Wasatch Weather Weenies

I enjoyed my ride to work this morning as there was a wonderful view of the snow-covered Wasatch capped by a lenticular cloud with the scene enhanced by fall colors in the valley.   We are truly blessed this October as there's ski touring in the mountains and, once the trails dry a bit more, mountain biking in the valley.  Our cup runneth over.  Let's take a quick look back at the storm.  Total snowfall based on automated observations from Alta-Collins was 14 inches with 1.98" of water.  Approximately 0.2" of that water fell as rain before snow levels dropped with the arrival of the cold front.  One aspect of the storm that I did not anticipate correctly was yesterday's snow density.  With decreasing temperatures, I thought we would see a transition to lower density snow, but the post-frontal environment was too unstable and convective.  There's an example below with four strong cells moving into or over the central Wasatch range from the nor...

It was quite a night last night with many sites in the Salt Lake Valley recording over an inch of rain.  Amounts were greatest in the Olympus cove area where just over 1.5 inches fell.  Even the airport recorded 1.21 inches.  And it's still raining. Meanwhile in the mountains we're up to about 8 inches of dense snow based on the Alta-Collins and the Snowbird snow stakes, the latter below.   Source: Snowbird.  Downloaded 7:38 AM 26 Oct 2021. Observations from Alta-Collins show that precipitation began between 1600 and 1700 MDT yesterday with temperatures near 40˚F.  About 0.18" or so fell at temperatures above 34˚F.  Temperatures reached 34˚F at 2000 MDT afterwhich snow fell at this elevation (it wasn't long after that it was falling to the Town of Alta.  I guestimate that the snow 8" of snow contains about an inch of water, for an 8:1 snow-to-liquid ratio or a 12.5% water content.  Total precipitation 1.22".   The GFS-derived f...

The forecast has largely been locked in now for a couple of days so there are few changes to report this morning.  Thus, this post is deja vu all over again.   Today brings the warm before the storm, as well as wind.  At 7 AM this morning it was a balmy 60˚F at the University of Utah and 35˚F on Mt. Baldy (11,000 ft).  I bet you forgot it's only late October.  Overnight wind gusts through 8 am include 87 mph on Ogden Peak, 77 on Mt. Baldy, 73 at the top of Jupiter Bowl (PCMR), and 73 near Point of the Mountain, the latter a lowland site.   Expect more of the same today as we are in the warm and windy southerly flow ahead of cold front.  Below is the HRRR forecast valid 0000 UTC 26 November (1800 MDT Monday) showing what has been expected for some time with strong southerly pre-frontal fro, a very well defined cold front, and heavy precipitation near and behind the front.  In the Salt Lake City area, pre-frontal precipitation is limited, ...

Not much change from yesterday's forecasts except for a skiff of precip later today and a bit later cold-front arrival tomorrow evening.  Below his this morning's (Sunday) 12Z Sunday GFS-derived forecast for ltitle Cottonowood.  Temperatures on Mt. Baldy climb to > 35˚F through tonight and remain warm through tomorrow.  There's a bit of precipitation associated with a weak-warm-front-like feature moving through this afternoon.  The GFS generates about 0.16" of water at Alta Collins and just about a half inch of snow due to snow changing to rain during the passage of the feature, even at 9700 feet.   The GFS continues to keep things mainly dry until the approach of the cold front Monday evening.  That's when temperatures and snow levels drop and precipitation gets going.  Expect snow levels to fall pretty rapidly and, perhaps after a bit of rain to start, the event to be mainly snow at upper elevations with snow levels eventually falling to belo...

Everything remains on track for the big storm in California and eventually more October snow for the central Wasatch.   Current forecasts show a very well-defined precipitation system moving across California and the Great Basin Sunday and Monday.  The GFS forecast valid 0600 UTC 25 October (0000 MDT Monday) shows heavy precipitation (>1" water equivalent in 3 h) in the Tahoe area, to the southeast along the Sierra Nevada, and then along the cold front in the northern Great Basin.    By 1800 UTC 25 October (1200 MDT Monday), the action has shifted southeastward to the southern Sierra and Mammoth Mountain and to central Nevada.  It is impressive to see a continuous precipitation band extending across the Sierra and into the Great Basin.  We will see if that verifies, but often there's a break due to shadowing downstream of the Sierra.   Much has been made of the "atmospheric river" conditions that will penetrate into the Great Basin and ind...

It's good to see the Pacific Storm Track angry again.  Below is the situation at 1200 UTC (0600 MDT) this morning.  The Pacific Jet is oriented predominantly zonally (i.e., from west to east) and quite intense with a maximum speed in the western Pacific of over 90 meters per second (180 knots, yellow to orange color fill).  Just ahead of and beneath the jet-speed maximum, called a jet streak by meteorologists, is a surface cyclone (circled in red) just south of the western Aleutians.  Ahead of that system are two cyclones and associated frontal systems (cold fronts identified with blue lines and triangles).   The easternmost of those frontal systemswill give us a bit of valley rain and mountain snow tomorrow.  The one further west will decay and pass to the north.  However, all hell breaks lose further upstream as the upper-level pattern amplifies and the surface cyclone that is currently south of the western Aleutians rounds the base of the ampli...

The Pacific Storm Track is angry my friends and about to unleash some serious weather on the western United States.   I'll focus on one component of the forecast, precipitation, and will use the downscaled NAEFS-derived forecast below to summarize. Over the next 7 days, the average amount of precipitation (liquid equivalent) produced by the 52 NAEFS members in the mountains of northern California exceeds 10 inches in portions of the Coastal Mountains and Cascades/far northern Sierra.  At these "favored" locations the driest members produce over 5 inches and the wettest over 20 inches.   The plume for the Mt. Shasta Ski Bowl at 8,000 ft elevation shows a mean of almost 8" with most of the NAEFS members clustered between 6 and 8.5", although the full range of all members is about 3.5 to 12".  Our snowfall algorithm is known to overestimate snow-to-liquid ratio in this part of the world so be cautious interpreting the snowfall plume.  Snow levels will be ...

October is coming through in spades for skiers.  As of 7 am this morning, the latest storm has produced 0.85" of water and 9" of snow at Alta-Collins, bringing the snow depth up to a solid for mid-October 25".  The HRRR analysis for 1200 UTC (6 AM MDT) this morning shows wrap around precipitation over the Wasatch and Wasatch Front, which is a pretty close match to reality.   The forecast for 1500 UTC (9 AM MDT) shows the continuous precipitation shield breaking up into more scattered precipitation.   Thus, I expect the snowfall in the central Wasatch to transition to snow showers that become widely scattered by later this morning.  Storm totals should be in the 12" range, give or take.   It's been a mighty long time since we've had this much snow so early in the season.  Below are traces of the snowpack water equivalent at Snowbird every water year (beginning October 1) since the 2000 water year.  This year is in blue, showing about...

 If you asked me a week ago about the prospects for snow tonight and tomorrow, I would have said unlikely.  However, as discussed a few days ago in the Dropout Forecasts  blog post, a closed low suddenly appeared in the model runs late last week and over the past few days the models have trended toward a snowier forecast for the central Wasatch.  Will the model trend be our friend? The 0600 UTC GFS suggests this will be a two part storm. Part I arrives later this afternoon as cold, moist southerly to southwesterly flow ahead of the upper-level trough pushes into northern Utah. Part II comes overnight as the trough moves downstream and "wraparound" precipitation in the westerly to northwesterly flow behind it gives another period of mountain snowfall.   Our experimental GFS-derived forecast guidance product for Little Cottonwood Canyon shows temperatures dropping precipitously this afternoon (upper left panel).  The bottom panels show two periods of hea...

Currently temperatures at upper elevations in the central Wasatch are in the teens, but that will be changing rapidly through tomorrow as temperatures climb with the approach of an upper-level ridge.   Our GFS-derived forecast guidance for Little Cottonwood Canyon shows temperatures on Mt. Baldy (11,000') climbing from the low teens this morning to 40˚F by tomorrow (Saturday) afternoon! Free atmosphere 700-mb (10,000 ft) temperatures reach 6˚C by late tomorrow as well.  Balmy.   Be aware of this change if you will be ski touring today or tomorrow, especially on south aspects today and all but high north tomorrow. 

Every now and then, the forecast skill of a computer modeling system plummets, leading to a brief period of poor performance.  Such periods are sometimes referred to as dropout forecasts. I've identified some examples in the plot below, which presents one measure of forecast skill, the standardized anomaly correlation of the 5-day (120-hour) sea level pressure forecast relative to the National Weather Service Weather Prediction Center sea level pressure analysis for the contiguous United States.  Without getting into details, the higher the standardized anomaly correlation, the better the forecast.  The graph presents the standardized anomaly correlation over the past 91 days from several models including the GFS (red circles), UK Met Office Model (brown diamonds), and ECMWF Integrated Forecast System (green Y).   Source: NCEP One can see that model skill varies daily, but there are times when one or more models suddenly produce much lower standardized anomaly c...

If you are in the mountains today consider yourself blessed.  It will surely be the most beautiful morning of the season so far with clearing skies and a fresh coat of new snow.   The view from Snowbird at sunrise is spectacular. Observations from Alta-Collins indicate 2 inches of snow overnight and a storm total of 16 inches, bringing the total depth to 21 inches.  Snowpack water equivalent at the Snowbird SNOTEL sits very near 3 inches.   High clouds will, however, be moving in this afternoon in advance of the next storm system.  The weak surface front associated with that system is forecast by the GFS to move into the Salt Lake Valley early Thursday morning and bring a period of unstable, moist, west-northwesterly flow. Below is our GFS-derived forecast guidance for Little Cottonwood Canyon showing the bulk of the precipitation at Alta mainly from 5 AM Thursday morning to 6 PM Thursday afternoon.  During that period, the GFS produces about 0.4 ...

Pretty impressive storm last night with something for everyone.  Here's what I've been able to piece together for accumulations based on automated measurements through 7 AM MDT this morning: Alta-Collins: 1.15" liquid precipitation equivalent/13" Snow Solitude Summit: 12-13" Snow Thaynes Canyon SNOTEL: 1.2-1.3"/12-13" Deer Valley Ontario: 1.14"/8-9" Mill D North SNOTEL: 1.2"/10" You can see from those numbers that the water equivalents are remarkably even across the central Wasatch and I suspect you'll find at least 8" of new snow throughout the range at elevations near and above 9000 feet, with more in favored areas.  Mountain snow showers will continue at times today and tonight, followed by a break Wednesday, with another trough moving in Wednesday night and giving the mountains snow showers at times through Thursday night.  The numbers from the 6Z GFS are below with 4.6" forecast for Alta Collins for late last night...