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TD 16E Bringing Heavy Rain to Southwest; Tropical Storm Ida Stalling at Sea

By: Jeff Masters and Bob Henson 3:43 PM GMT on September 21, 2015

Tropical Depression 16E, which formed on Sunday just southwest of Baja California, crossed the peninsula on Sunday night and made a second landfall Monday morning on the east side of Mexico’s Sonoran state, near Tiburón Island. The low- and upper-level circulations associated with TD 16E have decoupled in the grip of southwesterly wind shear, and the depression will become a post-tropical/remnant low in the next few hours as it moves northeast across Mexico, eventually reaching east-central Arizona by Tuesday morning. Meanwhile, a weak upper-level low roughly 300 miles southwest of Los Angeles has produced a separate cluster of thunderstorms with prolific lightning. This complex of storms was located just offshore of Southern California on Monday morning, moving northwest parallel to the coast.

Figure 1. This water-vapor satellite image from 1330Z (9:30 am EDT) on Monday, September 22, 2015, shows two distinct rain producers: an upper-level low (left, southwest of California) and Tropical Depression 16E (right, over northwest Mexico): Image credit: NHC.

TD 16E dumped several inches of rain as it passed over Baja California. Geary's webcam on the Sea of Cortez in central Baja captured dramatic images of the storminess on Sunday afternoon. Geary’s PWS recorded 0.73” of rain on Saturday; 2.36” on Sunday; and 0.40” on Monday morning up through 10 AM EDT. The convection associated with TD 16E is now drenching Sonora, with isolated amounts of up to 12” possible in mountainous areas.

Figure 2. MODIS image of TD 16-E approaching Mexico's Baja Peninsula as seen from NASA's Aqua satellite on Sunday, September 20, 2015. Image credit: NASA.

Figure 3. Flooding in the Central Baja town of San Ignacio, and between Constitucion and La Paz on Sunday, September 20, 2015, from TD 16-E. Image credit: Geary Ritchie. Geary's webcam on the Sea of Cortez in central Baja is capturing some good images of the storm, and his PWS had recorded a 3-day total of 3.49" of rain as of 10 am EDT Monday.

Much of the Southwest U.S. is plastered with flash flood watches for today and/or Tuesday, but the juxtaposition of the upper low with TD 16E complicates the rainfall outlook for the Southwest. Both rainmakers are now tracking a bit southeast of earlier forecasts, which reduces the odds of a widespread soaking rain for the Los Angeles area. From San Diego eastward, the deserts of far southern California could still notch 1-2” from scattered thunderstorms as the upper low swings closer to the area this afternoon into Tuesday. Meanwhile, the focused area of convection associated with TD 16E will bring very heavy rain into southeast Arizona and southwest New Mexico on Monday night into Tuesday, with widespread 1-3” amounts and localized totals of up to 8” where the circulation impinges on south- and east-facing slopes. The only time Phoenix has seen more than 3” of rain in a calendar day was just last year, when 3.30” fell on September 8, 2014, goosed by moisture from former Hurricane Norbert.

As TD 16E deteriorates, the weakening upper low will approach it, ingesting some of its moisture and setting the stage for scattered but intense thunderstorms from southeast California into Arizona, southern Utah, and New Mexico on Tuesday. These rains will be less focused than Monday’s, but pockets of torrential rain, downburst wind, and even small hail can be expected. Recreationalists will need to be especially vigilant on Tuesday: in some areas, it could seem that conditions are improving on Tuesday morning before dangerous weather develops later in the day.

Figure 4. Predicted precipitation for the 5-day period ending Saturday, September 26, 2015. TD 16-E is predicted to bring rainfall amounts of up to four inches to Arizona, with even higher amounts possible in isolated spots. Image credit: National Weather Service.

Tropical Storm Ida no threat to land
Tropical Storm Ida slowed down its forward speed to 6 mph on Monday morning, and is about to essentially stall out for 3 - 4 days over the Central Atlantic, well away from any land areas. Satellite images on Monday morning showed that Ida continued to struggle against moderate wind shear, with the center of circulation partially exposed to view, and all of Ida's heavy thunderstorms limited to the southeast side of the center. Conditions over the next few days favor intermittent strengthening, and Ida could be a hurricane by the end of the week. By the end of the week, a trough of low pressure passing to its north will likely pull Ida to the north, and it appears unlikely that Ida will pose a long-range risk to North America.

Figure 5. Latest satellite image of Tropical Storm Ida.

Gulf of Mexico development next week?
A southerly flow of moisture from the Western Caribbean and Southern Gulf of Mexico towards the northern Gulf of Mexico will develop this weekend, and the long-range forecasts from the GFS and European models are advertising the possibility of a tropical depression forming near Mexico's Yucatan Peninsula by next Monday. Wind shear is likely to be moderate to strong over the region, limiting the potential for this system to strengthen. The eventual track of such a storm so far in the future is highly uncertain; it could potentially stay trapped in Mexico's Bay of Campeche and only affect Mexico, or get pulled northwards to affect the U.S. coast from Louisiana to Florida. Stay tuned.

Tropical Storm Malia forms in the Central Pacific
Tropical Storm Malia formed on Monday morning in the waters several hundred miles west of the Hawaiian Islands. A Tropical Storm Warning has been issued for central portions of the Papahanaumokuakea Marine National Monument, from French Frigate Shoals to Maro Reef to Lisianski Island. Malia is headed north-northeastwards at 11 mph on a path that will take it midway between Hawaii and Alaska's Aleutian islands late this week.

Malia is the record 6th named storm to form in 2015 in the North Central Pacific (between 140°W and the Date Line.) According to wunderblogger Dr. Phil Klotzbach, prior to 2015, the previous record for named storms in the North Central Pacific for an entire season was four set in 1982. The other named storms that formed in the North Central Pacific in 2015 were Halola, Ela, Iune, Kilo and Loke. This year's record activity has been due to unusually low wind shear and record-warm ocean temperatures caused by the strong El Niño event underway.

Jeff Masters and Bob Henson


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.