Anchorage Ski Outlook - 12.7.2022

Dirty Details
Of course the big story is the band of snowfall that focused on Anchorage:

Figure: National Weather Service

I was wrong in my forecast last week when I predicted light snowfall favoring the mountains closest to the Prince William Sound. Not only was the snowfall not light, it didn't favor that zone! In my defense these heavy precipitation deformation zone events are really hard to predict a week out partly because they are such small-scale events. The storm is really visible in the sudden jump in the Anchorage Hillside SNOTEL:


I feel like a broken record here (yes that was a high school nickname), but leading up to the weekend we're gonna get hit by more gaps winds. These winds will pound the usual victims of Valdez/Seward/Whittier/Portage, but as shown below they will also hit the heart of our Turnagain ski terrain and the Mat Valley. When the Mat Valley winds blow like this they'll bleed over into the Arcose Ridge ski terrain as well.


Archived Outlook - 11.30.2022

THURS - FRI: sunny, windy, moderate temps (high confidence).
Strong winds (peaking Thurs) for Thompson Pass/Valdez, Seward, Portage/Whittier, the Anchorage Front Range, Crow Pass, and to a lesser extent Turnagain, Summit, and the Mat Valley. Temps rising towards the weekend with inversions for Turnagain/Summit, East Anchorage, and the Mint Valley.

WEEKEND: dry, warm, calm (moderate confidence).
Partly cloudy for all of our mountains. Moderate confidence because of uncertainty on how cloudy and when those clouds will be. Sunday might be clearer, as might be Thompson Pass.  Light ridgetop winds.

EARLY NEXT WEEK: warm, cloudy, light snow (low confidence).
Snow showers and cloudy for all our mountains. Confidence is low because timing of these showers is uncertain, but accumulation will be light. Most precip for mountains closest to the Prince William Sound (Valdez, Seward, Whittier). Ridgetop winds. No rain concerns.

Dirty Details

These midwinter clear and cold periods often bring wind. This is because the cold, dense and high pressure air of interior AK flows "downhill" towards the relatively warm and low pressure air in the Gulf of AK. These winds favor terrain channels and gaps (i.e. Thompson Pass & Whittier); hence Gap Winds. Here's a cool figure from UBC illustrating this:


The key to skiing during these wind events is to avoid the channeled terrain along the road corridors and long valleys where the wind can easily flow (i.e. the west face of Magnum vs. Goldpan).

This same cold weather also works to facet the snow as the moisture is sucked out. Just like most everything else in the world, faceting is driven by gradients (differences over distance or time). The difference between the relatively warm/wet air in the snow and the cold/dry atmospheric air produces a pressure gradient that drives water vapor through the snowpack and changes round snow crystals into square ones. Avalanche.org has a fun animation that describes the process:

Ski season is definitely here and some people have definitely been getting after it (specifically Colin Gordon & William Kincaid). A little bit more snow would always be nice though, so I was interested in how this season-to-date compares to normal for this time of year. Plotting up SNOTEL data for Turnagain and Hatcher, looks like we are off to a pretty average start:

Turnagain (Center Ridge SNOTEL):


Hatcher (Independence Mine SNOTEL):


Archived Outlook - 11.23.2022

Thurs - Fri: moderate temps, light snow, light winds (high confidence).
Thursday starts out clear and calm before snow and clouds arrive lasting into Friday. Clouds/snow arrive first for Thompson Pass/Valdez then the rest of our mountains as the storm moves west. Moderate snow for Thompson Pass with generally light snow elsewhere. No rain concerns. Winds ramp up Friday afternoon as the storm departs.

Weekend: cold, clear, windy (high confidence).
Temps drop thru the weekend with inversions for Granite Creek, East Anchorage, and the Mint Valley. Clearing least complete for Thompson Pass. Strong winds (especially Saturday) for Portage/Whittier, Seward, Thompson Pass/Valdez, the Anchorage Front Range, and Turnagain ridgetops.

Early Next Week: cold, dry, windy (moderate confidence).
Temps keep dropping with a break in the wind Monday. Slight chance of light snow Monday for Turnagain, Portage, and Whittier. Strong winds Tuesday for Valdez/Thompson Pass and to a lesser extent Whitter/Portage and Seward. Ridgetop winds elsewhere. Strong inversions for Granite Creek, East Anchorage, the Mint Valley, and Portage.
 
This is my first outlook of the winter, so here's how I go about figuring out where to ski:

1 Minute
5 Minutes
Use Meteoblue Multimodel to see differences in magnitude and timing between models for precip/sun, wind, and temp. For wind, different models have different resolution - think number of pixels in a picture - resolution affects wind forecast, so just look how relatively windy your day of interest is versus the rest of the week. Confidence in your forecast will be higher if the different models agree in strength and timing of weather. Check back on Meteoblue every day to see if the models are consistently forecasting the same weather - if the forecast is different each time you check then the confidence in your forecast will decrease.
Use the Forecast Discussion within the NWS Avalanche Weather Information for the Rain/Snow line - its a new product, so we'll see how we like it.
 
15 Minutes
  • Avalanche Weather Information Forecast Discussion - mountain-specific short-term forecast discussion.
  • Short Term Forecast Southcentral AK - most useful, provides specific weather.
  • Model Discussion - usually useful, informs confidence in your forecast.
  • Long Term Forecast - sometimes useful, provides possible trends.
  • Analysis and Upper Levels, Marine Gulf of AK - details and learning opportunities.
  • New Snow -  Use to figure out what area is favored. Hatcher Pass, Anchorage Front Range, and Turnagain Arm storms are better captured by high resolution models (NAM & ECMWF). Note, Windy uses a 10X conversion from precip to snow, with cold storms (like the recent ones), the Snow/Water ratio will be 15X or higher.
  • Winds @ 3000 ft - Surface winds. These require high res, so NAM is best, then ICON and ECMWF, with GFS the worst.
  • Winds @ 250 hPa - Jet stream elevation - the weather conveyor belt. When it hits us from the SE it brings precip to western Prince William Sound mountains. As it rotates to the SW it favors precip for Hatcher, Anchorage Front Range, and Thompson Pass. Look to see if the jet stream is coming from as far south as Hawaii for warm Pineapple Express storms. From the N it brings cold and often outflow winds.
Long Term - Should I get ahead on work to ski next week???
  • Windy's 10 Day ECMWF and GFS Forecasts - Do the two models agree? Is the forecast changing each time you pull it up? For higher confidence in your forecast look for agreement between the two models and between runs of each model.
  • NWS Forecast Discussion (referenced above) - This product is produced twice daily. Read the long term forecast. Does each new forecast discussion say generally the same thing or it changing? Look for the terms "confidence", "agreement" and "differences" throughout the forecast discussion - this will let you how much the Weather Service is trusting the models.
  • Meteoblue Ensemble - Farther into the future the model spread will generally grow. The bigger spreads there are for temp, precip, and wind, the less confidence you have in your forecast.
Note: the National Weather Service has discontinued their "Southcentral Alaska Mountain Forecast" replacing it with "Avalanche Weather Information". The old site was an imperfect product, but did have some short term forecasting value especially for rain line forecasting. The value in the new site appears to be in the Forecast Discussion in the "Overview" tab - I'm looking forward to seeing how it goes!

1 comment:

  1. Thanks for introducing me to Meteoblu, this seems to improve on Windy's forecast comparison tool.

    ReplyDelete