Colorado Weather Forecast #197: Thursday Update | June 16, 2022

Widespread thunderstorms from Friday to Monday, especially in southwestern Colorado. This will also draw smoke into Colorado. A weak system Tuesday, then another possible moisture surge the following weekend.

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Updated Thu Jun 16, 2022 3:00pm MST | Published Thu Jun 16, 2022

By Thomas Horner

Summary

  • As southerly flow sets up over the desert southwest, smoke from New Mexico and Arizona will be transported into Colorado today/tonight.
  • Deep southerly flow into Monday will drive widespread, frequent thunderstorms in Colorado’s high country, particularly the San Juans. There will be some action to the east of the Divide.
  • A weak system will impact the region from Monday PM to Tuesday. Slight cooldown. This could help surge moisture into New Mexico and southern Colorado, in addition to driving severe storms in the high plains on Tuesday.
  • Warming and fairly dry from Wednesday to Thursday.
  • Another moisture surge next weekend. Prepare for another stormy weekend.

Lightning Risk Grids

Forecast Discussion - This Weekend

Satellite imagery this morning shows some dispersed smoke over the desert southwest as Arizona and New Mexico face ongoing, historic wildfires.

Otherwise, clear skies are abundant, as a dry airmass behind our last system remains over the area. That brought a badly needed cooldown, but temperatures are already climbing rapidly. And unfortunately for locales outside of Colorado, that system caused historic floods near Yellowstone and helped drive extreme fire behavior near Flagstaff, Arizona.

The Hermits Peak fire in New Mexico has also flared up pretty badly again and will be responsible for making our skies smoky as we progress into Friday and the weekend. We’re expecting some pockets of heavy smoke to wander into Colorado tonight.

This smoke will loft over much of the western United States as southerly flow carries it into the northern Great Plains this weekend.

The southerly flow over our region is due to a large area of high pressure setting up over the central United States and a upper-level trough dropping into the Pacific Northwest and Great Basin.

These southerly winds will carry subtropical moisture into the desert southwest, which is badly needed.

Looking at fire weather indicators, the increased humidity and cloud cover will help mitigate fire growth, even if precipitation chances aren’t too exciting for Arizona. Hopefully, we wont see too much more smoke by the end of the weekend if the fires aren’t undergoing explosive growth.

The above image shows just how deep moisture will be over the desert southwest this weekend. Unsurprisingly, the precipitation forecast looks similar.

In Colorado, the San Juan mountains will be taking the brunt of the moisture, but widespread thunderstorm development will occur over most of the high country. Due to south-southwesterly winds, these storms will have a difficult time wandering onto the High Plains, but the I-25 corridor should still see some action every afternoon.

If you’re doing things above treeline, or really anything outdoors, it cannot be overstated that this will be a wet weekend with lots of lightning flying around. Take a look at simulated radar:

It’s more likely than not that many areas in the mountains will see some lightning striking the ground each afternoon, particularly on Saturday.

It’s possible there will be some isolated cells spitting out lightning by 11am on Saturday, so give yourself some extra time. Also be aware that these storms will be traveling mostly from south to north, so keep an eye to the south as opposed to the west. They’ll be moving slowly, which could cause flash flooding over burn scars.

Is this the monsoon?

That’s an interesting question! In our last forecast we did use the term “monsoon” to describe this moisture, which some National Weather Service offices have used as well. However, others have avoided the term. What gives?

Defining “The Monsoon”

Well, let’s define the monsoon first. Note that in Colorado it’s never “the monsoons” or “monsoons,” which is a common misnomer. Other incorrect usages of the term “monsoon” include:

  • Afternoon thunderstorms
  • Several days of afternoon thunderstorms

When we use the term “monsoon” we are specifically referring to the North American Monsoon.

A monsoon is merely a large-scale reversal of wind directions. We are mostly concerned with monsoons that occur over a large area and for a decent chunk of time (several months) and reoccur every year. There are various different notable monsoons across the globe such as the Southwest/Indian Monsoon.

The North American Monsoon occurs when near-surface winds over the Mexican Plateau and Sonoran Desert flip from flowing towards the ocean to flowing inland (generally, from the southwest). This occurs in spring and summer as the sun heats the desert land to a hot enough temperature compared to the more-constant ocean temperature. The Mexican Plateau averages almost 6,000ft. in elevation, so it soaks quite a bit of solar radiation during the longer days of spring and summer.

In general, when we use the term the monsoon we are referring to monsoonal winds / a monsoonal moisture pulse. “The monsoon will drive thunderstorms next weekend” really means “a pulse of monsoonal moisture will drive thunderstorms next weekend.” Got it?

Monsoon Timings and Impacts

Though the North American Monsoon usually occurs by May/June, the moisture remains locked up in Mexico and doesn’t reach the desert southwest (Arizona, New Mexico) until the start of July most years. Utah and Colorado are also impacted by the monsoon, but to a lesser degree, especially in northern Colorado. The monsoon is usually over by the end of September.

There are some other large-scale changes that occur in the atmosphere over our region, namely the development of surface low pressure near the Gulf of California and high pressure over the Mexican Plateau (which migrates northwards). The existence and location of these features help determine whether or not moist southerly monsoonal winds actually make it into the desert southwest.

Region of the North American monsoon (NAM - pink shading) after Comrie and Glenn [1998] and the approximate limit of strong (wet) monsoons after Harrison et al. [2003] (pink dashed line). Low-level moisture (red arrows) from the Gulf of California (GoC) and Gulf of Mexico (GoM) is channeled northward along the slopes of the central Mexico highlands. Mid-level moisture (gray, dashed arrows) flows around high-pressure systems (H) over the North Pacific and Texas panhandle. NAM moisture moves toward a low-pressure system (L) that develops over Arizona. (Figure 1 from Barron 2012)
Region of the North American monsoon (NAM - pink shading) after Comrie and Glenn [1998] and the approximate limit of strong (wet) monsoons after Harrison et al. [2003] (pink dashed line). Low-level moisture (red arrows) from the Gulf of California (GoC) and Gulf of Mexico (GoM) is channeled northward along the slopes of the central Mexico highlands. Mid-level moisture (gray, dashed arrows) flows around high-pressure systems (H) over the North Pacific and Texas panhandle. NAM moisture moves toward a low-pressure system (L) that develops over Arizona. (Figure 1 from Barron 2012)

In general, the large-scale pattern evolves over the course of the summer and every year is a bit different. For optimal moisture transport into the desert southwest, we want the monsoonal high pressure to migrate northeast into the southern midwest of the United States (Texas is a fine spot), but this doesn’t always occur.

For instance, in 2020, high pressure associated with the monsoon initially set up to our east which allowed for more monsoonal moisture to reach the desert southwest in early July. However, this broke down and high pressure emerged over the Four Corners region which blocked the transport of moisture into the area and made it harder for thunderstorms to develop. It ended up being a very dry summer.

In 2021, high pressure ended up consistently further east, and Arizona saw one of the wettest monsoons on record.

Monsoonal moisture typically arrives in “pulses,” so there are still big differences in the day-to-day forecast that forecasters must evaluate. The surges of moisture generally last a few days, interspersed with periods of drier weather. This moisture drives thunderstorm development which is the primary mechanism for precipitation during the summer, and also has a cooling effect.

It is these “pulses” that are often colloquially described as “the monsoon” or “monsoons” – but now you know the correct description is “a pulse of monsoonal moisture.”

The Monsoon vs. Other Causes of Storms

Over the past month we saw several systems move through the region which resulted in several days of soggy weather. If monsoonal moisture pulses also result in several days of soggy weather, why do we bother making the distinction?

Well, the systems that brought moisture to Colorado over the past month are no different than what cause big winter storms. Large kinks in the jet stream wrap around an area of low pressure and cold airmass. The upper-level dynamics associated with the curvature of the jet stream help turn moisture (which is often not subtropical in origin) into precipitation.

In contrast, the monsoon is more of a low to mid level phenomena (near the surface). Moist, unstable subtropical air carried up from the Gulf of California, Gulf of Mexico, and Pacific Ocean meets sun-baked mountainous terrain, causing thunderstorms to develop. The practical difference with this mechanism is:

  1. We don’t usually get significant cold fronts during periods of wetter weather in summer.
  2. Without a jet max nearby, we don’t get sustained high winds over a large swath of the state. That said, collapsing thunderstorms are windy, but far more random.
  3. Storm motion tends to mostly be from south to north, often without moving much from west to east. Sometimes, the monsoon will result in storms moving from southeast to northwest, which can catch people off guard. Storms can also move very slowly, causing flash flooding and damage. In comparison, thunderstorms driven by larger upper-level systems tend to move quickly from southwest to northeast or northwest to southeast.
  4. The warm subtropical air can carry a lot of moisture. Moister air is more unstable. Under the right conditions in the mountains, cloud-to-ground lightning can be a concern at morning and night, not just in the afternoon.

On the other hand, the potential for very severe storms in eastern Colorado tends to be limited in mid to late summer (but not always) since there is less interesting stuff going on in the mid and upper troposphere. No wonder late spring is often Colorado’s best season for stormchasing.

In summer, the jet stream migrates to the north which means we rarely see the larger sorts of upper-level systems that impacted us this spring and early summer. The monsoon thus becomes one of the most important factors driving thunderstorm development in Colorado, though we’ll still get an unrelated storm system every now and then, especially remnants of tropical storms.

So is this weekend the monsoon? Using the most pedantic description available: no, not exactly.

The jet stream is helping set up the deep southerly flow this weekend, as opposed to low-level monsoonal winds doing most of the work. We do see southerly winds at the surface (typically associated with the monsoon), but these are partially coupled with the mid and upper level southerly flow. Were the jet not there, we’d probably still see some southerly monsoonal winds in place, but moisture transport would be in question.

This means the enhanced moisture over our region is only partially due to the monsoon. However, the end result and overall weather characteristics are close enough to what we see during a good monsoonal moisture pulse that using the term “monsoon” is just fine outside of academic circles. Technically though, this is just sort of a “monsoon preview.”

Next weekend’s pattern actually looks more like the monsoon, or more pedantically, a proper monsoonal moisture pulse, though there’s still some mid and upper level support.

Anyways, since we’re talking about the jet stream, let’s take a look at the system that will hit us late Monday / Tuesday.

Forecast Discussion: Next Week and Weekend

The trough that is in place over the Pacific Northwest will lift to the east. Though it’s potent right now, it will be actively lifting as it gets closer to us which means it will only be a glancing blow at best.

This may help shunt additional moisture into New Mexico and southern Colorado on Monday and Tuesday, and could set up dynamics in northeastern Colorado / the high plains that would be conducive for severe storms.

For many of us, this may just be a bit of a cooldown as weak cold front passes through northern and western Colorado. Precipitation chances certainly look lower than the weekend.

We’ll start warming back up again on Wednesday and Thursday as weak ridging develops over the western United States. This, along with dry air behind the system, will limit precipitation, though it looks like there will be at least some isolated thunderstorm activity.

By next weekend, it appears likely that high pressure will again set up to our east which could promote moist southerly flow again.

This would of course have a larger impact on the higher terrain of southern Colorado than northern Colorado or the high plains. Additionally, New Mexico could get a impressive amount of rain.

This is a decent signal in the ensembles so plan for another stormy weekend after this upcoming one!

The Climate Prediction Center is anticipating similar impacts.

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