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Wednesday, September 25, 2013
Roanoke reached its normal annual rainfall more than a month ago, on Aug. 18.
Since then, there hasn’t been much water in the bucket.
Looks like I picked a good time for a weather column and blog hiatus … or rather, my newborn son did.
After 2013 rainfall crossed the 12-month normal rainfall threshold of 41.25 inches on Aug. 18, Roanoke got less than an inch the remainder of August, then pitched a shutout on rainfall through the first 19 days of September before getting 1.11 inches on Friday and Saturday. The year’s total now stands at 43.42 inches.
Blacksburg, meanwhile, is sitting at 40.65 inches, almost a quarter-inch short of its normal annual rainfall of 40.89.
The flip from a wet summer to a dry late summer/early autumn has been brought on by alternating patterns more typical of mid-summer and mid-autumn.
Cold fronts moving in from the northwest at fairly regular intervals, perhaps just a bit ahead of the seasonal norm, have cut off the flow of Gulf of Mexico moisture and dried out many layers of the atmosphere, especially those higher above the surface.
Between the cold fronts, we had one period of summerlike warmth build in from the southwest as a heat dome bringing record high temperatures to some of the Midwest expanded eastward. This brought us a taste of the common midsummer dryness we didn’t experience in midsummer.
The same pattern, however, led to dense moisture flow along its back side into the Front Range of the Rockies, leading to the historic flooding in and around Boulder, Colo. Conversely, much of the West was hot and dry while we were getting inundated back in early July.
Another significant factor in our dryness is the short-circuiting of the Atlantic hurricane season.
There have been nine named storms but only two relatively minor hurricanes, one far from any shoreline, the other so close to the Mexican coast that it quickly came inland and didn’t have any time to develop further over open water.
Widespread regions of dry and sinking air have, so far, stymied the Atlantic hurricane season. Of course, as evidenced by Hurricane Sandy, which became Superstorm Sandy late last October, and Hurricane Juan, which sent its remnants to our region and caused epic flooding in early November 1985, it’s too early to entirely let down our guard on this.
But the lack of tropical systems so far has taken out a frequent component in late summer/early fall rains for our region.
The widespread rain we had Saturday did have some tropical sources to it, including Hurricane Ingrid in the western Gulf of Mexico, Hurricane Manuel in the eastern Pacific and another tropical system in the western Gulf that came up short of developing into a named tropical storm.
Whether we get more rain similar to Friday and Saturday, or slip into continued dryness, will depend on how many of the storm systems sliding in from the west and northwest can dig abundant Gulf of Mexico moisture and throw it over us.
It appears a system on Wednesday will be a near-miss for widespread significant rain, staying mainly to our south, but it could bring some light rain and drizzle.
And we’ll keep an eye on the Atlantic for tropical systems. If we’ve learned anything this year in local weather, it’s how seasons like to show up fashionably late.
Weather Journal runs on Wednesdays.
Weather JournalEarly mix, then ice storm Sunday