Potential rain totals from Sunday, October 12th through Friday, October 17th 2008
Wet Weather Returning to the Valley
Early October Drying Disappearing Under More Rain

After a nearly two week period of relatively dry weather, interrupted only by an afternoon of soaking rains over Cameron and Willacy County on the 7th, yet another prolonged period of generally cloudy, and increasingly wet, weather is headed into the Lower Rio Grande Valley and Deep South Texas, beginning during the afternoon of the 14th and continuing through at least Friday, October 17th. Total rainfall across the area will likely range from 2 to 4 inches, though some areas in the Rio Grande Plains of Starr and Zapata County could see 5 inches by the end of the week. Farther east, particularly in Cameron County, more than 6 inches is possible in locations that experience stronger cells, since over an inch had already fallen in some areas along the Federal Highway 77 corridor between Brownsville and Harlingen before October 14th.

Why Here, Why Now?
The graphic below explains, in part, the interaction of weather systems that will result in the expected rains. Through Wednesday, October 15th, advancing Caribbean moisture, associated with a remnant tropical wave, will be lifted by a weak surface trough of low pressure (not shown), which will become nearly stationary along the coast. Showers and perhaps a thunderstorm will develop, and increase in coverage, from the afternoon of the 14th into the morning of the 15th, potentially dropping 1 to 2 inches of rainfall, with perhaps higher totals in heavier bands, near the coast from eastern Hidalgo and Brooks County into Kenedy, Willacy, and Cameron County. During the day on the 15th, numerous showers and a possible thunderstorm will develop into the Rio Grande Plains and all of Deep South Texas, where up to an inch of rain or more in stronger cells is expected before sunset on the 15th.

As the original tropical wave dissipates overnight on the 15th into the 16th, the deep tropical moisture left behind will hook up with a stream of Pacific moisture, which will "tip over" from a southwest to northeast axis to a more east to west axis by the 16th, then edge southward into the Lower Rio Grande Valley

later Thursday and continuing into Friday, aided by a weakening low level cool front which will aid in lifting the combined moisture sources into a full overcast, as well as occasional rain, through the 17th. Heavier rains are possible just about anywhere in the Valley on the 16th; precipitation may lighten a bit on Friday as a slight cooler air mass overtakes the original Caribbean sourced air.

Watching for Floods
The recent dry period has helped decrease surface moisture saturation somewhat; the Flash Flood Guidance indicated that 3 hour rainfall totals needed to create widespread flash flooding was between 4 and 5 inches for the Rio Grande Plains and Deep South Texas, and 3 to 4 inches for the Lower Rio Grande Valley, as of the afternoon of October 14th. Note: Updated guidance can be found here. That said, despite a lack of vigorous atmospheric forcing necessary to produce these types of heavy rains in a short period of time over a large area, some locations may receive enough rain in a short period of time to induce at least minor flooding of poor drainage areas, with an outside possibility of a more life threatening flash flood. Keep tuned to this website or NOAA Weather Radio for possible watches, warnings, or advisories through the end of the week.

The Rio Grande continues to run at or near "action stage", a result of very heavy rainfall in the Upper Rio Grande Valley, Big Bend region of Texas, and locations in northern Mexico since mid summer. The rains resulted in both releases of water and levee breaks from the Mexican side of the border, and forced additional releases at the jointly maintained Amistad Dam at Del Rio. While current data suggest the heaviest rains will remain south of the affected dams and levees through the 17th, the combination of any additional rainfall with the continued slow rise of the Rio Grande from San Benito to Brownsville will need to be monitored. For the latest in river stage information, along with explanations and potential impacts from Falcon Lake to Brownsville, check out our Advanced Hydrologic Prediction Service page.

Atmospheric Rainmakers, week of October 12-17, 2008
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