Brian Bledsoe

I want to immediately couch this post as a post that contains quite a bit of
uncertainty. However, it's time to take a stab at a forecast that involves
whether or not we monsoon this summer. The map below adeptly explains what are
referring to , when we talk about the monsoon.

The past couple of summers, we have largely missed out on the monsoon. The maps
below show how dry the past two July/August periods have been, which is the core
of the "monsoon season" for the Southwest US.

2019 July-August

2020 July-August

Missing out on this 60 day period of average rainfall can be crippling to this
area, especially if it is preceded by a dry winter and spring. Well, we
certainly had that for Western Colorado and most of the Southwest US. In fact,
drought which has recently been erased for the Western High Plains, continues to
intensify for much of the Intermountain West and The Southwest US.

The latest drought monitor and outlook available here for Pro subscribers:

Drought update & the outlook: Thursday, June 3rd, 2021More than 95 million
residents in the Lower 48 are currently in drought areas,
with little improvement in total drought area reported since last week. Here is
the updated monitor: The most notable improved areas recently have been
Colorado’s eastern plains and
parts of Kansas, Oklahoma, and Texas.…Matt MakensJun 03, 2021Weather5280
[https://www.weather5280.com/2021/06/03/drought-update-the-outlook-thursday-june-3rd-2021]
This should be concerning for a couple of reasons. 1) This area is already dry,
getting drier, and now starting to get hot. 2) The weather patterns that
produced such a trend still exist, and are not showing any signs of breaking
down. In fact, if anything the window of opportunity for drought to relax in The
Southwest US has effectively closed for now. That begs the question about the
monsoon potential...

Where Does The Summer Ridge Set Up?

Who sees the monsoon is largely dependent on where the summer ridge of high
pressure sets up across the south/southwest part of the US. Let's look at the
monsoon explainer map again...

You can see that the subtropical H is perfectly placed over Western Arkansas,
Eastern Oklahoma, Northeast Texas, etc., and that allows a very nice flow of
subtropical moisture (monsoonal moisture) to flow northward into The Southwest
US...including Colorado and states to the north. If that subtropical H is
located too far east or too far to the west, it has huge implications for the
moisture flow and subsequent rainfall. Clear so far? Good...because here is when
it gets a little sketchy. The H will have a propensity to set up and persist
over areas that are largely void of soil moisture. When this happens, it sets in
motion a positive feedback mechanism in the atmosphere that reinforces the
dryness and heat. As of right now, this is what the soil moisture looks like for
our region, and the entire US.

Things are great in the Plains right now, with an obvious surplus of soil
moisture. However, many other areas are void of soil moisture...especially, the
Four Corners states and most of The West. The Northern Plains and parts of the
Midwest are in bad shape too, but those areas at least the POTENTIAL to see more
frequent storms, due to the storm track favoring them during the summer. This
doesn't guarantee that they see adequate rainfall, but it gives them a better
chance than areas to the south. So, let's see where some modeling positions the
H to see if it makes sense. The animation below shows the upper level weather
pattern for the next two weeks.

Notice where the core of strong ridge of high pressure develops early next week,
and persists...The Southwest US.

Let's look beyond the next two weeks, with the EURO Weeklies (the next 45 days).
I'll be the first to admit that the weeklies have their shortcomings, but this
time of year the weather pattern is more stable. This leads to a relatively
better accuracy...if nothing else, in how the upper pattern is forecast.

See where the ridge persists? Again, over The Southwest US... With this type of
ridge position, it makes it very tough to get a sustained and adequate feed of
moisture into the Four Corners area...including Colorado.

The map above shows rainfall anomalies in inches, during the next month and a
half. While there is some signal of a monsoon tap coming northward through
Mexico (green shading), it rapidly weakens as you move northward. This
particular run of the model suggests that the dry stay dry, and the wet stay
wet, with one exception...The Western High Plains. This is something we aren't
totally sold on just yet, but if the first two weeks of June are going to be any
indication of what is to come, then this dry signal does have some merit.

Sea Surface Temperature Anomalies

This is another element of the equation, and one that is likely the most
important. Notice the colder than average water off the southwest coast of
California and The Baja.

This is a feature that can largely squash the monsoon from happening, as it
largely promotes sinking air and promotes the ridge development.  While the
Pacific Ocean is largely in an ENSO neutral phase (neither El Niño and La Niña),
the ENSO regions are weakly cool, and could possibly turn cooler later this
summer and fall. That could portend another La Niña later this year...which
we'll discuss as the time gets closer.

As I said, there is still some uncertainty as to how this unfolds. The long
range models will be fully updated by early next week, and I'll do a follow up
post on what they are saying. However, the CANSIPS (Canadian long range model)
came out with this July-September precipitation forecast earlier this week.

Looks a lot like what we have discussed above.

Monsoon or "nonsoon" this summer?