Monthly Archives: July 2016

Where is the North American monsoon? …and the anniversary of big floods

3:00 PM, Friday, July 29, 2016

Nineteen years ago yesterday Fort Collins suffered a devastating and deadly flash flood, especially along Spring Creek, when 10 inches of rain deluged the city in 5 hours. Thirty-one years ago Monday Cheyenne had it’s biggest rainstorm when more than 6 inches of rain and a whole lot of hail fell in a few hours resulting in deadly flash flooding. This weekend marks the 40th anniversary of the Big Thompson canyon flash flood of July 31, 1976 when 12 inches of rain in under 5 hours sent a wave of water down the Big Thompson canyon that claimed 144 lives (104 in vehicles) on the eve of the state’s centennial celebration. By comparison, the September 2013 flood that also devastated the Big Thompson canyon generally had peak flows that were much lower than in 1976, but lasted much longer resulting in a great deal of erosion and riverbank collapses.


Why is late July and early August such a common period for flash floods? You may have heard of the North American monsoon (sometimes called the Southwest monsoon or the Mexican monsoon). During June and July numerous tropical thunderstorms along the mountains of western Mexico pump large amounts of moisture into the atmosphere. As summer wears on the weather pattern sometimes permits that “monsoon” moisture to surge into the southwestern United States, in some years all the way to Colorado. July through September is when this could happen, with mid July to mid August being the prime time for monsoon impacts in Colorado. And yes, the September 2013 floods did occur in an active monsoon pattern.


The monsoon this year has not had much northward extension so far. There is more moisture around today (but not really from the monsoon), and a few localities may experience heavy rain and hail, but a widespread soaking rain appears unlikely. Warm with only small chances for afternoon thunderstorms is expected this weekend. Only subtle changes in the weather pattern can allow a monsoon surge into the Front Range region, so just because we’ve made it to late July without widespread monsoon rains does not eliminate that possibility in August.


Have a nice weekend!

Worst of heat wave to the east of Colorado

1:50 PM, Tuesday, July 19.2016

The weather nerds out there have probably noticed a whole bunch of heat advisories and warnings in the mid-section of the U.S. For the remainder of the week there is likely to be some impressive heat and humidity through much of the Great Plains, Midwest, and Great Lakes regions.


The Front Range region is in this expansive area of heat too, but we are on the western edge where a little moisture has managed to slip into the area and increase the chance for clouds and thunderstorms in the afternoons. That pattern will continue, although the coverage of thunderstorms may decrease a bit in the second half of the week. Daytime highs in Denver, Boulder, Fort Collins, and Colorado Springs should reach 90 each day and likely in the mid 90s for some locations through Friday. Some locations will get a good quick soaking from thunderstorms while others will see little or nothing. So the fire danger will continue even though it may be somewhat reduced in a few local areas.


Less hot and generally dry weather looks like a good bet for this coming weekend.

Hot and mainly dry pattern to continue

12:30 PM, Tuesday, July 12, 2016

The good news: 1) This week will be a little less hot than this past weekend. 2) There may be enough moisture by Thursday and Friday for a small chance of afternoon thunderstorms.


The bad news: 1) No substantially cooler or widespread wet weather appears to be on the way. 2) If thunderstorms develop Thu or Fri, some may be the dry kind with more lightning than rain.


Looking ahead: It appears that a major heat wave may be developing by next week for the central U.S., and the Front Range region may very well be within the west side of the heat wave area. One thing we could hope for is that the center of the heat wave shifts a little east and allows some moisture from the tropical Pacific to move into the Colorado region later next week (a pattern known as the North American monsoon). But right now it appears we should be ready for more hot and dry weather next week.

Hot weekend, getting breezy Sunday

10:30 AM, Saturday, July 9, 2016

Our hot, dry weekend may start turning windy in parts of the Front Range region late Sunday ahead of a cool front. After the cool front it will likely continue to be breezy into Monday in some areas, especially above tree line. The cool front won’t bring in “cool” air, but it should make the highs on Monday at least 10 degrees cooler than those on Sunday.


There is very little chance for rain through all of this. But as usual, be prepared for an isolated thunderstorm in the high country. Mainly dry, very warm weather is likely at least into midweek.

Unusually moist atmosphere today

8:30 AM, Friday, July 1, 2016

The atmosphere over the central Rockies is about as moist as it gets for July 1st. Using a value called precipitable water, this morning’s 1.20 inches of precipitable water at Denver is nearly twice the average of 0.65 inch for July 1st.


Moisture is an important ingredient for heavy rain, but it’s not the only ingredient. For heavy rain we also need rising motion either through convection as the sun warms the air, or through organized upslope (easterly) flow into the mountains. Because there are a lot of clouds around today limiting the sunshine and the upslope flow is not that organized, that may put a limit on the coverage of heavy rain. But I think we should expect that some local areas will receive heavy rain and some flooding while most areas will get at least some rain.


Saturday should trend toward less cloudy and less cool, but still with the chance of thunderstorms. Sunday and Monday should see much more seasonably warm weather with a small chance for thunderstorms late in the day.