A little more atmospheric moisture Thursday & Friday; explaining the “monsoon”

12:55 PM MDT, Wednesday, July 11, 2018


Although it would be an exaggeration to say wet and cool weather is coming Thursday & Friday, it’s accurate to say that the Front Range region should be less hot with a little boost in the chance for clouds and thunderstorms. The Southwest monsoon is resulting in increased thundershower activity in western Colorado and down in the desert, and some of that moisture is likely to move over the Front Range region, at least in the mid levels. Significant rainfall Thursday and Friday will probably be spotty, but gusty wind and brief showers will probably affect a larger portion of the region.


After a return to generally hot and dry weather on Saturday, a cool front on Sunday will probably boost the chance for periods of clouds and thunderstorms again.


What is the monsoon?

The Southwest Monsoon (also called the North American Monsoon, or the Mexican Monsoon) is not nearly as strong as its Asian counterpart, but it can have an important influence on weather in Colorado during mid and late summer.

It begins when the strong June sun warms up the high plateaus of Mexico and the southwestern U.S. The heated air become buoyant and rises, and moisture-laden air from the tropical Pacific (and some from the Gulf of Mexico) moves in and triggers torrential thundershowers in the Sierras of northern Mexico. As summer progresses some of that moisture moves up into the southwestern U.S. and enhances thundershower development there too.  The monsoon dies off in the late summer or early autumn as the strength of the sun weakens.

Colorado is on the edge of the monsoon influence. Some years see very little impact and in other years the pattern can allow surges of monsoon moisture that trigger slow moving thunderstorms with high rainfall rates. The monsoon is not continuous, but it can come in spurts from July to September, with mid July to mid August being the climatological peak. This week’s monsoon surge is not that impressive for the Front Range, but we still have the main season ahead of us.

I should point out that not all rain in July and August is from a monsoon pattern, but extreme rainfall is often connected to the monsoon pattern.


Today is the best chance for rain in the next 4 days

12:00 Noon MDT, Thursday, July 5, 2018

Moderate amounts of atmospheric moisture and a low-level push of moist air from the east should boost the chance for afternoon/evening thunderstorms today. There will almost certainly be parts of the Front Range region that miss out, but where the thunderstorms do develop there could be some brief heavy rain and hail.


The overall pattern is still a hot and mainly dry one for the central Rockies. So after today It looks like the weekend will be mainly dry with above average temperatures. thunderstorms that do develop may produce only small areas of rain with larger areas of brief gusty winds. Temperatures exceeding 95 degrees in portions of the Front Range urban corridor are likely.

A little better chance for thunderstorms on July 4th

12:50 PM MDT, Monday, July 2, 2018

Hot weather should continue this week, but not as hot as last week. There may even be a cold front passage in eastern Colorado on the 4th that brings the temperature down to average (mid and upper 80s in the Denver-Boulder area). There are areas of smoke floating around the region too, and we may continue to have periods of haze over the next few days.


Rain: Atmospheric moisture will likely increase a little bit Wednesday and Thursday and that should increase the chance for afternoon/evening showers and thunderstorms. This is not appearing to be a pattern for widespread rain, so many parts of the state will likely remain dry (or nearly dry).


Records: June 28th brought records to Cheyenne (99), Boulder (98), Denver (105), and Colorado Springs (100). Then the morning of July 1st came within 1 degree of record lows in Boulder (46), Denver (47), and Colorado Springs (44). No records are expected this week.



Unsettled weather Sunday, hot and drier by Tuesday

8:30 AM MDT, Saturday, June 23, 2018

A cold front will bring cooler weather for Sunday along with a better chance for thunderstorms. Hot and mainly dry weather is likely the become re-established over the region by Tuesday.


Today and Sunday

The cold front is likely to move through this evening or overnight. There is a slight chance of thunderstorms today and a better chance on Sunday. The best chance is east of Denver where some might be severe. Temperatures in the lower elevations should be in the 80s today but no better than the 70s on Sunday.


Next Week

Mainly dry and warmer weather appears to be on the way. In fact by Tuesday low elevation temperatures are likely to reach daily highs of 90 or more.

Trend toward drier and warmer midweek

6:00 AM MDT, Tuesday, June 19, 2018

For those that got hailed on Monday, there may be more severe thunderstorms in the region today, especially east of Denver.  But on Wednesday you should expect that the sunny periods will be longer and thunderstorms more localized. By Thursday and Friday the chance for thunderstorms should become quite small and warm, summer weather will be back.  There may be another cool front late in the weekend, but I’ll update that possibility later in the week. The overall trend for the next 10 days appears to be for warmer and drier (after today).

Less hot and more moist is on the way

1:00 PM MDT, Thursday, June 14, 2018

We are likely to enter a period this weekend with cooler temperatures and a better chance for periods of clouds and thunderstorms across the central Rockies, including the Front Range region. The change might start out slowly Friday or Saturday and become more noticeable Sunday and Monday.


It should still be quite warm in most areas, but some moisture from the tropical Pacific will increase the chance of late day cloudy periods and some localized showers or thunderstorms. The pattern is an early taste of the Southwest monsoon.


A cool front from the north may arrive by late Sunday bringing in cooler weather. Cloudy periods late Sunday into early next week may be more numerous and there may be a late day period with more organized coverage of showers and thunderstorms on Sunday or Monday.

Hot weekend, cool front Sunday night

5:45 AM, Friday, June 8, 2018

We have a hot summer weekend coming up with low elevation temperatures topping 90 and probably reaching mid 90s in a number of areas. A cool front Sunday night should trim about 10 degrees off the high temperature for Monday, but that’s still a warm day.


No organized areas of rainfall are expected and even isolated showers will be hard to come by. The main weather may be increasingly windy conditions both before and after the cool front Sunday afternoon and Monday. Those winds may enhance the risk of wildfires especially in the western and southern parts of the state where the vegetation is drier.