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.
Flash Flood Watches are in effect for 23 states (+DC) due to heavy rain expected from the remnants of Hurricanes #Ida (East) & #Nora (West). Western Colorado is already included. Uncertainty is higher near the Front Range, but will prob see some watches posted soon in Mtns #COwx pic.twitter.com/OPL2EriHKI
— BoulderCAST Weather (@BoulderCAST) August 31, 2021
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.
The flow of thick smoke from California into Colorado is seemingly never ending! New fires ignited yesterday! Air quality & visibility will remain poor through Thursday w/ slight improvement expected on Friday #COwx #CAwx #Smoke #CaldorFire
The latest: https://t.co/gcXXztAvZa pic.twitter.com/gbgB5pngQI
— BoulderCAST Weather (@BoulderCAST) September 8, 2021
Another day, more Air Quality Alerts for Colorado. Today they have expanded to include the central and northern Mountains as well where thick smoke will be present… #COwx #Smoke #AirQualityAlerts #Pollution pic.twitter.com/7B0thxz3in
— BoulderCAST Weather (@BoulderCAST) September 8, 2021
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.
Despite quite a bit of smoke, #Boulder was still able to set a new record high temperature on Thursday reaching 95°F in the late afternoon hours. Likely would have been +1-2°F warmer without the smoke. The prior record was 94°F set in 1983 #COwx #RecordHeat #smoke #Boulderwx pic.twitter.com/dSQodIPmbu
— BoulderCAST Weather (@BoulderCAST) September 10, 2021
(1/2) The official high temperature in #Boulder on Friday was 99°F, shattering the existing daily record of 94°F from 2012. This is the latest 99°F day the city has seen. #COwx pic.twitter.com/1aTuMY3I8Z
— BoulderCAST Weather (@BoulderCAST) September 11, 2021
For the THIRD scorching day in a row, #Boulder set a new daily record on Saturday at 95°F. This comfortably tops the prior record of 92°F set in 2016. #COWx #Heatwave pic.twitter.com/LIkKNoB5el
— BoulderCAST Weather (@BoulderCAST) September 12, 2021
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.
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!
Get ready to get bombarded with "It's snowing in the Colorado Mountains in September!" photos and videos this morning.
That said, here's our small contribution from the Alpine Visitor Center in RMNP where 1-2" of fresh snow has fallen… #COwx #Snow pic.twitter.com/l4oKGm9Rks
— BoulderCAST Weather (@BoulderCAST) September 20, 2021
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.
We're starting our very chilly this morning, but if you can believe it we'll be nearing 80 degrees later this afternoon! #COwx #Brr #Frost #Autumn #Colorado pic.twitter.com/3aON5INqMJ
— BoulderCAST Weather (@BoulderCAST) September 22, 2021
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.
Over the last 125 years, #Boulder had never reached 90°F on September 26th. That changed yesterday…🔥#COwx #RecordHeat pic.twitter.com/gIak3eUHie
— BoulderCAST Weather (@BoulderCAST) September 27, 2021
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.
It's a chilly & gloomy morning in #Boulder at 45° right now. We should eventually see peeks of sun this afternoon with a high in the upper 50's #COwx pic.twitter.com/KC22soHSDg
— BoulderCAST Weather (@BoulderCAST) September 30, 2021
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.
September 2021 Recap Graphics:
Where does September 2021 fit into the last 12 months?
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Featured image from @AspenLodging of Twitter.
