Last week’s atmosphere loaded with moisture

Published 7:00 am Tuesday, April 21, 2015

Wasn’t Sunday a gorgeous day? Sunny, blue skies, gentle breezes, temperatures in the 70s rising into the mid-80s, and humidity decreasing throughout the day.
Maybe it seemed especially pretty because it was the first time in ten days that it didn’t rain somewhere in Pearl River County. Much of the county received six to eight inches of rain during that long, cloudy, wet stretch.
A large low pressure circulation in the upper atmosphere, several miles aloft over the southwest U.S. kept pumping moist air over us at upper levels, while surface winds were generally from the Gulf. A number of disturbances in the southern jet stream spun out of the upper low and moved from the southwest across our area.
Meteorologists calculate a location’s total moisture content from the surface upward to the high, dry altitudes near the top of the atmosphere. They call this number “precipitable water.” It is equivalent to the depth of water if all of the water vapor in the column were condensed into liquid and fell as rain. Several days last week the precipitable water over us was nearly two inches, setting new records for those dates in April.
(We have our maximum “precipitable water” amounts in the summer, because warm air can hold more water vapor than cooler air.)
Higher-than-normal precipitable water means that rain clouds, as moist air converges toward them and builds thunderstorms, can become very efficient rain makers. That was the case last Tuesday morning when many of us received over three inches of rain in just a few hours.
Of course, this meant that local creeks and branches approached or briefly exceeded flood stage. The smaller streams have now receded as they have dumped their excess into the Pearl River. Hydrologists at the National Weather Service Office in Slidell have issued a flood warning for the Pearl River for minor flooding this week.
Thankfully, the upper low over the southwest U.S. lifted out to the northeast and dragged a spring cool front through our area yesterday. The dry, cool dome of surface high pressure behind the front will give us sunny, mild weather today.
The ridge of high pressure will quickly slide to our east, which means by Wednesday winds will veer around to a more southerly direction. Humidity will begin to climb along with the chances of widely scattered showers and thunderstorms on Wednesday and Thursday. A cool front approaching from the northwest on Friday will further increase our shower chances. At least until the two most reliable global computer models come into better agreement about how quickly the front passes our area, forecasters will keep a chance of showers in Saturday’s forecast.
The good news: forecasters are very confident that rain totals later this week will be much less than last week.
Enjoy the spring weather. It won’t be long before some of us will be looking back wistfully on this week’s highs in the low to mid 80s. Arriving sometime in the next three to six weeks is the season that dominates our annual weather cycle here in the Deep South: summer, with its 90+ degree days and sweltering, sauna-like humidity.

By Skip Rigney

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