September 8-9, 2020 Early Season Snowfall & Record Cold

Overview

A strong upper level low brought an end to record heat, and provided Denver its second earliest measurable snowfall on record. Numerous heat records were set leading up to the snowfall, and several new snowfall and cold records were also broken in this abrupt bout with winter.

On September 5, 2020, Denver set its all time record high for September, reaching 101 degrees during the afternoon. This was also the latest date a 100 degree reading has ever been observed in Denver. Another daily record high was then tied on September 6 when Denver hit 97 degrees. September 7th was the last day of heat across the eastern plains of Colorado, when Denver's high temperature reached 93. That tied us for the record for the number of 90 degree days for a year (73), and was also the warmest temperature ever recorded before a day of measurable snowfall.

By the evening of September 7th, a series of cold fronts progressed southward from Wyoming into Colorado, dropping the temperature down to the low 30s by early Tuesday morning. Snow developed across the Front Range and foothills overnight, while a mix of rain and snow developed along the I-25 metro corridor. A few locations picked up light snow accumulations in the morning. Accumulating snow was mostly confined to the higher elevations much of the day Tuesday, before spreading across the plains during the late afternoon and evening. Snow spread across the far eastern plains overnight, and tapered off by Wednesday morning.

Total snowfall accumulations were 1 to 3 inches across much of the eastern plains, 3 to 6 inches for cities along the mountains (including Boulder and the western portions of Fort Collins), and 4 to 10 inches for the mountains and foothills of the Front Range. A total of 5.6 inches of snow was measured at the NWS Boulder office, while Denver officially measured a total of 1.0 inches.

Snow on the NOAA DSRC Campus Tuesday evening

Records Set
Photos
Radar & Satellite
Storm Reports
Environment

Records Set

Heat

Record high of 101°F set on 9/5. Highest temperature ever recorded in September, and latest date of 100° reading.
Record high of 97° set on 9/6.

Cold

Tied earliest freeze on record (Previously set September 8th, 1962)
Tied record low on 9/8 (31°F, last set 1962)
Tied record low on 9/9 (31°F, last set 1962)
Record coldest maximum temperature on 9/9 (43°F). Previous record was 53°F set in 1989.
Tied record low on 9/10 (35°F, last set 1898)
Largest 2 calendar day temperature drop in September (62°F drop). Previous record was 59°F set in 1993.

Snow

Tied the second earliest snowfall on record for Denver (9/8). Earliest snowfall on record is 9/3/1961.
Daily snowfall record set for 9/8 (1.0 inch fell). Previous record was 0.7" in 1962.
Daily snowfall record tied for 9/9 (Trace). Previous record was a Trace in 1962.
Snapped a streak of 19 consecutive years without September snow in Denver.

Other Notable Records

Warmest temperature ever recorded the day before a measurable snowfall (93°F on 9/7). Previous high was 92°F on September 12, 1993.
Shortest number of days between a 100 degree day and measurable snowfall (3). Previous record was 38 days in 2019.
Number of days (2) between a record high and record low in Denver (September 6 to September 8). Ties previous record from August 15 -17 in 1960.

Photos

Overview


Trail Ridge Road in Rocky Mountain National Park Photo via Twitter (@RockyNPS)	Several tree branches and small trees were knocked down across Boulder Photo via NWS Boulder	Snow at the NOAA campus in Boulder Photo via NWS Boulder	Snowfall measured in Boulder Photo via NWS Boulder

Radar & Satellite

Overview


Water Vapor Satellite loop from GOES-16. Images from CIRA-CSU RAMMB-Slider	GeoColor Satellite view of the storm near peak intensity, Tuesday evening. Image from CIRA-CSU	Radar at 7:30 PM Tuesday. This was near the time the heaviest snow fell in both Boulder and Denver.

Storm Reports

Storm total snowfall for Northeast Colorado. Snowfall analysis from NOHRSC.



...SNOWFALL REPORTS FOR SPECIFIC LOCATIONS...

Location                     Amount    Time/Date                            

...Colorado...

...Adams County...
2 NE Thornton                2.2 in    0503 AM 09/09                        
2 NW Strasburg               2.0 in    1058 AM 09/09                        
3 SW Todd Creek              2.0 in    1000 AM 09/09                        
1 ENE Westminster            2.0 in    0935 AM 09/09                        
1 NNW Federal Heights        2.0 in    0737 AM 09/09                        
Thornton                     1.8 in    0928 AM 09/09                        
1 NE Northglenn              1.7 in    0700 AM 09/09                        
4 NNW Aurora                 1.5 in    0700 AM 09/09                        
5 S Barr Lake                1.0 in    0630 AM 09/09                        

...Arapahoe County...
5 ESE Arapahoe Park          4.0 in    0727 AM 09/09                        
4 ESE Foxfield               2.1 in    0600 AM 09/09                        
Aurora 2.7 WSW               1.8 in    0519 AM 09/09                        
4 SSW Buckley AFB            1.5 in    0700 AM 09/09                        
2 ESE Buckley AFB            1.3 in    0700 AM 09/09                        
Denver 6.3 SSE               0.8 in    0630 AM 09/09                        

...Boulder County...
1 WNW Eldorado Springs       8.9 in    0724 AM 09/09                        
3 SSW Boulder                5.6 in    0945 AM 09/09                        
3 SSW Boulder                5.5 in    0745 AM 09/09                        
2 E Sunshine                 5.3 in    0739 AM 09/09                        
2 NE Rollinsville            4.2 in    0700 AM 09/09                        
2 WNW Boulder                4.0 in    0700 AM 09/09                        
2 N Longmont                 1.7 in    0700 AM 09/09                        
1 NW Longmont                1.7 in    0627 AM 09/09                        
1 WNW Niwot                  1.6 in    0802 AM 09/09                        
3 NE Longmont                1.5 in    0800 AM 09/09                        
1 NNW Louisville             1.5 in    0730 AM 09/09                        
3 SSE Hygiene                1.5 in    0700 AM 09/09                        
3 SW Niwot                   1.5 in    0700 AM 09/09                        
Longmont 1 E                 1.4 in    0707 AM 09/09                        

...Broomfield County...
1 NW Broomfield              1.8 in    0824 AM 09/09                        
3 E Lafayette                1.7 in    0700 AM 09/09                        

...Denver County...
1 ENE Mountain View          3.0 in    0758 AM 09/09                        
2 NW Sheridan                2.5 in    0700 AM 09/09                        
Denver 5.3 SW                2.5 in    0607 AM 09/09                        
2 ESE Denver                 1.7 in    0700 AM 09/09                        
5 SE Commerce City           1.0 in    0700 AM 09/09                        
1 SSE Denver Intl Airport    1.0 in    0600 AM 09/09                        
2 N Englewood                0.7 in    0700 AM 09/09                        

...Douglas County...
3 WSW Roxborough Park        6.0 in    0700 AM 09/09                        
4 S Greenland                5.5 in    0721 AM 09/09                        
1 SSE Kassler                3.5 in    0800 AM 09/09                        
3 NE Castle Rock             2.8 in    0838 AM 09/09                        
2 WSW Lone Tree              2.7 in    0740 AM 09/09                        
1 ESE The Pinery             2.5 in    0645 AM 09/09                        

...Elbert County...
1 WNW Ponderosa Park         4.0 in    0657 AM 09/09                        

...Gilpin County...
3 NNE Aspen Springs          3.8 in    1004 AM 09/09                        

...Jefferson County...
1 SSW Crescent Village       9.0 in    0928 AM 09/09                        
2 SW Golden                  7.1 in    0700 AM 09/09                        
1 NW Genesee                 6.0 in    0630 AM 09/09                        
2 ESE Evergreen              5.0 in    0736 AM 09/09                        
4 WNW Arvada                 4.7 in    0945 AM 09/09                        
Wah Keeney Park              4.5 in    0954 AM 09/09                        
3 NW Arvada                  4.4 in    0700 AM 09/09                        
3 NW Arvada                  4.4 in    0659 AM 09/09                        
1 ESE Ken Caryl              4.0 in    0600 AM 09/09                        
2 ESE Lakewood               3.8 in    0600 AM 09/09                        
1 NNE Evergreen              3.3 in    0700 AM 09/09                        
Evergreen 1.8 N              3.3 in    0600 AM 09/09                        
1 SSE Evergreen              3.0 in    0818 AM 09/09                        
2 NNW Golden                 2.8 in    0830 AM 09/09                        
4 W Arvada                   2.6 in    0715 AM 09/09                        
1 NE Arvada                  2.0 in    0755 AM 09/09                        
1.2 SW Kittredge             2.0 in    0500 AM 09/09                        

...Larimer County...
2 S Estes Park               6.5 in    0700 AM 09/09                        
Estes Park                   6.0 in    0630 AM 09/09                        
1 SSE Estes Park             6.0 in    0630 AM 09/09                        
1 NW Pinewood Springs        5.8 in    0700 AM 09/09                        
2 SSE Estes Park             5.5 in    0657 AM 09/09                        
1 NNE Estes Park             5.0 in    1040 AM 09/09                        
4 W Pinewood Springs         4.5 in    0700 AM 09/09                        
Fort Collins 2.8 W           4.2 in    0400 AM 09/09                        
2 SW Fort Collins            4.0 in    0700 AM 09/09                        
Fort Collins                 2.7 in    0700 AM 09/09                        
2 NNW Loveland               2.4 in    0600 AM 09/09                        
3 ENE Laporte                2.1 in    0700 AM 09/09                        
Campion                      2.0 in    0700 AM 09/09                        
4 NNE Timnath                2.0 in    0700 AM 09/09                        
2 N Campion                  2.0 in    0700 AM 09/09                        
4 E Laporte                  0.8 in    0700 AM 09/09                        
2 NNE Loveland               0.1 in    0700 AM 09/09                        

...Lincoln County...
1 NW Karval                  4.0 in    0700 AM 09/09                        

...Logan County...
1 SSW Sterling               2.0 in    0739 AM 09/09                        

...Morgan County...
0.8 E Gary                   0.3 in    0600 AM 09/09                        

...Park County...
2 E Antero Junction          5.5 in    0846 AM 09/09                        

...Phillips County...
3 SW Holyoke                 2.1 in    0700 AM 09/09                        

...Summit County...
2 NNE Frisco                 4.0 in    0700 AM 09/09                        
8 SSW Copper Mountain        3.0 in    0800 AM 09/09                        

...Weld County...
New Raymer 21 N              2.0 in    0600 AM 09/09                        
7 NE Nunn                    1.8 in    0700 AM 09/09                        
2 E Galeton                  1.1 in    0926 AM 09/09                        
4 NE Firestone               1.0 in    0700 AM 09/09                        
2 NNE Erie                   0.8 in    0800 AM 09/09                        
1 NE Dacono                  0.8 in    0730 AM 09/09                        
1 W Firestone                0.8 in    0700 AM 09/09                        
&&

Environment

A strong shortwave dove southward from the Canadian Rockies into the Four Corners region. This upper low was quite rare for the area, as evidenced by standardized anomalies of over -4 sigma!


Figure 1: 500mb Heights & Standardized Anomalies	Figure 2: 250mb Heights, Wind, & Standardized Wind Anomalies	Figure 3: NAEFS Mean 700mb Temperature and Climatological Percentile. Note that much of Colorado is at a record minimum for the climate period.

Observed Soundings near Denver:


Figure 7: Denver sounding released at 0000 UTC September 8th (Monday evening)	Figure 8: Denver sounding released at 1200 UTC September 8th (Tuesday Morning)	Figure 9: Denver sounding released at 0000 UTC September 9th (Tuesday Evening)

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