The Weather Wire

December 2016 Volume 23 Number 12

Contents: A Look Back: December 2006 Snowstorms

Current Colorado and West-Wide Snowpack

Drought Monitor

November Summary/Statistics

December Preview

Snowfall Totals

A Look Back: December 2006 Snowstorms

This month will mark the 10th anniversary of one of the great snowstorms in Colorado’s history. December 2006 ended up as the third snowiest December ever recorded in Denver with 29.7” of snow, with much higher totals across other parts of the metro area. A large portion of the month’s snow fell during a blizzard that paralyzed northeast Colorado on December 20-21, just in time for the busy holiday second, but then a second impressive system brought another round of significant snowfall on the 28th.

Below is a surface analysis overlaid with satellite and radar from December 20, 2006, which depicts a surface low pressure center over southeast Colorado – a prime location to tap into Gulf of Mexico moisture, which combined with colder air arriving from the north and upslope flow created the perfect recipe for a major snow event in northeast Colorado.

The December 20-21st storm would go down as the 9th largest snowstorm in Denver’s history with 20.7” falling at DIA. This was also the third largest snowstorm ever in December, behind only the great blizzards of 1913 and 1982. Snowfall totals across the Denver/Boulder/Ft. Collins metro areas generally ranged from 1.5-3 feet, with some locations in the foothills of Boulder and Larimer Counties in excess of 3 feet. Below is a map of the snowfall totals across Northeast Colorado from this storm, created by NWS Denver/Boulder.

Just as the region was beginning to dig out from the December 20-21 blizzard around the Christmas holiday, a second impressive storm loomed just one week later. While it is not unusual to have back-to-back storms, it is somewhat unusual to have back-to-back storms of such an impressive magnitude in December, and to have a lack of melting between storms.

Typically for the lower elevations along the Front Range, there is a melting period before the next storm system moves into the area, as south/southwesterly winds ahead of approaching systems bring warmer air into the region, and result in localized downslope conditions from the higher elevations in which air is further warmed through compression. However, things were a little different between these two storms. The low solar angle of late December combined with a strong inversion due to heavy snow cover allowed cold temperatures to persist across the lower elevations before the second system arrived.

The second storm was actually much weaker in intensity than the first storm, but still produced an impressive 8.0” at DIA with up to 2 feet falling in/near the foothills, including the Boulder area. As if this storm weren’t enough, another respectable system in early January brought widespread accumulating snow and set the stage for a cold winter with unusually persistent snowpack. Overall, Denver would have 61 consecutive days of snow cover, 2nd longest on record and just missing out on the longest streak of 63 days!

Current Colorado and West-Wide Snowpack

Snowpack is currently well below average across all of Colorado, due to an unseasonably warm and dry fall that persisted through mid/late November. The pattern has become colder and more active over the past couple of weeks, which has allowed the mountain snowpack to make up ground since mid November. Although snowpack is below average, it is still very early in the snow season, and with an active pattern expected heading into December, mountain snowpack should continue to recover toward average.

Like Colorado, much of the Western U.S. experienced a slow start to the snow season as persistent high pressure dominated the first half of November. Snowpack is currently below average across most of the West as a result, but snowfall has been picking up over the past two weeks for many areas, with some areas now above average, such as Arizona, New Mexico, southern Utah, and the Sierra Nevada in California. The Black Hills of eastern Wyoming are now well above average following a major snow event in late November.

Drought Update

Areas of moderate drought have now expanded to include nearly the entire I-25 corridor in Colorado, Front Range foothills, and much of the eastern plains. Areas of severe drought have also developed over much of Larimer County. Elsewhere, areas of extreme to exceptional drought continue across parts of central/southern California, and extreme to exceptional drought has also expanded across much of the southeastern U.S. where large wildfires have been burning recently.

The map below shows forecasted temperature deviances for December 2016. There is a a moderate bias toward below normal temperatures across northern and western Colorado, and a slight bias toward below normal temperatures across most of southern and eastern Colorado.

The map below shows forecasted precipitation deviances for December 2016. There is a slight bias toward above normal precipitation across most of Colorado, except for the far northern mountains where there is a stronger bias toward above normal precipitation, and the southeast plains where there are equal chances of above or below normal precipitation.

Drought conditions are expected to persist across the Front Range and eastern plains of Colorado, as well as across California and western Nevada. Drought improvement is expected across much of the southeastern U.S.

November Summary

November 2016 was much warmer than average and snowfall was well below average. Persistent high pressure dominated the weather pattern through the first half of the month, continuing the trend of unseasonable warmth from October. A colder and more active pattern over the second half of the month brought more seasonal weather at times, but overall this month ended up as the 10th warmest November on record. The average high for the month was 59.9, which is 7.8 degrees above normal, and the average low for the month was 30.4, which is 5.9 degrees above normal. A stretch of unseasonable warmth during the middle of the month resulted in a high temperature of 80 degrees at DIA on the 16th. This was the latest 80-degree day ever officially recorded in Denver (keep in mind, the official records were moved from Stapleton to DIA less than 20 years ago), and also tied the all-time record high for November, which was previously set in 2006. The weather pattern finally changed the very next day, with the first snow of the season occurring on the 17th with 1.7” falling at DIA. This would be the only measurable snow to fall during the month, well below the November average of 8.7”. This was also one of the latest first snowfalls ever recorded in Denver, but avoided matching the record latest date which is on November 21st. For the season, DIA now has a snowfall deficit of 12.3”. Overall, precipitation was slightly below average with a total of 0.52” for the month. The wettest day of the month occurred on the 22nd with 0.30” of precipitation, most of which fell as rain with just a trace of snow. Colder temperatures would arrive at the end of the month, with the low temperature for the month of 10 occurring on the 30th.

November Stats

TEMPERATURE (IN DEGREES F)

AVERAGE MAX 59.9 NORMAL 52.1 DEPARTURE 7.8

AVERAGE MIN 30.4 NORMAL 24.5 DEPARTURE 5.9

MONTHLY MEAN 45.1 NORMAL 38.3 DEPARTURE 6.8

HIGHEST 80 on 11/16

LOWEST 10 on 10/30

DAYS WITH MAX 90 OR ABOVE 0 NORMAL 0.0

DAYS WITH MAX 32 OR BELOW 1 NORMAL 2.3

DAYS WITH MIN 32 OR BELOW 15 NORMAL 23.4

DAYS WITH MIN ZERO OR BELOW 0 NORMAL 0.6

TEMPERATURE RECORDS

11/16 – daily record high of 80 set, tied all-time monthly record high

HEATING DEGREE DAYS

MONTHLY TOTAL 586 NORMAL 801 DEPARTURE -215

SEASONAL TOTAL 896 NORMAL 1382 DEPARTURE -486

COOLING DEGREE DAYS

MONTHLY TOTAL 0 NORMAL 0 DEPARTURE 0

YEARLY TOTAL 878 NORMAL 769 DEPARTURE 109

PRECIPITATION (IN INCHES)

MONTHLY TOTAL 0.52 NORMAL 0.61 DEPARTURE -0.09

YEARLY TOTAL 11.07* NORMAL 13.95 DEPARTURE -2.88*

GREATEST IN 24 HOURS 0.30 on 11/22

DAYS WITH MEASURABLE PRECIP. 2

*Error on 3/23 due to undercatch from snow

SNOWFALL (IN INCHES)

MONTHLY TOTAL 1.7 NORMAL 8.7 DEPARTURE -7.0

SEASONAL TOTAL 1.7 NORMAL 14.0 DEPARTURE -12.3

GREATEST IN 24 HOURS 1.7 on 11/17

GREATEST DEPTH 2” on 11/18

WIND (IN MILES PER HOUR)

AVERAGE SPEED 9.8 mph

PEAK WIND GUST 43 mph from the W on 11/27

MISCELLANEOUS WEATHER

NUMBER OF DAYS WITH THUNDERSTORM

0 NORMAL 0

NUMBER OF DAYS WITH HEAVY FOG 1 NORMAL 1

NUMBER OF DAYS WITH HAIL 0

NUMBER OF SUNNY DAYS 14

NUMBER OF PARTLY CLOUDY DAYS 12

NUMBER OF CLOUDY DAYS 4

AVERAGE RELATIVE HUMIDITY 45%

December Preview

December is the first month of meteorological winter and is the coldest month of the year in Denver, with temperatures just slightly colder on average than January. December is also the third snowiest month of average in Denver, behind March and November, and some of the most notable snow events in Denver’s history have occurred during this month. December can feature a wide range of weather conditions, including arctic cold fronts that result in bitterly cold temperatures, although cold snaps in Denver tend to be relatively short-lived. Stretches of mild weather are also common during December, and it’s not unusual to experience days with highs in the 60s. The average high in Denver during December is 42.8 and the average low is 17.1. On average, there are six days with a high temperature of 32 or below and two days with temperatures below zero. The warmest temperature on record for the month is 79, which occurred in 1939, and the coldest temperature on record for the month is -25, which occurred in 1876 and 1990. The coldest month ever recorded in Denver occurred in December 1983 when the average overall temperature was a bitterly cold 17.4 degrees! Denver averages three days of snowfall greater than 1” in December, with an overall monthly average of 8.5” of snow. There have been some exceptional snow events in December in years’ past, most notably in 1913, which featured Denver’s biggest snowstorm on record with 45.7” from December 1-5! December 1913 was also the snowiest month on record in Denver with a total of 57.4”. Other notable Denver snowstorms include the 1982 Christmas Eve blizzard, which produced 23.8” and the December blizzard of 2006, which produced 20.7”. Snow that does fall in December tends to accumulate more easily to roadways and does not melt as quickly compared to other times of the year due to colder average temperatures and the lowest solar angle of any month. For December 2016, we are expecting colder than normal temperatures and above average snowfall as an active pattern sets up across the Western U.S., in stark contrast to what we experienced in November! Overall, we expect a total of 2-4 snow events. We may not end up with any major snow events, rather a higher frequency of light/moderate snow events accompanied by cold air masses that would create travel problems due to snow more easily accumulating on pavement.

DENVER'S DECEMBER CLIMATOLOGICALLY NORMAL

(NORMAL PERIOD 1981-2010 DIA Data)

TEMPERATURE

AVERAGE HIGH 42.8 AVERAGE LOW 17.1 MONTHLY MEAN 30.0 DAYS WITH HIGH 90 OR ABOVE 0 DAYS WITH HIGH 32 OR BELOW 6 DAYS WITH LOW 32 OR BELOW 29 DAYS WITH LOWS ZERO OR BELOW 2

PRECIPITATION

MONTHLY MEAN 0.35”

DAYS WITH MEASURABLE PRECIPITATION 4 AVERAGE SNOWFALL IN INCHES 8.5”

DAYS WITH 1.0 INCH OF SNOW OR MORE 3

MISCELLANEOUS AVERAGES

HEATING DEGREE DAYS 1086 COOLING DEGREE DAYS 0 WIND SPEED (MPH) 8.4mph WIND DIRECTION South DAYS WITH THUNDERSTORMS 0 DAYS WITH DENSE FOG 1 PERCENT OF SUNSHINE POSSIBLE 67%

EXTREMES

RECORD HIGH 79 on 12/5/1939 RECORD LOW -25 on 12/24/1876, 12/22/1990 WARMEST 43.8 in 1933 COLDEST 17.5 in 1983 WETTEST 5.21” in 1913 DRIEST 0.00” in 1881 SNOWIEST 57.4” in 1913 LEAST SNOWIEST 0.0” in 1995

Snowfall

October 2016 to April 2017

City Oct Nov Dec Jan Feb Mar Apr Total

Aurora (Central) 0.0 3.0 3.0

Boulder 0.0 2.8 2.8

Brighton 0.0 2.8 2.8

Broomfield 0.0 3.5 3.5

Castle Rock 0.0 5.2 5.2

Colo Sprgs Airport 0.0 TR 0.0

Denver DIA 0.0 1.7 1.7

Denver Downtown 0.0 3.0 3.0

Golden 0.0 4.5 4.5

Fort Collins 0.0 3.0 3.0

Highlands Ranch 0.0 3.7 3.7

Lakewood 0.0 2.5 2.5

Littleton 0.0 3.5 3.5

Parker 0.0 3.5 3.5

Sedalia - Hwy 67 0.0 4.0 4.0

Thornton 0.0 2.9 2.9

Westminster 0.0 4.0 4.0

Wheat Ridge 0.0 3.5 3.5

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