From the Lee Side

Tehuantepecer

Posted 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.

Lee

Categories: High Seas Analysis

Permalink | A A A

Top of Page

Reader Comments

Display: 0, 50, 100, 200

Sort: Newest First - Order Posted

Viewing: 1 - 12

Page: 1 — Blog Index

1. georgevandenberghe 8:51 PM GMT on January 04, 2013
Thanks for yet another great post. Although there are a lot of differences the Mistral in France has some similarities to this wind. I'd appreciate more comment on that from another poster.. I'm not an expert. Member Since: February 1, 2012 Posts: 3 Comments: 296

2. Lee Grenci, Retired Senior Lecturer and Forecaster 11:03 PM GMT on January 04, 2013
Quoting georgevandenberghe: Thanks for yet another great post. Although there are a lot of differences the Mistral in France has some similarities to this wind. I'd appreciate more comment on that from another poster.. I'm not an expert. Thanks George. Last year, Albuquerque, NM was one of the forecast cities in WxChallenge (run by the University of Oklahoma). The pattern set up for an easterly gap wind. I went high somewhere near 39 knots for max sustained winds, and, if my memory serves me, the forecast verified above 50 knots. Knowledge of gap winds is very important in weather forecasting. Member Since: October 24, 2012 Posts: 52 Comments: 444

3. NEWxSFC 2:25 AM GMT on January 05, 2013
A most interesting phenomenom. Thx for the analysis. Member Since: January 5, 2013 Posts: 0 Comments: 3

4. JNCali 2:37 AM GMT on January 05, 2013
Growing up in SoCal I got to travel through the San Gorgonio Pass which separates the San Gabriel and the San Jacinto mountain ranges as provides the gateway between the coastal communities and Palmsprings.. With mountain ranges over 10000' and a pass elevation of 1600' there is tremendous compression not only on a sunny summer afternoon but when a cut off low slides down the back side of the Sierra's or when a high pressure sets up a Santa Ana wind condition.. winds can reach hurricane force.. Member Since: September 9, 2010 Posts: 5 Comments: 1032

5. Lee Grenci, Retired Senior Lecturer and Forecaster 12:36 PM GMT on January 05, 2013
Quoting NEWxSFC: A most interesting phenomenom. Thx for the analysis. You're quite welcome. Member Since: October 24, 2012 Posts: 52 Comments: 444

6. Lee Grenci, Retired Senior Lecturer and Forecaster 12:36 PM GMT on January 05, 2013
Quoting JNCali: Growing up in SoCal I got to travel through the San Gorgonio Pass which separates the San Gabriel and the San Jacinto mountain ranges as provides the gateway between the coastal communities and Palmsprings.. With mountain ranges over 10000' and a pass elevation of 1600' there is tremendous compression not only on a sunny summer afternoon but when a cut off low slides down the back side of the Sierra's or when a high pressure sets up a Santa Ana wind condition.. winds can reach hurricane force.. Nice photograph. Thanks. Member Since: October 24, 2012 Posts: 52 Comments: 444

7. DocNDswamp 10:41 PM GMT on January 05, 2013
Howdy Lee, Thanks for another fine informative post. Indeed, quite the amazing topographical squeeze play involved with Tehuantepecers! Here's the same 1-3-13 0Z GFS wind analysis from NOMADS NCEP site, might illustrate slightly better detail - 1-3-13 0Z GFS 1000 mb Winds The effects of the winds streaking through the "gap" into Gulf of Tehuantepec also seen at 925 mb... and at 850 mb (depicting winds at layer height / altitude thru the canyon)... with perhaps the Venturi effect you mentioned. Interestingly, though not unexpected, the wind flow aloft is flowing opposite generally SW to NE (shifting from around 800-775 mb heights and above), reflecting the mid / upper subtropical jet... As seen in the GFS 700 mb wind analysis (same 1-3-13 0Z GFS), quite the difference from that at 850 mb. BTW, from my SE LA perspective, I'm already sick of the persistent "SW flow aloft" and it's associated dreariness.. as troughing over W / desert SW bumps against the stubborn, deep-layered Antilles-Caribbean high... LOL, darn thing sits there like a blocking linebacker, depriving me of Winter's coldest offerings! On the bright side of this (familiar) set-up, some who need it more to my W and N have received some rain/snow. Cheers! Member Since: September 21, 2005 Posts: 92 Comments: 4648

8. georgevandenberghe 11:09 PM GMT on January 05, 2013
In the DC area I'm still waiting for a truly killing freeze on Jan 5. This means something that kills bugs in the leaf litter and wipes out fall things in the garden once and for all. Normally we have had a few nights in the teens by this time. Not this year, lowest everywhere except unusual frost pockets was somewhere in the 20s. And it looks like I will not get it in the next week to January 12 Member Since: February 1, 2012 Posts: 3 Comments: 296

9. DocNDswamp 4:18 AM GMT on January 06, 2013
re: #8 Hi George. In a similar vein here, need to have a few mid-20's, only 3 light freezes / short duration so far at my locale. Told someone other day, we've had enough cold to feel miserable, not cold enough to do any good! Agree, another significant warm up ahead, likely near mid-month for next cold bout. Hope we get it then! Member Since: September 21, 2005 Posts: 92 Comments: 4648

10. Lee Grenci, Retired Senior Lecturer and Forecaster 12:58 PM GMT on January 06, 2013
Quoting DocNDswamp: Howdy Lee, Thanks for another fine informative post. Indeed, quite the amazing topographical squeeze play involved with Tehuantepecers! Here's the same 1-3-13 0Z GFS wind analysis from NOMADS NCEP site, might illustrate slightly better detail - 1-3-13 0Z GFS 1000 mb Winds The effects of the winds streaking through the "gap" into Gulf of Tehuantepec also seen at 925 mb... and at 850 mb (depicting winds at layer height / altitude thru the canyon)... with perhaps the Venturi effect you mentioned. Interestingly, though not unexpected, the wind flow aloft is flowing opposite generally SW to NE (shifting from around 800-775 mb heights and above), reflecting the mid / upper subtropical jet... As seen in the GFS 700 mb wind analysis (same 1-3-13 0Z GFS), quite the difference from that at 850 mb. BTW, from my SE LA perspective, I'm already sick of the persistent "SW flow aloft" and it's associated dreariness.. as troughing over W / desert SW bumps against the stubborn, deep-layered Antilles-Caribbean high... LOL, darn thing sits there like a blocking linebacker, depriving me of Winter's coldest offerings! On the bright side of this (familiar) set-up, some who need it more to my W and N have received some rain/snow. Cheers! Oh that graphic is superb. Well done. And thanks! Member Since: October 24, 2012 Posts: 52 Comments: 444

11. Lee Grenci, Retired Senior Lecturer and Forecaster 1:01 PM GMT on January 06, 2013
Quoting georgevandenberghe: In the DC area I'm still waiting for a truly killing freeze on Jan 5. This means something that kills bugs in the leaf litter and wipes out fall things in the garden once and for all. Normally we have had a few nights in the teens by this time. Not this year, lowest everywhere except unusual frost pockets was somewhere in the 20s. And it looks like I will not get it in the next week to January 12 George...did you read this piece? I don't want to get into a debate about climate change (I'm a weather guy), but it sort of relates to what you're talking about. Member Since: October 24, 2012 Posts: 52 Comments: 444

12. WunderAlertBot (Admin) 6:29 PM GMT on January 06, 2013
24hourprof has created a new entry.

Viewing: 1 - 12

Page: 1 — Blog Index

Top of Page

New Comment

Community Standards Policy Comments will take a few seconds to appear.

Post Your Comments

Please sign in to post comments.

or Join

Not only will you be able to leave comments on this blog, but you'll also have the ability to upload and share your photos in our Wunder Photos section.

About 24hourprof

Retired senior lecturer in the Department of Meteorology at Penn State, where he was lead faculty for PSU's online certificate in forecasting.

Tropical Blogs

Tropical Weather Stickers®

Contact This Author

Recent Photos

24hourprof's Wunder Photos

Previous Entries for 2013