Monsoon Flooding Returns to the Desert

The desert has been decidedly wet recently.

After seemingly sitting out much of July, the Southwestern monsoon sent moisture surging into Arizona this week and set the stage for flooding rain. Unusually humid air and a weak disturbance pushing through at the jet stream level was all that was needed to fire numerous heavy showers and thunderstorms across the state Tuesday and Wednesday.
 

Those storms resulted in water rescues on Tuesday evening along Interstate 17 just north of Phoenix before the road was closed due to the flooding.

One gauge measured 2.91 inches of rain from the complex of storms on the north side of town. While that may not seem to be a phenomenal amount of rain for places in the eastern U.S., that volume of rain is more than the baked desert soil can handle — the average August rainfall in Phoenix is only 1 inch.

With little vegetation to absorb rainfall, the water runs along the surface, collects, and floods. The urban environment worsens this effect, as the rainfall also races across non-porous surfaces like pavement and concrete.

Very slow steering winds kept the storms in nearly the same place once they developed, meaning the torrential rains kept affecting the same area. The rainfall rate with some of those storms was 2 inches per hour, which is unusually high for the region. The National Weather Service indicates that such a rate statistically occurs only once in 100 years in central Phoenix, meaning it has a 1 percent chance of occurring in any given year.

One particular site along I-17 got 2.7 inches of rain in an hour. According to National Oceanic and Atmospheric Administration data, that rate of rain is even more rare, occurring about once every 500 years (a 0.2 percent chance in any particular year). That does not mean either of those events cannot happen again before that time, it only indicates a historical probability.

One effect of the warming world from climate change is that heavy downpours are becoming more frequent, as a warmer atmosphere leads to more evaporation. This means there is more water available for precipitation in the storms that develop. This increase is a bit higher in the East, as the normal amount of annual precipitation is higher than in areas west of the Great Plains.

Credit: Climate Central

According to the NWS office in Phoenix, the measure of moisture in the atmosphere from their weather balloon launch on Aug. 3 broke a record for that date. Known as the precipitable water, it describes the amount of rain that would fall if of all of the water vapor in the atmosphere above a certain point were turned into liquid. That number was 2.16 inches in Phoenix on Wednesday, breaking the old daily record of 1.92 inches.

Gusty winds also came through with the storms as they developed on Wednesday, with wind gusts to 44 mph in Phoenix and 57 mph just east of town in Tortilla Flat.

Drier flow at the jet stream level will settle in for this weekend, and while isolated thunderstorms will develop, they will not be as numerous or as heavy as earlier this week. With fewer clouds and rain, the desert heat will no longer be tempered, meaning highs will again surge to between 105-110°F (40-43°C) this weekend and next week. But there still may be a few more chances at flooding storms this summer, as monsoon season continues until the end of September..