Wet Week Ahead for Gulf Coast, Arizona Deserts

With moist air predominant over much of the United States, several distinct areas of heavy rain will take aim on roadways, drainage systems, and people’s nerves over the coming week. The deluge has already begun across the central Gulf Coast, especially along the Florida coast from Tallahassee to Tampa, where totals of 3-6” were widespread from Sunday into Monday. These showers and thunderstorms are being fed by very high amounts of atmospheric moisture (more than 2” of precipitable water, the amount of water vapor in a column of air) across the southeastern U.S. The main trigger for development is a broad, weak mid-level low centered near the Florida Big Bend, together with an upper-level low off the east coast of Florida and a surface low over southern Alabama. This complex will trudge westward during the week, perhaps reaching Louisiana by around Friday.

The tilted structure of the low and its location near the coast will work against tropical development, but the slow movement will lead to prodigious rainfall totals. A broad swath of rains may total 7” to 15” by the end of the week along the central Gulf Coast (see Figure 1). Within this area, we’re bound to see localized weather features producing higher amounts--perhaps quite a bit higher--though the locations will be near-impossible to predict before the thunderstorms actually develop. We do know that the central Gulf Coast can get amazing amounts of rain in a short time when conditions are favorable. Historic flash floods were produced on April 29-30, 2014, as 10-15” of rain fell in less than 24 hours at both Mobile, AL, and Pensacola, FL. Pensacola racked up 20.47” in just 48 hours.


Figure 1. Total rainfall projected by NOAA’s Weather Prediction Center for the 7-day period from 8:00 am EDT Monday, August 8, 2016, to 8:00 am EDT Monday, August 15. Some localized amounts could exceed these projections by a considerable margin. Image credit: NOAA/NWS/WPC.

Big surge in the Southwest monsoon
Showers and thunderstorms typically increase across the U.S. Southwest during late July and August with the peak of the North American Monsoon, as streams of moisture head toward the region. Typically the low-level moisture arrives via the Gulf of California and the tropical East Pacific, with higher-level moisture imported from the Gulf of Mexico. As is often the case, this year’s monsoon has been sporadic and spotty, with pulses of heavy rain around the start of July followed by an extended monsoon “break” of more than two weeks. The action has picked up over the last few days, though. On Friday, August 5, the Phoenix area was socked by heavy rains and flash flooding. Close to 3” fell in Scottsdale in just three hours.


Figure 2. A burst of heavy rain falls across the Phoenix metro area on Friday, August 5, 2016. Photo credit: Bruce Haffner, courtesy NWS/Phoenix.

This week will bring additional potential for heavy Southwest rains, thanks in large part to a slug of moisture flowing up the Gulf of California around the east side of Tropical Storm Javier (see below). Precipitable water values may approach 1.75” - 2.00” in the Phoenix and Tucson areas, which would be among the highest levels on record for August. An upper-level trough extending unusually far south for August will also boost the potential for strong to severe thunderstorms packing high winds, especially on Wednesday. The juxtaposition of upper-level forcing with such rich moisture is quite unusual, suggesting that localized rainfall amounts--not everywhere, but in scattered locations--could be truly impressive. Scatered heavy rains may extend into parts of Utah, Colorado, and New Mexico by Thursday. Residents should take extra caution in areas of heavy rain and high water. The phrase “turn around, don’t drown” is especially apt in the U.S. Southwest, where dry creekbeds can quickly turn into raging torrents.

More soakings ahead for Upper Midwest
It’s been a soggy summer over much of the Midwest. Initial data from the forthcoming U.S. monthly climate summary (we’ll cover the full report on Thursday) show that last month ranked among the ten wettest Julys on record for Kentucky, Illinois, Missouri, Minnesota, and North Dakota. NOAA/WPC is projecting that showers and storms will drop widespread 2” - 4” rainfall amounts this week from eastern North Dakota to northern Minnesota and Wisconsin, as the potent-for-August upper-level trough mentioned above drags its way across the region.


Figure 3. Enhanced infrared satellite image of Tropical Storm Javier at 1930Z (2:30 pm EDT) Monday, August 8, 2016. Image credit: NOAA/NESDIS.

From Earl to Javier
There are no areas of potential development being highlighted by NOAA’s National Hurricane Center today, but traces of last week’s Hurricane Earl have made it into the East Pacific. After Earl dissipated over the mountains of southern Mexico on Saturday, its westward-moving remnants fed into an area of disturbed weather south of Mexico. The result was Tropical Storm Javier, which sprang to life on Sunday. As of the 2:00 pm EDT Monday update from NHC, Javier was located about 55 miles southeast of Cabo San Lucas, Mexico, with top sustained winds of 50 mph. Javier appeared to be holding steady in strength based on data from an Air Force Hurricane Hunter flight on Monday morning. Hurricane warnings have been dropped for parts of the Baja California coast, although tropical storm warnings remain in effect for both side of the peninsula south of about 25°N, with a tropical storm watch further north along the Pacific coast to Puerto San Andresito.

Javier is loosely organized, with its core of showers and thunderstorms pulsing and fading, and its northwestward path lies close enough to Baja California for some interference with its development. Javier does have some potential to strengthen over the next couple of days, as wind shear will be light (less than 10 knots) and the storm will be traveling over unusually warm waters (sea-surface temperatures of 26-28°C, about 1-2°C above average). From Wednesday onward, Javier will move over cooler waters (24-26°C). Together with land interaction, this should bring Javier below tropical storm strength. Models are in some disagreement on Javier’s path by later this week, but NHC projects Javier to continue northwestward, dissipating by Thursday. Javier’s remnants may flow into the Southwest U.S. by the weekend; before then, as noted above, the storm will be channeling moisture into the region.

Elsewhere in the tropics
Tropical Storm Ivette is on its last legs in the Central Pacific, where it is projected to dissipate amid high wind shear by Tuesday. In the Northwest Pacific, Tropical Depression Eight will gradually strengthen, perhaps becoming a typhoon late this week, while Tropical Depression Omais spins down east of Japan.

Bob Henson