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.
Wednesday, October 27, 2021
Tuesday, October 26, 2021
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".
Monday, October 25, 2021
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, although there could be a shower here or there mainly in the upper elevations.
Sunday, October 24, 2021
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.
If, however, you are like me and believe the key to a happy life is low expectations, then look to the ECMWF model which is on the lower end with about 1.5 inches.
Saturday, October 23, 2021
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.
|Source: Center for Western Weather and Water Extremes|
The time height section for Alta helps summarize the next few days and sheds some insights into the the atmospheric river forecast for Utah. Recall that time increases to the left in this diagram. First up is today's storm, which will bring 2-5" of wet-snow scraps to upper Little Cottonwood. On Monday, warm, moist southwesterly flow develops aloft. Freezing levels (thick blue line) rise to over 11,000 feet, which sounds scary, but the low levels are drier with lower relative humidity levels. This, and the lack of any well define warm-frontal feature to drive large scale lift, is why the GFS isn't generating much in the way of precipitation until the arrival of the cold front later in the day.
|Source: Rutz, Steenburgh, and Ralph (2014)|
Friday, October 22, 2021
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 amplifying upper-level trough and undergoes explosive cyclogenesis off the Pacific coast.
The result is a very powerful cyclone off the Pacific Northwest coast that will transport copious amounts of moisture into the western United States.
Below is the forecast of integrated vapor transport, a measure of the total water vapor transport throughout the atmosphere, at 1800 UTC 24 October (Noon Sunday MDT). Values > 250 kg/m/s (color shaded) are indicative of Atmospheric River Conditions. Values in an elongated plume that intersects the Bay area are greater than 1400 kg/m/s. Additionally, the transport vectors are oriented perpendicular to the coastal ranges and Sierra Nevada, which is optimal for precipitation enhancement.
|Source: Center for Western Weather and Water Extremes|
Below is our downscaled SREF forecast plume for Kirkwood. The ensemble mean accumulation from about 1200 UTC 24 October to 0000 UTC 26 October is a bit over 7 inches. Snowfall is about 30 inches, although this is a fairly warm storm and a lot is going to depend on elevation, temperature, and precipitation intensity. The "driest" ensemble members generate about 4 inches of water.
What will happen in the Wasatch? The GFS forecast currently calls for an extended period of Atmospheric River conditions from 2100 UTC 24 October (2 PM Sunday) through 2100 UTC 25 October (2 PM Monday). Unlike California, Atmospheric River conditions here are a bit more fickle and the GFS keeps things dry until the cold front approaches on Monday. One can see this general behavior in our downscaled NAEFS plumes for Alta. After a bit of snowfall on Saturday, things hold off until after 1200 UTC 25 October, with most members bringing in the precipitation later on Monday (although there are some that start earlier).
That ushers in a very heavy period of snowfall as the moisture associated with this system combines with the frontal forcing. I suspect the big snowfall numbers being put out by the NAEFS are over done, so don't let those bias your enthusiasm.
By and large, things look good for further building of base at the Cottonwood resorts. A few inches Saturday, a break with wind on Sunday, and then a big burst on Monday/Monday night. I'll try and do an update over the weekend as the details concerning this system come together.
Wednesday, October 20, 2021
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 pretty high through late morning Friday time, but significant snowfall is likely at upper elevations thereafter.
The scary aspect of this forecast for northern California is heavy rainfall on new burn scars. The potential for flooding and debris flows is high.
Meanwhile in Utah, the forecast is scary for meteorologists because of uncertainty. At Alta Collins, it's likely to stay mainly dry through Friday and then things get interesting. The spread in the NAEFS forecast for precipitation and snowfall amount through early next week is enormous and ranges from practically nothing to 6" of water and nearly 80" of snow through 0000 UTC 27 October (6 PM Tuesday).
In the NAEFS plume, it is often a member from the Canadian model that is the high outlier, but in this instance, both the Canadian ensemble and the American GEFS ensemble are exhibiting huge spread. Note that there are also differences in the timing of the heaviest precipitation periods.
The GFS sits about in the middle of the downscaled NAEFS distribution, so I'll use our GFS-derived forecast here as one possible outcome. The GFS produces two major storm periods. One is from late Saturday to Sunday morning. The other starts Monday night and extends into Tuesday night (recall that the NAEFS illustrates there are variations in timing depending on the ensemble member). Total water equivalent a bit over 2.5 inches for the entire period with total snowfall just over 25" at Alta Collins.
If you do the math on the snowfall and precipitation, that's a mean snow-to-liquid ratio of about 10:1, which is lower than average, meaning heavier snow. I see that as a good thing as paste is better than blower for base, but snow levels at times could be at or above 7000 feet depending on how everything comes together. There is also a major warmup between the two storms, with 11,000 ft temperatures reaching almost 40˚F.
If the storms come together, we will see a major increase in avalanche risk and I suspect that resorts like Alta may consider closing for a while to uphill to do avy control. Monitor resort and Utah Avalanche Center web sites, respect snow-safety closures, and remember that if open to uphill skiing, the resorts are still de facto backcountry.
Tuesday, October 19, 2021
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.
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 4" as of about midnight last night. The green line that surges to over 10 inches in late October is the 2005 water year and reflects the big October 2004 storm cycle. However, that didn't start as early as this one, although it delivered big time when it did.
The models are hinting at the possibility of another period of precipitation sometime this coming weekend to early next week.
Monday, October 18, 2021
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.
Friday, October 15, 2021
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.
Thursday, October 14, 2021
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).
Wednesday, October 13, 2021
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 inches of water and 6 inches of low density snow with snow-to-liquid ratios of around 15-17 to 1.
Given the cold conditions and northwesterly flow, there is also potential for lake effect. Odds are best Thursday morning when the moisture is deepest. Given the west-northwesterly flow, our GFS-based algorithms give the best chance to the Bountiful area.
These are the joys of forecasting post-frontal instability showers. I'm going to go for 4-8" at Alta-Collins for tonight through Thursday afternoon and will be praying for more. You should too.
Tuesday, October 12, 2021
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"
The downscaled SREF has a range of about 2 to 11 inches for the entire period. That's a lot of spread, and it likely reflects the scattered nature of precipitation features expected during the period.
I'm inclined to go with another 3-6" for Alta-Collins through early Wednesday morning and then another 3-6" for the Thursday storm.
Given the current 20" snow depth at Alta-Collins and 2.6" of snowpack water equivalent at the Snowbird SNOTEL (9177 ft), that will put upper Little Cottonwood near what I consider to be the lower limit for skiing in meadowy areas on a natural snowpack (24" base with about 3" of water).
Based on those numbers and the forecast, NOW IS THE TIME TO PRAY FOR SNOW. The extended forecast for the weekend and at least the first part of next week is not good, with precipitation odds scant. We need one of both of these systems to be an overproducer to push us into deeper territory.