Of course, not every spring storm takes on the characteristics of a tropical storm, but this one last night certainly came close. Usually, a fairly fast moving cold front moves through as winter’s last gasp creating those spring showers, but last night we had what is called a convergence zone that got stuck.
What’s that? Simply put, when winds from different directions meet, or converge, they have nowhere to go but up and they take their moisture up where it condenses into raindrops and falls. A lot of it falls. Especially when the zone gets stuck and then moves very slowly out of the area.
This is what happened when Tropical Storm Allison (2001) came on shore, and Hurricane Harvey (2017) and Tropical Storm Imelda (2019). They all got stuck and dropped huge amounts of rain.
Look at the synoptic maps from yesterday morning to this morning--24 hours. I’ve highlighted the High Pressure systems to our west, our east and our north. And you’ll notice the Low pressure systems and fronts inbetween are virtually in the same place. VERY little movement in 24 hours. That’s what getting stuck means.
So when this convergence zone set up north of Houston it could only very slowly move southeast toward the gulf which is the way most of these eventually move thanks to the spin of the earth. Allison, Harvey, and Imelda all finally moved southeast out of the area. Same pattern. Here’s last night’s radar from 9pm, then 3am and 6am this morning:
And as you might know, just like tropical systems, some areas had huge amounts of rain while others had very little. Across much of Houston: 1-2″ with western areas like Katy getting .06″! But north and east of us, 4-11″. Here’s a look:
So while we had no named storm coming off the Gulf, we did have a very healthy Low Pressure system that could sustain winds of 40mph (strong enough to be called tropical wind force) and if you see today’s cover photo certainly even stronger straight line winds occurred. What would be the main difference from last night and a tropical system? An actual tropical storm is capable of holding three times the moisture of a land based Low--and, as you may well know, that 11″ of rain near Groveton could easily have been 33″.
I bring all this up because just last week the National Hurricane Center was watching a tropical flare up in the far Atlantic and now here we are still in April with the makings of very heavy rain. Good reminders to be prepared for the tropical season ahead.
Frank
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