Friday, April 21, 2017

Some Truth about Lake Effect

Another day, another front, another gasp of lake effect.  Radar imagery this morning shows some lake-effect precipitation over the Salt Lake Valley and the central Wasatch.


Alta-Collins is now up to a storm total of 8 inches.  Some of that is from yesterday's and yesterday evening's non-lake-effect convection, but some of it is lake effect.  Might be some decent spring skiing up there today.

The fact we've had a couple of brief lake-effect episodes this month shouldn't be a surprise.  We are in the heart of the spring lake-effect season.  A curious aspect of lake effect produced by the Great Salt Lake is that it's not most common during the heart of the ski season, but instead during the shoulder seasons.  The primary peak in lake-effect frequency (and amount) is in November, and the secondary peak is in March and April (depending on if you are looking at amount or frequency).

Average number and amount of precipitation produced at Snowbird by Great Salt Lake events by month.  Source: Secrets of the Greatest Snow on Earth.
Fall events tend to last longer and produce more precipitation in November than March and April.  One reason for this is that the sun is much lower and the days shorter in November, which allows for stronger, more persistent land-breeze circulations at night and into the morning.  Such circulations play an important role in lake-effect initiation and maintenance during many events.  

The two seasons of lake effect produced by the Great Salt Lake is unusual compared to larger bodies of water.  For example, downstream of Lake Ontario, there is a single peak in December and January (note change in x-axis from above).  

Source: Veals and Steenburgh (2015)
This difference is closely related to lake depth.  The Great Salt Lake is a very shallow lake and warms rapidly in the spring, enabling a 2nd peak of lake effect.  In contrast, Lake Ontario is a deeper lake and exhibits a strong lag in lake temperature relative to the seasons.  As a result, lake-effect peters out as one gets into the late winter and spring.

The importance of lake-effect for skiing in Utah is massively overstated by many ski writers.  It represents only about 5% of the cool-season precipitation (on average) in the Cottonwoods.  It can, however, occasionally provide enough at the end of a storm to vastly improve the skiing.  I suspect that might be the case today.  Without the lake effect, we were clearly looking at a dust-on-crust event this morning with perhaps 4-6".  Putting 2-4" more on top of that surely will help the skiing a bit.

Thursday, April 20, 2017

This Radar Loop Is for the Birds

Meteorological radars are designed to detect precipitation, but any objects of sufficient size and number concentration will give you a return.

This morning's radar loop, for examples, shows returns that are almost certainty from birds leaving their roosts along the wetlands surrounding the Great Salt Lake.  Note in particular the plume-like development and dispersion of echoes from near the southeast shore of Farmington Bay and the Bear River Migratory Bird Refuge.


To the west, you can see some echoes created by precipitation.  Some April showers are on tap for today.  

Wednesday, April 19, 2017

About Last Night

April showers bring May flowers.  The combination of abundant moisture and good large-scale forcing yielded a solid frontal precipitation band that swept through northern Utah last night.  Below is the KMTX radar image from about 0100 UTC (1900 MDT) yesterday evening.  Something for everyone.
Source: NCAR/RAL
About all we missed out on was severe thunderstorms.  There were some lightning strikes in the area, as indicated below, but I didn't see any strong wind or hail reports on the SPC web site this morning.  That's probably for the best.  It is only in the warped mind of a meteorologist that one is disappointed when severe weather doesn't materialize.

Source: lightningmaps.org
Rainfall reports reported to the National Weather Service show accumulations over .9 inches at several sites along the east bench.  The airport came in with 0.65 inches.  Those are good totals for a relatively brief storm.  

As of 7 am, Alta-Collins has observed precisely 1.00" of water and 7 inches of snow.  I suspect that the first tenth of an inch or so of water fell as rain as temperatures at that location (9662 ft) were in the 40s until 6 PM.  After that, cream on crust.  The snow depth is back up to the 125" US-unit psyche point.  Nice, but for those of you attending the March for Science this weekend, that's 317.5 cm.

Tuesday, April 18, 2017

A Spring Airmass Best Described as "Juicy"

I got some odd stares this morning carrying my umbrella on the bus, but I know better.  A juicy airmass is in place over the Great Basin and, with good dynamics later today, we're in for a wet period later today and tonight.

First, let's take a look at the moisture that we have in place upstream.  The 1200 UTC (0600 MDT) sounding from Elko shows a near saturated atmosphere through most of the troposphere, with the exception of a dry layer just above 500 mb.

Source: SPC
The total precipitable water in this sounding is 15 mm (0.6 inches).  The sounding climatology shows values of 0.6" are near, but not quite at the upper-limit of prior observations at Elko.  There are a few days in mid-to-late April with values reaching about 0.65-0.67".  Nevertheless, 0.6" is pretty high for April.

Source: SPC

If we look at the return interval of the precipitable water forecast produced by the North American Ensemble Forecast System for 1200 UTC (0600 MDT) this morning, we're looking at something that maximizes between 1 day every 5 years and 1 day every 10 years in a strip extending across central Nevada and northwest Utah.

Source: NWS
By 0000 UTC (1800 MDT) this afternoon, that strip is parked over the Salt Lake Valley.

Source: NWS
A juicy airmass isn't enough, however, to ensure precipitation.  One also needs lift.  In other words, something to initiate clouds and precipitation,  Better yet if the stability is weak.  We have both today along a developing trough.  The HRRR forecast shows this trough centerd over northwest Utah with extensive precipitation, some heavy, over much of the area.

  We will have to see how this all plays out, but rain and thunderstorms look likely to develop during the day today.  Below is a summary issued by the National Weather Service.


I'll add that the Storm Prediction Center has our region in a marginal risk category for severe thunderstorms this afternoon and evening.

Bottom line: Keep and eye on the sky and the radar.  Things might just get interesting...

Sunday, April 16, 2017

The End of an Era

Today marked the last day of lift served skiing at Alta and the end of an era for the resort as General Manager Onno Wieringa retires after this season.


Onno was a strong supporter of weather research and forecasting efforts in Little Cottonwood Canyon and the Wasatch Range.  Alta was the first ski area to begin supplying weather observations to the "Utah Mesonet," predecessor to MesoWest (special thanks as well to Alta Ski Patrol, especially Howie Howlett and Titus Case).  Before the advent of MesoWest, weather observations were sparse to non-existent across most of the Wasatch Range.  Today, we get observations from nearly all of the ski resorts and they make a huge difference.  The Alta-Collins site, in particular, provides very high quality snowfall and snow water equivalent measurements that are used by everyone who forecasts in the Wasatch Mountains including the Utah Avalanche Center, National Weather Service, UDOT, utahskiweather.com, opensnow.com, and wasatchsnowforecast.com.  During storms, I probably look at those observations 50 times a day (even though they come in only once an hour!).

Onno also generously provided the wonderful snow observations collected by Howie, Titus, and the Alta Ski Patrol to develop algorithms for forecasting snow density during winter storms in the Wasatch Range, observations that we ultimately used not only for science, but also to set the record straight on why Utah snow is so great (er, Alta snow) in an article for the Bulletin of the American Meteorological Society and also my book, Secrets of the Greatest Snow on Earth.

Much thanks to Onno for all he has done for the weather forecasting community of Utah.

Onno's retirement isn't, however, the only one at Alta this year.  The Supreme Chair is being replaced (as is Cecret) with a high-speed quad that will run from near Alf's Restaurant.  If I remember right, the old triple was the Germania triple prior to the installation of the Collins high-speed quad, so it has a long history at Alta.


In my view, a perfect closing day is sunny, warm, and corn filled and that was the case today, and the Frank World Classic really packed them in.  You don't see lines like that for Wildcat every day.


I elected to lap Collins.  I missed out on the Frank debauchery (such a shame), but did enjoy some entertainment at times from Collins.


Looking forward to the Frank YouTube video...