What Drives Snowstorms and Seasonal Snowfall Amounts in Colorado?

What drives snowstorms and seasonal snowfall amounts in Colorado?

Russ S. Schumacher Colorado State Climatologist Director, Colorado Climate Center Department of Atmospheric Science, Colorado State University

Along with: Becky Bolinger, Peter Goble, Zach Schwalbe, Nolan Doesken

Western Snow and Ice Conference 27 September 2018 Annual average precipitation

Data: PRISM climate group prism.oregonstate.edu

COLORADO CLIMATE CENTER Cheyenne Ridge

Platte Valley

Palmer Divide

Arkansas Valley

Raton Mesa

http://www.joeandfrede.com/colorado/misc_trip_reports/colorado_topo_med_res.png COLORADO CLIMATE CENTER What are our data sources?

COLORADO CLIMATE CENTER NWS Cooperative observer program – long-term measurements

COLORADO CLIMATE CENTER COLORADO CLIMATE CENTER National Weather Service automated stations

Photo of an ASOS station

Map showing NWS stations (from mesowest.utah.edu) COLORADO CLIMATE CENTER “Mesonets” Oklahoma Mesonet http://www.mesonet.org/

COLORADO CLIMATE CENTER COLORADO CLIMATE CENTER CoAgMET

❑ 75 stations ❑ 10 coming soon on west slope ❑ 44 5-minute stations

❑ interactive mapping through eRAMS ❑ includes ❑ time series charts ❑ site photos coagmet.colostate.edu Note: precipitation data is only good in the warm season! COLORADO CLIMATE CENTER CoAgMET

❑ 75 stations ❑ 10 coming soon on west slope ❑ 44 5-minute stations

❑ interactive mapping through eRAMS ❑ includes ❑ time series charts ❑ site photos coagmet.colostate.edu COLORADO CLIMATE CENTER CoAgMET

❑ 75 stations ❑ 10 coming soon on west slope ❑ 44 5-minute stations

❑ interactive mapping through eRAMS ❑ includes ❑ time series charts ❑ site photos coagmet.colostate.edu National Weather Service automated stations

Map showing NWS stations (from mesowest.utah.edu) COLORADO CLIMATE CENTER NWS stations + CoAgMET + other networks

(from mesowest.utah.edu)

COLORADO CLIMATE CENTER The problem with most of these networks: no reliable measurements of frozen precipitation!

COLORADO CLIMATE CENTER SNOTEL (Snow telemetry)

Tower SNOTEL site (10,500 ft, near Steamboat) https://www.wcc.nrcs.usda.gov/siteimages/825.jpg

Note: not current data!! COLORADO CLIMATE CENTER COLORADO CLIMATE CENTER

Precipitation from the Tower SNOTEL site

Nearly 60” of precipitation (liquid equivalent) each year! Almost all comes from snow at this elevation Plus the “old-fashioned way” A different kind of snowboard

https://denver.cbslocal.com/2015/12/15/theres-more-to- measuring-snow-than-just-finding-out-how-many-inches- fell/

COLORADO CLIMATE CENTER Putting it all together…

Data: PRISM climate group prism.oregonstate.edu

COLORADO CLIMATE CENTER In every month, it’s the wettest month somewhere in Colorado!

Data: PRISM climate group prism.oregonstate.edu

COLORADO CLIMATE CENTER What do we want to know about the snow? • In Colorado, we’re usually most concerned about snow-water equivalent (SWE): this is what provides the water for rivers, reservoirs, agriculture, drinking water, etc., etc. • The general public mostly wants to know how deep it will be • But in your business, surely it’s not just the depth, but also other characteristics: – Wet or dry – How fast is it accumulating? – Drifting? • And these things are difficult to measure, and even harder to predict!

COLORADO CLIMATE CENTER COLORADO CLIMATE The “forecast funnel” CENTER The “forecast funnel”

• In weather forecasting, we are always thinking about the different “scales” (both time and space) that are important to whatever it is we’re trying to predict • We’ll usually start at the very large scales, and then “funnel” down to the smaller scales • Let’s use that approach today for understanding snowstorms… COLORADO CLIMATE CENTER Scales of motion in the atmosphere

Planetary scale: broad, often slow-moving waves through the middle and high latitudes But fast-moving winds in the jet streams through those waves!

earth.nullschool.net COLORADO CLIMATE CENTER Scales of motion in the atmosphere

Synoptic scale: the jet stream, large storm systems (cyclones)

earth.nullschool.net COLORADO CLIMATE CENTER Scales of motion in the atmosphere

Mesoscale (middle scale): fronts; thunderstorms; interactions with mountain ranges and land/sea contrasts

weather.cod.edu/satrad/exper COLORADO CLIMATE CENTER Scales of motion in the atmosphere

Microscale: local effects owing to small variations in terrain; land cover; instabilities in the atmosphere; etc. Seasonal drivers: El Niño-Southern Oscillation (ENSO) • One of the biggest drivers of seasonal climate over North America is ENSO, the periodic variation of ocean temperatures in the Pacific Ocean • Its effect is to shift where the jet stream tends to be, how wavy it is, etc. • We are currently in neutral conditions, but most forecasts point to El Niño conditions being in place by late fall/early winter

COLORADO CLIMATE CENTER COLORADO Example of a strong El Niño (1997-98) CLIMATE CENTER Seasonal drivers: El Niño-Southern Oscillation (ENSO)

El Niño refers to the waters in the central and eastern Pacific being warmer than normal (La Niña is when waters in this region are cooler than normal).

In turn, this affects where large thunderstorm complexes develop in the tropics, which then affects the jet stream pattern over North America COLORADO CLIMATE CENTER Seasonal drivers: El Niño-Southern Oscillation (ENSO)

El Niño refers to the waters in the central and eastern Pacific being warmer than normal (La Niña is when waters in this region are cooler than normal).

In turn, this affects where large thunderstorm complexes develop in the tropics, which then affects the jet stream pattern over North America What about for Colorado?

• We sit in between the more robust signals of warm/dry to the north, and cool/wet to the south • Therefore, the ENSO phase just isn’t that reliable of a predictor for our seasonal snowfall totals – If the jet stream position (on average) pushes slightly one direction or the other, it can have a big influence on our winter! • Last winter was La Niña conditions, and it turned out to follow the usual pattern: relatively cool and snowy to the north, and warm and dry to the south • But El Niño winters seem to have even more variability...

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Correlation of ENSO phase to seasonal snowfall

Fort Collins: a slight tendency for snowier winters during El Niño, but lots of variability COLORADO CLIMATE CENTER

Correlation of ENSO phase to seasonal snowfall

Similar at Denver: slight tendency for snowier winters during El Niño, but lots of variability COLORADO CLIMATE CENTER

Correlation of ENSO phase to seasonal snowfall

In most mountain areas, the correlation is even weaker At Copper Mountain, a very slight tendency toward less-snowy winters during El Niño COLORADO CLIMATE CENTER

Correlation of ENSO phase to seasonal snowfall

The one mountain area with a reasonably strong signal is in the Sangre de Cristos, which usually see more snow in El Niño winters COLORADO Averaged over the Upper Colorado River Basin CLIMATE CENTER

Wet years tend to come during El Niño, but it’s no guarantee... ENSO phase

“A case study of a wet and a dry year (with similar ENSO/PDO conditions) shows that the occurrence of a few large accumulating events is what drives the seasonal variability” ENSO phase -Bolinger et al. (2014, Journal of Climate) COLORADO CLIMATE CENTER What about big snowstorms along the Front Range?

At Fort Collins (and similar for other Front Range locations), big snowstorms occur more frequently during El Niño winters, but the relationship isn’t particularly strong El Niño summary

• The phase of ENSO does slightly tilt the odds in terms of snow seasons in Colorado: – Southern Colorado tends to be cooler and snowier than average – The Front Range and Plains are somewhat more likely to see a big snowstorm – The relationship in the central mountains is not especially strong • But there are many more factors than ENSO that affect the frequency of snowstorms, and the seasonal snow accumulations in Colorado (as opposed to north and south of us, where connections are stronger)

COLORADO CLIMATE CENTER COLORADO CLIMATE CENTER

Winter outlook from Climate Prediction Center

December-January-February outlook shows slightly increased chances of warmer and wetter conditions compared to normal COLORADO CLIMATE CENTER

Winter outlook from Climate Prediction Center

December-January-February outlook shows slightly increased chances of warmer and wetter conditions compared to normal COLORADO CLIMATE CENTER

Synoptic-scale drivers: how does a big Front Range snowstorm come together?

Let’s look at the March 2003 snowstorm: the biggest one in recent memory Top: upper- level winds and vorticity (“spin”) March 14 (3 days before):

Warm on the plains, no sign yet of the storm Bottom: lower- at upper levels level winds and temperature Top: upper- level winds and vorticity March 15 (2 (“spin”) days before):

Still warm on the plains, the first signs of the wave that will develop Bottom: lower- into the big level winds storm and temperature Top: upper- level winds and vorticity March 16 (1 (“spin”) day before):

Still warm on the plains, wave just starting to develop Bottom: lower- level winds and temperature Top: upper- level winds March 17 (day and vorticity the storm (“spin”) starts):

Upper-level storm just now developed over the 4 corners

Bottom: lower- Surface level winds cyclone over and southeast temperature Colorado Top: upper- level winds March 18 and vorticity (snow (“spin”) continues):

Upper-level low has hardly moved

Surface Bottom: lower- cyclone level winds remains over and southeast temperature Colorado Water vapor loop of March 2003 storm, courtesy Dan Lindsey COLORADO CLIMATE CENTER “Plunger” lows Upper-level low over 4 corners Surface low over southeast CO Although the March 2003 storm was a particularly extreme case, situations where the upper-level low pressure “plunges” southward are the most common pattern for Front Range snowstorms The fact that the storm system doesn’t even exist 1- 2 days before the snow flies makes the forecasting difficult! COLORADO CLIMATE CENTER CONTRASTING SURFACE PRESSURE PATTERNS FOR DENVER AND FORT COLLINS SNOWSTORMS Local effects

FCL experiences FCL experiences downslope flow off upslope flow south of We have many localized Cheyenne Ridge SURFACE PRESSURE Cheyenne Ridge influences on snowfall in Colorado because of our FCL FCL mountains and other ● ● DEN DEN terrain variations! DEN experiences upslope north of Palmer Lake Divide DEN experiences downslope off Palmer Lake Divide

DEN SNOWSTORM FCL SNOWSTORM

Wesley et al. NWA Journal of Operational Meteorology 30 April 2013

Surface observations at 1800 UTC 17 March are shown in Fig. 12a, at a time when blocking of easterly winds (over eastern Colorado and southeastern Wyoming) was already in place and northerly flow was developing along the Colorado Front Range. Meanwhile, a surge of colder northerly flow was evident over eastern Wyoming, with a nose of stronger winds oriented at 360º. Postfrontal cold advection within the northerly flow characterized the surface observations from about 1800 UTC 17 March to 0500 UTC 18 March [unlike the Marwitz and Toth (1993) event, where little to no cold advection was measured] as well as the 800-mb model-analyzed fields through about 0600 UTC 19 March (a representative 800-mb analysis was shown in Fig. 10b). Representative calculations of temperature advection ranged from 0.7 to 2.5°C h-1 at the surface and 800 mb between 1800 UTC 17 March and 0000 UTC 19 March. The Barrier jets and cold-air damming cold advection complicated the situation and c•learSometimes,ly cold air near the surface will pile up east of the Front Range, and lift of the did not indicate that the northerly flow regime wasupslope a flow over the cold air can provide an additional source of ascent farther out on classic barrier jet, as in the Parish (1982) study. the Plains; along with sufficient cooling to allow for snow The surface-based cold-air dome, which developed late on 17 March, and the enhanced northerly flow were in place by 0000 UTC 18 March. This structure was dominant in the METAR observations shown in Fig.10a at 0000 UTC 19 March (note the urban corridor and foothill observations of enhanced flow parallel to the north south barrier). Most surface observations along the urban corridor at that time indicated sustained northerly winds around 5 10 m s-1 (10 20 kt). Generally, northerly winds of 5 13 m s-1 (10 25 kt) occurred during the period of extremely heavy snowfall from 0000 UTC 18 March through FiguFromre 12 Wesley. (a, to pet) Sal.u rf(2013),ace M studyETAR of o thebse Marchrvatio n2003s for snowstorm1800 UTC 17 1200 UTC 19 March, with locally stronger gusts. As March 2003. (b, bottom) Schematic of the cold-air-damming the storm progressed, the eastward extent of the structure present from about 0600 UTC 18 March to 0600 UTC 19 northerly winds increased slowly. Also notable was March along Colorado’s northern Front Range, based on that local wind-prone areas just east of the foothills observations and NCEP model analyses. This is a vertical cross- section, oriented west-to-east (latitude 40.5°N), with the shaded experienced surface northerly wind gusts in the 13 21 area representing the blocked layer. Other important components -1 m s (25 40 kt) range associated with this blocked are the high moisture content upstream, northerly flow within the structure, causing extensive blowing and drifting blocked layer, and uplift generated by the blocked layer. Cross- snow. Meanwhile, well out on the plains, east- hatched region is the terrain. -1 northeasterly flow at 5 10 m s (10 20 kt) was -1 common through the period [see the Akron (AKO) March. Generally, easterly flow at 13 26 m s (25 50 wind observation in Fig. 10a]. kt) was present in the deep layer from 700 400 mb, The evolution of the wind profile at Platteville, starting around 0600 UTC 18 March. Below 700 mb Colorado (PTL, a location about 35 km east of the the wind profile exhibited significant directional shear, foothills, see Fig. 3a), captured the low-level northerly with a layer of northerly flow near the surface to about 750 mb. The magnitude of this northerly flow was flow well and indicated deepening surface-based -1 northerlies with time, as well as strong overrunning 13 21 m s (25 40 kt) after 0600 UTC. This easterly flow. Figure 8b shows the evolution of this represented the blocked layer. Note that an elevated layer from 0000 UTC 18 March through 0000 UTC 19 wind maximum is not readily obvious in the PTL wind

ISSN 2325-6184, Vol. 1, No. 4 35 “The quiet revolution of numerical weather prediction”

Forecast skill (500-hPa height anomaly correlation)

From Bauer et al. (2015)

COLORADO CLIMATE CENTER Weather forecasting and chaos

One example of a chaotic system Lorenz explains is a ski run with moguls – he suggested Cat’s Meow at Loveland http://www.karlkelman.com/ski- pictures/loveland/lovebyrun/chairone/cats-meow/middle-cats- meow-loveland-basin-2009-04-27.jpg

COLORADO CLIMATE CENTER Develop a model for a ski or board pushed from the top of the slope at a specified velocity

From Lorenz, The Essence of Chaos Space the boards 10 cm apart and push down the slope at identical velocity

From Lorenz, The Essence of Chaos Now space them 1 millimeter apart

It takes more time/distance, but the boards eventually diverge even in this situation

From Lorenz, The Essence of Chaos “Ensemble” weather forecasts

COLORADO CLIMATE CENTER “Ensemble” weather forecasts

COLORADO CLIMATE CENTER COLORADO CLIMATE CENTER Forecast skill for the February and March 2016 Front Range snowstorms Forecasts of snowstorms

In 2016, we had two big snowstorms along the Front Range, one in early February and one in mid- March Forecasts of the first one were spot-on: about as good as we could hope for! The second one...not so For this skill score, higher numbers are better, 1 is a perfect forecast much. Example of National Weather Service snow forecast scenarios, December 2016 Low-end: “Expect at least this much” Most likely High-end: “Potential for this much”

https://www.weather.gov/bou/winter

COLORADO CLIMATE CENTER And finally, the all-important question: “Do you have a rain gauge?” (and a snowboard!)

COLORADO CLIMATE CENTER If you are interested in weather and the variations in precipitation, please join the Community Collaborative Rain, Hail and Snow Network http://www.cocorahs.org or see me today CoCoRaHS snowfall data, 23 March 2016 Front Range snowstorm

Some of the sharp changes from place to place are apparent – and we always need more observers!

COLORADO CLIMATE CENTER CoCoRaHS snowfall data, 23 March 2016 Front Range snowstorm

Some of the sharp changes from place to place are apparent – and we always need more observers!

(prototype interactive maps, coming soon!)

COLORADO CLIMATE CENTER CoCoRaHS data in Hurricane Florence, September 2018

35.05” in 7 days Swansboro, NC

COLORADO CLIMATE CENTER CoCoRaHS data in Hurricane Florence, September 2018

35.05” in 7 days Swansboro, NC

COLORADO CLIMATE CENTER Thank you! http://climate.colostate.edu/ [email protected]