One of the most potent autumn severe-weather episodes in years was taking shape on Sunday over Oklahoma and north Texas, where residents were bracing for a localized but intense round of tornadic supercell thunderstorms. A much larger part of the southern and central Great Plains will experience another round of severe weather late Tuesday, with widespread damaging winds possible.
As of midday Sunday, a moderate risk of severe weather extended across much of southern and central Oklahoma, including Oklahoma City, according to the National Weather Service’s Storm Prediction Center. Despite its mild tone, a “moderate” risk is the second highest of the center’s five risk categories, behind only “high.” Moderate-risk areas are not uncommon in spring, but they’re less frequent during autumn, when the annual “second season” of severe weather is typically more spotty than the springtime peak. This is the center’s first moderate risk issued in October for any part of the nation since 2014.
This week’s May-like setup for severe weather will be triggered by a large upper-level trough of low pressure across the western United States. One packet of upper-level energy will rotate around the trough and across Oklahoma on Sunday evening, with very warm and moist low-level air streaming into the state. The result will be a fast-evolving set-up just ahead of a cold front and dryline, with highly unstable air and strong vertical wind shear (winds shifting and strengthening with height).
One or more rotating, long-lived supercell thunderstorms can be expected to form in southwest Oklahoma and race east-northeast by late Sunday afternoon, potentially affecting much of the Interstate 35 corridor between Oklahoma City and the Dallas–Fort Worth area. The Storm Prediction Center said any of these storms could produce one or more strong tornadoes – defined as at least EF2 on the Enhanced Fujita Scale (top winds of 113-157 mph). Hail larger than golf-ball size (2 inches in diameter) is also a distinct possibility with these supercells.
High-resolution forecast models began flagging Sunday’s risk of tornadic storms on Friday, and subsequent runs have boosted confidence in the outlook.
Another round of severe weather expected on Tuesday
A larger push of energy will swing through the upper-level trough into the Plains by late Tuesday, leading to a broader area of intense storms. An enhanced risk of severe weather (third highest level) is already in place for Tuesday from northwest Oklahoma to southern Nebraska.
The stronger upper-level forcing on Tuesday versus Sunday will likely favor an overnight squall line, as opposed to distinct rotating supercell storms. However, widespread severe wind gusts are expected, and a few tornadoes will still be possible.
Tropical Depression 16-E a dangerous threat to Mexico
Tropical Depression 16-E, formed in the warm waters off the Pacific coast of Mexico at 5 a.m. EDT Sunday, appears destined to become a dangerous landfalling hurricane for Mexico by mid-week. The next name of the Eastern Pacific list of storms is Pamela.
At 11 a.m. EDT Sunday, TD 16-E was about 590 miles south of Mazatlan, Mexico. The depression was headed west-northwest at 18 mph, with top winds of 35 mph and a central pressure of 1006 mb. Conditions were favorable for development, with light wind shear near 10 knots, a moist atmosphere, and very warm waters near 30 Celsius (86°F). These favorable conditions likely will persist through Tuesday, when TD 16-E is expected to be a hurricane recurving to the northeast just south of the southern tip of the Baja Peninsula. It currently appears that the core of the storm will pass to the south of Baja, but in its 11 a.m. EDT Sunday wind probability forecast, the National Hurricane Center gave San Jose del Cabo a 15% chance of receiving hurricane-force winds, and a 67% chance of tropical storm-force winds.
As TD 16-E approaches the coast of mainland Mexico on Wednesday, the waters beneath it will warm to a scorching 31 degrees Celsius (88°F) – over one degree Celsius above average. This ocean warmth is due, in large part, to an exceptional heat wave that has brought near-record heat to western Mexico in recent weeks. Warm water along TD 16-E’s path extends to a substantial depth, with a total ocean heat content in excess of 100 kilojoules per square centimeter, a value commonly associated with rapid intensification of hurricanes.
Although this hot water will favor intensification, TD 16-E will be wrapping a substantial amount of dry air off the deserts of Mexico into its circulation on Wednesday, and there may be enough wind shear present to allow this dry air to penetrate the core of the storm and weaken it. Many of the top intensity models show TD 16-E intensifying to a category 2 or 3 hurricane, and category 4 strength is not out of the question. TD 16-E is predicted to make landfall Wednesday night on the coast of Jalisco or Nayarit states, within 100 miles of Mazatlan.
Low pressure system 92L bringing heavy rains to North Carolina
A large low-pressure system centered just south of North Carolina’s Outer Banks was bringing heavy rain to much of coastal North Carolina on Sunday afternoon, as seen on radar images. An Air Force hurricane hunter aircraft investigating the low, designated 92L. on Sunday morning found top surface winds near 35 mph and a well-organized surface circulation with a central pressure of 1005 mb. However, there was not enough heavy thunderstorm activity near the center of 92L for it to get a name, and it remains unclear if this will eventually become Subtropical Storm Wanda.
92L is expected to move slowly north-northeastward toward the coast of North Carolina through Monday morning, then head northeastward, out to sea, on Monday. Wind shear will remain high, near 20 knots, over the next two days, slowing any development. In its 2 p.m. EDT Tropical Weather Outlook, NHC gave 92L 2-day and 5-day odds of development of 20%.
Disturbance 93L in tropical Atlantic headed toward the Lesser Antilles
A tropical disturbance located near 10°N, 50°W, at 8 a.m. EDT Sunday, was headed northwest at 10-15 mph. This system, designated 93L, had favorable conditions for development, with moderate wind shear of 10-15 knots, a moist atmosphere, and warm waters near 29 Celsius (84°F). Satellite images on Sunday afternoon showed 93L had developed considerable rotation at mid-levels of the atmosphere, but no surface circulation was evident. The system had few heavy thunderstorms.
93L is headed into an area of high wind shear, which should begin tearing up the system by Monday morning. By Tuesday, when 93L will be nearing the Leeward Islands, steering currents are predicted to collapse, resulting in a very slow and erratic motion of less than 5 mph. High wind shear of more than 20 knots will continue to tear at the system on Tuesday and Wednesday, and 93L has little model support for development. In its 2 p.m. EDT Tropical Weather Outlook, NHC gave 93L 2-day odds of development of 20% and 5-day odds of 30%.
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