A two day sting of high heat and humidity made out to be our hottest stretch we've seen in five years! While the temperatures weren't abnormally high (don't get me wrong, low 90s are still hot) it was the humidity that made things so uncomfortable.
We first let you know last Friday that a potent hot stretch of days was coming. That promise came to fruition as an abnormally warm ridge of high pressure built in across the nation's mid section. This area of high pressure helps to promote sinking air motion in our atmosphere. Generally speaking this keeps areas under it's grip sun-filled and very warm.
Being we were on the leading edge, that left Minnesota and Iowa with a middle ground with the threat of more rain and clouds. For the most part, we avoided the rain in this two day stretch, sans a couple of minor bubblings tonight. But the clouds did nip us both days in regards to temperatures both Thursday and Friday. Truth be told, this was a very difficult forecasting week we had, with a bust of a Flash Flood Watch on Tuesday night/Wednesday morning, a close call with a very potent complex of storms Thursday morning which did widespread damage across northern Minnesota and Wisconsin and those added clouds which were difficult to pin down. All of this made difficult by very poor performing guidance. Through all of that, the heat did materialize and it was a potent batch too.
The following will be an overload of statistics, many of which you'll likely find interesting.
Temps were in the upper 80s and low 90s both Thursday and Friday, far from extreme but considerable because of our cool summers we have experienced in 2014 and 2015. What made it so hot, the dew points which spiked to some pretty rare levels. Dew Points were commonly found in the 75-82° range (random sample shown below). 80s, are very rare around our parts and have only happened on a handful of days in the last 30 years. The reason they were so high locally, the pattern was aligned as such that we had a river of wind pulling in an airmass directly off the Gulf of Mexico... hence our Tropical Airmass
So the humidity was the key factor here with this recent heat streak. Those dew points, coupled with highs in the 80s and 90s both afternoons lead to heat indices as high as 118°. The worst of the two days was Thursday, but both were significant.
So, the hottest in five years, but not by temperature. In fact we registered 93° earlier this year. This is the first time we've seen the heat index as high since July of 2011. For official records we turn to the Rochester International Airport for the record books. Both afternoons saw a peak heat index of 106°. That makes for the warmest such reading since July 19th, 2011.
For maximum air temperature, it's not quite as long of a drought. The average maximum air temperature from July 20-22 was 88.3° (85/90/90) for Rochester. Looking at the warmest 3 day stretches in recent history paints this stretch as the warmest since August 27th-29th, 2013. So in regards to the last three days, it was the warmest overall stretch in nearly three years.
The highs the last two afternoons (90/90) were far off breaking any records. But the fact we reached back to back 90s was significant. The last two summers have been pretty weak, some might consider this summer the same up to recent. In 2014, we failed to reach 90°. We only reached it twice in 2015. The last time we saw consecutive days reach 90 or better was in August of 2013
Even though the high temperatures weren't record setting, the lows were breaking records. That can largely be attributed to so much moisture being in the air. You can't cool the temperature past the dew point... so we saw some impressive overnight temps. Thursday's low temperature of 78 was the warmest low temperature recorded on July 21st in history AND the 3rd warmest low temperature EVER recorded in the history of observations in Rochester. If the low of 75 holds for Friday (which it should) it would also mark the warmest low temperature on July 22nd in history.
So many stats! Long and short of it... it was hot. We're not quite done just yet either. Temperatures on Saturday are expected in the mid-upper 80s with still plenty of humidity. That should leave heat indices in the 90-100 range. The big difference here, is we are in transition to a cooler pattern. A storm system sweeping through is kicking that ridge of high pressure out and we're beginning to show signs of cooling back down. And rain will certainly be a heat buster when it falls Saturday, which looks to take aim mostly on the latter stages of the day. Finally our heat and humidity breaks on Sunday.
And one last graphic because it certainly illustrates the extremes we see living in Minnesota and Iowa. The upper midwest sees some of the most extreme temperature (and feels like temperature) differences in the nation. In this year alone, we've seen a difference in wind chill to heat index of 147°!
You have to be a hardy bunch to live here. That is one of the very reasons why I love the weather and why I love living here.
Storm Tracker 6 Chief Meteorologist