Dateline: Vail, CO
During the months of May and June, in many parts of the Nation, you might start hearing talk about the soon-to-arrive, “monsoon.” But, do you really know what the monsoon is?
The North American Monsoon provides summer thunderstorms from California to Colorado and from Mexico to Montana. The area of coverage is basically the southwestern U.S. and Northwest Mexico.
It is sometimes referred to as the Southwest monsoon or the Mexican Monsoon.
Not to be excluded, Florida experiences a similar monsoon.
When it comes to describing a monsoon, a lot of people tend to think about the torrential flooding rains that typically occur in India and Southeast Asia each summer. The U.S.’ summer monsoon is not quite as dramatic, but it does provide lots of precipitation. Flooding rains, due to slow moving thunderstorms and/or repeated thunderstorm transits, are often part of the “wet” monsoon season.
Yet, the word monsoon (Arabic origin) simply means a seasonal wind reversal. Rainfall is not the driver for the monsoon; rather, it is the result!
Hence, what starts as a “wet” event (linked to a certain wind flow pattern), ends with the onset of a “dry” event (with a different wind flow pattern). Both are monsoon events, even though most folks key on the “wet” portion only.
Figure 1 can be used to describe all three of the above-mentioned different geographically-based seasonal monsoon patterns. Note how the locations of pressure systems flip-flop between land and water locations based on solar heating variations and land-water differences. This, in turn, results in significant wind direction shifts seasonally (even if not day-to-day). Wind shifts allow different types of air masses to move into or away from certain areas.
Linked to all these ground level (or “surface”) wind patterns, there are accompanying upper level pressure and wind patterns. For example, consider the upper level high-pressure system (5 to 10 miles above the Earth’s surface) that had parked itself across Arizona, Nevada and California for much of June. That high has now shifted eastward, allowing the upper level monsoonal wind circulation to transition. In the case of Colorado, my home base this week, the high’s movement to a location south of Colorado has allowed thunderstorms to finally develop across western parts of the state as well as much of the intermountain western states (Fig. 2).
According to Julie Malingowski of the National Weather Service’s Grand Junction Forecast Office, “Synoptically, the classic monsoon pattern in Colorado is defined with high-pressure over the Southern Great Plains. This allows a clockwise/southeasterly tap from the Gulf of Mexico.” Then, moisture-laden easterly waves on the south side of the high ride the high into the Four Corners. This week, there is no closed high-pressure system, but the upper level ridge has moved to the south of Colorado. This is allowing Pacific moisture to move into the southwest U.S. and Colorado (Fig. 3).
Thunderstorms are now mentioned in statewide forecasts for the southwest U.S. for most of the next four days. Then, the monsoon is expected to relax somewhat next week. However, the 7-day quantitative precipitation forecast map clearly shows that the upper high will be relocating even further to the east (Fig. 4), suggesting a quick return to monsoonal conditions.
Since the “wet” monsoon is summer-season based, it is not unusual for record-breaking heat to herald its arrival. Such was the case across much of the southwest U.S. earlier in June. Places like Denver and Grand Junction, CO, Flagstaff and Phoenix, AZ, Albuquerque, NM and Los Angeles, CA all reported very hot days just before the onset of the monsoon.
However, even with media hype about the excessive, sometimes record-breaking, warmth, temperature averages for the first 28 days of June, at all of the above inland locations, were only in the 3.7 to 4.7 above average range. Both Los Angeles airport and downtown locations reported readings only a couple of degrees above average.
Now that the monsoon has begun in many locations, temperatures have dropped to near to just slightly above seasonal averages.
© 2016 H. Michael Mogil
Originally posted 6/30/16