The storm is the 16th of the 2021 eastern Pacific hurricane season, but could have effects on parts of the Lower 48. Pamela’s remnant moisture is expected to slide northeast, spurring heavy flooding rains in Texas and Oklahoma that could dump as much as 8 inches by Thursday.
Flash flood watches stretch from the Rio Grande through the Lone Star State and into Southeast Oklahoma, with the bull’s eye of heaviest rains anticipated to fall in a corridor that includes the Dallas-Fort Worth metroplex.
Thereafter, Pamela’s remnant moisture will become swept up in a cold front and enhance a line of brief heavy downpours that will trek toward the East Coast by Saturday.
Pockets of heavy rain were already rolling though north Central Texas and parts of Oklahoma east of Interstate 35 on Wednesday morning, drenching the ground and setting the stage for flooding. Antecedent conditions have, fortunately, been comparatively dry, but the amount of rain and the rapidity with which it will fall is likely to become problematic.
Pamela makes landfall
Pamela made landfall in Sinaloa with a “life-threatening storm surge and dangerous hurricane-force winds” midmorning Wednesday, according to the National Hurricane Center. It had sustained winds of 75 mph, just over the minimum threshold for a Category 1 hurricane. It was centered just northwest of Mazatlán, home to about a half-million people. That would have placed the city under the right side of the eyewall — usually the strongest part of the storm — which means Mazatlán probably saw winds gusting to at least 60 mph.
Scenes from social media showed downed trees and some structural damage in the city and very rough surf at the coast.
The wind threat was diminishing some, since Pamela had become a tropical storm as of 11 a.m. with 65 mph, but the flood threat was increasing in the Mexican states of Sinaloa, western Durango and northern Nayarit.
Reuters noted that Sinaloa is “the country’s top grower of corn, Mexico’s staple grain, as well as a major producer of tomatoes and other fruits that figure prominently in the country’s agricultural exports to the United States.”
Heavy rainfall will tally 4 to 8 inches with localized spots exceeding a foot. The heaviest will fall on the windward side of the Sierra Madre Occidental mountains, resulting in mudslides that could pose a serious hazard to remote rural communities.
Pamela will weaken further as it shifts inland, the storm losing steam as it becomes removed from its oceanic heat source. The Hurricane Center notes the storm will probably “lose its identity” and wither in its entirety within 36 hours of making landfall. The rugged terrain of western Mexico will hasten its demise, and it will probably be a mere tropical depression by early Thursday.
U.S. flood threat
Though the circulation associated with Pamela is not long for this world, the atmosphere surrounding it will remain waterlogged as that moisture advances northeast. Much of that moisture will precede a cold front, which will enhance the risk of heavy downpours.
The setup bears some resemblance to a “predecessor rain event,” during which moisture streams out ahead of a tropical system and is concentrated by a cold front. That’s part of the reason heavy rain will break out by early Wednesday afternoon from roughly San Angelo northeast to near Wichita Falls and the Red River. That initial band will expand in areal coverage and intensity overnight into Thursday morning before collapsing southeast toward the coastline.
Rainfall rates in the deeply tropical air mass will surpass an inch and a half per hour. That means any thunderstorms that train, or move repeatedly over the same areas, run the risk of producing locally excessive rainfall. In convective, or thunderstorm-related, environments, it’s impossible to pinpoint in advance where exactly the heaviest rain will fall.
Where the Pacific and Atlantic hurricane seasons stand
So far, 16 named storms have formed in the eastern Pacific compared with 20 named storms in the Atlantic. Ordinarily the eastern Pacific should see about 50 percent more than the Atlantic.
The 2021 Atlantic season had been robust through September, but has since flatlined — at least temporarily. Only one name remains on the season’s “conventional” rotating list of Atlantic storm names before a new supplemental list of names will be utilized.
Meteorologist can gauge how active a season has been through a metric called ACE, or Accumulated Cyclone Energy. It’s a measure of how much atmospheric energy storms expend on their winds and is proportional to storm intensity and duration. The Atlantic is running about 56 percent ahead of typical values for this point in the season thanks to long-lived storms like Sam and Larry, while the eastern Pacific sits about 28 percent behind.
Ordinarily one ocean basin will balance out the other due to large-scale overturning circulations in the atmosphere. Air rising over one basin and enhancing storm development has to subside and sink somewhere, which often occurs over the other basin and suppresses storm growth.