It's not uncommon to hear Coloradoans referring to "the monsoon" in the late summer
when afternoon thunderstorms become an almost daily occurrence. But what exactly is this
monsoon, why is it there, and how much does it actually impact Colorado?
What causes the NAM?
The North American Monsoon (or NAM) is a shift in the wind pattern that allows for continuous
moisture to flow from the Gulf of California into the normally arid southwest region of
the country. This usually happens when a strategically centered high pressure (with clockwise
flow around it) and low pressure (with counterclockwise flow around it) settle in over the
region, like the first picture shown.
How do we know when it's monsoon season?
Over the southwestern United States, the NAM typically ramps up in July and persists
through August. While you might observe
some occasional wet days before it arrives, or some drier days after it's arrived, a
fairly persistent, large-scale pattern will emerge. For example, you may notice that dewpoints throughout
the southwest are more often in the upper 40s and 50s (but earlier in the summer had
more often been much lower, in the 30s or below!).
Click on the surface map images to the right, and check out the differences in dewpoints throughout Arizona, New
Mexico, and around the Four Corners region. The first surface map is from June 2015 and
shows dewpoints in the teens and 20s. The second map, two months later, shows dewpoints
in the 50s, with a high pressure centered over the Four Corners and a low pressure along
the California-Arizona border.
Does the NAM really impact Colorado?
The NAM indeed does play a role in Colorado's late summer weather pattern! A typical
monsoon pattern shows the greatest precipitation anomalies extending from Arizona and New Mexico
and past the Four Corners (your geography lesson for the day: Colorado makes up the northeast corner
of the Four Corners). Depending on the strength of the NAM, and the location of
where it ends up, the vast majority of the state has, at one time or another, seen the
effects of the monsoon.
Give me some proof!
In the image to the left, monthly averaged precipitation values are shown for three
different stations around the state. In the first example, from southwest Colorado, the
presence of the NAM is quite clear, with the highest precipitation months being July, August,
and September. The second example shows precipitation averages in Denver. While the NAM
is not as obvious, you can see two maxima. The highest precipitation month is in the late spring.
But a secondary peak pops up around the time of... you guessed it, the NAM! In the third
example, from northeast Colorado, we can see that the effects of the NAM are not evident,
with peak precipitation occurring in the early summer and waning after that.