Saturday, October 27, 2007
Weather columnist Kevin Myatt: Soaking was much-needed fluke
Kevin Myatt is The Roanoke Times' weather columnist.
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So we've had this desperately needed three-day rain of 3 to 10 inches across Southwest Virginia.
Where do we go from here?
The drought, in terms of long-term rainfall deficit, is not over. In Roanoke, we managed to get back to less than 10 inches below our normal rainfall for the year to date, but that still leaves us plenty behind. And remember, 2005 and 2006 were each below-normal rainfall years, too.
The reservoirs and streams will get a nice boost and the fire, danger will be quelled significantly for a while with all the surface moisture.
But getting out of a drought is more about getting 1-inch rains at something resembling regular intervals. There is simply no signal that such a pattern is about to develop. In fact, it looks warm and mostly dry into the foreseeable future.
This week's rain was something of a meteorological fluke.
The jet stream made a dive southward, which is what generally happens this time of year as cool air begins moving south. But the overall atmospheric pattern wasn't conducive to sustaining that southward dip. So as the jet stream retreated back north, a piece of it became cut off in a persistent whirl over the southern United States.
That upper-level low's counterclockwise spin pulled in lots of moisture from both the Gulf of Mexico and the Atlantic Ocean.
More than a week before, there were signs that this cut-off low was going to develop. However, its exact position was uncertain.
It ended up setting up perfectly to spin the rain over us for three days. It was not set up right to give much rain to Georgia, though, where some reservoirs could dry up within three months if there isn't significant rain.
So let's count our blessings with this rain, but not forget that others near us are still desperately dry, and our moisture isn't yet where it needs to be.
California burns again
Speaking of places that need moisture badly, the big weather story nationally has been the destructive and deadly wildfires in southern California, fueled by Santa Ana winds.
Four years ago, I wrote a column about a similar outbreak of fires in California. Below is an excerpt from my Oct. 31, 2003, column explaining Santa Ana winds:
"The weather phenomenon responsible for spreading the fires last weekend and early this week is known as the Santa Ana winds. They recur on an intermittent basis when a strong high pressure system sets up to the north of the region. Circulating clockwise, the high pressure system drives winds from east to west out of the desert into southern California, rather than the typical westerly breezes off the Pacific Ocean that usually make California such a pleasant place to visit.
"Persistent winds from one of the hottest, driest places on the planet would be bad enough for fire danger, but southern California's topography turns the Santa Ana winds into a pyrotechnic tempest.
"The winds rise and fall over several waves of mountains. If there were any moisture at all in the desert winds, it would be squeezed out when it's lifted over the first mountain. When air descends down a mountain's slope, it dries out, and also heats up as it is compressed. So already hot, dry winds become hotter and drier.
"Then, the winds become faster. The winds are channeled through tight valleys in the mountains, and this accelerates them to speeds commonly as high as 60 or even 70 mph."




