The Weather Wire

April 2017 Volume 24 Number 4

Contents: • What happened to the snow this winter?

• Current Colorado and West-Wide Snowpack

• Drought Monitor

• March Summary/Statistics

• April Preview

• Snowfall Totals

What happened to the snow this winter?

Although we still have at least a month left of our snowfall season, it’s safe to assume at this point that snowfall for the 2016-2017 season will end up well below average across Eastern Colorado and the I-25 corridor. This has been a much different winter compared to last year, when snowfall was well above average for Eastern Colorado. But this does show that we can experience significant variation year-to-year in snowfall amounts, and occasionally some winters turn out to drier than normal, such as this one.

Outside of eastern Colorado, most of the Western U.S. has actually experienced a snowier than average winter, essentially erasing the drought west of the Continental Divide and also creating snow excellent conditions for skiers and snowboarders from the Rockies to the Sierra Nevada. So why has Eastern Colorado experienced such a poor winter for snowfall, when the rest of the west had a good winter?

There are a few reasons. During the fall season, when the first accumulating snows typically occur along the Front Range, abnormally warm and dry conditions persisted through the end of November as high pressure dominated the weather pattern, putting all of Colorado and the Rocky Mountains into a snow deficit to start the season.

From December through early February, a very active west-to-east oriented jet stream brought frequent, heavy snowfall events to the mountains west of the Divide, including Colorado, but these systems were fast-moving and typically “skipped” over the Front Range quickly, putting the I-25 corridor in a drier downslope flow, as opposed to the wetter/snowier upslope flow that occurs when a system slows down east of the Continental Divide. While we did get some snowfall events associated with arctic fronts during mid-winter, it remained dry overall across the Front Range, while the mountains along and west of the Continental Divide received well above average snowfall.

During the first three weeks of March, unseasonably warm and dry conditions were present all across Colorado as high pressure dominated the Southern Rockies. Over the final 7-10 days of the month, a more active spring pattern finally developed, bringing a

series of storms that provided much needed moisture to the I-25 corridor, but temperatures remained warmer than normal during all of these events (relative to what is typical for March storms), and as a result much of the precipitation was lost to rain or non-accumulating snow for most of the I-25 corridor outside of the higher elevation Palmer Divide.

Here is a look at snowfall this year compared to the average snowfall over the past 10 years. At DIA, only 19.3” of snow has been recorded so far this season through March 31st, and this actually goes into the record books as the least snowy October-March period on record in Denver! However, it should be noted that official records weren’t moved to DIA until the mid-2000’s, and DIA tends to underrepresent snowfall for metro Denver since it is located on the drier northeastern side.

Snowfall 2016-17 So Far 10-Year Average

Denver DIA 19.3” 52.0”

Denver Downtown 26.4” 66.7”

Golden 38.9” 96.6”

Boulder 45.8” 95.9”

Castle Rock 31.7” 89.0”

Monument Hill 42.3” 116.9”

CO Springs North 18.1” 50.6”

CO Springs Airport 15.1” 30.9”

Next, let’s take a look at this year’s snowfall compared to each of the previous 10 years for a few locations. First is for downtown Denver. Notice how much less this year’s total is compared to the previous years. The other low year, 2010-11, was similar to this year when the mountains received heavy snowfall but the plains received very little.

85.0

55.049.8

87.2

32.4

74.1 73.9

51.4

70.9

87.6

26.4

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

Downtown Denver Annual Snowfall10-Year Average: 58.8"

Next, we’ll take a look at more snowfall graphs for Boulder, Castle Rock, and northern Colorado Springs dating back to 2006-07, and will see similar distributions compared to Denver with 2010-11 the other low year similar to this year. Heavy snowfall years in the mountains don’t always mean low years on the plains and vice versa, but occasionally the pattern does set up that way, with this winter being one of those. Otherwise, we have actually see some healthy snow years in the previous 10 years aside from the two obvious low years.

116.1

72.8 75.0

123.1

41.7

93.9

123.0

90.7

115.7107.0

45.8

0.0

20.0

40.0

60.0

80.0

100.0

120.0

140.0

Boulder Annual Snowfall10-Year Average: 95.9"

141.0

71.876.8

107.0

43.7

91.5

69.1 73.5

90.8

125.2

31.7

0.0

20.0

40.0

60.0

80.0

100.0

120.0

140.0

160.0

Castle Rock Annual Snowfall10-Year Average: 89.0"

What do our snowfall forecast ranges actually mean?

During our recent storm systems involving tight snowfall gradients due to borderline temperatures for rain vs. snow, some questions have come up regarding the snowfall ranges we issue in our forecasts. For example when if we forecast “TR-2” of snow” for an area, that doesn’t just mean we expect “some areas to get exactly a trace and some areas to get exactly 2”. Instead, this range indicates we expect the majority of locations within the specified forecast zone to receive snowfall amounts falling anywhere in between those numbers, which means most areas will receive less than the high number. This is true with our other forecast amounts as well, if we forecast” 3-6” of snow then we expect the majority of locations to be within the 3-6 range.

When we specify a “trace” on the low end, that means your location may very well end up with no snow accumulating on the ground. According to the National Weather Service’s official definition of a trace: “If snow continually melts as it lands, and accumulation never reaches 0.1 inches on your measuring surface, snowfall should be recorded as a trace”, thus, be definition, even if no snow accumulates on the ground, and all snow melts that falls, it would still be a trace.

Current Colorado and West-Wide Snowpack

Warmer and drier than normal weather through most of March reduced Colorado’s mountain snowpack quite a bit, compared to mid-winter when snowpack values were well above average. As of the end of March, snowpack has decreased to near average across Northern Colorado, while snowpack remains 110-130% of average across the southern Colorado mountains, who benefited more from the late March storms.

57.8

50.8 50.2

62.1

23.8

38.235.8

54.560.0

72.6

18.1

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

Colorado Springs North Annual Snowfall10-Year Average: 50.6"

Overall, snowpack remains healthy across most of the Western U.S., but a warmer and drier than average March across the Southern Rockies has reduced the snowpack across much of Utah and Colorado compared to one month ago. Snowpack remains well above average across California, central Idaho, and western Wyoming, and snowpack is near to above average across the Pacific Northwest andNorthern Rockies.

Drought Update

Drought has intensified over the past month across much of the Front Range urban region, and the I-25 corridor from Denver north to the Wyoming border is now under a severe drought. Drought improvement did occur across portions of the eastern plains that received better moisture toward the end of March. Western Colorado remains drought free, as does most of the Western U.S. west of the Contiental Divide.

The map below shows forecasted temperature deviances for April 2017. There is a slight bias toward above normal temperatures across most of Colorado, except for the northwestern corner where there are equal chances for above or below normal temperatures.

The map below shows forecasted precipitation deviances for April 2017. There are equal chances for above or below normal precipitation across all of Colorado.

Drought conditions are expected to improve across the I-25 corridor this month, while drought conditions are expected to persist across the southeast plains of Colorado. Drought removal is also likely across the Black Hills of South Dakota, while most of the remainder of the Western U.S. should remain drought-free.

March Summary

March 2017 was the third warmest March on record and tied for the least snowiest March on record in Denver, although precipitation ended up close to average. A strong ridge of high pressure was the dominant weather feature over the first three weeks of the month, which resulted in abnormally warm and dry weather. The final 8 days of the month featured a pattern change to much more active weather, with several systems bringing much-needed moisture to eastern Colorado. However, cold air was lacking with all of these systems, which resulted in marginal temperatures for snow at the lower elevations. As a result, most of the precipitation fell as rain or non-accumulating snow. For the month, only a trace of snow was recorded at DIA, which was tied with 2012 for the least amount ever in March for Denver. Although DIA is not very representative of records due to its location outside of the metro area and due to the change in official records in the mid-2000’s, no measurable snowfall occurred in the city of Denver either, which is significant given that March is the snowiest month on average. For the season, DIA has only recorded 19.3” of snow, bringing the seasonal snowfall deficit to 26.6”. Temperatures for the month of March were well above average with a monthly high of 81 occurring on the 18th. The average high for the month was 64.0, which is 9.6 degrees above normal, and the average low for the month was 31.9, which is 5.5 degrees above normal. Despite the dry first three weeks of the month, a wet finish to the month resulted in 0.90” of precipitation for the month, which is right around average. South of Denver metro, areas in Douglas County experienced light snowfall amounts for the month with up to 6.0” in Castle Rock, which is still a bit below average. For monthly/annual snowfall amounts in our table at the end of the newsletter, the March totals do not include the snow event on March 31-April 1, which will instead be included for April snowfall.

March Stats

TEMPERATURE (IN DEGREES F)

AVERAGE MAX 64.0 NORMAL 54.4 DEPARTURE 9.6

AVERAGE MIN 31.9 NORMAL 26.4 DEPARTURE 5.5

MONTHLY MEAN 47.9 NORMAL 40.4 DEPARTURE 7.5

HIGHEST 81 on 3/18

LOWEST 14 on 3/1

DAYS WITH MAX 90 OR ABOVE 0 NORMAL 0.0

DAYS WITH MAX 32 OR BELOW 0 NORMAL 1.9

DAYS WITH MIN 32 OR BELOW 16 NORMAL 23.6

DAYS WITH MIN ZERO OR BELOW 0 NORMAL 0.1

TEMPERATURE RECORDS

3/23 – daily record high of 77 set

HEATING DEGREE DAYS

MONTHLY TOTAL 523 NORMAL 763 DEPARTURE -240

SEASONAL TOTAL 4325 NORMAL 5202 DEPARTURE -877

COOLING DEGREE DAYS

MONTHLY TOTAL 0 NORMAL 0 DEPARTURE 0

YEARLY TOTAL 0 NORMAL 0 DEPARTURE 0

PRECIPITATION (IN INCHES)

MONTHLY TOTAL 0.90 NORMAL 0.92 DEPARTURE -0.02

YEARLY TOTAL 1.67 NORMAL 1.70 DEPARTURE -0.03

GREATEST IN 24 HOURS 0.45 on 3/23-24

DAYS WITH MEASURABLE PRECIP. 6

SNOWFALL (IN INCHES)

MONTHLY TOTAL TR NORMAL 10.7 DEPARTURE -10.7

SEASONAL TOTAL 19.3 NORMAL 45.9 DEPARTURE -26.6

GREATEST IN 24 HOURS Tr on mult dates

GREATEST DEPTH 0

WIND (IN MILES PER HOUR)

AVERAGE SPEED 12.0 mph

PEAK WIND GUST 60 mph from the W on 3/7

MISCELLANEOUS WEATHER

NUMBER OF DAYS WITH THUNDERSTORM

0 NORMAL 0

NUMBER OF DAYS WITH HEAVY FOG 2 NORMAL 1

NUMBER OF DAYS WITH HAIL 0

NUMBER OF SUNNY DAYS 3

NUMBER OF PARTLY CLOUDY DAYS 22

NUMBER OF CLOUDY DAYS 6

AVERAGE RELATIVE HUMIDITY 43%

April Preview

April is a transition month from winter to spring in Denver, but is also one of the wettest months of the year with an average monthly precipitation of 1.71”. Periods of warm, sunny weather are often interspersed with cool, wet, and sometimes nowy storm systems that can bring beneficial moisture to eastern Colorado. April is Denver’s 5th snowiest month on average with a long-term average of 6.8”. Much of the snow that falls in Denver in April tends to be limited to grassy surfaces due to warmer average temperatures and a stronger spring solar angle. However, during more significant snow events with heavy snowfall rates (1”/hour or more), meaningful pavement accumulations can easily occur even during April. Snowfall amounts tend to vary from year to year in April and significant snowfalls are much more likely in early to mid April than over the final 10 days of the month. Last year was a big April for snowfall with 15.6” falling at DIA, and most of this actually fell in mid to late April when the pattern was more active. Prior to 2016, the next most recent April with significant snowfall occurred in 2013 with a monthly amount of 20.8” at DIA. For temperatures, the average high in Denver is 61.1 and the average low is 33.3 with a monthly average of 13 days below freezing. Temperatures can vary significantly as the record high for the month is 90 and the record low for the month is -2. For April 2017, we are expecting warmer than average temperatures and near average precipitation. Snowfall amounts are likely to end up a little below average due to the warmer temperatures expected, but odds would favor one or two accumulating snowfall events in Denver, unlike in March when most of Denver ended up with no meaningful snow.

DENVER'S APRIL CLIMATOLOGICALLY NORMAL (NORMAL PERIOD 1981-2010 DIA Data)

TEMPERATURE

AVERAGE HIGH 61.1 AVERAGE LOW 33.3 MONTHLY MEAN 47.4 DAYS WITH HIGH 90 OR ABOVE 0 DAYS WITH HIGH 32 OR BELOW 0 DAYS WITH LOW 32 OR BELOW 13 DAYS WITH LOWS ZERO OR BELOW 0

PRECIPITATION

MONTHLY MEAN 1.71”

DAYS WITH MEASURABLE PRECIPITATION 9 AVERAGE SNOWFALL IN INCHES 6.8” DAYS WITH 1.0 INCH OF SNOW OR MORE N/A

MISCELLANEOUS AVERAGES

HEATING DEGREE DAYS 529 COOLING DEGREE DAYS 1 WIND SPEED (MPH) 10.0mph WIND DIRECTION South DAYS WITH THUNDERSTORMS 2 DAYS WITH DENSE FOG 1 PERCENT OF SUNSHINE POSSIBLE 67%

EXTREMES

RECORD HIGH 90 on 4/30/1992 RECORD LOW -2 on 4/2/1975 WARMEST 56.4 in 1946, 1981 COLDEST 38.8 in 1920 WETTEST 8.42” in 1900 DRIEST 0.03” in 1963 SNOWIEST 33.8” in 1933 LEAST SNOWIEST 0.0” in 1888, 1930, 1943, 1992

Snowfall

October 2016 to April 2017

City Oct Nov Dec Jan Feb Mar Apr Total

Aurora (Central) 0.0 3.0 9.5 8.9 1.3 0.5 23.2

Boulder 0.0 4.4 13.0 18.5 9.9 0.0 45.8

Brighton 0.0 2.8 9.4 8.4 1.2 0.0 21.3

Broomfield 0.0 3.5 9.9 14.6 3.6 0.0 31.6

Castle Rock 0.0 5.2 7.1 7.8 5.6 8.0 33.7

Colo Sprgs Airport 0.0 0.0 6.4 6.3 0.7 1.7 15.1

Denver DIA 0.0 1.7 9.7 6.3 1.6 0.0 19.3

Denver Downtown 0.0 3.0 10.4 11.9 1.1 0.0 26.4

Golden 0.0 4.5 16.0 11.9 5.5 1.1 39.0

Fort Collins 0.0 3.0 8.2 11.1 4.0 0.0 26.3

Highlands Ranch 0.0 3.7 7.3 8.4 3.8 1.1 24.3

Lakewood 0.0 2.5 12.5 8.8 2.6 0.0 26.4

Littleton 0.0 3.5 11.2 9.0 4.5 0.0 28.2

Parker 0.0 3.5 8.1 6.9 4.0 3.9 26.4

Sedalia - Hwy 67 0.0 4.0 11.8 10.5 5.8 4.9 37.0

Thornton 0.0 2.9 11.9 10.5 2.9 0.0 28.2

Westminster 0.0 4.0 12.0 13.6 3.0 0.0 32.6

Wheat Ridge 0.0 3.5 14.4 13.1 1.6 0.0 32.6

Skyview Weather 2350 N Rocky View Rd Castle Rock, CO 80108 Phone: (303) 688-9175 Fax: (303) 380-3338 E-mail: Tim@SkyviewWeather.com On the web at www.SkyviewWeather.com

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