Forecast Summary:

Thursday was awesome.  Very light snow on Saturday and Sunday.  Low confidence in any forecast beyond Sunday.

Forward (Why This Week’s Storms were Awesome and Why We’ll Actually Take Advantage of Them):

Before we jump into the forecast discussion, I wanted to start with a brief discussion of how awesome the last week has been snow-wise, and why despite the current insanity in the world we will be able to take advantage of this recent snow.  I know that there are many other fellow patrollers that would agree that the best ski month in our patrol zone is not any month during our patrol season schedule, but rather the month of June.  June is when the velvet corn peaks, when avalanche danger mostly subsides, and the biggest and best lines in the Indian Peaks are in their top condition.  Mt. Neva, Mt. Toll, Challenger Glacier, Skyscraper Glacier, and Radiobeacon Mountain, all in our patrol zone, hit their peak skiing potential in June (or early July).  And hopefully the current stay-at-home orders will be only a painful memory by then.

98% of my forecasts are based upon meteorology, and only 2% rely upon climatology.  But today, let’s step back and take a brief look at climatology.  Contrary to popular opinion, the highest year-to-year correlation on the size of our glaciers in the summer is not based upon winter snowfall.  Indeed, the amount of winter snowfall is not even a statistically significant predictor of summer snow.  (For you non-science types, don’t get bored by the big words.  Hang in there, this is really important if you love to ski in the summer like I do.) 

Rather, the number one correlation between weather and summer glacier size is summer temperatures.  The number two correlation is the amount of spring snow.  So, if you want to have an awesome summer of skiing in our patrol zone (and RMNP) – ignore the winter snowfall data and at this point of the year, hope for great spring snows. 

And guess what?  We’ve just had a truly extraordinary week of spring snows!  Boulder passed its all-time record snowfall.  The average snow depth over the last number of years at the Lake Eldora Snotel site peaks in the first week of April at 12.5” SWE (i.e. Snow Water Equivalent).  This year, the Snotel at Lake Eldora had seemingly peaked on March 31, 2020 at 15” SWE, but early this morning (April 17) it hit 15.1” SWE.  That is a VERY good sign for our summer skiing potential, especially if the snow continues.  Rejoice my fellow patrollers. 

As an aside, if you want to read the key paper on climatology and our summer snowpack, it is: Hoffman, Fountain, and Achuff, 20th-century variations in area of cirque glaciers and glacierets, Rocky Mountain National Park, Rocky Mountains, Colorado, USA, Annals of Glaciology 2007.  It’s easy to find for free online.  You can’t understand when good summer skiing occurs without understanding the basic science of how our snow sticks around in the summer.

Okay, enough science.  Let’s return to the pseudo-science of weather forecasting.

Forecast Discussion:

Friday afternoon looks to be sunny, but a small shot of snow returns on Saturday afternoon.  Dusting per WRF Model, 1” per American Model, and 2” per Canadian Model. 

Then, a dusting to an inch of snow on Sunday afternoon. 

After Sunday, the models start to diverge significantly.

The American Model has an inch on Tuesday, and then 6” on Friday.  The Canadian Model has 8.5” between Sunday night and Wednesday morning.  Then, the Canadian Model has another 9” between Thursday mid-day and Saturday mid-day.  The European Model only has 4” Tuesday to Saturday. 

In other words, don’t trust any forecast beyond Monday.  Finger’s crossed the Canadian Model is the one that turns out to be the best prognosticator, as it has the snowiest solution.

Retrospective Discussion:

In my Tuesday Forecast, I lay out the model predictions (30” WRF Model, 18” Canadian Model, 13” NAM Model, 9.5” American Model, and 8” European Model) for the Thursday storm, and I had enough hubris to make my own prediction of 12-18”.  Bear in mind that this snowstorm was partially fueled by the jet streak, which is notoriously tough to predict.  So, what happened?

The Lake Eldora Snotel picked up 1.6” of SWE.  Conservatively, that means 20” of snow.  Hooray!  Actual field reports of snow have 20” near Nederland, 20” near St. Mary’s Glacier, and 22” and 17” reports near Allenspark.  (Near Jamestown seems to be the biggest winner with 30” of snow.)

It looks like my forecast of 12-18” was a bit low.  The most accurate model was the Canadian one.  The WRF Model was a bit high, and the NAM Model was too low.  But, the biggest failures were happily the pessimistic solutions from the American Model and European Models.

As the Canadian Model did the best on the Thursday storm, let’s hope its optimistic solution for next week is correct. 

K.B.O. everyone.

-Jordan (published on Friday morning, but mostly written Thursday night based upon Thursday afternoon model runs)

Note: Unless otherwise noted, all forecasts are for 10,000’ in exposed areas.  References to American Model are the American (GFS) Model.  References to the Canadian Model are the Canadian (GDPS) Model.  References to the WRF Model are the CAIC WRF Hi-Res Model.  References to the European Model are the European (ECMWF) Model.