California Dreaming

KT-22@Squaw Valley.  Source: Unofficial Networks

There is no snow on the ground presently in the northern Sierra, and they struggled mightily last year, but that's about to change. 

Below is the five day accumulated precipitation forecast (snow-water equivalent in inches) from this morning's GFS, valid for 1200 UTC 19 Oct – 1200 UTC 24 Oct.  

Source: NCEP

Note the bull's eye in the northern Sierra where forecast snow-water equivalent amounts are greater than 1.5 inches.  The pounding is heaviest on Monday and Monday night.

These forecasts have me California dreaming.  Snow also looks to be in the cards for the Wasatch next week, but it's too soon to say just how much.

Rolling Dice with Ensembles

Source: http://www.onlinecasinosdir.com.  Of course the Wasatch Weather Weenies do not endorse gambling, except when it comes to the weather.

Our inability to know exactly what the atmosphere looks like at a given time limits how well we can forecast the weather.  Modelers call this "initial condition uncertainty." Because the atmosphere is a chaotic system, errors in the initial conditions can grow with time, and wreak havoc on the weather forecast.  As a result, there are a range of possibilities in any forecast, and typically that range gets larger with increasing forecast lead time.

Ensemble forecast systems (see previous post "Forecast Tools: Ensemble Forecast Systems") produce  many forecasts with slightly different initial conditions to get a handle on the range of future possibilities.  For example, below are all the 168 hour 500-mb geopotential height forecasts produced by yesterday afternoon's run of the NCEP Global Ensemble Forecast System.

Source: Penn State E-wall

These forecasts, which are valid at 0000 UTC Thursday 25 Oct all produce a trough over either the western US (e.g., middle right) or along the west coast (most of the other members).  The consistency across these forecasts gives us some confidence that a large-scale pattern change will occur, but the details of what happens in northern Utah is less clear since our weather depends strongly on the position and intensity of the trough, as well as smaller scale effects related to the positioning of surface fronts and the like.

Unfortunately, we still have a problem.  First, since we don't know the initial conditions precisely, we don't know how to generate all the differing initial conditions for the ensemble precisely either, so there is a very real possibility that the range of possibilities differs from that indicated above.  Further, the model used for those forecasts isn't perfect either, and that is an additional source of error.  As a result, we tend to use ensembles of ensembles for long-range forecasts.  So, in addition to the GEFS, we look at the ensemble produced by the Canadian Meteorological Center (and in practice, others as well).

In the Canadian Meteorological Center ensemble forecast valid for the same time, we see a bit more spread.  A few of the forecasts put a trough over western North America or along the west coast, but there are others that put a ridge over western North America (e.g., upper right).

Source: Penn State E wall

Using these ensembles, meteorologists try to put odds on the weather.  One could think of this like rolling the dice, except how strongly the dice are weighted varies from day to day.  Some patterns are more predictable than others and we can have greater confidence in longer range forecasts.  Other patterns are less predictable, and the future is more uncertain.

Possible Snow...Or Not

It's that time of year when a cold upper-level trough can bring the goods and enable some early season turns.  Because early snow often rots into weak, faceted snow that can create dangerous avalanche conditions (see last year's post "Patience Young Jedi Knight"), I typically hope that Mother Nature holds off until November.  However, I know many of you are chopping at the bit for the first dump of the year.

The latest runs from the GFS and ECMWF forecast models call for an upper-level trough to move into the western United States next week.  Here are a couple of snippets from the GFS just to whet your appetite.

If that forecast verifies, chances are good that many will be hiking for turns late next week, especially those with a high tolerance for bony conditions.  

Or not.  The extended range forecasts have been entirely unreliable the past week or two.  Conditions over and downstream of the Pacific Ocean have simply been highly unpredictable at long lead times.  Each day I come to the office, pull up the extended range forecast, and scratch my head because it has changed so much.

Thus, keep an eye on the forecasts and keep the mountain bike tuned while you get the skis waxed.   Perhaps things will come together, but if not, single track still beckons.  

Thinking Globally

Weather forecasting is inherently a global problem that requires international cooperation between countries around the world.  Forecasts in the United States are better because of our partnerships with the EU, India, China, Japan, and many other countries.

Let's use today for example.  We have a cold front coming into Utah this afternoon, but we can trace the impetus for this event upstream to Japan.  As can be seen below, a very weak cyclone off the coast of Japan five days ago meanders all the way across the Pacific Ocean and moves into western Canada, driving cold air into the Pacific Northwest last night.  This cold air will be moving into Utah today.

0000 UTC 11 Oct – 1200 UTC 16 Oct IR satellite imagery, sea level
pressure (black contours), and low-level (925-mb) temperature (color contours). 

From this perspective, one can see our cold-frontal passage as being connected to a weather system that has travelled 1/3 of the way around the Earth.  "Think globally, act locally" is very appropriate for weather forecasting.

Changes on the Playa

I've spent the last two weekends collecting field observations over the playa of the Great Salt Lake Desert.  The playa is an incredibly fascinating land surface.  One might even argue that it lives and breaths as it undergoes incredible changes due to weather, seasons, climate variability, climate change, and human disturbance.

For example, the storm on Friday and Friday night caused some noticeable changes to the brightness, texture, and wetness of the playa.  Compare the two photos below, which were taken last Sunday and yesterday.  They were taken in slightly different locations, but generally reflect the characteristics of the playa in the area in which we were operating.

Sunday 7 October

Sunday 14 October

Playas are known to have a strong influence on local circulations, not just in Utah, but other dryland regions of the world.  It will be interesting to see how the changes above affected the surface energy balance (e.g., the absorption of solar radiation, fluxes of heat and moisture to the atmosphere, etc.) and if those changes have an influence on airflows in and around the Great Salt Lake Desert.

It wasn't all hard work.  We did find time early yesterday afternoon to bag Volcano Peak just north of Wendover in the Silver Island Mountains.

Space Matters

Someone once told me that when it comes to science, people are most interested in three things: Space, weather, and dinosaurs.

With regards to the former, check out the crowds watching Endeavor's final mission through the streets of LA.

Photos: CNN/Getty Images