First heat wave of summer coming into sight

We have thus far escaped any real hot weather this spring and summer, with just a day or two here and there. In fact, June temperatures averaged below-normal and our first official 90 degree day came weeks late.

The next 5-10 days or so temperatures will be NORMAL or lower than normal on average with highs in the 80s but the humidity will make up for it a lot of the time.

The average or normal high temp this time of year is 89.

However, there have been suggestions of a heat wave in the models for a week or two now for the longer range, and now there are other non-model  signals (such as ENSO/AAM/PMM ) that have joined to indicate that our first true heat wave of the summer is foreseeable but in the distance, starting around July 10th give or take a couple days.

There’s nothing unusual about hot weather in the summer. The question is always is it NORMAL level heat or ABOVE normal level heat and does it last longer than typical.

Research shows that as heat waves go beyond 3 days heat-related deaths and illness increase especially for vulnerable groups. Warm nights and high humidity of course increase the stress on everyones bodies and increase our air-conditioning bill.

To qualify as a true “heat ridge” aka subtropical high pressure area (anticyclone) at the 500mb level (about 18,000 feet) it must have a height/pressure of at least 588dm. Once readings measure at that level high temperatures at the surface will often be above 90F and typically last from many days to sometimes many weeks in a row. If the 500mb level reaches or exceed 594dm then a critical heat wave is probable with readings of 95-100 if it’s not cloudy, which it usually won’t be under that kind of strong ridge.

To date the jet stream has been too active for much heat with frequent upper-level troughs of low pressure over the Southeast part of the nation. These type patterns favor more clouds and thundershowers and more humidity preventing temperatures from soaring. For more sun and heat you need to be under an upper-level ridge of high pressure:

NOTE the forecast change in the upper-level pattern prognosticated by the two primary Global NWP Models the American GFS and the European ECMWF:

Both (above) show an expansive conjoined Sonoran-Sargasso Sea/Bermuda heat ridge with upper-level highs of 591 in our area and 594 or above to our West and North which is where the worst of the hot and dry weather would be centered. So the real blow torch looks to be in the Great Plains and Midwest/Great Lakes.

The trend is supported by the GWO/AAM and the PMM correlations for July:

The temperature departures from normal shown above are pretty consistent with the GFS ensemble analog derived guidance:

The expectation would be for a streak of 5 or more days in a row with a max temperature of 90 or higher. Analog guidance is indicating an 85% chance of air temperatures above 90 and a 74% chance of a Heat Index above 100F during the hot spell.

The operational deterministic GFS model is projecting widespread 100s across the Southeast including Georgia with a max one day of 103F in Atlanta.  I am not ready to go that far yet, I think the model is way too hot.

I’ll let ya know if I change my mind.


Dew point is a measure of the moisture content of the air and is a better indicator of comfort or discomfort than relative humidity, because relative humidity is RELATIVE to the temperature so it goes down as the temp goes up even if the moisture level holds constant.

For more follow me on Twitter @MellishMeterWSB.

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