Monthly Archives: September 2020

Colder Sunday

6 AM MDT, Saturday, September 26, 2020

Today will be the last in a string of unseasonably warm days. It will also be dry with breezy conditions at times, so be careful with fire.

A cold front will probably have moved through the Front Range region by Sunday morning, and a second cold front will probably move north-to-south through the area during the day. Expect the high temperatures on Sunday and Monday to be 20 degrees cooler than today. It will be mainly dry, but a few areas of clouds and some sprinkles may develop late Sunday. The chance for that is greatest south of Denver, and even there it’s a small chance.

Both Monday and Tuesday mornings could bring some local frosts in some of the colder spots along the Front Range.

Beautiful weekend weather

8:00 AM MDT, Saturday, September 12, 2020

It’s a great weekend for outdoor activity. Chilly mornings will be followed by warm days, around 80 today and into the 80s (lower elevations) beginning Sunday and continuing until at least midweek.

There is still a lot of smoke in the air along the West Coast, and some of that may work its way in our direction after a few days. But the local sources of smoke have been tamed by the storm earlier this week. There is some hopeful news for the West Coast. Some areas of rain or drizzle may move into Washing and Oregon this coming week, and maybe even parts of northern California.

Accumulation likely this evening

12:30 PM MDT, Tuesday, September 8, 2020

If you were expecting to wake up to whitened ground in the Denver-Bouder-Fort Collins corridor, so was I. Although as I indicated yesterday, the full accumulation probably would’t be over until tonight.

The storm is farther west than expected. Moderate and some heavy rain and snow (along with some thunder snow), is occurring in western and central parts of Colorado, which includes the major fire areas.

It appears likely that a period of snow accumulation is likely this evening along the Front Range, with intermittent light snow later tonight. Although the amounts forecast yesterday are probably a little too high, there could still be some tree/power line damage, and record cold temperatures are still expected.

Cold and snow coming, may be slower to leave than originally forecast

9:50 AM MDT, Monday, September 7, 2020

The main change from yesterday is that the storm progression is slower, so the chance for periods of accumulating snow will likely extend well into Tuesday night. Before the cold front this evening, some hot, dry westerly winds may develop again along the foothills.

Storm progression

The strong cold front is likely to move through this evening before nightfall north of I-70, and around or after sunset as you go south. Clouds and precipitation are likely by about midnight (snow above 7500 feet) along the foothills from the western Denver suburbs northward, and spread south and east during the late night.

Rain will probably change to snow before 3 AM in Cheyenne, between 4 and 7 AM from Fort Collins to Denver, and after sunrise as you move south. Snow may be heavy at times Tuesday and become more intermittent as it continues through part or all of Tuesday night.

Peak depth in grassy areas through Tue night

Accumulation this time of year is more efficient at night, but accumulation during the day can occur quickly during periods of heavy snow, and then melting can dominate during lighter snow periods. For that reason, snow depth at the lower elevations is likely to be quite variable.

Above 8000 feet: 10-18 inches

6000-8000: 6-12 inches

Fort Collins (west side), Boulder, Broomfield, west & south sides of metro Denver, Castle Rock: 4-9″

Greeley, Fort Collins (east side), Longmont, DIA, Aurora, Colorado Springs: 1-5″

*NOTE: the official snowfall may be greater than the peak depth on the ground, especially in the lower elevations because the snow depth may increase and decrease more than once during the storm


Prepare for power outages and damage to vegetation.

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.


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.

Near record heat, then near record cold

12:15 PM MDT, Thursday, September 3, 2020


Labor Day weekend is looking hot and dry, at least through Sunday, but maybe Monday too. The record highs for Saturday-Sunday-Monday are 98, 97, 95 in Denver, and 97, 97, 93 in Boulder. We should at least get close to those, perhaps with a dry wind at times, so be aware of the fire danger.

Cold snap

We may get an initial weak cold front on Monday, but right now it is looking more likely that much of that day will be quite warm too. Late Monday or Monday night is when a strong cold front is forecast to move down the Front Range bringing unseasonably cold temperatures for a couple days.

There are increasing indications that the cold air mass may combine with a developing storm system over western Colorado, drawing moisture in from the plains, for the first “upslope” storm of the season. The result could be the first real snow for the Front Range Mountains and places like Estes Park and Nederland. Depending on the intensity and duration of the precipitation, and the time of day, a rain-to-snow progression can’t be completely ruled out for lower elevation areas like Denver, Boulder, and Fort Collins on Tuesday. I will certainly plan to update that possibility. At the very least, it’s looking likely that Tuesday will have some cold rain.

September Snow?

For those who moved to this area in the last 20 years, September snow in Boulder and Denver may seem like science fiction. But in Boulder, for example, the season’s first measurable snow occurred in September about 25% of the time between 1898 and 2000. September produced the first snow of the season 5 times in the 1990s. Occasionally a big wet snow (8 inches or so) caused considerable damage to trees since they are still in their vulnerable leafed-out condition. That occurred in 1995 with 8.6″ in Boulder on September 20-21 (6-10 inches throughout much of the Denver area). But since 2000, measurable snow at the Boulder station only occurred once when a mere 0.5″ fell in the predawn hours of September 12, 2014, and melted almost as quickly as it fell.

If it does snow next Tuesday, the earliness of the date would be highly unusual, but not unheard of. Measurable snow occurred around the Denver-Boulder area on September 3, 1961. In Boulder the snowfall on that date is officially listed as a “trace,” but local accounts at the time report the ground and cars were whitened. Denver (Stapleton) reported 4.2″ with that one.