Tuesday, November 24, 2015

How Much Should You Trust High Resolution Models?

The forecast for tomorrow morning provides a good example of one of the problems we have as meteorologists using higher resolution models.

For the 3-hour period ending at 1800 UTC (1100 MST) tomorrow, the NAM is generating just a bit of light precipitation over the northern Wasatch.

In contrast, the high-resolution 4-km NAM goes for considerably more precipitation, amounting to 0.16" of water in three hours at Alta-Collins.  Those aren't huge numbers, but they add up.

I'm seeing similar behavior in the NCAR ensemble for tomorrow.  The largest precipitation rates are being produced in the southerly flow ahead of the front at Alta-Collins.

High resolution forecasts like these are alluring as we know the central Wasatch can have strong effects on precipitation.  The problem, however, is that these models produce heavy mountain precipitation far more frequently than Mother Nature does, so the issue in a situation like this is how much to believe such a forecast?  If you bite on a forecast like this every time, you're going to have a lot of false alarms, and nothing annoys a forecast consumer more than false alarms.  Hitting a home run every now and then, but striking out frequently is not a path to success in the forecast business.

It will be interesting to see how things verify tomorrow.  Odds are that the precipitation rates being advertised for the central Wasatch by the high-resolution models are too high during the morning.

In the afternoon and evening, the front is expected to traverse northern Utah and bring snow to all elevations.  Although accumulations will probably be in the 1-4" range at most valley and bench locations, keep an eye on the forecast and adjust travel plans as needed as the currently advertised timing couldn't be worse.

Monday, November 23, 2015

Shades of Predictability

Predictability describes the extent to which the weather or an aspect of the weather can be predicted with existing tools and knowledge.  Its close partner is uncertainty, a measure of the ambiguity or lack of precision in a forecast. In any given forecast, some aspects can be relatively predictable, while  others are less predictable.  A good example is the forecast for this Thanksgiving week.

Over the next few days, a Rossby wave breaking event will occur over the north Pacific and western North America, as can be seen in the GFS dynamic tropopause (jet-stream level) loop below.  The wave breaking features the amplification of the high-amplitude ridge over the Gulf of Alaska and northwest North America and the near simultaneous amplification of the upper level trough over the western United States.

There is very strong agreement amongst the models and ensembles on this large-scale evolution.  Below are forecasts from several members of the Global Ensemble Forecast System (GEFS) valid Friday afternoon and they all show a large-scale ridge over western Canada and a trough over the western US.

Source: Penn State e-wall
Based on the correspondence between these models, the shift in the large-scale pattern over the western U.S. is relatively predictable with limited uncertainty.  It's going to happen.  

Precipitation, however, is another story.  There are differences in the strength and location of the trough and in turn the areas of precipitation pinwheeling around it, and these differences matter a lot for local accumulations.  If we look at our downscaled accumulated precipitation plumes from the North American Ensemble Forecast System (NAEFS), which includes the GEFS and ensembles from the Canadian model), we see huge spread in the forecasts for Alta-Collins.  Through 1200 UTC 27 Nov (Friday morning, when hoards of skiers will be looking for snow), accumulations range from as little as 0.1" of water equivalent to as much as 2.4".  This about as much spread as I've seen in these forecasts since we started producing them last year.  Usually, there is stronger clustering.  Thus, the precipitation predictability for any given location is low and the uncertainty high.

The meat of the precipitation plumes lie in the range from about 0.8" to 1.6" through Friday morning (most of that precipitation falls late Wednesday or Wednesday night).  If you need a most likely scenario, that's it.  If I had to pick a most likely side that an alternative result could fall on, it would be the low side (sorry, but regular readers of this column know that I'm a realistic pessimist and like to be surprised by more snow than I expected).  

The bottom line is if you are traveling Wednesday or Thursday, keep a close eye on the weather forecast and be prepared for the possibility of winter driving conditions.  

Sunday, November 22, 2015

The Trough a Week Club Continues to Deliver

We have been in quite a cycle the past month with a trough passing through northern Utah early in the work week each of the past four weeks.  Each trough is similar in that it brings a temperature drop, followed by a recovery.  Check out the temperature ranges below for each day in October and November.  After a long stretch of quiescent weather in October, we had a trough passage and associated temperature drop around Oct 29, Nov 4, Nov 10, and Nov 16.

Source: NWS
The trough of the week club will continue to deliver this week, bringing the warm before the storm through Tuesday,

and then a return to more winter-like temperatures on Wednesday.

As was the case with the previous storms, we're dealing with a crap shoot for what will happen for mountain and possibly valley snow.  Contrasts in the track, strength, and shape of the trough can make a big difference for precipitation around here, so I'm still taking a wait and see approach until we get a big closer to the event.  The uncertainty is reflected in the large spread being produced by our downscaled North American Ensemble Forecast System (NAEFS) guidance for Alta, which shows members producing from as little as about 0.4" of water to as much as 2.5".

It would be great to get the latter.  A once a week storm is better than nothing, but we remain on the shy of what I'll call "real" skiing and the stretches of warmer, sunnier weather are hell on the south facing aspects.  On the plus side, the regular storms keep the valley stirred up and the inversion at bay.

Friday, November 20, 2015

Locally Dense Snow and Globally High Temperatures

There are some numbers coming out of the high elevations of the central Wasatch that I really like.  How about 3 inches of snow with 0.6 inches of water at Alta-Collins through 7 am.  That's 20% water content, nasty by even Cascadian standards, but the sort of stuff we need to build up a base.  I'll take it gladly over cold smoke at this stage.

Less encouraging are the outrageously high global mean temperatures for October.  Talk about off the charts.  Thank you El Nino and global warming.
Source: NCEI
And for those keeping score at home for the year to date, it's still 2015 1, the rest of the instrumented record 0.

Source: NCEI
Two predictions for the future.  2015 will be the warmest year on record by a wide margin (an easy forecast).  When there is an inevitable temporary drop in global temperatures following this El Nino, such as occurred following the last Uber El Nino in 1997, you will hear people claiming a global warming hiatus once again.

Thursday, November 19, 2015

Funky Clouds, Tonight's Storm

There was a beautiful cloud display this morning as the altostratus covered the sky like a wavy blanket.  The morning sunrise tickled its soft white underbelly.

Here's another view later.  Pity my cell phone camera is such a piece of junk.

Clouds that cover the sky like a blanket and have large wave-like undulations are now called undulatus asperatus, the first cloud type to be officially recognized in many years.  I'm not sure if the amplitude of the waves this morning was sufficient for to qualify as that cloud type, but it was a pretty scene.

We are very close to having forecasts from the National Center for Atmospheric Research 3-km ensemble available on our web site.  It's based on the Weather Research and Forecast model and has 10 members.  We have no idea how well it will work, so I'm interested in seeing how it does over the next several storms, including tonights, which looks to be a windy, orographic affair with strong crest-level flow shifting from westerly to northwesterly overnight.

The plot below shows the 48-hour precipitation accumulation (water equivalent) produced by the ensemble from 0000 UTC (5 PM MST) yesterday afternoon through 0000 UTC (5 PM MST) Friday afternoon.  The ensemble mean, maximum, and minimum precipitation is along the top row, showing you the range of what the 10 members are producing, with the probability of .01, 1, and 2" on the bottom row.  The peak water equivalent in the central Wasatch varies from just over .4" of water equivalent to over an inch.
The plume for Alta-Collins shows significant variations in the timing and amount of precipitation.  The members that generate precipitation early are also the wettest (see the top panel) producing about 0.9 to 1.35 inches of water.  Those that generate precipitation late are relatively dry (see the red lines) and generate .4 to .6 inches of water.

The hourly precipitation graph at the bottom is something we cooked up based on the input of colleagues in the avalanche business who really want to know what periods have the greatest potential for heavy precipitation.  The red line represents the median or middle of the 10 ensemble members, the "box" captures the middle half of the members, and the blue lines extend to the lowest and highest member.  In this plot, you can see that the model produces the greatest odds of heavy precipitation from about 11–15 UTC (4–8 AM MST) tomorrow morning, although there's at least one member at each hour during that period that produces very little precipitation.

At this point we have very little clue concerning the performance of this modeling systems, including the biases in amounts and the spread of the ensemble.  High-resolution ensembles like this are, however, the future of weather forecasting, so I look forward to assessing the strengths and weaknesses of this ensemble during the coming winter.  

Wednesday, November 18, 2015

Will This Be Utah's Hottest Year on Record?

The hottest year on record in Utah remains 1934, a black-swan year if ever there was one.
Source: NCDC
However, we are making a run at it this year.  For the year-to-date through the end of October, 1934 and 2015 are in a near dead heat with statewide average temperatures of 54.8ºF and 54.9ºF, respectively.  
Source: NCDC
November and December 1934 brought temperatures that were 2.6ºF above average, so that's the number for November and December 2015 to aim for.  Lots can happen in November and December, so I'm not making any predictions.  I do suspect that unless we get really cold in December, something in the top 5 is virtually assured.