One of the most anticipated seasonal weather changes during the northern hemisphere summer is the North American Monsoon. This monsoonal circulation produces significant rainfall over western Mexico and the Southwestern United States in response to summer heating over the higher terrain. While the monsoon brings beneficial rain, it can produce hazardous conditions over the desert. Monsoon thunderstorms may produce frequent lightning, hail, powerful flash floods, and blowing dust.
The onset of the monsoon is strongly influenced by the positioning of a 500mb area of high pressure. When this high is positioned over the Four Corners region, winds at this level turn more easterly over Arizona. Easterly flow allows inverted troughs to move west across the area, providing greater instability for thunderstorm development. Of greater significance is the resulting deep moisture advection from the Gulf of California. In July, mid-level winds over the Gulf of California are south to southeasterly, rounding seasonal high pressure that extends westward across the Gulf of Mexico during summer. Surface winds over the same region are southerly, transporting significant low-level moisture from both the eastern Pacific and Gulf of California. Lower wind speeds and weaker high and low pressure circulations are expected during summer as a result of less temperature variations. This often creates slow-moving monsoon thunderstorms, bringing an elevated risk of flash flooding.
Sunday, July 16th was the first very active day of the 2017 American monsoon season with nearly ideal conditions for hazardous weather over Arizona. With monsoon moisture on the rise, a Flash Flood Watch was issued for south central Arizona, including Phoenix. The forecast called for numerous thunderstorms over the higher terrain, propagating west to southwest across the lower elevations through the evening hours. NWS Storm Prediction Center issued a mesoscale discussion that afternoon detailing the threat of severe winds. Prime ingredients downstream included 20-25 knot easterly mid-level flow and sufficient instability as most unstable CAPE approached 2000 J/kg. Several severe thunderstorm warnings were issued by the National Weather Service forecast office in Phoenix that evening. At 6:16pm, a blowing dust advisory was issued for Northwestern Pinal County into southern Maricopa County until 8pm.
Severe thunderstorm warnings continued through the evening and early overnight hours. Flash Flood Warnings were issued for 30 minute rainfall totals exceeding 0.5 inch north and west of Scottsdale, according to the Flood Control District of Maricopa County (FCDMC). It is common for lower elevation areas and valleys to experience their heaviest rains at night.
According to NWS Phoenix, this activity brought temperatures down to 74°F, which was the first time the area had experienced 70° temperatures in nearly 30 days. It is typical of the monsoon to onset quickly, causing an abrupt change in weather conditions. Earlier this month, the Phoenix area was setting record high temperatures of 118-119°F.
Here are some strong wind reports from the storms this day:
More wind reports can be found here.