Monthly Archives: July 2018

Sunday: cooler and a chance for thunderstorms

5PM MDT, Saturday, July 14, 2018

A cold front pushing south near the Montana/Wyoming border will likely reach Cheyenne in the late night hours and push through the northeastern plains & foothills  of Colorado Sunday morning to midday (with little or no impact west of the continental divide). For the Front Range region that should take about 10 degrees off the high temperatures for Sunday. Areas of showers and thunderstorms in the afternoon will cool it down more in local areas. Although it’s not a sure bet, at least some rain will probably occur over more than half of the Front Range region and high plains, and that’s as good as it gets this month!

 

There has not been a major pattern shift, so more hot and mainly dry weather will likely return in the week ahead.

 

90-degree days

For those wondering, both Denver and Pueblo are seeing the most 90-degree days in their records for the season through July 14th (47 at Pueblo and 32 at Denver/DIA). In Boulder we have recorded 22 90-degree days through today. Since the current climate station was established in Boulder in 1990, this year’s number of 90-degree days through July 14th falls short of the years 2000 (25 days), 2002 (24 days) and 2012 (24 days).

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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.