Flash Floods Likely in Arizona and California From 91E; Atlantic Threats are Minimal

By: Jeff Masters , 4:44PM,GMT on September 20,2015

An area of disturbed weather off the coast of Mexico's Baja Peninsula (Invest 91E) is headed north at 15 mph, and will likely make landfall on the central Baja coast on Monday morning. Satellite images of 91E on Sunday morning showed plenty of moisture and heavy thunderstorms were associated with 91E, though this activity was not well-organized enough to be declared a tropical depression. In their 8 am EDT Sunday Tropical Weather Outlook, NHC gave the disturbance 2-day and 5-day odds of development of 70%. Moisture from this system will produce heavy rains across portions of the Baja California Peninsula and northwestern mainland Mexico through Tuesday, potentially causing dangerous flash flooding and mudslides. An impressive surge of moisture 91E will bring some of the highest levels of water vapor ever recorded in September to Southern California and Arizona beginning on Monday and extending into Tuesday. This rich tropical moisture will very likely cause dangerous flash flooding; the NWS is predicting that up to four inches of rain could fall in portions of Arizona, which is a very extreme amount of rain for this desert region. Daniel Swain's excellent California Weather Blog noted on Saturday evening that this week's rains from 91E could cause September 2015 to set some all-time monthly precipitation records in Southern California, thanks to rains last week from the remnants of Hurricane Linda. Those rains led to flash flooding that killed 19 people in the Utah/Arizona border region.

Geary's webcam on the Sea of Cortez in central Baja is capturing some good images of the storm, and his PWS has recorded 1.03" of rain as of 5 pm EDT Sunday.


Figure 1. Latest satellite image of 91E.


Figure 2. Predicted precipitation for the 7-day period ending Sunday, September 30, 2015. 91E is predicted to bring rainfall amounts of up to four inches to Arizona. Image credit: National Weather Service.

Invest 96L off Southeast U.S. coast little threat
An area of disturbed weather off the coast of North Carolina (Invest 96L) is bringing heavy rains to the waters more than 500 miles offshore of the U.S. East Coast, but is not a major wind or heavy rain threat to the coast. The disturbance is under high wind shear of 20 - 30 knots, and there is plenty of dry air around it, which is inhibiting development. Satellite loops on Sunday morning showed that 96L had a weak and very elongated surface circulation, and the storm's heavy thunderstorms were poorly organized and far from the center of circulation. Our three reliable models for predicting tropical cyclone genesis showed no development, and NHC has stopped tracking this system as an "Invest" (area of interest]. In their 8 am EDT Sunday Tropical Weather Outlook, NHC gave the disturbance 2-day and 5-day odds of development of 10%. Strong winds from the system will bring high surf to the Outer Banks of North Carolina much of the week.


Figure 3. VIIRS image of the action in the Atlantic as seen by the Suomi satellite taken on Saturday, September 19, 2015. Tropical Depression 9 (TD 9, center) was downgraded to a remnant low by the National Hurricane Center on Saturday afternoon. Image credit: NASA.

Tropical Storm Ida no threat to land
Tropical Storm Ida continued to head west-northwest at 12 mph on Sunday morning, well away from any land areas. Satellite images on Sunday morning showed that Ida was struggling against moderate wind shear, which had exposed the center of circulation to view and kept all of Ida's heavy thunderstorms limited to the east side of the center. Conditions over the next few days favor slow strengthening, with the 12Z Sunday run of the SHIPS model predicting moderate wind shear of 5 - 15 knots, warm ocean temperatures near 28.5°C (83°F), and a moist atmosphere. The long-range fate of Ida is unclear, since the storm will experience a collapse in its steering currents from Monday through Wednesday that will cause a very slow, erratic motion. A strong trough of low pressure passing to its north may be able to pull Ida to the north late in the week, or the storm may stay trapped in the Central Atlantic by weak steering currents.

Tropical Depression 5-C in the Central Pacific
Tropical Depression 5-C is moving north-northeastwards on a path that will take it several hundred miles west of the Hawaiian Islands. Although TD 5-C is fighting high wind shear of 25 - 30 knots, just a slight increase in organization would bring it to tropical storm status, and 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. A Tropical Storm Watch has been issued for the area from Lisianski Island to Pearl and Hermes.

Jeff Masters