Alvin's remnants bringing unusual pre-monsoon rainfall over Desert Southwest
In Arizona, a Flood Warning was in place for Boulder View and Wildcat burn-scar areas on Monday morning in Maricopa County. In New Mexico, a Flood Watch is in place for Hermits Peak and Calf Canyon burn scars through Monday evening.
Remnant moisture from Eastern Pacific driving flood threat in Southwest
Remnants from Tropical Storm Alvin sent moisture surging into the Southwest U.S. this week prompting flash flood alerts for areas near Phoenix.
PHOENIX – A surge of moisture, due in part to the placement of an upper-level low and the tropical moisture field from the Northern Hemisphere’s first tropical cyclone of the year, will lead to increased rain chances across the Southwest over the next several days.
Moisture from the remnants of what was Tropical Storm Alvin will help increase humidity levels and eventually rain chances for cities such as Phoenix and Yuma, Arizona, and potentially as far west as Las Vegas and Palm Springs, California. Alvin was downgraded to a post-tropical cyclone on Saturday morning.
24-hour rain estimates across parts of Arizona and New Mexico.
Computer forecast models generally expect less than a quarter-inch of rainfall across southeastern California, while parts of Arizona could see totals closer to a half-inch through Monday.
Forecasters caution that rainfall amounts could be enhanced over higher terrain, where totals of up to an inch are possible, potentially leading to minor flooding in downstream areas.
An isolated thunderstorm cannot be ruled out, but the main impact will come from the unusual rainfall for this time of year.
The National Weather Service issued flood alerts for burn scars in Arizona and New Mexico. Water can quickly accumulate in these areas, leading to flash flooding.
In Arizona, a Flood Warning was in place for Boulder View and Wildcat burn-scar areas on Monday morning in Maricopa County. In New Mexico, a Flood Watch is in place for Hermits Peak and Calf Canyon burn scars through Monday evening.
SOUTHWEST MONSOON SEASON IS HERE: WHAT YOU NEED TO KNOW
According to National Weather Service historical data, Phoenix Sky Harbor Airport has only recorded measurable rainfall 38 times during the last week of May and the first week of June.
Rainfall during this period is even rarer in Yuma, which has only recorded measurable rain 11 times during the same window since the late 1800s.
The FOX Forecast Center stresses that the increased humidity and rainfall are not part of the annual monsoon, which typically begins around June 15 and lasts through Sept. 30.
Shown on the map is the typical weather pattern that dominates during much of the North American monsoon season. (FOX Weather)
Cities in the Southwest accumulate roughly half of their annual precipitation during the months of June, July, August and September, though amounts can vary significantly.
The monsoon pattern is closely linked to the status of the El Niño-Southern Oscillation, or what is commonly called the ENSO.
During La Niña events, the monsoon signal is typically more expansive and leads to heavier rainfall, while El Niño conditions can delay the start of the rainy season.
The unusual early-season rainfall will bring major benefits, including reduced wildfire potential and cooler temperatures.
Highs during the second half of the weekend are expected to only reach the lower 90s, a significant drop from recent temperatures reaching 100-110 degrees.
Any relief is expected to be temporary, as a building heat dome late this week and into the second week of June could bring the hottest temperatures of the year so far.