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Wednesday, July 24, 2013
Heavy rains were hitting Southern California on Monday.
In itself, that’s not really all that relevant to Southwest Virginia, other than people there sharing some of our misery. But in this case, the same upper-level low that was triggering some flash floods there had done the same for us 10 days ago.
A storm system crossed almost the entire width of continental United States from east to west, opposite of what is almost constantly a west to east upper flow across the U.S.
Such is the bizarre summer of 2013.
Retrograde weather systems, those that buck the prevailing atmospheric flow and move the opposite direction of what is typical, aren’t unprecedented. Most years have a few of these. Superstorm Sandy, which stirred waves on Lake Michigan after demolishing the New Jersey coast, was an example.
Summer is a peak time to see these kind of events, when the fast upper-level winds retreat north over Canada and leave sluggish wind flow over the U.S.
But it is highly unusual to see a storm system move from West Virginia to off the West Coast.
What happened in this case was a game of atmospheric billiards bouncing the upper-low westward.
The westward expansion of a strong high pressure system off the East Coast brought a heat wave to the Northeast. We had a week’s worth of somewhat above-normal temperatures, a little short of what we would really call a “heat wave.”
The low, sitting just west of us spinning up tropical moisture a week and a half ago, got pushed westward, then southwestward, by the expanding high.
It got shoved into Texas, where it delivered much-needed rains in regions experiencing extreme drought, as well as ridiculously low mid summer temperatures. While New England was seeing mid-90s to near 100, places like Dallas, San Antonio and El Paso had days that stayed below 80.
The low eventually drifted westward across the Desert Southwest and northern Mexico, then got bounced westward by newly expanding high pressure in the West. It was offshore on Monday, spinning Pacific moisture into California.
Residents of Southwest Virginia may have noticed storms moving southward, southwestward and westward at times last week.
It’s not unusual to see slowly meandering storms in odd directions during summer with typically weak upper-air flow, but some of the storms actually moved with some speed, 25 mph or so.
This had to do with the “heat dome” high that expanded westward becoming centered over the Ohio Valley, and its clockwise circulation pushing storms around it to the west and southwest.
Retrograde patterns, inevitably, break down. West to east upper-air flow to our north eventually edged southward, and this eroded the heat dome high and brought a cold front down to break the Northeast heat wave.
For us, it’s still sticky like it has been for weeks, with scattered afternoon storms, some with heavy rain, but the motion of those storms has been restored to the typical eastward and northeastward directions we would expect.
Backward and forward, it’s still a wet summer for us.
Weather Journal runs on Wednesdays.
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