Weather pattern transitioning to summer

Published 7:00 am Tuesday, May 12, 2015

Our transition from spring to summer is underway, which is not surprising for mid-May. The weather map is taking on more of a summer look, and it’s beginning to feel more like summer as we move into the month.
The polar jet stream in the upper atmosphere is to our north, and we may not see another major dip of the jet down into the Gulf South until next fall. That means that surface cool fronts don’t have much southward push by the time they make it down to the Gulf Coast. Those fronts stall and become stationary near us, and then gradually dissipate.
Such a stationary front is in our area today and will help increase our chances for showers and thunderstorms into the 50 percent range.
The front dissipates by Wednesday, and a ridge of high pressure in the upper levels of the atmosphere will strengthen over the region. The ridge will keep a lid on our rain chances making showers in our area more of the widely scattered variety rather widespread on Wednesday and Thursday.
Why are ridges of high pressure in the upper levels between about three and eight miles up associated with drier weather?
In the lowest part of the atmosphere, temperatures generally decrease as you go up. This region is called the troposphere, and in our area this time of year the troposphere is about eight miles deep. However, there are often several layers in the troposphere where warm air is on top of cooler air. These are called temperature inversions. High pressure, especially several miles up in the atmosphere, causes very slow sinking of air from the middle and upper levels over very large areas. As the air sinks, it is compressed and warms. This resulting layer of warm air, called a subsidence temperature inversion, acts as a lid or cap on air rising from below. If the rising air from below doesn’t have enough momentum to punch through the cap, then any clouds that do form just can’t rise high enough to become rain showers.
The strength of upper level high pressure and the strength of the main associated subsidence temperature inversion is a major factor during summer as to whether the Gulf South experiences just isolated afternoon thundershowers (strong capping inversion), or more numerous thunderstorms (weak or no capping inversion).
By Wednesday and Thursday, the strengthening ridge will produce a capping inversion strong enough to lower our shower chances to the 20 to 30 percent range.
As we head into Friday and the weekend the upper ridge and cap weaken. At the same time, at the surface, we will be on the western periphery of the Bermuda High, another feature whose strength and position greatly influences our weather every summer. The circulation around the Bermuda high will be pumping plenty of warm, humid air into our area in the lower and middle levels of the troposphere. This, in combination with the weakening cap should make for a summery weekend with scattered, mainly afternoon thundershowers.
Temperatures throughout the week will be in the middle to upper 60s early in the mornings rising to highs in the middle to upper 80s in the afternoons.

By Skip Rigney

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