Warm, tropical moisture is surging into the Southwest U.S., on Monday, poised to rain down on the drought-inflicted region through Tuesday and raising concern for dangerous flash flooding from southern California to New Mexico.
Tropical Depression Sixteen in the eastern Pacific Ocean is serving as the tropical tap of moisture, which formed on Sunday afternoon just off the west coast of the Baja California Peninsula and made landfall shortly there after early Monday morning. Now, the broad area of low pressure is drifting north and pumping warm, moist air into the Southwest U.S. from southern California to New Mexico.
A large plume of atmospheric moisture will spread north over the region through Tuesday afternoon, enhanced in no small part by the extremely warm ocean water off the coasts of southern California and the Baja. Precipitable water — a measure of how much air moisture is available to turn into rain — has climbed to 200 to 300 percent of normal for this time of year from Southern California to New Mexico.
The morning weather balloon launch over Tucson, Ariz., measured precipitable water values well above normal for September, and record-high for Sept. 21.
— Greg Diamond (@gdimeweather) September 21, 2015
Over the next 36 hours, that moisture will wring out of the atmosphere in the form of rain as an upper level low digs east through the Southwest. Models are forecasting widespread rainfall totals of 1 to 2 inches across southeast Arizona and the high elevations of New Mexico. Isolated amounts in excess of 4 inches are possible in southeast Arizona and in the areas where the strongest thunderstorms develop.
The highest elevations southeast of Los Angeles could 1 to 2 inches of rain, but unfortunately this system does not seem likely to the widespread, blockbuster rainfall totals that were previously in the forecast for the area.
Flash flood watches are in effect into Tuesday across a large part of the Southwest. Locally heavy rainfall rates could cause flash flooding on foothills and mountain slopes, as well as the dry river washes and arroyos and recent burn scars.