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September 7, 2011 / Dave Gorham

South-Central U.S. Drought Possible Through Next Spring. Here’s Why.

ImpactWeather StormWatch Manager Fred Schmude weighs in today on SciGuy Eric Berger’s blog with the cryin’ shame not-so-great news that much of the south-central U.S. might not see any significant rain throughout the fall or winter.  The news isn’t good but it’s a great read and more than a little educational, including the update posted earlier this afternoon:

A question many people are raising right now is when will the current intense drought end?  Before we answer that question we really need to figure out what caused the current dry spell and see if there are any changes forecast for the near or distant future. The drought really intensified last winter thanks in large part to a very strong La Niña over the eastern Tropical Pacific. La Niñas typically result in a weaker than normal sub tropical storm track limiting the amount of rain producing weather disturbances that affect the Texas area during a normal winter.

For this year that phenomena started in February and intensified during March and April with very few rain producing weather systems. By the later part of spring and early summer the La Niña phenomena weakened and became a non player, however the very dry soil over the Texas region intensified the drought and allowed a semi-permanent upper-level high pressure to build over the area keeping most of the rain bearing tropical disturbances either south or east of the Texas Gulf Coast.  As a result, most of Texas was left high and dry resulting in the current 20-25 inch rain deficit we are currently experiencing in the Houston area and the increased fire danger as wildfires quickly spread across central and east Texas.

Unfortunately the main players that caused the very intense drought last winter are coming back this winter in the form of another potential La Niña condition. We currently are favoring a redeveloping moderate to strong La Niña this winter based largely on another weather phenomena called the Pacific Decadal Oscillation, or PDO.  The PDO is a large scale weather cycle over the northern Pacific Ocean which demonstrates alternating periods of cold and warm cycles, which can typically last 10 to 20 years.  When the PDO is in its cold phase La Niñas typically are more frequent and intense while the reverse is true during the warm phase of the PDO.  For this year the PDO has shifted toward the negative phase, which is signaling a redeveloping La Niña later this fall as colder than normal water over the eastern Pacific Ocean is funneled southward into the eastern Tropical Pacific.

As a result, we should see the current drought persist through next spring over most of Texas, including the greater Houston area as La Niña intensifies.  Yes, Texas will likely see some welcomed wet periods at times during the fall and early winter as the polar storm track occasionally shifts south bringing quick bursts of precipitation associated with cold fronts and other fast moving disturbances; however, below to well-below-normal precipitation will likely by the dominant weather trend over most of the state though next May. We can always hope for some type of a weak tropical system during the latter half of September into October, but even that scenario is looking less likely with time.

By the way our three driest years on record in the Houston area occurred in 1917 (17.66”), 1988 (22.93”) and 1901 (27.09”).  In a normal year the Houston area will typically receive around 45.00 to 50.00” of rainfall, so you can see in 1917 we had a little more than third of the normal rainfall while in 1988 we had roughly half.  This year we have currently tallied 11.00” for the year, while we should be at 32.58”, meaning so far this year we have received only about a third of our normal rainfall and are on a pace to tie or unfortunately surpass the all time 1917 record.

And in case you’re wondering what was going on in the eastern Pacific Ocean in 1917, there was a strong La Niña in place along with a negative phase of the PDO. Yes, it’s indeed amazing how history repeat itself, not only in life but with the weather.

UPDATE: Several readers, including Zack, have asked how dry soil contributes to furthering our hot and dry summer, as Fred mentioned above. Here’s his answer:

That’s a great question and can be best answered by a rule of thermodynamics that basically states dry soil heats up faster than wet soil. The reason being is when moisture is evaporated out of wet soil cooling results since evaporation is a cooling process. Dry soil does not have that same benefit and as a result with the near absence of significant evaporation (a cooling process) the air will tend to heat up faster.

Why is this important in regards to pressure? Well, in the world of weather pressure drops off more rapidly with height in cold air and more slowly with height in warm air. Since the soil has been so dry and warm over Texas the past 6 months the end result is a tendency for higher pressure aloft (~500 millibars or 18,000 feet) to sit over the Lone Star State, which creates a sinking and stable environment limiting clouds and rain bearing weather systems.

In essence I blame the dry soil for the most intense part of the drought we are now experiencing. The dry soil has a direct feedback on the strength and endurance of the upper high that is parked over Texas. We need a nice soaking rain that will moisten the ground over a large part of Texas and change the weather pattern for the late fall, winter and especially next spring. If the soil does not moisten up, we could easily see that strong upper high build right back over the area for next spring.

I’m tempted to wring some type of silver lining out of this but any reference to a cloud, at this point, would just be sad.

At least the region is getting an early taste of fall temps already.

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