Nadine, Norman, and Tropical Plume Impacts

Although we have gone a while now without any direct threat of 'tropical' weather in the mainland United States, there are a couple of interesting cases currently unfolding. Far out in the Atlantic Ocean, Nadine has again strengthened to hurricane strength. This indecisive storm has been wandering around since September 11. Although initially following a reasonable mid-oceanic track, it has behaved more erratically over the last 10 days or so. Not only did it suddenly swerve away from the Azores, but it transitioned briefly to a subtropical storm, then a post-tropical storm, before again becoming a tropical storm. After looping and zigzagging its way back westward, it has just now again become a hurricane.

Looking toward the Pacific basin, Miriam dissipated to a remnant low last night west of Baja California, while just to the east of Baja a new storm formed, Tropical Storm Norman. Although Miriam swerved west instead of making landfall in Baja California and Norman is doomed to a very short existence by its proximity to land, the two storms may actually have a significant impact . . . on Texas. As occasionally happens with Pacific hurricanes, the tropical moisture that they have transported up into the mid and upper troposphere is being transported northeastward across the plateaus and mountains of northern Mexico into south and west Texas. Such 'tropical plumes' have been responsible for some of the most devastating flash flood events in the Texas Hill Country. Accordingly, the National Weather Service is warning of the possibility of widespread rainfall in South Texas that may be as high as 8 inches locally.

To add to the intrigue, the numerical model guidance gives good reason to expect that some of the mid-level circulation/vorticity from Norman will make it across Mexico over the weekend and then help to intensify the approaching shortwave trough. The resulting cyclogenesis will then produce a distinct surface low in the northwestern Gulf of Mexico. This is one of the relatively rare scenarios to bring significant widespread rain over Mississippi and surrounding states in October. Since this will be forming in conjunction with a mid-latitude shortwave trough and a weak pre-existing surface front, there is very little reason to think that the Gulf low could attain 'tropical' characteristics. (In the extremely rare cases when tropical storms do move from the Pacific into the Atlantic basin while maintaining their tropical character, they receive a new Atlantic name instead of keeping their Pacific one.)