Record high, records lows, and early snow

10:15 AM MDT, Sunday, September 6, 2020

Dry, smoky, record hot weather continues until a strong cold front late Monday. Then we expect snow at high elevations and rain changing to snow at low elevations with record lows by Tuesday morning.

Before the cold front

Records today in Denver and Boulder are 97 in both places and we are well on our way. Yesterday (Sep 5) Denver set an all-time September record of 101, while Boulder’s record of 99 was the hottest for so late in the year. A dry wind will continue to create extreme fire danger, and the Cameron Peak fire may flare up again as it did yesterday. The winds above mountain top level are more northwesterly today, so more smoke may work its way into the Denver metro.

A weak cold front tonight should cool things off a little and temporarily stop the downslope wind, but hot, dry winds could re-develop again during the day Monday ahead of the very strong cold front late Monday afternoon or early evening.

After the cold front

Temperature will fall rapidly late Monday afternoon/early evening. Clouds should develop by midnight with snow above 8000 feet and rain or drizzle below 8000 feet changing to all snow even in the urban corridor before daybreak Tuesday. Melting rates at the ground will be high, so the maximum accumulation is likely on vegetation and cars during periods of heavy snow in the morning, even though the snow may continue for much of the day.

The record lows for Tuesday and Wednesday are 35 and 32 in Boulder, and 31 both days in Denver. All are at risk of being broken. It’s impressive enough to go from record high to record low in such a short period, but in this case we may go from the “hottest so latest in the season” to the “coldest so early in the season” in 48 hours.

Most likely accumulation (maximum on the ground at the peak depth)

Above 8000 feet: 7-12 inches.

6000-8000 feet: 5-10 inches

Below 6000 feet (urban corridor from metro Denver northward): 2-6 inches (probably on the lower end in Colorado Springs)

A small chance exists for more the 6 inches in Denver-Boulder, mainly if the snow starts before 3 AM, which seems unlikely at this time. Will update tomorrow.

Impact

Except for some brief slush accumulation on roads during peak snowfall rates, roads will be mainly wet. The real issue will be the heavy weight of wet snow on trees and power lines. Prepare for power outages and tree damage by Tuesday morning.

Not all impacts will be bad. The extreme fire danger and ongoing fires will be tamed by nature. This storm will do nothing to quench the fires in California, so there may still be some smoky skies in our future, but at least during the storm and for several days afterward we should have clean air.

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