Following an exceptionally dry 2017-2018 winter season, snowfall has rebounded across the state of Colorado this winter. This was especially true in February, when snowfall was well above average across most of the state. As it stands now, the snowpack in the Colorado mountains is decidedly above average state-wide (image source: NRCS).
Across Eastern Colorado, season-to-date snowfall is close to average across most Denver metro area locations. This has been quite an improvement compared to the past two winters, and after a dry start to this winter as well.
Now that March has arrived, it’s time to start taking a look at the spring season, which historically is the wettest season in Colorado. Meteorological spring is defined as the period from March through May.
The first few days of March came in like a lion this year, with widespread snowfall statewide (very heavy amounts in the high country) and record cold temperatures across the Front Range.
For the long range forecasts, we have taken a look at past years with similar climate variables to what we are currently experiencing, and came up with several “analog” years to give us some direction.
Currently, we are in a weak El Nino phase. This is something that has been advertised since last fall, but only in recent weeks have ocean temperature anomalies in eastern El Nino Pacific started to take on a true El Nino signature. Weak El Nino conditions are expected to persist into the spring and possibly linger into the summer as well (image source: NOAA). In past years, El Nino springs have often resulted in an active storm track across the west central and southwest U.S. with wetter and snowier than average conditions for the Front Range.
An interesting twist to the outlook this year is the cooler than average waters of the North Pacific off the west coast of Canada and the U.S. This has been indicative of a negative Pacific Decadal Oscillation phase (or -PDO), and has created a favorable environment for colder than average temperatures across the Western U.S. recently along with frequent low pressure troughs pushing into the west coast. See below (image source: NOAA).
Given the favorable pattern outlined above for an active pattern across the Western U.S., this raises the odds for a snowy spring across Colorado’s Front Range, including Denver.
Another factor we look at in the spring forecast is western U.S. snowpack and current drought conditions. Moisture (or a lack of) at the surface tends to play a feedback role in the atmosphere, especially later in the spring when convective rains/thunderstorms become more common. Surface moisture is also a sign of recent weather patterns, which can be tough to break without a large scale change in the upper atmospheric flow pattern.
Currently, drought conditions still remain over portions of the Southwest U.S. (image source: NOAA). However, compared to last summer and fall, the drought situation has substantially improved thanks to a snowy winter.
The next image shows current snowpack percent of average, in terms of snow water equivalent, across the Western U.S. (image source: NRCS). This is a great look for the Western U.S. with widespread above average snowpack for all but the far northwest. Places that previously were experiencing serious drought, such as the southwest and California, have received well above average snowfall this winter. This means more moisture available later this spring as low pressure systems track across the West.
Looking at past years with similar El Nino and PDO regimes that also had above average winter snowpack, we came up with four analog years: 1995, 1993, 1978, and 1969. We also came up with two other weaker analog years (1953 and 1979) that were factored into our outlook to a lesser degree than the four stronger analog years.
Here are the average precipitation and temperature anomalies for the above mentioned years.
While there are always factors that will occur that cannot be predicted in advance, these are fairly strong signals toward an active spring season across Colorado.
As a result, we are anticipating above average precipitation, above average snowfall, and colder than average temperatures across Colorado this spring. The odds for a big snowfall event or two will be elevated across Denver and the Colorado Front Range in March and April this year, compared to recent winters.
Later this spring, if the active pattern persists then we could see elevated thunderstorm potential and elevated severe weather potential over the second half of May and into June. Also, flooding could become a concern in May for creeks and streams along the eastern slopes of the Front Range and adjacent plains, due to snowmelt from an above average mountain snowpack, and perhaps from heavy spring precipitation events as well.
There is always a degree of uncertainty with long range forecasts, but we believe based on past evidence that the odds favor an active spring.
Here are our precipitation and temperature odds for March through May:
Precipitation (including snowfall) Odds:
Above Average: 50%
Near Average: 30%
Below Average: 20%
Above Average: 20%
Near Average: 30%
Below Average: 50%
As for the summer season, it’s a bit too early to say how that will play out, but we will see how things progress this spring and will release a summer outlook down the road.