Friday, October 31, 2014

Not So Scary Weather

As of 11:30 AM this halloween, it's already 68ºF at the Salt Lake City Airport.  The record high for the date is 76ºF and it's looking like we will make a serious run at that.

Other than the fact that it may be windy later today, you can't ask for better Halloween weather.  That's great news for trick-or-treaters who won't have to cover up their costumes with bulky layers to keep warm.

Tomorrow looks to be an exciting weather day with the potential for some thunderstorms, strong winds, and a vigorous frontal passage.  After that, much depends on the details of the postfrontal flows and the whims of the lake.  I still see a lot of spread in the model forecasts, although most are leaning towards periods of snow beginning tomorrow afternoon through Monday.  My most likely accumulation for the central Wasatch is 4 to 8 inches, less if the NAM wins (drier post frontal environment),  more of the GFS wins (wtter post frontal environment) and we also get the lake going.

Thursday, October 30, 2014

Some Things We Know, Some Things We Don't

One of the most important papers in the atmospheric sciences was published by Ed Lorenz, the Father of Chaos Theory, in the journal Tellus in 1969.

The key finding of the paper is summarized in the second paragraph of the abstract:
It is found that each scale of motion possesses an intrinsic finite range of predictability..."cumulus scale" motions can be predicted about one hour, "synoptic-scale" motions a few days, and the largest scales a few weeks in advance.
Although we have learned that in some instances the predictability limits for atmospheric phenomenon may be longer than suggested by Lorenz in that paper, the general result that larger-scale weather phenomenon (e.g., cyclones, upper-level waves) can be predicted with greater lead time than mesoscale weather phenomenon (e.g., fronts and mountain waves) and especially smaller scale phenomenon (e.g., precipitation generated by convection) is critical to consider in weather forecasting.

A great example is provided by the forecast for this weekend.  We have known for a couple of days that an upper-level trough will be moving through Utah this weekend and that this would be accompanied by a dramatic airmass change and cooler weather.  Both the ECMWF and GEFS ensemble forecast systems have shown this for the past couple of days and continue to show it today, as illustrated by the GEFS forecast panels below.

The lower right-hand panel, in particular, is known as a spaghetti diagram.  It shows contours of 500-mb height from all 21 GEFS forecasts.  Where the lines are close, there is good agreement.  Where you see spaghetti, there is lower agreement.  All members call for a trough to be approaching northern Utah on noon Saturday, but the spaghetti is indicative of a range of forecasts for the position and intensity of the trough.

So, this is an illustration of Lorenz's theory in action.  The large-scale characteristics are relatively predictable, but the smaller-scale characteristics are less predictable.

Things get even more problematic if we ask how much precipitation will fall?  The precipitation forecast is dependent not only on the trough structure, but also the frontal structure and dynamics (a smaller scale process), the associated cloud processes, and interactions with the topography.  These smaller scale processes have a shorter predictability horizon than the large-scale trough.

This is why we can now anticipate a large-scale airmass change many days in advance, and indicate that precipitation is likely, but nailing down rainfall and snowfall timing and amounts is more problematic.  In addition, many storms in Utah are very small in scale.  As a result, the predictability horizon for rainfall and snowfall is even shorter here than found in other parts of the country where cool-season storms are larger in scale, especially the eastern United States.

This case is especially problematic since the various models and their ensembles have yet to tell a consistent story with regards to the gory details.  My best guess is that the mountains are going to see a few inches late Saturday and Sunday, but it won't be a huge event and it will probably leave us in hiker and skier Purgatory when the ridge builds back in next week.  

Wednesday, October 29, 2014

Slipping Further Behind

My graduate advisor, Cliff Mass, provides yet another sobering assessment of U.S. numerical weather prediction in his blog today.  Be sure to give it a read.  Many of you are well aware that the European Center for Medium Range Weather Forecasting (ECMWF) is currently the gold standard for numerical weather prediction.  There are a host of reasons for this, but inadequate computer power for operational numerical weather prediction in the U.S. is one of the more significant.  Further, while other nations are plowing forward with major computer hardware investments, the U.S. has failed to even purchase a computer with the $25 million that was provided to NOAA by Congress a year and a half ago.


U.S. National Centers for Environmental Prediction
OK, those images exaggerate the situation, but this is serious business since advances in computer power and modeling capabilities offer significant potential to protect lives and property in the United States.  Other countries aren't simply throwing money at the problem for national pride. They recognize the economic advantages of advances in weather forecasting, not to mention better anticipation of a good powder day.

Announcement: Porter Fox Utah Talks

Porter Fox, Powder Magazine Editor and author of Deep: The Story of Skiing and the Future of Snow, will be giving three talks in Utah next week.

  • 12:00 PM 4 November, Weber State University, Shepherd Union Building, Wildcat Theater
  • 11:30 AM 5 November, Utah State University, Taggart Student Center
  • 7:00 PM 6 November, Westminster College, Vive Gore Auditorium
I believe all three talks are free and open to the public.

Tuesday, October 28, 2014

Snowstorm Possibilities Through the Weekend

One storm is currently "raging" in the Wasatch, with another one possible this weekend.

The storm presently underway is of the artificial variety as the resorts lay down a base of artificial snow in preparation for the coming ski season.  Below is a view of Alta this morning.

I'm guessing someone will try to ski the white ribbon of death in the near future, if they haven't already.

The second storm is fortunately of the natural variety.  Most of the medium range ensemble model forecasts are calling for an upper-level trough and surface front passage this weekend.  This is true of the GEFS ensemble below, as well as the gold-standard ECMWF ensemble.
Source: NWS
There is, however, some variability and timing and amount.  Some forecasts are calling for the action to get going early Saturday, others later Saturday.  Some suggest a few inches for the mountains, others a more significant event.  We've passed the so called predictability horizon with regards to the big picture (i.e., there will be a trough/front passage this weekend), but not for the gory details.  We'll see how things come together in the coming days.

Monday, October 27, 2014

Orographic Convection

Looking southeast at orographic convection over the central Wasatch @ 7:45 AM this morning
There's a lot to be learned by observing the weather during periods that are relatively benign.  Big storms are great, but you can't always peer into them and see the underlying processes that are contributing to precipitation generation.   Weak events, however, enable you to see some of the phenomena that ultimately contribute to big events.

Early this morning provided such an example.  We had weakly unstable northwesterly flow impinging on the central Wasatch.  This is led to the development of orographic convection over the central Wasatch and adjoining Traverse Range (see photo above).  Orographic convection features strong updrafts that are initiated as unstable or potentially unstable flow is forced over a mountain range.  Typically the intensity, depth, frequency, and or spatial coverage of the updrafts is greater over the mountains than the upstream lowlands, resulting in more widespread and deeper clouds.  

In some instances, such convection produces or enhances precipitation rates over the mountains, sometimes resulting in heavy snowfall or rainfall.  Today, however, the airmass is relatively dry, so for the most part we are seeing non-precipitating cumulus clouds developing and persisting over the mountains, with just a few showers at times.

Here are a couple of views, the first looking southward from near Red Butte Canyon.


And a second looking eastward from the western Salt Lake Valley.


Note in particular the greater depth and persistence of cumulus clouds over the Wasatch Mountains compared to the upstream lowlands.

This provides a weak example of what often happens in unstable, post-frontal, northwesterly flow over northern Utah.  Lifting by the mountains helps to initiate or intensify orographic convection, enabling storms to rage on.