Thursday, April 2, 2020

The Seeder-Feeder Effect

There was a great visual example of the seeder-feeder effect this morning over the central Wasatch.  The seeder-feeder effect involves the fallout of precipitation from seeder cloud aloft into a lower-level cloud generated by flow over a mountain barrier.  This can result in the growth of ice crystals that originated in the seeder cloud as they fall through the feeder cloud, leading to precipitation enhancement over the mountains, as illustrated schematically below.

Source: Secrets of the Greatest Snow on Earth
Below is a photo I took about 7:10 AM MDT this morning.  Thin, mid level "seeder" clouds were producing falling ice crystals aloft.  These crystals then fell into a feeder cloud associated with orographic lift over the central Wasatch. 

At this time, precipitation rates were very like and I suspect if you were on, for example, Lone Peak you probably would have noticed just a few flakes.  Nevertheless, this is a nice illustration of what happens during seeder-feeder, which during precipitation events often can't be "seen" without the use of a radar.

Wednesday, April 1, 2020

Buds are Breaking

We are at the start of a dramatic transformation of the Salt Lake Valley as the "urban forest" that covers much of the Salt Lake Valley undergoes leaf out.

The urban forest is especially dense over the older neighborhoods of the northeast Salt Lake Valley, including the Avenues, Sugarhouse, Mill Creek, and environs.  This morning, that forest still looks brown as most trees are just beginning to bud. 

The view will, however, change dramatically in the next two weeks as buds break and the trees sprout leaves.  Signs of spring are definitely evident.  One example is that my apricot tree is beginning to bloom.

Take a few moments to enjoy the clear skies, gaze at an early season bloom, inhale the clean air, and ponder the words of Alexander Pope:

“Hope springs eternal in the human breast;
Man never Is, but always To be blest.
The soul, uneasy, and confin'd from home,
Rests and expatiates in a life to come.”

Monday, March 30, 2020

Snowpack Status

April 1st is commonly used as the day when the snowpack water equivalent (SWE) is at it's peak in the mountains of the western United States, although in reality it varies depending on aspect and elevation.  For example, peak snowpack on north aspects at 9500 feet isn't reached until late April.  Nevertheless, we'll take a look at where things stand today, a couple of days early, as an indicator of how the snow season has gone.

Data from NRCS SNOTEL stations shows all basins in Montana, Wyoming, Colrado, and Utah between 98 and 135% of median SWE.  The fattest snowpack relative to median is in the upper elevations of the Lower Colorado-Lake Mean watershed, which gets most of its water from the mountains of far southwest Utah.  Other Utah watersheds are 100-117% of median.

Source: NRCS
Watersheds in California, Nevada, southern Oregon, southern Idaho, and New Mexico are generally at or below median, with the Salt and Upper Gila basins of Arizona running the farthest below average.  Note that the southern Sierra are sampled by the State of California and are not included in the NRCS analysis above.  

In the Wasatch range, SWE as a percentage of median varies from 70% at Ben Lomond Trail in the Ogden Valley to 121% at Snowbird.  To the east, most sites in the Uintas are at or above median, as are sites in the Oquirrh and Stansbury Mountains to the west.

Source: NRCS
Let's take a closer look at the two Wasatch extremes relative to median: Ben Lomond Trail and Snowbird.  Ben Lomond Trail is low in elevation (5829 ft) and thus the snowpack evolution there is strongly influenced by temperature and precipitation type (rain or snow) in addition to total accumulated precipitation.  In mid February, this season's SWE (blue) was very close to median (purple), but then it flatlined for 2-3 weeks before declining through mid March and recovering slightly in the last few days.

Here, the warmth of march took its toll, as it did elsewhere in the other mountain valleys of the Wasatch. 

In contrast, at higher elevation (and north aspect) Snowbird, although the rate of increase in SWE has been lower than it was through early February, there have been no loses and SWE currently sits very near the peak median for the season (which occurs around May 1st).  

Source CBRFC
My view is that it has been a solid but not exceptional snow season by long-term historical standards.  Positives are the fact that much of the snow fell during the part of the season when the sun angle is low (i.e., December and January) and that it was a Steenburgh winter, with 100" of base reached at Alta before February 10th.  Negatives are that the spigot closed a bit from mid February to mid March. 

Sunday, March 29, 2020

Northern Hemisphere Ozone Hole

For the most part, there has been good news concerning the ozone layer in recent years as efforts to reduce ozone-depleting chemicals appear to be working. 

However, unusually low stratospheric ozone concentrations are presently found over the high latitudes of the northern hemisphere.  Below is the total ozone over the Northern Hemisphere on Friday showing a clear ozone hole.   

One can compare that to the long-term climatology to illustrate that those concentrations are unusually low. 

Another way to look at it is to compare to past observations made during the satellite era (beginning in 1979.  The graphs below show the range of highest and lowest values of stratospheric ozone in the polar region (top) and temperature (middle) illustrating that both stratospheric ozone and temperature have been exceptionally low and occasionally at or below the lowest on record for the respective day of the year over the past 2-3 months.  

I suspect the record low ozone and temperatures are closely related.  The low temperatures enable the formation of polar stratospheric clouds, which in the presence of CFCs enable the reactions to occur that lead to ozone depletion.  These observations are also consistent with the remarkably strong polar vortex that persisted for much of the winter.  The low ozone, low temperatures, and strong polar vortex are, however, all related like chicken-and-egg, so I'm forced to wait until the stratospheric chemistry and dynamics experts weigh in to know why this has happened this winter.   For a bit more detailed discussion, see this article in Nature.  

Thursday, March 26, 2020

The "Perfect Storm"

I love spring storms that bring snow to the mountains, accumulating snow to my yard, but little to no accumulation on the sidewalks and roads.  Last night's nailed it in the upper Avenues, pictured below. 
Valley areas where snowfall rates were higher probably are seeing some accumulations on roads, but like politics, all weather is local, and for me, this storm is perfect.

Through 7 am, Alta-Collins shows 8 inches on the stake since late yesterday.  The Utah Avalanche Center reports a total snowfall for the week of 12-21 inches in the central Wasatch and just over 2 feet in the northern Wasatch.  I suspect the riding conditions today will be outstanding and some of the best of the season.  Enjoy but be careful and keep your distance out there.

Wednesday, March 25, 2020

Complexities of Great Basin Weather in Spring

It was a wet night and morning in Salt Lake City, with a bit of the white stuff dusting the ground earlier this morning in the upper Avenues.

Snowfall during the 24-hour period ending at noon today appears to be about 11 inches at Alta Collins.  Thusfar, it's a pretty good event, with more on the way.

It's really interesting to look at what has been happening and what will happen from yesterday through tomorrow morning.  During this period, a large-scale upper-level trough is digging southward off the Pacific coast, resuting in southwesterly large-scale flow across the Great Basin.

At 2100 UTC (1500 MDT) yesterday, the 700-mb analysis showed evidence of flow splitting around the southern high Sierra, with confluent flow over central Nevada where there was a weak wind shift from SW flow to the south to west-southwest flow to the north.

Precipitation was not organized along that wind shift, but instead was widely scattered as convection developed due to heating of the weak stability airmass.

In the absence of surface heating overnight, however, the precipitation became more continuous along the wind shift, and this morning the resulting precipitation band was centered over the Salt Lake Valley and central Wasatch.

This band weakened as one moved southwestward toward Nevada.  There were some showers near Elko, but otherwise, the action was along the wind shift where there was also a weak temperature gradient.  One might call this a front, but a meteorologist might instead call it a baroclinic trough.

However, with surface heating today, we are seeing a breakup of the precipitation band and a transition to scattered convective showers.  This is a nice example of the diurnal modulation of precipitation processes in the presence of weak stability and daytime surface heating.

Throughout this period, the confluent wind pattern remained over the Great Basin.  In fact, it has moved perhaps 100 km southward.

Overnight, forecasts show that it remains stationary through 0000 UTC (1800 MDT) this afternoon, although it begins to move southward near the Sierra Nevada by 0600 UTC (0000 MDT) tonight.

Note how the NAM forecast above calls for the precipitation to redevelop along/near the wind shift overnight in the absence of surface heating, as happened last night.

Even by 1800 UTC tomorrow, the 700 mb wind shift remains pretty much over the Salt Lake Valley.

Thus, this slow moving feature will continue to dominate our weather through tomorrow, with transitions in the characteristics of the precipitation.  With temperatures slowly decreasing during the period, I suspect ski conditions tomorrow will be quite good, if it doesn't get too deep as it appears that another 8 to 16 inches are likely in upper Little Cottonwood through tomorrow afternoon.  Respect uphill closures at the result, make adjustments for avy conditions, and, as noted in the video below, "this is not the time to get sendy."