Matt Makens

It is the time of year when the Southwestern U.S. typically sees the development
of the monsoon, and this will be a critical one for the Four Corners Region that
is hit so hard with drought. Although, the Southwest is not the only area
currently seeing a significant shortage of moisture.

A well-developed monsoon can also spread beneficial rains to the Central and
Northern Rockies and can even help the Northern High Plains. So, this year’s
monsoon will be critical to drought conditions and rainfall chances across a
large portion of the country. For those across the South and Southeastern U.S.,
the moisture outlook is somewhat favorable regardless of how well the monsoon
plays out this summer to the west.

What’s the North American Monsoon, and how does it impact Colorado?The North
American Monsoon (NAM) is a seasonal weather pattern that develops a
persistent wind flow. The wind flow is specific and brings tropical moisture
into Mexico, Arizona, New Mexico, Utah, Colorado, and sometimes Texas. Using
this image from the Boulder NWS office, note the high-pressure are…Brian
Bledsoe
Weather5280
[https://www.weather5280.com/blog/2020/07/22/whats-the-north-american-monsoon-and-how-does-it-impact-colorado]
The North American Monsoon is a seasonal weather pattern that develops a
persistent wind flow. That pattern brings tropical moisture into Mexico,
Arizona, New Mexico, Utah, Colorado and west to south Texas. Further, Wyoming,
Montana, the Dakotas, Nebraska, Minnesota, Iowa, Wisconsin, and Illinois can
also see an uptick in precipitation if the monsoonal flow establishes strongly
enough.

The development of the monsoon begins, on average, in early to late June across
Mexico. From there, moisture increases across the Four Corners region throughout
July – again, on average.  The arrival of the monsoon means increased humidity
and, therefore, a rainier pattern.

For the Southwest, the monsoon’s development is critical for yearly moisture
totals. For example, for parts of Arizona, Colorado and New Mexico, an average
monsoon is responsible for one-third to one-half of the yearly total
precipitation (based on climatology from 1981 to 2010 using PRISM precipitation
data). Therefore, the Southwest is in a critical time of year for rain.

Recently, this pattern has been a disappointment; the past few years have not
seen the monsoon develop strongly enough to bring that much-needed water.  This
map indicates the percentage of average precipitation received in the past five
years during July and August.

The outlook for the monsoon in 2021 remains unfavorable, meaning it is unlikely
we will see a well-established pattern capable of spreading the needed amount of
rainfall from the Southwest to the northern states. This year may see the best
rainfall in July for Arizona and New Mexico, with sporadically placed rainfall
events in July and August from the Four Corners to the Northern Plains.

Without a persistent stormy pattern, it is unlikely we see a significant benefit
from a drought perspective. With that said, the most likely scenario this year
is a drier than average outlook for the July to September timeframe across the
classic monsoon areas.

The drought, to date, is an element of the forecast. “Drought feeds drought,”
and that is a challenge for the monsoon to be able to establish itself as a
long-term benefit. A stronger than forecast monsoon would be of great benefit to
the drought-stricken areas. Yet, that remains unlikely.

As a quick aside: In addition to a strong monsoon, wet winters and springs are
best suited to improve drought conditions for the west and that remains to be
seen for next year. As we transition from this summer to fall, we will begin to
focus again on the status of La Niña and its possible re-development as a factor
in the colder months and what, if any, drought improvement we can expect.

Currently, the outlook for the country from July to September includes drought
growth in parts of the western U.S. and Northern Plains.

Per region, this is the most likely scenario through the end of Summer:

Northwest:
This region is to have drier than average conditions persist most of the period.
Historically, a stronger push of moisture will develop late in the season and
carry into fall for the extreme Pacific Northwest. However, that is dependent on
a change to the Pacific Ocean which is less likely this year. Meaning, plan on a
much drier than average period here. An active wildfire season is a safe
assumption.

West:
Near to drier than average conditions are most probable for the period. The hope
of moisture will rely on the monsoon’s ultimate development and depends on a
more western placement of the monsoon than what is currently forecast. However,
the current drought likely creates a dome of heat and dryness that will remain
present over this region. Active wildfires are probable in this zone as well.

Northern Rockies and Plains:
As discussed in June’s National Cattlemen, this region should be watched closely
for drought conditions to persist. The area is likely to remain drier than
average for the period, and therefore see increased drought-related issues. The
most promising area for potential moisture will be the eastern sections of the
region. Active wildfires are probable here, too.

Southwest:
In order for this region to receive the amount of moisture needed, the monsoon
will need to overperform what is forecast. There will be an uptick in rainfall
frequency throughout this region, but the overall impact will most likely leave
the region drier than average for the period. The most promising area for
moisture will be the south, nearer that tropical flow of moisture close to
Mexico. Active wildfires are probable.

South:
This zone will see quite a difference in weather conditions. The wettest weather
is more likely for the eastern half of this region; a wet late spring and early
summer here may continue through the next couple of months. The western half of
this region will be drier than average, barring an unforeseen change to the
strength of the monsoon. Of course, tropical systems will need to be watched
closely for flooding along the Gulf Coast and Southern Mississippi.
Historically, this region (in total) remains drier than average when compared to
similar years.

Upper Midwest:
Likely to miss out on any well-established moisture this year, the western areas
of this region are expected to be the driest and therefore, drought concerns
continue. The eastern areas of this region, and those right near the Great
Lakes, have a near-average precipitation expectation for the period.
Historically, this region is much more frequently dry versus wet in similar
years.

Ohio Valley:
Odds (historically and in model projections) support a drier than average
period. Moisture may stay south and east of the region as well as just north of
it. This area is placed between the best setup for moisture as is currently
forecast. A stronger push of humidity from the South and Southeast regions may
benefit parts of this zone, but that is of low probability barring an active
hurricane season.

Southeast:
As discussed in June’s National Cattlemen, this region will see favorable summer
precipitation. However, for the period headed into fall, the area is likely to
see a drier trend takeover. Of course, tropical systems will need to be watched
closely to reverse that forecast.

Northeast:
Much will depend on spreading moisture from the South and Southeast region into
the Northeast, as well as tropical activity. Odds are a favorable flow will
continue into fall, leading to a wetter than average forecast for much of the
region. The moisture is more likely to increase later in the period and
currently it is forecast that a wet fall will be in place to follow this
increase in moisture late this summer.

Summer moisture outlook and likelihood of the monsoon