Persistent Heat Brings String of 90's

It's been somewhat of a broken record forecast lately with a string of hot days to greet the start of Fall. From the 20th of September through the 25th of September, we've had five days in the 90's and one at 89°. The average high over the last six days is 91°, almost twenty degrees above average for this time of year.

When we think about why it's been so hot, a few factors come into play. The first is the pattern of the upper level winds, or as we call them, our jet stream winds. A large trough or dip in the jet stream across the Pacific Northwest and west coast, brought a relief from the record breaking heat they experienced this summer. That in turn brought stronger winds moving from the southwest to the northeast over the Rockies. This is considered the more 'active jet' where storm development happens a little more frequently. This kept a more active pattern from the Rockies to the Northern Plains, and a reinforced ridge across the Midwest and East Coast. With the ridge and blocking high pressure system in place, temperatures were able to build and rise across the Midwest.

There's another reason though, that the heat has lasted this long, and it's not something you'd expect. The multiple hurricanes in the Atlantic have actually kept the hot and humid weather in place. The multiple strong hurricanes amplified the upper level pattern, and kept it from moving east.

There's one more culprit that helped the string of 90's stick around, and its actually the lack of rain that we've had. Dry ground warms more efficiently compared to saturated soil. So essentially the dry ground reinforces the dry and hot weather over the area.

As you've heard, the pattern is finally starting to change, with more fall like temperatures arriving Wednesday.