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Alvin weakening, may become post-tropical cyclone sooner than expected
By: Civicane49 , 4:43 AM GMT on May 17, 2013
East Pacific’s Tropical Storm Alvin is weakening. The combination of strong west-southwesterly shear and interaction of the monsoon trough has disrupted the cyclone from organizing further. The latest National Hurricane Center (NHC) advisory stated that Alvin is barely a tropical storm with winds of 40 mph and central pressure of 1006 mb. Satellite images depict a poorly organized tropical storm with a disorganized cloud pattern. Satellite imagery also shows a cyclone interacting with the monsoon trough. The storm appears to be in the process of becoming a post-tropical cyclone or an open trough. In fact, the latest ASCAT pass suggests that the surface center has become an open circulation. If it remains that way for several hours, the NHC will likely end the advisories of this cyclone. (One of the requirements of a tropical cyclone is that it must have a closed surface circulation center).
Figure 1. Evening infrared satellite image of Tropical Storm Alvin. Image credit: NOAA/RAMMB.
Forecast for Alvin
Alvin should continue to meet its demise. An upper-level trough located just off of the western coast of Baja California has resulted strong west-southwesterly shear of 20 knots over the storm, and the shear is not expected to abate significantly in the next few days. In addition, the interaction of the monsoon trough will keep Alvin from organizing further but rather weakening it. Thus, I expect additional weakening of the storm before degenerating into an open trough by 72 hours. Though, I would not be surprised to see Alvin becoming an open trough sooner than the NHC expected. The system is currently moving west-northwestward and should remain in this motion for couple of days under the influence of a mid-level ridge over southwestern Mexico. Global models forecast the low-level and mid-level centers of the cyclone to split when the storm will degenerate. The low-level center is predicted to move westward within the easterly flow, while the mid-level center is expected to shift poleward toward the trough. Alvin is not anticipated to threaten any land masses.
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