Last week, Tropical Storm (and former Hurricane) Hilary impacted the Western U.S. and the remnants of this storm resulted in significant rainfall.

Believe it or not, another post-tropical cyclone will impact parts of the West late this week. But this time, the remnants will be coming from a tropical storm in the Atlantic Basin, or more specifically, the Gulf of Mexico.

Tropical Storm Harold made landfall in South Texas on Tuesday and has since weakened into a post-tropical depression. However, the remnants of this storm have become entrained into the North American Monsoon circulation across the Southwest.

This will result in more widespread showers and thunderstorms along with locally heavy rain across the Four Corners region from Wednesday to Saturday.

Pattern Overview and Projected Track of Post-Tropical Storm Harold

An area of low pressure associated with the remnants of Harold will rotate around the Four Corners states late this week, eventually slowing down as it moves across Northern Colorado/Southern Wyoming on Friday and Saturday. Moisture from this system will enhance the flow of monsoonal moisture over the next several days.

The visible satellite image below shows significant cloud cover across New Mexico, Eastern Arizona, and Southern Colorado. The low pressure center associated with post-tropical storm Harold is denoted by the red "L" with the projected track displayed for the days to follow.

The low pressure center is located near the New Mexico/Arizona/Mexico border on Wednesday. The low will then move northward into Southern Utah by Thursday afternoon and Northern Colorado by Friday afternoon. Associated impacts will shift northward and eastward with time as a result.

Let's dive into the day-to-day details...

Wednesday Afternoon and Wednesday Night:

The heaviest and most widespread rainfall on Wednesday has been occurring (and will continue to occur) across Southern and Central New Mexico. Lightning activity has been limited across New Mexico due to limited instability.

Scattered thunderstorms have developed across Arizona and Utah with more frequent lightning activity, while storm activity has been more isolated across Southwest Colorado. Locally heavy rain will be possible with storms through the afternoon and evening hours, and showers will remain possible overnight as well.

The rainfall will be welcome in most areas of the Southwest, but there is a higher risk of flash flooding (level 2 out of 4 risk) across Southern New Mexico and Southwest Texas, with slot canyons, dry washes, and burn scars most susceptible. A risk of slot canyon flooding exists across Arizona and Utah as well.

Forecast → Grand Canyon South Rim

Thursday to Thursday Night:

Shower and thunderstorm activity will favor Northern Arizona, Southern/Central Utah, and Western/Central Colorado with a decrease in coverage across Southern New Mexico. Most of the action will happen during the afternoon and evening hours, but Colorado could see some late-night showers as well.

Thunderstorms, rainfall, and lightning activity will favor the Flagstaff/Grand Canyon region, Utah national parks, and the San Juan Range in Colorado though all areas west of the Divide in Colorado will be susceptible to lightning. 

Thunderstorm activity can also be expected across the Wasatch and Uinta Ranges in Northern Utah.

Locally heavy rain will be possible with storms throughout these regions. The highest threat of flash flooding is expected across the canyon regions of Southern Utah, especially in slot canyons and dry washes.

The National Weather Service in Salt Lake City issues a Flash Flood Outlook each day for Southern Utah, and this is a good resource if you plan on recreating in this region:

And as always, be sure to check our point-specific forecasts for any location/elevation for rainfall probability, rainfall amounts, lightning potential, and more under the "Weather" tab for each location.

Forecast → Zion National Park

Forecast → Mt. Timpanogos, UT

Forecast → Mt. Wilson, CO

Friday to Friday Night:

The low pressure system associated with the remnants of Harold will slow down as it tracks across Northern Colorado on Friday. Meanwhile, a cold front arriving from the northeast will bank up against the Front Range from the plains, resulting in easterly upslope winds and favorable orographics along the eastern slopes of the Continental Divide. 

The result will be a setup conducive to widespread significant rainfall across the Colorado Front Range and Southeast Wyoming, with the potential for heavy rain and flash flooding.

Models are still struggling to get a handle on the placement of the heaviest rainfall totals, but all areas of the Front Range and I-25 corridor have a chance of receiving heavy rain.

The European Model (pictured below) and German Models are favoring the Northern Front Range and Southeast Wyoming, including Rocky Mountain National Park.

The American GFS Model and Canadian Model (pictured below) are favoring heavier rainfall further south toward the Palmer Divide and Pikes Peak region.

While the Front Range looks to take the brunt of this system, areas west of the Divide will see plenty of showers and thunderstorms as well, with activity possible over the higher elevations prior to noon.

NOAA's Weather Prediction Center has highlighted the Front Range, NE Colorado, and SE Wyoming under a level 2 out of 4 risk of flash flooding on Friday. Western and Southern Colorado has a level 1 out of 4 risk.

The amount of water vapor in the atmosphere (known as precipitable water) is expected to approach some of the highest levels on record in Denver, as measured by weather balloons launched twice per day. 

Forecast → Rocky Mountain National Park

Forecast → Pikes Peak

Saturday to Saturday Night:

Lingering moisture will result in another active day across the Colorado Front Range on Saturday with a good chance of showers and thunderstorms, especially in the afternoon.

Rainfall amounts are expected to be lighter compared to Friday, but heavy rainfall rates are still possible which could lead to flash flooding since soils will already be saturated.

Thunderstorms will be more scattered and "routine" in nature west of the Continental Divide in Colorado. Heading further south, a relative uptick in thunderstorm activity and rainfall amounts can be expected across Northern New Mexico compared to prior days.

Moving forward, be sure to take advantage of our Forecast Anywhere feature to stay up to date with the latest forecasts for any location.

Also, check back in for our next Western U.S. Daily Summit forecast update on Friday morning.

Alan Smith, Meteorologist