Retired senior lecturer in the Department of Meteorology at Penn State, where he was lead faculty for PSU's online certificate in forecasting.
By: Lee Grenci , 7:15 PM GMT on January 04, 2013
The Tropical Analysis and Forecast Branch (TAFB) of the National Hurricane Center routinely issues maritime forecasts and warnings for portions of the Atlantic and Pacific Oceans, including the Gulf of Tehuantepec (check out TAFB's high-seas domain).
The color relief map below, which is a close-up of the the Isthmus of Tehuantepec (the narrowest area of land between the Gulf of Mexico and the Pacific Ocean), shows a gap (Chivela Pass) in the Sierra Madre Mountains. For reference, here's the same image with latitude/longitude. As it turns out, Chivela Pass plays an important role in generating gap winds that eventually impact the Gulf of Tehuantepec. These Tehuantepecers, which can reach storm-force speeds (48 knots or greater), typically occur from November to March. Not surprisingly, Tehuantepecers are observed more frequently in this region than equivalently strong winds produced by tropical cyclones during hurricane season.
A relief map of the Isthmus of Tehuantepec. Image with latitude/longitude. Here's the same image with lat/long references. Courtesy of Ray Sterner of Johns Hopkins Applied Physics Laboratory.
In the context of Chivela Pass, a gap wind forms in concert with outbreaks of cold air over the Southern Plains. More specifically, a post-frontal high-pressure system typically builds over Texas (or thereabouts) and ridges southward, producing a northerly flow over the western Gulf of Mexico. This northerly flow is eventually blocked by the Sierra Madre Mountains (except, of course, for the cold air that channels through the 40-kilometer wide Chivela Pass). In turn, the build-up of relatively dense cold air along the northern foothills of the Sierra Madre (revisit the color relief map of the Isthmus of Tehuantepec) causes surface pressures to increase, strengthening the north-south pressure gradient and driving cold air through the mountain gap. I suspect that the accelerating air flow might also be enhanced by the Venturi effect (the flow of air speeds up through the constricting gap).
Wednesday evening (January 2), I happened to look at the meteogram for Minatitlan, Mexico, and noticed the strong, gusty northerly flow during the afternoon. Sure enough, there was a post-frontal high-pressure system centered over Texas (1026 mb) on the 00Z GFS model analysis of MSL isobars (00Z is 6 P.M. CST). The corresponding 00Z GFS model analysis of streamlines confirmed the pronounced northerly flow over the western Gulf. So I generated the 00Z GFS model analysis of 1000-mb isotachs below (color-filled in knots, larger image).
The 00Z GFS model analysis of 1000-mb isotachs (color-filled in knots) on January 3, 2013 (the evening of January 2). Larger image. Courtesy of Penn State.
I'm sure the rather coarse resolution of the GFS did not completely resolve the faster winds on the northern side of the Gulf of Tehuantepec (annotated image of 1000-mb isotachs). But the rather small area affected is quite believable, given the decelerating, divergent nature of the flow over the northern Gulf of Tehuantepec (annotated image of streamlines). This ASCAT image of surface winds on January 3 showed surface winds of 25 to 30 knots in this small region (wind speeds were likely stronger earlier on the evening of January 2). By the way, ASCAT stands for the Advanced Scatterometer, which is mounted on the METOP-A satellite (the data shown on the image were measured on the ascending pass of the satellite).
The bottom line here is that NHC's Tropical Analysis and Forecast Branch were aware of the Tehuantepecer, and issued this analysis for significant wave heights (the average of the highest third of the waves) at 00Z on January 3 (the evening of January 2). An average height of 18 feet for the highest third of the waves was not too shabby.
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