Posted: 7:48 am Wednesday, July 5th, 2017

Different kind of summer in Atlanta 

By Kirk Mellish

This certainly is not the kind of spring and summer we’ve grown accustomed to in Georgia in recent decades with frequent droughts. More often than not, like JUST LAST YEAR, Summer started in April or May and did not relent until September or October with long streaks of 90+ days, multiple 95-100+ days, lots of sun and fewer than normal pop up storms.

Instead, we have ONLY hit 90 5 times this year and we just went above 90 for the FIRST time July 5th, the 6th latest on record to have a high above 90!

So in many ways, June and the start of July have been something of a “throw back” to old fashioned southern summer weather, instead of the ABNORMAL excessive heat and drought we’ve become accustomed to experiencing.

This year the big strong upper level ridges of high pressure that often sets up across the Southern or Southeast USA has not formed. Only for brief spells of a few days has either a Sonoran high ridge built in from the West or a Bermuda high built in from the East. There has thus been a notable lack of high pressure centered over the Great Smoky Mountains and adjacent regions. These type ridges bring dry sun and heat waves.

And when there has been some riding aloft over Georgia it has been with either some troughing, or with an unstable west or southwesterly flow containing ripples or waves of energy in the jet stream to lift the humid air into clouds and showers or storms, as the 500mb chart shows past 24 plus hours:

Screen Shot 2017-07-06 at 5.43.07 AM

In the absence of a strong high pressure ridge aloft it’s difficult to get hot sunny dry weather to last.

Generally speaking air is sinking downward and drying under a ridge of high pressure, while under a trough (troff) of low pressure air is generally rising upward and moistening.

Modeling suggests more of the same ahead with the worst heat west:

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We start with troughs, then get some ridging but not overpowering, and then weakness shows up in the ridge before it tries to strengthen again.

So as I’ve tweeted and blogged so many times over the past month, we get come and go heat and dry spells measured more in days rather than weeks or months.

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And the 46-day precipitation departure from normal reveals more of the same as well:

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It’s hard to be rain-free when the air mass is so moist. Positive feedback loops also play a role. Dry ground helps the air heat up and moist ground holds temperatures down. So drought begets more drought and heat and wet “attracts” more wet and below normal temperatures on average.

The models have on several occasions the past couple months, suggested that a big hot dry heat ridge dome would build over the South and/or Midwest, but those projections keep failing to verify.

Eventually, it probably will happen before summer is over as it usually does. BUT, until we see “the whites of its eyes” I will expect only come and go heat or dry conditions and be skeptical of a wholesale pattern flip or reversal.

 

 

 

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