System could bring beneficial rain to drought-stricken Texas

Satellite image of Invest 90L, likely to become a tropical depression some time today. (NASA)

2:15 p.m. update: Tropical storm Don is nigh. The National Hurricane Center writes: “Satellite imagery indicates a tropical depression or tropical storm could be forming about 90 miles north of Cancun, Mexico. . . . This system has a high chance...near 100 percent...of becoming a tropical cyclone during the next 48 hours.”

From earlier: With almost half the state of Texas afflicted by exceptional drought, a big dose of tropical rains would provide considerable relief. A fledgling tropical disturbance crossing the Yucatan Channel - if it takes the right track - could wash ashore in the parched state, answering many prayers. Although we expect this system to strengthen into a depression and probably a tropical storm - whose name would be Don, it is unlikely to become a destructive hurricane.

Over the last several days, this broad swirl in the easterly trade flow at about 10,000 feet moved across the tropical Atlantic with little meteorological fanfare. But over the last day or so, it has grown into a rather strong and well-defined mid-level low pressure system. Cruising northwestward at 15 mph across the Yucatan Channel, its well-defined rotary motion is clearly evident in animations of satellite pictures west of Cuba.

Though not currently at tropical depression strength (because no closed circulation has been found near the surface), Invest 90L, as it is called, shows signs it will soon earn that designation.

Surrounded by a relatively humid environment with low wind shear, and located over warm oceanic conditions highly supportive of further growth, much of 90L’s near-term development will be tied to internal processes not obvious in the satellite imagery.

For example, the sustenance of thunderstorm clusters longer than a few hours very near the rotation is evidence that 90L is maturing within these favorable surroundings.

Nonetheless, there has been little indication from the models so far that significant intensification is likely. In fact, as suggested by the model guidance, the mid-level vortex is not expected develop downward to the surface in a way that will trigger tropical storm development.

Despite the models’ bearish prospects for development, we think the warm sea surface temperature and relative lack of wind shear will allow the system to reach tropical storm intensity, with the upper limit for intensity before landfall probably a weak hurricane.

Steering currents in the Gulf of Mexico should steer tropical disturbance towards Texas (Courtesy U. Wyoming)

This scenario could provide at least some rainfall to regions of Texas and perhaps other areas of the south central U.S. that have been devastated by drought.

Here are several staggering drought stats aggregated by Star Tribune meteorologist Paul Douglas:

* Texas has officially experienced the driest nine-month period in the states history between October of 2010 and June 2011. This beat the previous record of June 1917 to February 1918.

Model track forecasts for tropical disturbance 90L. (Southwest Florida Water Management District)

* The dry conditions are also leading to a record wildfire season. According to the Texas Forest Service, a record 3.3 million acres of land have been burned since mid-November by more than 15,000 wildfires.

In addition to large parts of Texas, significant sections of western Louisiana and Oklahoma are contending with exceptional drought. While rains from this system would no doubt be beneficial, wetter grounds could also lead to the undesired consequence of significantly raising the humidity/heat index in places that will be over 100° regardless. And if the storm stagnates over the same area when it comes inland, flash flooding could become an issue.