Tuesday, October 17, 2017

Perplexing Probabilities

There are a host of challenges posed in the forecast communications business.  One that I thought of this morning as I surveyed the ensemble forecasts is the low probability, high impact weather event.

Fair weather looks to predominate over northern through Thursday, but on Friday, an upper-level trough moves across the northwest U.S. with the trailing cold front racing across Utah.  The NAM calls for precipitation accompanying the front to be relatively light.  Perhaps some valley rain showers and mountain snow showers, but nothing for skiers to get excited about.  

If we look at our downscaled forecasts for Alta based on the Short Range Ensemble Forecast System (SREF) we find that most members are producing very light accumulations of 0.25" of water equivalent or less through 6 PM Friday (0000 UTC 21 October).  Again, nothing to get excited about.  However, 2 of the 26 members are going bigger and putting out about 0.7" of water or so.  
If we look at our downscaled NAEFS forecasts for Alta, most members producing light accumulations, a few in the 0.4" to 0.7" range, but then two outliers that go absolutely huge, generating about 2.5 inches of water and around 25 inches of snow.  Skiing anyone?

Such outliers are unusual, but not unheard.  However, I don't know of any studies that have attempted to look specifically at the reliability of such low probability, high impact forecasts.  The NAEFS forecast above, if taken literally, would yield about a 10% chance of 20" of snow or more on Friday, but a 90% chance of 7 inches or less.  Is that a reasonable forecast of the probabilities?  In addition, if that was a reasonable forecast of the possible outcomes, how best to communicate that to the public and forecast customers?  "Well, we think that there will be some snow showers.  Odds are it won't add up to much, but there's a slight chance of 20."  That should go over well.

I don't have answers for these questions.  We need better validation studies of our ensembles and, as ensembles improve, better ways to both extract and communicate probabilistic forecast information in a way that is useful to the end user.  

Monday, October 16, 2017

Impacts of Post-Tropical Ophelia on Ireland

Following up on yesterday's post, here's a few tweets from Ireland showing the impacts of Ophelia.

Sunday, October 15, 2017

Irish Eyes Aren't Smiling

Hurricane Ophelia has had an unusual life cycle in the northeastern Atlantic Ocean and as of 11 AM AST this morning, was still a category 1 hurricane off the coast of Portugal. 
Source: National Hurricane Center
Over the next 24 hours, Ophelia is expected to undergo what is known as extratropical transition, the transformation from a tropical cyclone into an extratropical cyclone.  It is expected to track northeastward and bring strong winds to Ireland and Scotland on Monday. 

The GFS sea-level pressure and wind speed (meters per second) forecast from the GFS is below and it shows the storm strengthening and broadening just southeast of Ireland, before weakening just a bit prior to landfall. 

Nevertheless, the system is quite strong at landfall.  The areas in yellow feature sustained winds of 28 m/s (56 knots) and orange around 35 m/s (70 knots), the latter are hurricane force.  These areas are found over water.  Winds are weaker over land, but still quite strong. 

The Irish Meteorological Service, Met Éireann, has issued a National Weather Warning and is expecting sustained winds of 80 km/h (43 knots) and gusts in excess of 130 km/h (70 knots) in the southern half of the country.

Source: Met Éireann, 9:30 MDT 15 Oct 2017
Much will depend on the precise track of the storm, but it looks like this will be a strong windstorm for Ireland, and perhaps Scotland and other portions of the UK. 

Saturday, October 14, 2017

The Waiting Game Begins

Last night's trough passage brought a couple inches of snow to the upper elevations of the central Wasatch.

Source: Snowbird
Even at my place, there was a trace of snow. 

The waiting game now begins for the start of the ski season.  As things stand now, the next five days look dry.  Although cold today, we should see marvelous fall weather beginning tomorrow through at least Wednesday and probably Thursday as well.  

After that, we shall see.  The good news is that the GEFS is calling for troughiness over the western US late next week.  

However, snowfall in the Wasatch is greatly dependent on all sorts of factors that cannot be nailed down so far in advance.  Some of those solutions would probably give us a pretty good dump, others next to nothing.  

Thus, at this stage, it's best not to buy into any click bait based on extended range forecasts. 

Need a Recommendation

I've discovered some minor damage to my carbon fiber mountain bike frame.  Recommendations for affordable repair shops or individuals greatly appreciated. 

Friday, October 13, 2017

Abrupt Changes This Weekend

Mid October arrives on Sunday, meaning that I begin to take a closer interest in potential storms that could bring the start of ski season.  

The trough moving through tonight and tomorrow won't bring more than some light accumulations to the central Wasatch, so the ski watch continues, but it will bring some rapid changes in upper-elevation temperatures.

While we have fall-like weather today, by 1200 UTC  (6 AM) tomorrow morning, a fast moving upper-level trough ushers in some legitimately winter-like air, with 700-mb (10,000 ft) temperatures at or below -10ºC by tomorrow morning.  That's pretty frigid for October.

But by 1200 UTC (6 AM) Sunday morning, we've rebounded nicely to +2ºC. 

Here's the yo-yo as illustrated by our automated temperature forecast derived from the NAM for the summit of Mt. Baldy (11,000 feet).  Temperatures today in the low to mid 20s (the station currently shows 25ºF), but plummeting overnight to 8ºF by 8 AM.  Temperatures climb, however, as the trough moves out and the ridge moves in and by Sunday morning, they are pushing 40ºF.  

Bottom line: Tomorrow is good for sleeping in, enjoying brunch, and shopping for skis.  Save higher altitude adventures for Sunday.  Those adventures, however, won't involve skiing, at least in the central Wasatch.  

Tuesday, October 10, 2017

Sad Reminder that Early Snow Is Not Necessarily Safe Snow

It's very early in the ski season, yet a backcountry skier died in an avalanche on Imp Peak in the Madison Range of Montana on Saturday.

Source: Friends of the Gallatin National Forest Avalanche Center
Some basic information is available in this article from the Bozeman Daily Chronicle.

This is a sad reminder that early snow is not necessarily safe snow.  The central Wasatch has had early season fatalities in the past, including within resort terrain, which is de facto backcountry during the preseason.  Keep this in mind when we start to see snow piling up again.