SPM 09-26-2018: Cooler Air Arrives, but no Frost yet for the Lowlands

Issue Date: Wednesday, September 26th, 2018
Issue Time: 09:40 AM MDT


With dry weather continuing to dominate across Colorado, our attention begins to turn towards the fast-approaching end to the growing season. Many mountain valley locations have had several freezes already, and some of the high mountain peaks have seen dustings of snow over the last week or two. However, in the lower elevations, many are still awaiting their first freeze. The maps below show the median date of the first 32°F or lower temperature of fall at a number of stations across the north-central U.S., compared to the dates those stations hit that threshold this year (many have yet to do so). In Colorado so far, we can see that while many mountain locations have seen their first freeze already, most of us on the plains and western slope are still awaiting the end of the growing season (right panel of figure below). For the Front Range and eastern plains, this typically happens anytime from now (late September) through mid-October; on the western slope, the dates are more variable, but generally run from mid-September to late October (left panel of figure below).

The lack of frost hasn’t stopped the onset of fall foliage changes, however. Aspens across the mountains have been changing color since late August in spots and are now expected to approach peak color in the next two weeks. Meanwhile, in the lower elevations, we’re beginning to see patchy color appear, especially in the foothills and along creek and river valleys (where the coolest air tends to pool at night). A cool interactive map that shows the predicted evolution of fall color for each week of the fall can be found at https://smokymountains.com/fall-foliage-map/, and shown below is the prediction for October 1st (next Monday).


Beyond frost and foliage, fall also portends the return of snow to much of Colorado. While there doesn’t appear to be any significant chance of snow even in the mountains in the next few days, the dates of typical first snowfalls are rapidly approaching. From the NWS offices around the state, here are the average first measurable snowfall dates listed for a handful of major cities:

  • Denver: October 18th
  • Colorado Springs: October 19th
  • Alamosa: October 29th
  • Pueblo: November 4th
  • Grand Junction: late November

If you’re a fan of the cold and snow, then the time is almost here. Hopefully we can begin getting some high-elevation snow soon to bring a close to fire season and allow the resorts to prepare for ski season.

Flooding was not reported on Tuesday. For rainfall estimates in your area, check out our State Precipitation Map below.

Click Here For Map Overview

The map below shows radar-estimated, rainfall gage-adjusted Quantitative Precipitation Estimates (QPE) across Colorado. The map is updated daily during the operational season (May 1 – Sep 30) by 11AM. The following six layers are currently available: 24-hour, 48-hour and 72-hour total precipitation, as well as maximum 1-hour, 2-hour and 6-hour precipitation over the past 24 hour period (to estimate where flash flooding may have occurred). The 24-hour, 48-hour, and 72-hour total precipitation contain bias corrections that are not disaggregated into the hourly estimates, so there will likely be some differences. The accumulation ending time is 7AM of the date shown in the bottom right corner. Also shown optionally are fire burn areas (post 2012), which are updated throughout the season to include new burn areas. The home button in the top left corner resets the map to the original zoom.