Drenching storms to increase risk of localized flash flooding in southern US into late July

By Alex Sosnowski, AccuWeather senior meteorologist

July 15,2017, 2:54:07PM,EDT

 

 

 While humidity levels will remain high overall, rounds of drenching thunderstorms will help limit long-lasting extreme heat over much of the southeastern United States into late July.

A slight southward dip in the jet stream will be the catalyst for the storms and slight cooling.
The jet stream is a high-speed river of air at the level in the atmosphere where jets cruise at.
During the summer, when the jet stream dips into the southern U.S., it creates conditions in the atmosphere that favor eruptions of showers and thunderstorms.
"We anticipate a northward bulge to be a semi-permanent feature over the western half of the nation for the balance of this summer," according to AccuWeather Lead Long-Range Meteorologist Paul Pastelok.

"The heat produced by this pattern will shift back and forth from the Western states to part of the Central states, but will be largely absent farther east," Pastelok said.
The northward bulge in the jet stream over the West will help create the corresponding opposite southward dip in the East.
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Strong sunshine, high humidity and cool air aloft associated with the dip in the jet stream will fuel the storms.
"Since only a slight southward dip will occur, any push of dry air will be weak and generally limited to areas adjacent to the Ohio Valley, central Appalachians and mid-Atlantic region," Pastelok said.
Ahead of these pushes of slightly drier air, heavy storms can erupt.
One such example will be this weekend, especially from parts of the lower Mississippi and Tennessee valleys to the Carolinas.

Any storm that pops up in the southern U.S. during the summer can be briefly heavy.
However, storms can be more robust than average in this pattern with incidents of flash flooding and isolated damaging wind gusts.
When the dry air settles into some northerly locations, storms are likely to be briefly suppressed.
Other occasional lulls in the storms may develop in parts of Georgia, upstate South Carolina and central North Carolina, where a slight breeze occasionally blows downhill and dries out the air a bit from the Appalachians. Storms may also be scoured out at times near and west of the Mississippi River, when warm air moves in aloft.
Humidity levels will remain high over much of the Southeast, especially along the Gulf and southern Atlantic coasts with storms likely almost every day.

In terms of temperature, highs will generally be within a few degrees of average, since the cool air aloft will only briefly work its way down to the ground during some of the storms.
Highs generally range from the upper 80s to the middle 90s over the Southeastern states from the middle of July to early August. This is the time of the year often referred to at the "dog days of summer."
Strong sunshine this time of the year will quickly cancel out any brief cooling from the downpours so it will still feel hot.
Brief spikes of extreme heat can occur on occasion just about anywhere in the South this time of the year.
"Areas most likely to bunch multiple days together with highs in the upper 90s will be across the central and southern Plains to the middle and lower Mississippi Valley," Pastelok said.
When the sun is out, with the southern high humidity, AccuWeather RealFeel Temperatures can surge well past 100 degrees Fahrenheit and conditions can be difficult to dangerous for strenuous outdoor activity. Such activities should be limited to the early morning or evening hours.