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

CoAgMET COLORADO CLIMATE CENTER 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! CoAgMET COLORADO CLIMATE CENTER 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

CoAgMET COLORADO CLIMATE CENTER 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 COLORADO CLIMATE CENTER (from mesowest.utah.edu)

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 COLORADO CLIMATE CENTER Note: not current data!!

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-tomeasuring-snow-than-just-finding-out-how-many-inchesfell/ 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

The forecast funnel COLORADO CLIMATE 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

Example of a strong El Niño (1997-98) 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

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... COLORADO CLIMATE CENTER

COLORADO CLIMATE CENTER 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

ENSO phase ENSO phase Averaged over the Upper Colorado River Basin Wet years tend to come during El Niño, but it s no guarantee... 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 -Bolinger et al. (2014, Journal of Climate) COLORADO CLIMATE CENTER

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 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: upperlevel winds and vorticity ( spin ) Bottom: lowerlevel winds and temperature March 14 (3 days before): Warm on the plains, no sign yet of the storm at upper levels

Top: upperlevel winds and vorticity ( spin ) Bottom: lowerlevel winds and temperature March 15 (2 days before): Still warm on the plains, the first signs of the wave that will develop into the big storm

Top: upperlevel winds and vorticity ( spin ) Bottom: lowerlevel winds and temperature March 16 (1 day before): Still warm on the plains, wave just starting to develop

Top: upperlevel winds and vorticity ( spin ) March 17 (day the storm starts): Upper-level storm just now developed over the 4 corners Bottom: lowerlevel winds and temperature Surface cyclone over southeast Colorado

Top: upperlevel winds and vorticity ( spin ) March 18 (snow continues): Upper-level low has hardly moved Bottom: lowerlevel winds and temperature Surface cyclone remains over southeast Colorado

Water vapor loop of March 2003 storm, courtesy Dan Lindsey

Upper-level low over 4 corners Surface low over southeast CO Plunger lows COLORADO CLIMATE CENTER 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!

CONTRASTING SURFACE PRESSURE PATTERNS FOR DENVER AND FORT COLLINS SNOWSTORMS Local effects COLORADO CLIMATE CENTER FCL experiences downslope flow off Cheyenne Ridge DEN experiences upslope north of Palmer Lake Divide FCL DEN SURFACE PRESSURE FCL DEN FCL experiences upslope flow south of Cheyenne Ridge DEN experiences downslope off Palmer Lake Divide We have many localized influences on snowfall in Colorado because of our mountains and other terrain variations! DEN SNOWSTORM FCL SNOWSTORM

b between arch. The and clearly gime was a dy. developed therly flow is structure s shown in the urban anced flow ost surface t that time 5 10 m s -1 5 13 m s -1 extremely ch through r gusts. As Barrier jets and cold-air damming Sometimes, cold air near the surface will pile up east of the Front Range, and lift of the upslope flow over the cold air can provide an additional source of ascent farther out on the Plains; along with sufficient cooling to allow for snow Figure From 12. Wesley (a, top) et Surface al. (2013), METAR study of observations the March 2003 for snowstorm 1800 UTC 17 March 2003. (b, bottom) Schematic of the cold-air-damming

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/skipictures/loveland/lovebyrun/chairone/cats-meow/middle-catsmeow-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

Forecast skill for the February and March 2016 Front Range snowstorms COLORADO CLIMATE CENTER 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! For this skill score, higher numbers are better, 1 is a perfect forecast The second one...not so 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

Thank you! http://climate.colostate.edu/ russ.schumacher@colostate.edu Follow us on Facebook and Twitter! @ColoradoClimate

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