9:30 AM MDT, Saturday, October 24, 2020
But first, the fires. Gusty and dry west winds are already occurring above 7500 feet and will continue through today creating conditions for spreading wildfire. Those winds make break through the denser cold air in some places below 7500 feet as well. Then, a record cold air mass and snow should quiet the fires tonight and Sunday.
The cold front will likely be through the Cheyenne area in the late afternoon, the Boulder-Denver area early this evening, and Colorado Springs a couple hours after nightfall. We are not likely to see temperature rise above freezing again until Tuesday afternoon. Snow is likely to begin during the late night in Cheyenne and spread southward during Sunday morning. Sunday and Sunday evening are expected to see the most snow, but some areas of snow could linger into Monday morning.
The upslope (northeasterly winds into the mountains) will favor areas in and along the foothills and the north side of the Palmer Divide for the heaviest amounts. But there will be another important element with this storm. Bands of heavy snow oriented southwest-to-northeast about 10-20 miles wide will probably develop and extend 100 miles or so out onto the plains. Snowfall rates in these bands could exceed an inch per hour. Some areas might only see those heavier rates for about an hour, while other areas could experience one or more multi-hour periods of moderate or heavy snowfall rates.
Most likely accumulation:
3-7 inches: northeast plains (DIA, east side of Denver metro, Weld County, Longmont), with the heaviest amounts where those heavy bands occur.
6-10 inches: in and near the foothills (west and south sides of Denver metro, Castle Rock, Boulder, Nederland, Estes Park, Cheyenne)
12 inches: in some locations where the heavy bands are more persistent (probably about 10% of the area)
Snowfall records are not expected since the storm is occurring on the same dates as the October blizzard of 1997 that left 15-50″ throughout the region (including 29.8″ in Boulder).
Record cold is very likely. Sunday and Monday high temperatures should remain below freezing, and possibly in the teens on Sunday. Monday and Tuesday mornings will probably range from the single digits to low teens.