Cooler for the weekend

6:00 AM, Friday, September 15, 2016

A cold front today will result in a significant change in temperature but not a big increase in the chance for rain. It may not reach the Denver area until late afternoon, so it should still be a warm day until then. But the cooler air will probably sweep into the northern Front Range (including Cheyenne) a bit sooner. Cooler weather over the weekend won’t be too bad, and afternoons may be quite pleasant.

 

This system will bring some much needed rain and snow to parts of Montana and northern Wyoming. For the Front Range there is a slightly increased chance for clouds and a few showers late today through Saturday morning, but for the most part the overall dry spell has a good chance of continuing.

 

Warmer weather should return early next week ahead of another cold front around in the Tuesday-Wednesday time frame.

 

 

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Turning much cooler Monday evening

9:30 AM, Sunday, September 3, 2017

Today, Sunday, should be the hottest day of the Labor Day weekend. Temperatures should be around record levels. The record in Denver is 95 (from 1985) and in Boulder it is 94 (from 2013). Monday will continue the very warm weather, but a cold front sweeping in from the north Monday evening will make it 20-30 degrees cooler on Tuesday, It’s possible that the temperatures will start to cool off by late afternoon Monday along the northern Front Range.

 

A widespread rain is not expected with the upslope flow behind the front. But there may be a period of low clouds and some patchy drizzle Monday night or Tuesday morning. Fire danger will remain elevated, especially north of I-70 where the summer monsoon was not as generous.

 

Beyond that the weather in Colorado will not be too exciting (other than the fire danger), and average to above average temperatures are likely to return later in the week. But weather may be making national news again by next weekend as Hurricane Irma approaches the eastern seaboard, an extreme fire danger continue the plague parts of the West.

Nice late summer weekend, for us

3:00 PM, Friday, August 25, 2017

We are in for a warm and dry (or nearly dry) late summer weekend. Saturday is likely to be the warmer day of the weekend, with high temperatures around 90 (average for Denver and Boulder is around 83-85). This pattern of warm and generally dry weather should continue for at least the next 5 days or so.

 

Weather will be making the news this weekend, but not in Colorado. Hurricane Harvey will bring dangerous conditions to the mid and lower Gulf Coast of Texas this evening. But it’s impact won’t end this evening. The storm is likely to hang around south Texas for days, somewhat weaker in terms of wind but very wet. Flooding is likely to be historic in some places of southeastern Texas, possibly as far inland as the hill country around San Antonio or Austin.

 

 

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: https://www.washingtonpost.com/news/capital-weather-gang/wp/2017/08/17/total-solar-eclipse-weather-forecast-as-of-aug-17/?utm_term=.41adcb6ae557

 

 

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.