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Eclipse viewing: we are not necessarily in the clear

12:00 Noon, Thursday, August 17, 2017

We are just 4 days away from the total eclipse and I wish the potential for clear skies was greater than it is. This blog will focus on Colorado (where a partial eclipse will occur), and the narrow totality path across Wyoming and Nebraska where most Front Range folks seeking totality will end up on Monday.


Let me start by saying that the chance for midday clear skies this time of year according to climatology is on the order of 80%. It’s a bit less than 80% directly over the mountains where daytime cumulus clouds get their start, and also a little less in central and eastern Nebraska where it is typically more humid.


So what I look for is whether the atmospheric moisture is likely to be below average, average, or above average. When I wrote last week it appeared that atmospheric moisture would be near or below average on the 21st. But now it appears that our flow will be from the south and southwest at different levels of the atmosphere which will likely increase the moisture content, especially on the plains. Let me point out that atmospheric moisture does not necessarily result in clouds (water vapor is invisible), but it makes it easier for clouds to form. A moister atmosphere may result in early development of thundershowers over the mountains on Monday and lingering patches of cirrus clouds from the previous day’s thunderstorms across any part of the region. The worst case scenario is a frontal boundary that could trigger development of a broad area of clouds. Current indications are that there may be an east-west frontal boundary somewhere in the northern Kansas-Nebraska-South Dakota region extending westward toward the Front Range. There is a chance the front will be weak, or north of the eclipse path, but for now this is what I see as the likely scenario:

  • western & central Wyoming has a good chance of clear skies (similar to climatology)
  • eastern Wyoming and Colorado have less than the climatological chance for clear skies, but still a little better than 50%
  • as you go eastward along the total eclipse path in Nebraska, the chance of clear skies slips below 50%


There are of course a lot of opinions about the complicated cloud forecast. For another opinion see the Washington Post’s Capital Weather Gang:



Typical late summer weather, eclipse outlook

1:00 PM, Friday, August 11, 2017

A small trend toward warmer/drier

It was quite a cool week, but not one that was record setting. The risk of afternoon showers and thunderstorms won’t go away this weekend, but it’s a slightly smaller chance. Daytime temperatures are likely to rise into the low 80s (average this time of year for Denver-Boulder-Fort Collins is 84-85, and a couple degrees warmer at DIA).  There is indication that next week will be fairly warm and mostly dry.


Will clouds eclipse the eclipse?

Of course it’s much too early to forecast August 21st cloud cover in Wyoming/Nebraska. Clouds can be even more difficult to forecast than precipitation. Forecasting clouds when a big organized storm system is coming is much easier than forecasting the transitory nature of summertime cumulus and cirrus. At this point I am looking for signs that we will be in a somewhat westerly flow aloft with an atmospheric ridge in the West because that increases the chances of a relative dry continental atmosphere. Southerly flows from the Gulf of Mexico or the Southwest monsoon would increase the risk of clouds.

So the good news is that the long range model forecasts are suggesting a pattern more conducive to a dry atmosphere above the high plains and northern Rockies. But that is a 10-day forecast, so the reliability is fairly low.

Unseasonably cool and damp to start the week

1:00 PM, Sunday, August 6, 2017

A cooler air mass has moved into the region as forecast (mainly east of the continental divide) and we are in for several days with below-average temperatures, a few showers or thundershowers, and maybe even a multi-hour period with steady drizzle or rain.


It’s likely that Monday will be the coolest day with the best chance of rain. Monday’s temperatures may not make it out of the 60s even in the lower elevations. Sunny intervals by Tuesday should allow temperatures to warm back to the 70s, but we might remain below the average daytime high in the 80s most of this week.


Despite the long stretch of unseasonably cool weather, it’s possible we will get through without setting any record lows. The record lows in Denver on Monday and Tuesday are 50 and 49, and in Boulder they are 47 and 46. Denver has a better chance than Boulder of reaching one of those, but it’s only a small chance. The record low maximum temperature on Monday is 65 in Denver and 54 in Boulder, both from 1939. Denver has a shot, but Boulder will get warmer than 54.


Another cool-down Sunday-Tuesday

12:40 PM, Friday, August 4, 2017

Today and tomorrow (Saturday) will see a bit drier and warm weather with only a small chance for a thundershower. But the next cold front is likely to arrive Saturday evening or overnight and usher in a cooler than average period lasting at least through Tuesday. Along with the cooler weather we are likely to see an increased chance of cloudy periods and thundershowers. There may even be a period of low clouds and drizzle.


This is all part of a pattern that has resulted in record heat beneath an upper level high over the West Coast.  North and northwest upper level winds on the east side of the high are driving weak storm systems and cool airmasses into the high plains and central U.S. The cool spell will be more subtle west of the continental divide.

Warm & Dry, Then Cold Front late Wednesday

7:00 AM, Tuesday, August 1, 2017

The deep atmospheric moisture associated with the monsoon has been pushed to the south of Colorado. As a result today through at least midday Wednesday should see very warm and dry weather. Then a cold front from central Canada is likely late Wednesday afternoon or evening. Some moisture in the low levels will move into Colorado behind the front and increase the chances for cloudy intervals and some showers and thunderstorms Wednesday afternoon through Thursday. Daytime temperatures Thursday are likely to be about 15 degrees cooler than today.


The Front Range region from Denver north finished out July drier than average (with a few local exceptions). Together with a dry June we are seeing a big summer rainfall deficit at climate stations like Denver, Boulder, and Cheyenne. Across southern Colorado it is a very different story. And active monsoon brought a moist July to many locations, including over 6 inches in Colorado Springs. Even the high desert location of Alamosa had around 3 times its July average with over 3 inches.

Less hot, better chance for rain

6:55 AM, Tuesday, July 25, 2017

Beginning today and for the next few days we will see more afternoon cloudy periods, cooler daytime temperatures, and a better chance for showers and thunderstorms.


The 2017 North American monsoon has been active but it has been mainly south and west of the Denver-Boulder-Fort Collins-Cheyenne area. There were a few days late last week when a plume of moisture moved over the Front Range region, but it was mainly upper level moisture and so there were limited amounts of rain reaching the ground. This morning there are already showers across parts of western Colorado as another monsoon moisture surge moves north. That should increase chance for showers and thunderstorms this afternoon and tonight (and again Wednesday and maybe Thursday) for northeastern Colorado and the Front Range. There could even be some locally heavy rain.

The North American monsoon

12:35 PM, Wednesday, July 19, 2017

The North American monsoon, sometimes called the Southwest monsoon or the Mexican monsoon, is a seasonal weather pattern that in some years can bring enhanced rainfall across the southwestern U.S., including the Front Range region. The impacts for the Front Range are seen in the climate record from mid July through the late summer.  In a nutshell, showers and thunderstorms erupt daily along the Mexican Sierras as the tropical Pacific heats up and that pumps lots of moisture into the atmosphere. Some of that moisture makes its way north into the southwestern United States. In the Front Range region not every year sees the effects, and the surges of moisture can be intermittent.


The 2017 monsoon has arrived, but for the most part the moisture has been in the upper levels of the atmosphere and so rainfall has been sparse despite afternoon clouds (there has been more moisture and areas of rain from Colorado Springs southward). Over the next two or three days a moderate plume of moisture (mainly middle and upper levels) should result in some afternoon and evening showers and thunderstorms. It’s not a highly anomalous amount of moisture, but it’s the best we have seen in a while. The best chance for rain is over the mountains of western Colorado, but I am holding out some hope for the Front Range too.