September 2021 will go down as one of the overall hottest on record for the Front Range, and not much more. Other weather highlights during the month included a multi-day late-summer heatwave, continued smoky skies from the California wildfires, the first snow for many Mountain locations, dismal precipitation for much of the state, and one measly day that could pass for autumn. Here’s a quick and colorful recap of our weather during September and how it relates to climatology.

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Top Weather Highlights of September 2021:

HURRICANE REMNANTS FOR EVERYONE… BUT US! On the first day of September, the remnants of Hurricane Nora were impacting the Desert Southwest while Hurricane Ida’s remnants slammed the East Coast causing unprecedented flooding in New York City. Unfortunately this surge of tropical moisture largely bypassed the Front Range and we got next to nothing.

RELATED POST:
BoulderCAST Daily – Wed 09/01/21 | Underwhelming rainfall expected despite near-record moisture levels from Nora's remnants

MORE SMOKE: As some of the big fires in northern California started to come under control, new fires broke out such as the Caldor Fire near Lake Tahoe and the KNP Fire Complex further south in California. Needless to say, smoke from out-of-state wildfires was a mainstay for Colorado much of September as well, though things did improve considerably during the latter third of the month.

A TRIP TO THE OVEN! From September 9th to 11th, the Front Range endured one of the strongest late-season summer heatwaves on record. Boulder set new daily record high temperatures on all three days, as shown below.

The most striking facet of the heatwave was September 10th which ended up being the latest 99°F day on record and the third hottest September day ever observed in Boulder. This was despite thick smoke filling the skies which has a net cooling effect. Had it been a bluebird sunny day, temperatures would have been a degree or three warmer. Should we all band together and send California a “thank you” card??

Five of the six hottest September days ever observed in Boulder since the late 1800’s have occurred in the last three calendar years! Septembers of today are much different than just a few decades ago in northeast Colorado.

NO SURPRISE — SUMMER 2021 ENDED HOT & DRY: Meteorological Summer 2021 was Colorado’s 4th warmest and 33rd driest in the historical record. Northwest portions of the state had a record hot summer, while northeast Colorado had a record dry one. Boulder was a bit of an outlier: it was warmer than normal, but not exceptionally so, and also we had above normal rainfall thanks to a few localized heavy rainfall events. Denver (DIA) reported just 1.45″ of rainfall all summer long which was the city’s 5th driest summer. If you missed our in-depth recap post of the summer for Colorado, you can find it HERE.

RELATED POST:
Summer 2021: Crunching the numbers of yet another hot & mostly dry summer season for Colorado

THE MOUNTAINS TURN WHITE AGAIN! As the seasons begin to shift and temperatures trend downward, snow tends to become much more common across Colorado during the month of September. It didn’t snow in September in the Denver Metro area like it did last year, but we did get a little taste of winter weather in the Mountains in the final days of summer on September 20th. The sun rose with most mountain areas above 10,000 feet donning a light dusting of snow up to a few inches. Trail Ridge Road through Rocky Mountain National Park was temporarily closed while they worked to plow away 2 to 3-foot snow drifts!

FALL ARRIVED WITH LITTLE CHANGE: The first day of autumn on September 22nd brought the first Frost Advisories of the season to portions of northeast Colorado (not the Denver area though). While it was indeed a cold morning, sunshine quickly helped warm temperatures well above normal close to 80 degrees.

RELATED POST:
BoulderCAST Daily – Wed 09/22/21 | A cold morning, but warming up fast on this first day of autumn

ONE MORE 90-DEGREE DAY FOR GOOD MEASURE: One final summer-like pattern briefly led to a resurgence of 90-degree temperatures across the Front Range on September 26th. Both Boulder and Denver set new daily record high temperatures on the date.

With four new record highs set during the month of September, Boulder has now notched a total of nine for the year with three months still to go. The graphic below shows how 2021 compares to the rest of the 21st century in terms of record highs and lows set in Boulder.

September 26th was likely the last 90-degree day of 2021 which will end with 50 total days at or above 90 degrees, one less than last year but still way above normal.

HEAT RECORD THWARTED ON THE LAST DAY OF THE MONTH: Not surprisingly, Denver was on pace for its hottest September on record as of the 29th. However, the final day of the month was a gloomy and cold one with highs in the 50’s, sprinkles and overcast skies. This single day dropped the month to third place on the monthly temperature record list in Denver and 11th place in Boulder.

RELATED POST:
BoulderCAST Daily – Thu 09/30/21 | Staying cool today with A.M. sprinkles, but a gorgeous weekend is shaping up

NO RAIN, NO GAIN! You may have noticed a lack of much discussion of rainfall so far in this summary post. That’s because September was largely dry for the Front Range. Boulder reported just 0.54″ of rain during the month which is about 30% of average. The same fate struck the rest of the Denver Metro area as well.

RELATED POST:
August 2021 Weather Review: A month headlined by poor air quality and relentless smoke

September 2021 Recap Graphics:

Where does September 2021 fit into the last 12 months?

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Featured image from @AspenLodging of Twitter.

BoulderCAST Team

This post had major contributions from at least two members of our team. The more meteorologists the better, right?

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