Heat to Kick Western Monsoon into Gear
The heat making recent headlines in the Desert Southwest over the past week is key to understanding the development of what is known as the “western monsoon” season in the Desert Southwest and portions of the Southern and Central Rockies.
With the longest hours of sunlight near the summer solstice it should be no surprise that the deserts California, Arizona, Nevada and Utah heat up in June and triple digits should be expected.
The buildup of the desert heat is critical to help forming an area of low pressure over a large area of the west and southwest areas of the nation. Warm air is naturally less dense than colder air, with less air density over a large area, an area of low pressure forms.
Air moves naturally from high pressure to low pressure. During the heat of June and July, there is a big contrast in temperature between the deserts and the relatively cooler waters of the Pacific west of Mexico and the Gulf of Mexico. The air over the Pacific and Gulf of Mexico waters is cooler and that creates an area of high pressure. The air over the waters then wants to be attracted to the lower pressure (heat) over the Desert Southwest.
The graphic below highlights this well. The green line shows areas of moisture that move north into the west and southwest helping to trigger afternoon and evening thunderstorms in late June and continuing into August before waning.
The western monsoon is critical to bring rain and thunderstorm activity in the Desert Southwest and Rockies during the summer season.