Throwback pow

First up today is another reader-submitted photo. This was from over two weeks ago, but I just got a chance to post it. Pretty awesome to be skiing soft, deep pow in early November!

Tuesday recap

We saw sunshine in the morning but clouds covered the sky during the midday and afternoon hours. Temperatures climbed into the upper 20s to low 30s.

The upcoming storms are hard to forecast

People often ask me what makes some storms easy to forecast and what makes storms harder to forecast.

The easier and more confident forecasts are when a well-defined system moves quickly. Think about a strong cold front that moves through with 3-4 hours of intense snow and then it’s gone. Models do a good job with these storms and the forecast is often consistent for days before the system arrives.

The harder and less confident forecasts are when a less well-defined system moves through more slowly and changes strength as it moves. This is what we have coming up with the next three storms, so expect the forecast to continue to change even right up to the storm’s arrival.

Putting all of this another way, if a train is moving down the tracks at 60 mph, it’s easy to forecast where that train will be in the next minute. Compare that to a person that leaves a bar late at night and slowly trudges down the street, wobbling a little bit as they walk. Where will this person be in the next minute? That’s a less confident forecast!

Storm #1: Wednesday & Thursday

* Location. Most snow in central and northern mountains.

* Amount. I’ll stick with 2-6 inches. This might not seem impressive, but mid-week powder days are a gift and amounts could be higher in spots with good moisture in the air.

* Timing. Snow starts in waves on Wednesday afternoon. More consistent snow Wednesday night through Thursday midday. Best powder on Thursday morning and midday.

* Weather details…

The Wednesday morning radar shows plenty of precipitation over the western United States. The rain and snow in California (and some rain and snow in Utah that is not visible on radar due to lack of radar coverage) will hit Colorado late Wednesday through Thursday.

The good factors with this storm will be adequate moisture, reasonably strong vorticity (storm energy) which helps to lift the air and convert moisture to snow, and the jet stream which also helps to lift the air.

Let’s look at the jet stream for a moment. This is a fast-moving corridor of air at around 30,000 feet in elevation (around where commercial aircraft fly at cruising altitude).

Certain parts of the jet stream promote rising air. One of these areas is the left exit region of the jet stream.

The graphic below outlines Colorado, the colors illustrate the strongest winds that make up the jet stream, my purple line shows the center of the jet stream, and my purple oval shows the left exit region where the jet stream creates the most lift.

You can see that the most lift from the jet stream should be over the central and northern mountains of Colorado, which is why I think most models show favor the most snowfall in this area.

The negative factors with this system are a lack of instability (which helps air rise) and warmer temperatures (in the 20s), though the warmth allows us to have lots of moisture which is important.

Finally, the wind will blow from the west-southwest for most of the storm. This could favor areas from Crested Butte (maybe a hair too far south to get the most snow?), Snowmass and Sunlight, Beaver Creek, the highest elevations of Summit County, and Steamboat. That said, since most of the forcing for this storm will be from the jet stream and not from air hitting a mountain (orographic lift), the wind direction is less important.

Storm #2: Friday & Saturday

* Location. Most snow in the southern mountains but all areas should see snow.

* Amount. Southern mountains and Crested Butte 5-10 inches, central and northern mountains 3-8 inches.

* Timing. Snow falls heavily on Thursday night into Friday morning. Best powder in the south and Crested Butte likely on Friday. More snow spreads over central and northern mountains Friday midday through Saturday midday. Best powder in the central and northern mountains likely Saturday morning.

* Weather details…

Following a break in most snow on Thursday afternoon, the next round will start late on Thursday night.

The first part of the storm will bring the most intense snow to the southern mountains on Thursday night and then a wind from the west-southwest should keep snow going in the southern mountains and at Crested Butte on Friday midday. Temperatures will start warmer, so the snow quality could be thicker.

From Friday midday through Friday evening, there should be enough storm energy to bring snow to the central and northern mountains. Temperatures will be cooling through the day though a wind direction from the west-southwest isn’t great for the central and northern mountains.

On Friday night through Saturday, temperatures will continue to cool and the wind direction should be from some variety of northwest, west, and west-southwest depending on the exact mountain. This should allow the northern mountains to do well while potentially favoring the Aspen area (west wind is best) and snow could continue in some southern areas as Telluride can do well with a west and west-northwest wind.

I could go on and on about this system, but that’s enough detail for now. To recap, best snow in the south and Crested Butte on Friday, and most places will have fun powder on Saturday.

Total snow from Wednesday through Saturday

This graphic is just to give you an idea of what might happen. The details are too uncertain to trust the exact placement of the most intense snow.

Storm #3: Sunday – Tuesday

The forecast keeps changing in a major way and now it looks like this system could favor only the southern mountains with light snow elsewhere. The one thing that we can have a lot of confidence in is that the air will be cold. I want to wait at least another day before dealing with the details to see if the forecast stabilizes.

Storm #4: December 6-7

The latest models show another system moving through southern Colorado later next week. If these storms keep favoring the southern mountains it would bring their snowpack up to above average and in line with the rest of the state. I love it when the atmosphere even things out!

Dry: December 8-11

This part of the forecast hasn’t changed, at least not yet. Most models agree that we’ll see a break in the snow for at least a few days next weekend and into the following week.

Next system: December 12+

Amazingly, despite the uncertainty associated with the storms in the next 5 days, the forecast for 13-15 days from now has remained consistent with most models showing a return to storminess around December 12th. Let’s hope that’s the case!

Thanks for reading!

My next update will be on Thursday, November 29.

JOEL GRATZ

 My upcoming presentations about the winter forecast and tips for chasing pow! 

* November 28 in Vail at Walking Mountains Science Center. Free to attend. The talk starts at 630pm. Please RSVP here (currently there is a waitlist…sorry!).

* December 5 in Denver at the Denver Athletic Club. More details soon. 

 OpenSummit

We have an iPhone app that provides detailed weather forecasts for your hiking, biking, and climbing adventures. OpenSummit now includes forecasts for 1,000 of the highest and/or notable summits and hiking areas across the United States. Download OpenSummit (iPhone only) 

Geography Key

Northern Mountains
Steamboat, Granby, Beaver Creek, Vail, Ski Cooper, Copper, Breckenridge, Keystone, Loveland, Abasin, Winter Park, Berthoud Pass, Eldora, Rocky Mountain National Park, Cameron Pass

Along the Divide
Loveland, Arapahoe Basin, Winter Park, Berthoud Pass

East of the Divide
Eldora, Echo, Rocky Mountain National Park, Cameron Pass

Central Mountains
Aspen, Sunlight, Monarch, Crested Butte, Irwin, Powderhorn

Southern Mountains
Telluride, Silverton – north side of the southern mountains | Purgatory, Wolf Creek – south side of the southern mountains