Wake lows and blowing boxcars
Published 7:00 am Tuesday, May 5, 2015
Last Monday morning, April 27th, I was on my way to an appointment in Metairie. At about 11:15 a.m. I found myself on the I-10 high-rise in New Orleans in some of the heaviest rain and highest winds I have ever driven in.
A strong squall line was ripping through southeast Louisiana causing street flooding and wind damage. A New Orleans television reporter captured video of several boxcars being blown off the tracks of the Huey P. Long Bridge. The video has garnered nearly one million views on YouTube.
I safely made it to Metairie. Then I was in for my second interesting meteorological experience of the day. Even though the main storms were now well to my east, and the rain was much lighter, for the next 45 minutes the sustained winds blew at 35 miles per hour with occasional gusts to 50.
The initial strong winds had been caused by the main squall line, but what was causing the strong winds behind that system?
The culprit was a mesoscale phenomenon known as a “wake low.” (Mesoscale refers to horizontal scales in the atmosphere of ten to a couple of hundred miles, compared to the much larger “synoptic” systems covering hundreds to greater than one thousand miles that forecasters usually deal with.)
As parcels of air sink in the atmosphere, they warm. Wake lows sometimes form behind squall lines because the descending air there becomes warmer and less dense than the surrounding areas. On relatively rare occasions, quite large surface atmospheric pressure differences between the wake low and the surrounding higher pressures cause significant wind events as the air quickly moves to fill in the low and equalize the pressure field. The air was moving pretty quickly last Monday in New Orleans!
While the worst passed to the south, Pearl River County still had very gusty winds and rain totals of one-half to one inch.
As discussed in last week’s column, our April 2015 was wetter than normal.
The final numbers are in, courtesy of the Community Collaborative Rain, Hail, and Snow Network, or CoCoRaHS (www.cocorahs.org). CoCoRaHS is a nationwide network of volunteer precipitation observers. The data is organized and posted on the Internet by staff at the CoCoRaHS headquarters at Colorado State University’s Department of Atmospheric Science.
We have several CoCoRaHS observers here in Pearl River County. Three sent in observations to CoCoRaHS every day last month, yielding these totals for April:
5.6 miles east-northeast of Picayune: 12.74 inches
3.2 miles south-southwest of Carriere: 8.78 inches
5.9 miles north of Carriere: 9.37 inches
Poplarville had 6.58 inches of rain in April as reported by Mississippi State University’s South Mississippi Branch Experiment Station. Although not a part of CoCoRaHS, the Experiment Station also measures precipitation. They make their data available through the Southern Regional Climate Center at Louisiana State University (www.srcc.lsu.edu).
Average rainfall for April in Pearl River County is around five inches, so these numbers show how unusually wet April 2015 was in the southern part of the county.
In contrast, early May will continue to be dry with only the slightest chance of showers until Sunday or Monday of next week.
By Skip Rigney.