Wasatch Weather Weenies

After a fantastic couple of days of skiing, a bit of a pattern change is in store for this week.  In particular, over the next couple of days, cyclogenesis will occur over the eastern Pacific, leading to a surge of subtropical moisture into California and the Sierra Nevada late Tuesday and Wednesday.  As this occurs, we will see the large-scale flow over Utah shift to southwesterly with a strong connection into the subtropics.  Note the surge of high precipitable water air into California late in the loop below. 0600 UTC 28 Feb initialized GFS sea level pressure (black contours), preciptable water (color fill) and 250-mb (jet level) wind vectors (red). The Sierra in in store for a round of storminess for sure, although snow levels may rise to as high as 6000 feet near Lake Tahoe during the storm. Due to the large-scale southwesterly flow, this will be a good case to examine the transformation (drying and warming) of the airmass across the Sierra Nevada and the e...

Utah's top meteorologists took extensive observations today and report that the powder was not too thin, not too deep, but just right. The views weren't bad either.   Sometimes in Utah you need to pinch yourself to make sure you are conscious and not dreaming.  Today was one of those days.  

Here's a snow crystal image during the period of higher density snow at Alta on Friday night.  Note the crystal on the left is quite rimed, as is the right hand portion of the crystal on the right. Thus, it does appear there was increased riming that led to the higher density snow, as proposed by David in his comment to my post below.

The Utah Avalanche Center reports this morning that the snow that fell overnight in the Wasatch was much denser (about a 10% water content) compared to what fell yesterday (about a 5% water content).  As a result, the new snow is inverted , and, as most backcountry powder snobs know, that makes for more difficult skiing and trail breaking.  But we are so spoiled!  Skiers elsewhere are going to have little sympathy for us. At issue is why did the water content of the snow increase overnight?  Looking at the hourly data from Alta-Collins suggests that the density of the freshly fallen snow increased after about 0700 (12 AM MST) when precipitation rates increased dramatically with >0.1" of SWE falling each hour through 1100 UTC (4 AM MST).  During this period, the snow interval increased 7 inches, with .67" of snow-water equivalent, which is a water content of about 10% c ompared with the 4% that was falling yesterday . Yesterdays snow was produced by a ver...

I love days like today when I can just look out the window in amazement at how the Wasatch Mountains can create an intense snowstorm that pummels Alta and environs but does very little elsewhere. For most of the day today, it has been snowing in an area confined to the region immediately downstream of the Wasatch from about Provo Canyon to Big Cottonwood Canyon.  Radar echos do not begin over the initial westward slope of the Wasatch, but somewhat downstream.  This is evident in the radar image below and also longer loops covering the afternoon (loops not shown). Visually, this is what it looks like from the Avenues.  There is an incredible, shallow, cumuliform cloud banked up over Lone Peak.  This is the so-called Lone Peak cap cloud that appears frequently in southwesterly flow, but isn't always associated with heavy snowfall.   Note that it is not snowing at all at the mouth of Little Cottonwood Canyon, Big Cottonwood Canyon, or along the base of the W...

Earlier posts this week have discussed the uncertainty in forecasts of front trough and snowband position for this evening.  Here's a nice loop showing a sequence of forecasts from the NAM beginning with the 84 hour forecast from 1800 UTC 22 Feb and ending with the 12 hour forecast from 1800 UTC 25 Feb (today), all valid for 0600 UTC 26 Feb (11 PM tonight).  Meteorologists sometimes refer to this as "DModel/Dt".  Notice how the position of frontal trough and precip band jumps back and forth, from as far south as central Utah to near the Utah-Idaho border. Now, if you think that now we are dealing with a shorter lead time (24 hours or less), so presumably the forecast is more certain, think again.  The target area has narrowed, but for 1200 UTC 26 Feb (5 AM Saturday morning) the NAM puts the front right on top of Salt Lake City with heavy precipitation to the north, whereas the GFS puts the front roughly over Provo. NAM Forecast Valid 1200 UTC (0500 MST) 26 Feb...

It's time to spend some time looking at the Wasatch Mountains again and, in particular, the remarkable pounding that Alta has taken over the past 24 hours or so.  Looks like a total accumulation of about 17 inches, with only .66" of water.  That's a water content of only 4%.   C old Smoke! Although mountain snows were fairly widespread overnight, they have become more concentrated in the upper Cottonwoods over the past couple of hours, with some spillover across the southern Park City ridge line, as indicated by the 1847 UTC (1147 AM MST) radar image. There is also an incredible Lone Peak cap cloud.  Somebody from the U take a picture and send it to me!

Today provides an excellent opportunity to examine the differences between a western US "Intermountain" cyclone and an eastern US cyclone. Radar composite with RUC2 sea level pressure (white contours) and 850-mb temperature (red contours) at 1400 UTC 25 Feb 2011 In the eastern United States, the cyclone is an event .  Midlatitude cyclones are responsible for much of the cool-season precipitation in the east, including most of the major winter storms.  The precipitation shield accompanying today's cyclone is extensive, although those with a good eye may notice that to the south of the low center, precipitation is falling well ahead of the surface cold front.  This is not unusual in this part of the world and indeed overnight a very impressive pre-frontal squall line with numerous severe weather reports rumbled through the southeast . In contrast, your Intermountain cyclone is a sub-synoptic-scale event.  The frontal structure is presently quite disorganiz...

Steenburgh's Intermountain Cyclone Forecast System Photo: Wikipedia Commons I'm just about ready to give up.  As discussed in the previous two posts, there's quite a bit of uncertainty with regards to the details of the weather forecast over the next couple of days.  We know there's an Intermountain cyclogenesis event on tap, but the forecast models can't seem to settle on a track or frontal position, especially for tomorrow afternoon and evening. I was encouraged to see that the NAM and the GFS were converging on a solution this morning , but that doesn't necessarily mean they were converging on the right solution.  Indeed, the SREF showed that there was a lot of spread when one looked at all the available model solutions, so uncertainty remained. So, perhaps I shouldn't be surprised when I pull up the 1800 UTC 24 Feb NAM and GFS and find that they have shoved the frontal trough and precipitation band for tomorrow afternoon back to the north compar...

As discussed yesterday , we're on track for an Intermountain cyclogenesis event tomorrow and Saturday.  Yesterday the NAM and the GFS forecast cyclone tracks and frontal positions for the event that were quite different.  Today, they have converged on a solution that moves the low center across southern and central Utah. Over the next 24-36 hours, a surface front intensifies and slides eastward and southward into Nevada and northern Utah.  As shown by the 1200 UTC 24 Feb initialized NAM, the passage of this front through northern Utah (see the cyclonic wind shift in the image below) produces a good round of snow for the Salt Lake Valley and Wasatch mountains tomorrow afternoon. Concurrently, cyclogenesis occurs over central Nevada, downstream of the High Sierra along the western portion of the frontal boundary.   Although both the NAM and the GFS eventually move the cyclone across southern Utah, the real forecast problem for Friday night concerns the placemen...

One of the more remarkable transitions that occurs in Utah climate during the spring is the increase in the frequency of Intermountain cyclogenesis.  As shown by Jeglum et al. (2010)  using the ERA-Intermim, NARR, and NCEP/NCAR atmospheric reanalyses, the frequency of occurrence and genesis of Intermountain cyclones increases monotonically from a minimum in December or January to an absolute maximum in May. Mean monthly Intermountain cyclone frequency and genesis ( Jeglum et al. 2010 ). The intensity (or amplitude) of Intermountain cyclones also tends to be greater in the spring. Two-dimensional histogram of peak 850-mb Intermountain cyclone amplitude vs. month ( Jeglum et al. 2010 ). Not coincidentally, Shafer and Steenburgh (2008) found that the frequency of strong Intermountain cold frontal passages is also highest in the spring, although the peak is sharper and in June.  Monthly frequency of strong cold frontal passages over the Intermountain West (soli...

It's back.  The infamous shallow cumuliform cloud that sometimes drapes itself over Lone Peak and the Little Cottonwood-American Fork divide during periods of southerly and southwesterly flow.  The cloud, which was the subject of a Wasatch Weather Weenies post in October , made an appearance again this morning. Looking at Lone Peak from the U at 1600 UTC 22 Feb 2011 The display was quite spectacular this morning with snow spilling over Lone Peak into Little Cottonwood Canyon. The KSLC sounding from this morning provides some insight into the cloud dynamics.  The flow is southerly below 800 mb, but veers to southwesterly at 700 mb (roughly crest level.  Note the steep lapse rate from 750-600 mb, but also the inversion at 600 mb. Perhaps forced ascent is leading to convection over the barrier that is capped by the inversion?  The cloud-form is fascinating and deserving of a name.  Suggestions?  

Looking for that special something to bring an end to a great ski weekend?  Look no farther than Rogue Ale's Yellow Snow IPA. Photo: thefullpint.com There's even a Utah connection.  Rogue Ales says it was introduced for the 2000 (sic) Winter Olympics in Salt Lake City . I wonder if it is as good as my current favorite weather ale, Inversion IPA from the Deschutes Brewery in Bend, OR. Photo: Deschutes Brewery

Ryan Murphy, 13, gets the goods today at Solitude Sure, it's Presidents Weekend, traffic stunk because the canyons were closed at times, and it was packed at the ski areas.  But, you have to admit, the snow today was sublime.  In fact, it was one of the better faceshot days I've had this year.  My cheeks are still burning!   The Greatest Snow on Earth is back after a month of unusually high water content snow in Utah. Erik Steenburgh, 12, sampling the Greatest Snow on Earth. Yes, his Dad is raising him right. It takes special snow to make great face shots.  Yes, you need deep powder, but it also has to be light and dry so that it easily waffles up in your face when you make a turn or pop off a mogul or jump.  For faceshots, you need dendrites stacked deep . Dendrites are are snowflakes with lots of tree like branches.  The stellar dendrite serves as the official logo for Alta, and the unofficial ice crystal of Utah. Examples of stellar ...

Photo: Wikipedia Commons Last night I enjoyed great food and conversation at Alta's Shallow Shaft Restaurant with a group of meteorologists and snow-safety pros from Alta and the eastern United States.  There's nothing I like more than talking Wasatch weather and climate with locals and visitors who are passionate about snow and want to know more about the secrets of the Greatest Snow on Earth. After the weather and snow geek talk, our attention turned to the incredible moonlit slopes of upper Little Cottonwood Canyon.  Leaving the restaurant at 10 PM, one could easily have gone for a quick lap sans headlamp.  How I wish I had my gear!  I ended up slithering down the canyon to return home to Salt Lake, but wonder if my dinner companions ultimately opted for a lap or two. The full moon comes today, so what are the chances of moonlit laps tonight?  The clouds have moved in this morning and much (but not all) of the Intermountain West is currently overc...

Click here to see a great video showing the "Four Horsemen of the Weather Apocalypse" during yesterday's storm: Wind, Dust, Snow, and Lightning.

If you are reading this, you should be skiing. After yesterday's event, it's a bit of a let down to look at the satellite imagery, radar, and surface observations this morning, but there is something interesting in the forecast. As many of you know, we are in the midst of a La Nina winter.  During La Nina winters, there is a tendency for the southwest to experience below average precipitation during the heart of winter (Jan-Mar). Source: NOAA/Climate Prediction Center We have a saying in meteorology, however, that climate is what you expect, weather is what you get .  Over the next few days, a deep upper-level trough will impact the desert southwest, bringing another round of lowland rain and mountain snow to that region. The deep upper-level troughs that have moved across the southwest this winter provide a good example of why it is important to avoid over generalization.  Just because the "dice are loaded" for a dry winter, doesn't mean it is guaran...

Some remarkable wind observations were recorded as the front penetrated across northern Utah.  In particular check out this meteogram from the UDOT Lake Point I-80 site (UT9). There is a wind gust of 80 mph at 0040 UTC.  This is a prefrontal wind gust (SSW flow before, during and after) that is accompanied by a brief 4F temperature fall.  The front then comes through at 0110 UTC. I was ready to dismiss this as a bad ob, and then I looked at the traces for Lake Point (LAK) and KSLC.  They both show the same behavior, although the peak gusts are not as strong. How do we explain this unique southerly wind burst?  The 0000 UTC sounding at KSLC shows a classic inverted-V sounding with large values of downward convective available potential energy (DCAPE). Looking at the radar, one can clearly see a pre-frontal precipitation band with bow-echo-like structure moving over the observing sites during the period of peak gusts.  It appears the strong winds ...