Friday, July 29, 2011

Tropical Storm Don Eyes the Southwest

Tropical Storm Don presently moves toward the south Texas coast at about 22 km/hr.  Landfall is expected this evening.

Image: NOAA/National Hurricane Center
Although tropical cyclones are commonly thought not to impact the southwest United States, they frequently modulate and contribute to monsoon precipitation.  This may be the case with Don, which has been tracking to the west-northwest in the easterly flow on the equatorward side of the monster ridge that has been parked over the central and eastern US for the past few weeks.

48-h loop of 925-mb winds, precipitable water, and IR satelite
imagery ending 1500 UTC 29 July 2011. 
In particular, note the slug of high precipitable water accompanying Don across the Gulf of Mexico and how Don's present track is taking it toward northern Mexico, west Texas, and southern New Mexico and Arizona.

We can track the remnants of Don in the GFS forecast initialized 0600 UTC 29 July.  To do this, I've plotted 925-mb absolute vorticity and precipitable water.  The former is a measure of the circulation accompanying the storm, the latter the moisture.  I've also highlighted the 3.5 cm precipitable water contour.

During this period, precipitable water is generally high over the Gulf of Mexico and west of the Sierra Madre and along the Gulf of California.   Lower precipitable water over the Sierra Madre at least partly reflects the fact that there is no lower troposphere over this high terrain area.  As Don moves inland, it is accompanied by a surge of moisture into west Texas and northern Mexico.  A careful look at the loop shows a slug of higher precipitable water (>3.5 cm) penetrating across the northern Sierra Madre, tracking pretty much over Tucson, and merging with the tongue of high precipitable water on the west side of the Sierra Madre.  After that merger, the moisture associated with the remnants of Don is difficult to track.  

0600 UTC 29 Jul GFS forecast through 1800 UTC 1 Aug.
925-mb absolute vorticity color filled, precipitable water
contoured with 3.5 cm contour highlighted.
We'll have to keep an eye on this event and see how this Atlantic hurricane impacts precipitation over the monsoon region.  Ritchie et al. (2011) examine the influence of eastern Pacific tropical cyclones on monsoon precipitation, but I'm not aware of any papers that specifically investigate the influence of Atlantic cyclones.

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