Cooler for second half of the week

10:35 AM, Tuesday, August 23, 2016

Once again we have a cool air mass moving in from the north that is expected to bring cooler than average weather Wednesday through Friday. Daytime highs in the Denver-Boulder-Fort Collins area will probably stay below 80 Wednesday and Thursday, and maybe Friday too.


The last cool air mass on Friday and Saturday last week didn’t stay around long, but it did result in a record low of 43 in Boulder on Saturday morning (Aug 20), and Denver’s low of 47 missed their record by 1 degree.


An increase in atmospheric moisture across southern and western Colorado should result in numerous showers and a few thundershowers there this afternoon and tonight. The best chance for significant rain is south and west of Denver, but some parts of the Denver-Boulder area will probably see a shower or thundershower this afternoon and again Wednesday. There could even be some patches of low clouds during the night and early morning.


Enjoy the late summer cooler weather!

Strong cold front and rain, then nice weekend

12:35 PM, Friday, August 19, 2016


A strong cold front was just about to Cheyenne at noon, and should surge south through the Front Range this afternoon. It’s only 48 degrees with cloudy skies in Casper, WY at noon!


Expect the mid-late afternoon to turn much cooler with clouds and some showers and thunderstorms. The primary chance for rain should be over by late night, but low clouds and some areas of fog or drizzle may persist into early Saturday morning. The weekend should be nice after a chilly start on Saturday, with relatively cool temperatures Saturday afternoon and closer to average temperatures Sunday afternoon.

Strong cold front to end the week

12:30 PM, Wednesday, August 17, 2016

A strong cold front on Friday is expected to bring cooler weather late Friday through Sunday. There is also a good chance  for clouds and areas of rain and thunderstorms late Friday into early Saturday.


There may end up being more than one push of cool air from north to south on Friday, so it’s possible that the real cool down may not occur until late Friday. With northeasterly winds accompanying the arrival of the cooler air, some moisture may be pushed upslope into the Front Range increasing the chance for showers and thundershowers. There may even be a multi-hour period of overcast with some steady drizzle or rain late Friday into early Saturday.


Denver, Boulder, and Fort Collins have all been running drier than average for June-July-August (Colorado Springs has not been as dry). So if this push of cool air late Friday manages to trigger some widespread rain, that moisture would be welcome.

Cooler, showery through Saturday

1:15 PM, Thursday, August 4, 2016

A batch of mid and high level moisture has moved up from the south as forecast a few days ago, and the moister atmosphere should be with us through Saturday. For eastern Colorado, low levels of the atmosphere are still a bit dry, so there may be a lot of evaporation beneath the clouds when the first showers form today.


Numerous showers/thunderstorms should develop late today (Thursday). Almost everyone will probably see at least a little rain, and a few pockets may see locally heavy rain. The chance for heavy rain is a little greater in the mountains than it is on the plains. There is even indication that areas of showers may continue through the night. So don’t be too surprised if you hear some rain falling during the late night.


Friday and Saturday will likely see showers and thunderstorms mainly in the afternoons and evenings. If you have outdoor plans, you may very well be fine and it won’t be hot. But you should have a plan B in case of rain. Beginning Sunday we are likely to trend toward fewer showers and warmer temperatures.

Hot Wednesday; then a bit cooler and moister

4:05 PM, Tuesday, August 2, 2016

The Southwest monsoon of 2016 has been reluctant to leave its origins in Mexico even though this is the time of year when it sometimes surges into the southern or central Rockies. It now appears that may change, at least for a couple days, beginning late Thursday. The weather pattern should allow some tropical moisture from the south  associated with the monsoon and a cool front from the north to team up and give us a break from the heat and and dryness late Thursday, Friday, and maybe Saturday.


It’s too early to have a good handle on the magnitude of the rain chances. But during the period late Thursday through Saturday it appears there is a good chance for more widespread afternoon and evening showers and thundershowers, and some localized areas of heavy rain.  At this point it doesn’t appear to be a long-lasting change to the pattern.


For my colleagues at UCAR/NCAR planning their summer party and road race Friday afternoon…  the good news is that it won’t be as hot. The not-so-good news is that there is a better chance for rain than we have seen in recent days. As usual, there should be areas with no rainfall, but it appears likely that most areas will get some measurable rain.

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.