Late-Season Tropical Storm Sandra Could Strike Mexico and Drench Texas
Tropical Storm Sandra formed on Tuesday morning in the record-warm Pacific waters off the south coast of Mexico, about 570 miles south of Manzanillo. Sandra joins last week's Tropical Storm Rick as one of the latest-forming tropical storms in the history of the Eastern Pacific. Since accurate records began in 1949 (with higher-quality satellite records beginning in 1971), the Eastern Pacific has seen only six tropical storms form after November 18: December 5, 1983 (Winnie), November 27, 1971 (Sharon), November 27, 1951 (Unnamed), November 24, 2015 (Sandra), November 20, 2011 (Kenneth), and November 19, 2015 (Rick). None of these storms hit land. If Sandra hits land, it will be the latest landfalling Eastern Pacific storm on record.
Satellite images on Tuesday morning showed that Sandra had a large and steadily organizing area of heavy thunderstorms. Steering currents favor a path to the west or west-northwest parallel to the coast and far enough offshore to prevent heavy rains in Mexico through Thursday. On Friday, a trough of low pressure passing to the north of Sandra will turn the storm to the north and northeast, but will also bring high wind shear, weakening the storm as it approaches the tip of the Baja Peninsula.
Conditions are quite favorable for Sandra to intensify over the next couple of days, with record-warm sea surface temperatures of 29-30°C (84-86°F) and wind shear of less than 10 knots beneath an upper-level high arcing around the storm. The official NHC forecast issued at 10 am EST Tuesday brings Sandra to Category 2 strength, with peak winds of 100 mph by Thursday. NHC’s Rapid Intensification Index shows roughly a 50-50 chance that Sandra will strengthen by at least 30 knots in the next 24 hours. If Sandra does reach Category 2 strength, it will be the strongest Eastern Pacific hurricane known to exist so late in the year. All of the six cyclones above were tropical storms or Category 1 hurricanes except for Kenneth, which hit Category 4 strength on November 24, 2011. Kenneth is the latest-occurring major hurricane in Eastern Pacific records.
Figure 1. Latest satellite image of Sandra.
With Sandra virtually certain to be a tropical storm or hurricane on Thanksgiving Day, this year will mark the first time NHC has ever had to issue Thanksgiving Day advisories for an Eastern Pacific named storm. However, NHC has issued Thanksgiving Day advisories for Atlantic storms on a number of occasions. The last time was November 24 during the year--you guessed it--2005, during the insane Hurricane Season of 2005, when Tropical Storm Delta was active during Thanksgiving weekend in the far eastern Atlantic.
Sandra may complicate heavy rain threat over Southern Plains
The biggest impacts from Sandra may occur from Texas to Arkansas, where a prolonged heavy rain episode late this week seems likely to be enhanced by Sandra’s remnants. A strong upper-level low settling over the U.S. Southwest will help pull Sandra northeastward, and the storm’s moisture will flow atop a very shallow cold air mass that will spill across the Southern Plains later this week. Such setups involving Eastern Pacific hurricanes are notorious for giving the region some of its heaviest rains on record, but they are far more typical of September or October than late November. Localized flooding may become a major travel headache, as rainfall totals of 3-8” are expected from Thursday through Sunday over central and north Texas, southeast Oklahoma, southern Missouri, and northwest Arkansas (see Figure 2 below). These rains will fall over areas that have been doused repeatedly over the last few months. With 50.75” of rain for the year through Tuesday morning, Dallas-Fort Worth area may well break its all-time annual precipitation record of 53.54” (set in 1991) before November is done.
On the northwest edge of the heavy rain swath, there should be a parallel strip with low-level temperatures cold enough for mostly light but widespread freezing rain, sleet, and/or snow, with an initial round from Thanksgiving Day into Friday and perhaps a second batch over the weekend as another lobe rotates around the sprawling upper-level low. Frozen precipitation is most likely from the Texas Panhandle and northwest Oklahoma across much of Kansas to eastern Nebraska.
Wunderblogger Steve Gregory has a new Monday afternoon post, Stormy Thanksgiving / Potential Flash Flooding and Blizzard Conditions for Texas.
Jeff Masters and Bob Henson
Figure 2. Projected 5-day precipitation totals (rain and melted snow/sleet) for the period from 7 am EST Tuesday, November 24, through Sunday, November 29.
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Dr. Masters co-founded wunderground in 1995. He flew with the NOAA Hurricane Hunters from 1986-1990. Co-blogging with him: Bob Henson, @bhensonweather
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