s.  International Falls, Minn. set a record low of 39.

This cool air mass is somewhat fleeting and will gradually erode as it sinks southeast.  But, east of the Mississippi River, it won’t be replaced by anything particularly hot.

Animation of high temperature forecast difference from normal over next week, from GFS model (WeatherBell.com)

Washingtonians who deplore the heat should feel pretty good about how this summer has played out so far.  While the temperature have averaged slightly above normal, the number of 90+ days is slightly below normal year-to-date (actual year-to-date is 14, average is 16) and we’ve had no triple digit heat.

The best part?

After Friday, D.C.’s average temperature starts going down, as we commence the thermal descent towards the cold season.