A Westward Shift in Summer Heat

While I was on vacation last week our weather here in the Midwest transitioned from warm and humid to cool and rainy. High temperatures the last three days here in Illinois have been in the mid 70s, and prospects through the 4th of July are for continued cool weather. The reason for the change was the development of a strong upper level trough over the central U.S.  This contrasts remarkably with last year when the central U.S. was baking under a strong high pressure ridge and widespread highs in the upper 90s to 100s were recorded as we headed toward the 4th of July.

Mean 500 millibar map (~18,000 ft) for July 1-4, 2012

Mean 500 millibar map (~18,000 ft) for June27-30, 2013

The upper atmospheric pattern is much the same as it was this time last year, except the ridge is further west. The dry, sinking air beneath ridge is helping produce record temperatures throughout the southwest U.S., while a large trough over the central U.S. is responsible for generally cooler than normal conditions. Last year much of California and parts of the southwest were cooler than normal to near normal the last week of June and early July.

Record highs and record high minimum temperatures have been recorded through the west the past week.  Death Valley, CA reported the following high temperatures the past three days:

June 29       125°F
June 30       129°F
July 1          127°F

The temperature of 129°F on Sunday new monthly record and a tie for the hottest June temperature ever recorded in the U.S. This is preliminary and will be validated by a team of meteorologists and climatologists.

After three consecutive days with highs over 120°F, Needles, CA reached "only" 115°F today.

Record temperature locations for the last week. Credit: HAMWeather

It looks like the upper ridge over the west will start to break down by the end of this week. The will mean moderating temperatures for much of the west, although temperatures are still likely to run above normal.