A Menacing Frontal Passage

Some nasty looking clouds are accompanying the cold front as it moves across the University of Utah.  Radar shows precip is starting to move into the Salt Lake Valley, but the coverage is far spottier than I would like to see.  Nevertheless, the precip is about to move in and I just heard thunder. Whoot Whoot!

Salt Lake will get some snow out of this eventually, but the GFS has shifted the heaviest accumulations to the south just a bit compared to this morning's run, suggesting the big winners might be to the south.

We'll see how things come together tonight.

A Maalox Moment

Today is the kind of day when a meteorologist could use a bottle of Maalox.  The forecast through tomorrow is very difficult, with divergent views being produced by our two main forecast models: The GFS and the NAM.

To illustrate this, I've put together a paneled chart that shows the guidance being produced by the two models side by side.  The GFS is on the left, the NAM on the right.  The panels are the 6-h accumulated precipitation (snow-water equivalent) ending at the time indicated in the legend at color filled intervals of .01, .05, .10, .25, and .5 inches.  The forecasts start with this afternoon (1800–0000 UTC/1100–1700 MST) and end tomorrow afternoon.  Click to enlarge further.

These panels illustrate a common dilemma facing meteorologists in the Intermountain West.  The GFS (left-hand-side) is a lower resolution model.  It doesn't handle topographic effects very well.  For example, note the lack of structure in the precipitation forecast compared to the NAM on the right.  However, the GFS runs later, sometimes ingests data that the NAM hasn't, and frequently does a better job handling the movement of large scale features like fronts.  For this forecast, that could be very very important.

Notice how the GFS develops an elongated west-east oriented frontal precipitation band that sags very slowly through northern Utah and Nevada.  If such a forecast verifies, there would be major accumulations not just in the mountains, but also the lowlands of Salt Lake and Utah Counties, as well as portions of western Utah and central Nevada.

In contrast, the forecast produced by the NAM is very different.  Beyond the greater detail owing to its higher resolution, the frontal precipitation band is weaker, especially over western Utah and central Nevada, and moves more quickly into southern Utah.  Does one lean toward this given the better handling of topographic effects?

One thing is for sure.  Snow is coming.  Major accumulations are likely in the mountains, but let's hope the GFS verifies so that we get a pasting in the valleys too.

Pretty in Pink

Source: NWS

Pink that is for a winter storm warning.

Source: NWS

Looks like periods of snow through Monday, then the meat of the storm comes in Monday night and Tuesday.  In the Cottonwoods, we are now in the midst of a transition from marginal early season conditions to what will probably be a pretty good upper-elevation snowpack for the holidays.  Conditions outside the Cottonwoods, which have been quite poor, will also be on the upswing.  Whoot whoot!

Forecast Tools: Little Cottonwood Model Guidance

Alta and Upper Little Cottonwood Canyon from Mt. Superior
January 27, 2009

If one wishes to access specific forecasts for Little Cottonwood Canyons, you can always turn to the National Weather Service Cottonwood Canyons Forecast or utahskiweather.com.  But what if you want to make your own forecast or look under the hood of the forecast process?  You can always access the model forecasts at sites like weather.utah.edu or Weather Underground's Wundermap.  These sites are great, but if you are like me, you want to also look at numbers.  How much precipitation is the model producing?  What are the snow levels?  Etc.

An option is to turn to our experimental model guidance page for upper Little Cottonwood Canyon.  The data on this site, which was developed by Trevor Alcott when he was a graduate student here at the University of Utah, is based on model soundings obtained directly from the NAM and GFS models (known as "bufr" soundings).   For example, here's the guidance from the NAM model through noon tomorrow based on the Alta bufr sounding.  

Snow level is 500 feet lower than the NAM "wet-bulb zero" level.  Studies have shown that this is usually a good approximation for the snow level.  Snow ratio and water content are based on a non-linear version of the algorithm described by Steenburgh and Alcott (2010) applied to the NAM forecast.  The Mt. Baldy temperature, RH, and wind forecasts involve simple statistical adjustments to the NAM forecast, which does not adequately resolve local topographic effects, to produce a better point-specific forecast.  

The QPF (quantitative precipitation forecast) tells you how much snow-water equivalent the NAM is producing.  Snowfall amounts are based on the QPF forecast combined with our snow ratio algorithm.  

The web site also includes similar guidance from the GFS model, but uses the bufr sounding from the Salt Lake Airport since there is no GFS bufr sounding for Alta.  The GFS bufr data is also available at less frequent time intervals.  

The use of the GFS bufr sounding from Salt Lake City instead of Alta probably doesn't affect results much since the effective horizontal grid spacing of the GFS is only 25-km and the terrain is pretty pathetic.  Thus, there's probably not much difference in the forecast produced by the GFS for Salt Lake and Alta.  

It is important to recognize that this page provides access to forecast guidance and not a forecast.  A meteorologist would take a look at these numbers, as well as those from other models, and combine it with their situational awareness and past experience to produce a forecast.  One of the biggest problems is with the QPF and snowfall amount forecasts, which are frequently (but not always) underdone because neither the NAM nor the GFS adequately resolves the Wasatch Mountains, let alone the fine-scale details of the terrain in and around Little Cottonwood.  Use it with caution for your own forecasting purposes.  

Everyone to Get Some

The next seven days look to be fairly active across the western United States as we should see one of the more progressive patterns thusfar this season.  Like Santa Claus, the GFS gives something to everyone in the west.

Source: NCEP

There's enough variability that I'm reluctant to speculate on specifics, but I'm optimistic that we're going to see some much needed action during this period for the entire Wasatch Range.  Keep your fingers crossed.

A Big Record Will Likely Fall

Source: www.mattbors.com

This has been an exceptional year meteorologically in Utah.  One that lies outside the range of weather that has been recorded previously (since 1895) in the state.

For the year to date (Jan–Nov), our mean statewide temperature is 53.7ºF, the highest in the instrumented record.

Source: NCDC

A close second is 1934 at 53.4ºF (I'm not sure if the difference is statistically significant, but we'll go with 2012 as the leader).  1934 was a real outlier relative to the climate of Utah during the early 20th century.  In fact, despite warmth in recent years, none have exceeded it.  Will we finally top it this year?

Data from NCDC suggests that in December 1934 the statewide average temperature was 1.1ºF above the 1981–2010 statewide average temperature.  I don't have access to statewide statistics for the first 12 days of this December, but we can look at anomalies at specific stations.  At Salt Lake City we ran  7.6ºF above the 1981–2010 average.  At Cedar City we ran 8.0ºF above the 1981–2010 average.  Now that's a fever!

Thus, we head into the last half of December with a very real chance of recording the warmest calendar  year in Utah during the instrumented record.  That sounds impressive, but in the coming decades, this year won't seem all that unusual.