After the "Deluge"

After days of anticipation, yesterday's valley rain and skiff of mountain snow was a nice change, but pretty meh.  Water equivalent precipitation observations reported to the National Weather Service in the Salt Lake Valley maximized at 0.39" at the City Creek Water Plant and included 0.29" in the upper Avenues, 0.19" at the Salt Lake Airport, and 0.14" at Olympus Cove.  Pretty paltry amounts and enough to wet the upper soil layers, but it remains parched below.  

In the central Wasatch, Alta reported 1.3" of snow with 0.23" of water equivalent and Brighton 2" of snow with 0.35" of water.  Dust on dirt and probably for the best as below-average precipitation looks to persist as the storm track remains to our north the next couple of days and the western ridge redevelops thereafter.  Hopefully Mother Nature is saving it for November.  

Although it would have been nice to get a better soaker, yesterday afternoon was spectacular.

Smoke-free fall weather continues today.  Enjoy!

I Love October!

You'll be hard pressed to find a nicer Saturday than today.  Low humidity.  High temps in the 80s.  Low angle sun.  Fall colors at low elevation.  Spectacular.  

Just the pick me up I needed!  Get out and enjoy.  

Thoughts on Extended Forecasts and Pattern Changes

There's a saying amongst meteorologists that the severity of a storm is inversely proportional to the hype preceding it.  

I've seen tweets about this weekend's storm and a possible pattern shift going back about 10 days.  However, at that time, and as recently as a couple of days ago, the ensembles produced a wide range of possible outcomes from very little snow to quite a bit in the central Wasatch.  

The latest forecasts lean not toward a bit storm, but instead toward a solution with the heaviest heaviest precipitation going to our friends to the north including the Central Idaho Mountains and Teton Range.  We get a frontal passage and cooler air, but less precipitation.  For the 24-hour period ending 0000 UTC 12 Oct (1800 MDT Sunday), the ECMWF IFS gives valley locations in northern Utah only 0.2 to 0.3 inches of precipitation, which would fall as rain.  

Source: Pivotal Weather

That's one of the wetter models.  The GFS is between 0.1 and 0.25" for the Salt Lake Valley with a bit more for the mountains.  After we downscale and apply our snow-to-liquid ratio algorithms, the GFS yields 2-4" of snow above about 7000 feet and 4-8 inches at the highest elevations of the central Wasatch.  

The vast majority of downscaled ensemble forecasts favor a small event.  45/52 members of the NAEFS produce an inch or less of water and about 8 inches or less of snow for Alta.  6 members go for 1-2" of water and 12-18".  Then there's a plucky member of the Canadian ensemble going for over 50", but for reasons discussed earlier this week, discount that (I guess there's always hope).  

We can also look at the Short Range Ensemble Forecast system (SREF).  Its numbers are in line with the majority of the NAEFS members.  All SREF members are under 1" of water and 10" of snow.  Note that both the NAEFS and SREF have some members producing scant amounts of snow (2" or less).  

One shouldn't verify a forecast with a forecast, so we'll have to see how this ultimately ends up, but my view is it illustrates the importance of considering the full range of ensemble forecasts and not buying into any one possible outcome when several are presented.  

Additionally, if you look at the NAEFS, notice how the accumulated precipitation and snowfall traces are generally flat after Sunday's storm.  Although we some changes in the large-scale pattern in terms of a strengthening jet and storm track, that's happening largely to our north and the models suggest continued ridge dominated weather.  Something might slip through the net, but for the most part, extended forecasts suggest the dice are heavily loaded for continued dry weather.  

My view at this point is a best case scenario for Sunday would be some valley rain and very little mountain snow to allow upper-elevation hiking and biking to continue.  I think the odds of getting enough snow for skiing are nearly zero at this point, so a storm that lays down 12-18 inches would just be a waste.  

Flies in the Ointment

Chances are you've been hearing about the pattern change we are going to experience this weekend for about two weeks.  Chances are you've been dreaming of it for at least two months.  It's going to happen, but for mountain snowfall, there are some flies in the ointment.  

As discussed in the previous post, the medium range forecasts from a couple of days ago exhibited a good deal of spread for precipitation and snowfall at Alta.  The North American Ensemble Forecast System (NAEFS) exhibited three solution clusters, one that produced light accumulations (two inches or less), a second that produced moderate accumulations (about 6-12 inches), and a few others that produced considerably more.  This reflected uncertainty in the speed and depth of the upper-level trough as it moved through the western United States.

There has been some convergence of the large-scale solutions produced by the models since then.  As shown by the thumbnails of the 32 members of the GEFS ensemble below, most ensemble members now call for a lower-amplitude, faster moving trough to sweep across northern Utah.  

Source: Penn State e-wall

Such a solution is fairly close to what the Global Forecast System (GFS) has been advertising the past couple of days.  The European Center for Medium Range Weather Forecasting (ECMWF) Integrated Forecast System has been calling for a deeper trough and more precipitation for northern Utah, but even it has shifted toward a lower-amplitude trough, as illustrated below.

Source: Pivotal Weather

With a shift to a lower-amplitude, faster moving solution, most of the members of our downscaled NAEFS product are calling for an inch or less of water equivalent at Alta (upper left panel below) during the Sunday storm.  Six of the 52 members (12%) go for more than an inch, with one member of the Canadian ensemble generating over 4 inches.  Mean snowfall is about 8 inches, with a majority of members producing in the range from zero to 18 inches.  

I am sometimes asked about the outlier members of the Canadian Ensemble.  The Canadian ensemble has a couple of members that are prone to producing very large localized accumulations.  When this happens in northern Utah, we get a couple of members that produce unrealistic accumulations.  In this instance, the outlier member produces about 50" of snow in 24 hours, which is highly unrealistic for a situation like this.  

I am also asked if a range of 0 to 18 inches is reasonable.  Can't we nail it down more?  This is a 72-96 hour forecast and my answer to that is probably not with current observing and modeling abilities.  In this instance, the driest solutions feature a weak trough that passes to the north, producing just light precipitation in the central Wasatch.  A bit deeper trough and you get more precipitation in the middle of the range.  A bit deeper than that and you start getting totals > 12".  Thus, there are sensitivities in snowfall due to uncertainties in the large-scale trough.  If we included the European ensemble, which has more members favoring the deeper solution, we would probably see an increase in the odds of snowfall in the 6-15 inch range and a decrease in the odds of lower amounts.  However, even in the European ensemble, the greatest range of possible outcomes in the trough intensity is found over Utah, as indicated by the shading in the map below.  

Source: ECMWF

Thus, we can be confident of cooler weather on Sunday (temperature is more predictable than precipitation and snow), but we can't completely nail down the snowfall forecast.  I'm hoping we come in with some valley rain and limited mountain snowfall as this really isn't a long-term pattern change.  The extended forecasts suggest the dice are loaded for below average precipitation after this trough passage.  Without more storms, early snow becomes weak snow on north aspects, and that's a recipe for avalanche problems when the storms return. 

Improving Medium Range Forecasts

In late September, the National Weather Service upgraded its Global Ensemble Forecast System, GEFS, which should lead to improved medium range forecasts for ski season.  

The upgrades include the following:

• GEFS is now based on the new "FV3" dynamical core
• Horizontal grid spacing decreased from 33 to 25 km
• Members increased from 21 to 31 members
• Use of new techniques to produce initial conditions for the members

For more details, see this NOAA Press Release or this Service Change Announcement.  

Time will tell if these upgrades lead to significant improvements, but I'm optimistic they will help some.  The increase in membership and new techniques they are using to initialize those members should help improve the spread of the ensemble, providing better probabilities of precipitation thresholds.  

What we don't know is how these changes are going to affect GEFS biases at individual locations and, in turn, how those biases will affect our downscaled forecasts.  It will take a few storms (maybe more) to start getting a handle on this. 

We will have our first chance Sunday and Sunday night when the first major fall system of the season looks to push through northern Utah.  Our downscaled NAEFS product, which includes all 31 of the new GEFS members, indicates an ensemble mean of nearly an inch of water equivalent in "favored" Wasatch Mountain locations through 0000 UTC 12 October (6 PM MDT Sunday) and 6-12 inches of snow for upper elevations around the Cottonwoods.  

For Alta-Collins, the GEFS mean is just under 12 inches.  Curiously, that's quite a bit higher than the Canadian Ensemble mean, which is a bit unusual.  There are three major clusters of GEFS members, one that produces two inches or less, a second that produces about 6-12 inches, and a third that is in the 18-28 inch range (see lower-left panel).  One very excited member produces more than forty inches ;-).  

A big question that I have at this point is whether or not these forecasts exhibit more bias than they did last year.  Are they skewed toward higher amounts, for example, or is the model and downscaling reasonably calibrated.  Large spread in this instance doesn't surprise me based on the characteristics of the trough, which is digging and closing off over the western U.S.  Such situations typically exhibit significant uncertainties for local snowfall accumulations since much depends on if, where and how the low closes off.  

I look forward to cooler weather, but would rather have the snow hold off a bit longer, especially if it's only going to be a modest event.  If it goes big, I guess I'm more interested, but even in that situation, it will likely rot on north aspects and melt elsewhere, so let's save it until a bit later in the season.  

Alex Rakes France and Utah Radar Upgraded

Here are a couple of items to distract you from pandemic-fueled political coverage today.  

Storm Alex Rakes Northwest France

A powerful midlatitude cyclone named Alex (such storms are named in Europe) moved across northwest France overnight, bringing damaging winds.  Below is a Meteosat loop showing the development of the system and its movement Bretagne and Normandy.  

Source: CIRA

The UK MetOffice analysis for 0000 UTC 2 October shows the low centered over nothwest france with a central pressure of 969 mb. 

Source: UK MetOffice

Peak wind gusts relayed through some of my channels by Tim Hewson of the ECMWF include 186 km/h (116 mph) on Belle-Île, and island just off the south coast of Bretagne, 157 km/h (98 mph) in Groix, 142 km/h (88 mph) in Granville, and 135 km/h (84 mph) in Sarzeau.  These are sites on or very near the Atlantic or English Channel coasts.  However, even inland, Cholet had a gust of 129 km/h (80 mph).

Northern Utah Radar Upgrade

The KMTX (Promotory Point) National Weather Service radar was installed in 1995 and celebrated its 25th anniversary this summer (see Happy 20th Anniversary KMTX! from five years ago for some background on its history.  This week, however, it underwent a major upgrade, lowering the angle for the lowest elevation radar scan from 0.5˚ to 0˚.  That doesn't sound like much, but it's important for a radar that sits on a mountaintop because it means that the radar will be able to sample lower altitudes over the Great Salt Lake and surrounding valleys and basins.  This means improved detection of precipitation systems and some forms of severe weather in all seasons, as summarized by the NWS infographic below.  

Source: NWS

Many of us have wanted this to happen since the install of the radar, but the upgrade has been tied up largely by politics (and perhaps some technical matters) for some time.  

If you are wondering, this change probably won't affect radar coverage in the Wasatch much.  The 0˚ scan is severely blocked by the initial western face of the Wasatch, so higher angle tilts, such as the 0.5˚, will remain important.  Most of the radar images I share for the blog are based on the 0.5˚ tilt, but the ones you see elsewhere may be hybrid or composite images constructed with information from multiple tilts as the NWS radars perform a series of scans at multiple elevations.  

Below are a couple of videos from the UCAR/COMET program describing how radar works and the operation of National Weather Service Radars.