"May was another surprisingly cool and wet month for much of Arizona with an active weather pattern dominating the western U.S. Northern Arizona was under the gun much of the month as cool and wet storms moved in off the Pacific in a circulation pattern that looked much more like winter than late spring. Some of these storm systems did touch off thunderstorm activity across southern Arizona, bringing locally heavy rainfall to some locations.
The month started off relatively quiet as a weak and shallow ridge of high pressure tried to build in, but was quickly pushed back south as a broad trough of low pressure set up across the entire western U.S. starting on the 6th and persisting through the 11th. This brought a period of unusually cool and wet weather to Arizona for mid-May. Northern Arizona again saw most of the action during this period, with one RainLogger near the south rim of the Grand Canyon reporting precipitation every day with a total near 2”. RainLoggers from Sierra Vista up through Phoenix reported precipitation during this period, but in much lighter amounts with most observations in the 0.1-0.3” range.
Some lucky RainLoggers received very heavy precipitation amounts on the 11th as the upper level was moving through to the south of Arizona, helping to spark off thunderstorms that day. Southern parts of the Tucson metro area and down into Sahuarita were in the direct path of a slow-moving heavy thunderstorm that dumped almost two inches of rain. One RainLogger near the Tucson International Airport observed 1.8” on May 11th. The Airport received 1.10” of rainfall which was a daily record for May 11th.
Overall May was much below average temperature-wise and average to above-average for precipitation. Far northwest Arizona observed its record wettest and coolest May though, according to the Westwide Drought Tracker. Arizona is almost drought-free with only a small part of northeast Arizona under abnormally dry to moderate drought conditions.
We now turn our attention to the impending monsoon season. The official seasonal outlook issued by the NOAA Climate Prediction Center is concerned that the monsoon might have a late start, but confidence is low in this outlook. A lot of moving parts are at play including the expectation of a busier than normal east Pacific tropical storm season.
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