Unseasonable Sandra: Hurricane Threat for Mexico, Torrential Rain in Southern Plains
Holiday travel during the busy Saturday/Sunday Thanksgiving weekend across portions of the Southern Plains will get disrupted by a most unusual occurrence--flooding rains and a potential ice storm, enhanced by moisture from the strongest Eastern Pacific hurricane observed so late in the year. Declared a hurricane on Tuesday night, fast-strengthening Sandra is already making East Pacific history about 700 miles south of Cabo San Lucas. Sandra reached Category 2 strength in the National Hurricane Center’s 10 am EST Wednesday advisory, with top sustained winds estimated by satellite at 85 knots (100 mph). Sandra is the second-latest hurricane to form in the East Pacific since reliable satellite-based records began in 1971. Prior to Sandra, the strongest East Pacific hurricane so late in the year was 1983’s Winnie, which topped out on December 6 at 80 knots (about 90 mph). The king of November hurricanes in the East Pacific is 2011’s Kenneth, which attained top sustained winds of 125 knots (145 mph) on November 22--just two days before (U.S.) Thanksgiving Day.
Figure 1. Satellite image of Hurricane Sandra taken at 10 am EST November 25, 2015. At the image, Sandra was a Category 2 storm with 100 mph winds. Image credit: NASA/GSFC.
Even though it’s late November, conditions are unusually supportive for Sandra to continue intensifying into a major hurricane. A small eye has become evident on satellite imagery over the last few hours, with a compact zone of intense convection around it. Sea surface temperatures are at record highs for the time of year (29-30°C, or 84-86°F), and there is plenty of oceanic heat content below the surface (see Figure 2). Although Sandra is gradually moving into a region of higher vertical wind shear, the current values of 10 - 20 knots should allow continued strengthening. Things will change quickly by Thursday, however, as Sandra’s northwestward motion takes it into much stronger flow ahead of a powerful upper-level trough moving into the Southwest U.S. Cooler SSTs and higher shear will take its toll, weakening Sandra dramatically. There remains some spread in computer model guidance on Sandra’s intensity and track as it approaches the Mexican coastline, although a stronger, more intact Sandra would tend to recurve more quickly and further south due to greater influence of the upper-level steering flow. The 00Z ECMWF model brings Sandra into the Baja Peninsula as a very weak cyclone, while the 06Z runs of the HWRF, GFDL, and GFS models bring Sandra to the coast near or north of Mazatlan early Saturday as a stronger system, possibly a Category 1 hurricane. Hurricane or tropical storm watches or warnings may be issued as soon as Wednesday evening for parts of the Mexican coastline, and the Hurricane Hunters are on call to investigate Sandra on Friday afternoon. According to WU contributor Phil Klotzbach (Colorado State University), Sandra is on track to become the latest landfalling tropical cyclone on record for Mexico, beating out Tara (Nov. 12, 1961).
Figure 2. Oceanic heat content (in kilojoules per centimeter squared) along the path of Hurricane Sandra, beginning at 7 am EST Wednesday November 25. Sandra was over water with a lot of heat on Wednesday morning (yellow colors), but was forecast to move into a region of substantially less heat content by Wednesday night (12 hour forecast point over blue-green colors.) The oceanic heat content for Sandra is much lower than what Hurricane Patricia had to work with on October 23, 2015, when it rapidly intensified into a Category 5 storm with 200 mph winds. Image credit: NOAA/RAMMB.
A rare Thanksgiving Day hurricane
Assuming Sandra persists as a hurricane into Thursday as expected, it will be only the second Thanksgiving Day hurricane in modern records for the Atlantic or Eastern Pacific, and the strongest by far. The other was Hurricane Karl, which spun harmlessly as a minimal Category 1 hurricane far out in the central North Atlantic on Thanksgiving Day 1980. Several other weaker storms have had NHC forecasters issuing advisories on Thanksgiving Day. This includes 1987’s Tropical Storm Keith, which struck Florida as a tropical storm on Wednesday, November 23, and persisted as a strong tropical storm east of Florida until midday Thanksgiving Day (November 24). In 1998, minimal Tropical Storm Nicole weakened to a depression east of Bermuda early on Thanksgiving Day (November 26), with advisories discontinued at 10 am EST. Nicole did get a new lease on life several days later, becoming a hurricane on November 30 and persisting to become one of just five Atlantic hurricanes on record during the month of December. In the hyperactive Atlantic season of 2005, Tropical Storm Delta roamed the eastern Atlantic on Thanksgiving Day (November 24). And in 2011, a weakening Tropical Storm Keith well out to sea in the eastern Pacific prompted advisories on Thanksgiving Day (November 24). Prior to the establishment of NHC as we know it, an unnamed tropical storm dissipated on Thanksgiving Day 1953 (November 26) well east of Bermuda. Hawaii takes the cake for the worst U.S. hurricane-related impacts during Thanksgiving Week: Hurricane Iwa passed near Kauai on Tuesday, November 23, 1982, during the run-up to the “super” El Niño of 1982-83. Iwa caused one death and inflicted $250 million in damage in Kauai.
A banner year for Central and Eastern Pacific hurricanes
Assuming Sandra is our final tropical cyclone of 2015 in the Central and Eastern Pacific (not at all a sure thing in a year like this!), it will serve as a fitting capstone to an amazing year. The 16 hurricanes so far this year east of the International Date Line matches the record total set last year and in 1992. This year also ties with 1992 for the largest number of hurricanes--10--although Sandra could push us above that mark. NHC’s Eric Blake notes that the two-year total of 32 Central and Eastern Pacific hurricanes in 2014 and 2015 beats out the two-year record of 27 set in 1992 and 1993.
Figure 3. Projected 5-day precipitation totals (rain and melted snow/sleet) for the period from 7 am EST Wednesday, November 25, through Monday, November 30.
Thanksgiving travel troubles in store across the central U.S.
Some of the biggest impacts from Sandra may occur with a prolonged heavy rain episode in the Southern Plains, increased by Sandra’s remnants. The 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. Flash flood watches are already in effect from far north Texas to southwest Illinois, and localized flooding may become a major travel headache. Rainfall totals of 5-10” are expected from Thursday through Monday across north Texas, southeast Oklahoma, and much of Arkansas. 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. Warm surfaces, and plenty of warm air at upper levels, may cut into this storm’s ability to cause high-impact winter weather, but frozen precipitation appears likely from eastern New Mexico, the Texas Panhandle, and northwest Oklahoma into eastern Colorado and much of Kansas and Nebraska, as well as the higher elevations of Colorado and New Mexico. An ice storm warning may be issued for parts of the northwest half of Oklahoma as the event draws closer.
Stay safe, everyone, and have a great Thanksgiving!
Bob Henson and Jeff Masters
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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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