Friday, September 30, 2016

Weather Observations on the Salt Flats

The Bonneville Salt Flats, a remarkable geological wonder, in July 2008
The Bonneville Salt Flats are a remarkable geological wonder and well worth a visit and a hike of a nearby mountain.  Most people drive through them at a high rate of speed trying to get to Nevada or California as quickly as possible, but even a quick stop reveals a world completely different than any other you have experienced.

The Bonneville Salt Flats are perhaps best known for land-speed events that take place on the hard salt crust to the northeast of Wendover.  In recent years, however, that salt crust has thinned and degraded, forcing cancellations or restrictions in speed events and sparking controversy concerning management of the salt flats.

Brenda Bowen, a professor of Geology and Geophysics and director of the Global Change and Sustainability Center at the University of Utah, is leading a 3-year study to improve understanding of the Salt Flats and their recent change (see this Scientific American article for more information).  Yesterday, some intrepid members of our MesoWest team installed a weather station on the Salt Flats to help with the effort, and returned with some great photos.

These photos illustrate the highly dynamic nature of the Salt Flats.  It's not uncommon for portions of the salt flats to be covered with water, but thanks to recent storms, the coverage and depth is quite high.

Click here to access weather observations from the station.

Thursday, September 29, 2016

Are Recent Bad Snow Years at Alta Due to Climate Change?

Since the 2010/11 ski season, the Wasatch Range has experienced below average snowfall years.  Data from the Utah Avalanche Center shows an average seasonal (October-April) snowfall at Alta of 497 inches since 1945/45, but over the last five seasons we've received only 330, 383, 358, 268, and 393 inches.  

I am often asked if these poor snowfall years are due to climate change.  

The answer is no.  

Many people are surprised to hear this, but here's why I do not think the recent poor snowfall years at Alta are due to climate change.  

First, the base of Alta is at 8500 feet, well above the freezing levels of most storms during the winter.  While it is possible that you can experience a brief rain-on-snow or drizzle-on-snow event at Alta, nearly all of the precipitation that falls there during the cool season falls as snow.  The low numbers over the past five years are not due to more wintertime precipitation falling as rain, but instead less precipitation.

Second, at that elevation, the fraction of precipitation that falls as snow during the cool season is relatively insensitive to another 1ºC (about 1.8ºF) of warming.  The image below from my book Secrets of the Greatest Snow on Earth and based on work by Leigh Sturges and John Horel here at the University of Utah shows the percentage of snow that currently falls as snow that would instead fall as rain if the climate were 1, 2, 3, or 4ºC or warming.  Alta is blessed with high altitude and thus snowfall at that elevation is less sensitive to the first couple of degrees of warming.  

Percentage of snow that would instead fall as rain with 1, 2, 3, and 4ºC of warming
Third, we're not really sure if the cool-season precipitation in northern Utah will increase or decrease with global warming.  Some model projections suggest we'll be drier, others wetter.  For all future emissions scenarios most climate models lean to a wetter future (the blue color bars below indicate the average winter precipitation change amongst models, the whiskers the middle 50% of the projections, and the dots the middle 80%).  Bottom line is that at present there isn't a strong reason to expect global warming to cause a decrease in wintertime precipitation, although it can't be ruled out.  

Projected seasonal and annual precipitation change over the central Wasatch and surrounding region for the 30-year periods centered on 2040 (2026–2055), 2060 (2046–2075), and 2085 (2070–2099) relative to 1976-2005.  See Fig. 6 for emissions scenarios.  Winter is December through February, Spring is March through May, Summer is June through August, and Fall is September through October.  Bars show the average change across the model runs, whiskers the range between the 25th and 75th percentile of the model runs, and dots the range between the 10th and 90th percentile of the model runs. Source:
Instead, I see the recent poor snow years at Alta as largely a reflection of climate variability instead of climate change.  Shifts between wet and dry periods are evident in both the instrumented and paleoclimate (e.g., tree ring) records.  The causes for these shifts are not well understood and I hesitate to speculate as to the causes of recent poor snow years, but my view is that the climate-variability dice simply have not rolled in our favor the past five seasons.

That being said...

This analysis is for Alta, a high elevation site in the Wasatch Range.  It does not mean that climate change is not influencing snowfall and snowpack at lower elevations (e.g., base of PCMR, Mountain Dell, etc.) or that Alta and the upper elevations of the Wasatch will not suffer pain and agony in the future.  It also doesn't mean that it isn't getting warmer in Utah (it is).  Different snow and climate measures (e.g., temperature, snowfall, snowpack, etc.) respond differently to climate change depending on elevation, aspect, and regional and local climate (for example, see Western Snow Trends and Global Warming: Part I and Part II).   When it comes to snow, we simply need to be careful about generalizing across all climates, elevations, and aspects, and cautious about conflating climate variability with climate change.  


Shortly after posting this article, I remember I've written about this topic previously.  See Is This a Long-Term Trend.

Tuesday, September 27, 2016

Degraded Air Quality in Late September

Dude, I can barely see the Oquirrhs!
I hope this isn't a harbinger of things to come, but there is actually an elevated air quality event underway over Salt Lake with some characteristics similar to what we see during the winter.

As everyone is aware, late last week and last weekend and deep upper-level trough and associated cold-air intrusion rumbled through Utah.  The maximum temperature on Friday was only 51ºF.

As the trough moved downstream, a high-amplitude upper-level ridge built rapidly over the western United States.  This led to significant warming in the mid- and upper-levels of the atmosphere at a rate faster than solar heating could warm the colder air near the surface.

As a result, yesterday afternoon, the atmosphere over the Salt Lake Valley was capped by a series of stable layers.  The lowest of these stable layers was only a few hundred meters above the valley floor.

Source: Storm Prediction Center
Weather camera images yesterday showed a clear layer of gunk over the Salt Lake Valley with a pronounced top.  Look toward Lone Peak in the image below.

Source: MesoWest
And the buildup of pollution is evident in the time series below, which shows that we are now in the moderate air quality category for PM2.5.

Such conditions are a bit unusual for September, but there are probably three issues at play.  The first is the depth of the weekend trough and strength of the cold air, followed by the building of a strong ridge and associated rapid warming aloft.  Second, we received a great deal of rain Friday and Saturday, leading to high soil moistures.  As a result, a portion of the sun's energy that often would be partitioned into heating the ground and atmosphere is being used for evaporation and transpiration.  Finally, we simply have to realize that we have more people living here and driving farther than every before.  I suppose there might also be some smoke sources out there, although I'm unaware of any major incidents in the immediate area.


Monday, September 26, 2016

Looking Back at Summer

White capped mountains signal that fall is finally here
Meteorological summer ended earlier this month.  Astronomical summer ended on Friday.  Although temperatures will rebound some this week, summer is finally over.  Good riddance.  Don't let the door hit you in the butt on the way out.

The climate numbers for summer (June–August) were released last week by the National Centers for Environmental Information.  You already know that this summer was the 2nd hottest on record at the Salt Lake City airport, but how did things go statewide?

Well, it was a hot summer by old-timers standards, but not for the young.  With a statewide average temperature of 71.6ºF, this summer was tied for only the 5th warmest on record.  However, the four ahead of this year, as well as the other years tied with it, have all occurred since 1994.  

Source: NCEI
So, compared to the climate of the latter 19th century and most of the 20th century, this was a very hot summer statewide.  Compared to the climate since 1990, it was hot, but not exceptionally so.

Globally, Boreal summer (June–August, summer in the northern hemisphere) was the hottest on record, although from a statistical perspective, it's probably in a dead heat with last summer.

Source: NCEI
For the year to date, however, 2016 is easily the front runner.  It's a near virtual lock for hottest year on record unless something truly surprising or catastrophic happens in the last 4 months of the year.  

Source: NCEI
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Friday, September 23, 2016

Severe Convective Storms Yesterday, Mountain Snow Today and Tonight

Wow, what insanity.  After a meteorologically boring summer, fall comes in with a bang.  I'll go into rapid-fire mode for this one:

1. Ogden Supercell?: Yesterday afternoon, a long-lived thunderstorm with supercell-like characteristics developed near Dugway Proving Ground and tracked to the Ogden Area over about a 2.5-hour period.  It was a right-moving storm, in that it was moving somewhat to the right of the steering-layer flow and the tracks of other convective cells.  This is common characteristic of supercell thunderstorms, which are long-lived thunderstorms with rotating updrafts (note: there are also left-moving storms, but they are less common).  The Doppler velocity signature of the storm, however, wasn't especially pronounced, so I'll leave it to the severe convective storms experts to ascertain whether or not the storm qualifies for Supercell status.

2. Severe straight-line winds.  The storm brought strong straight-line winds to the Ogden area and the northern Wasatch Front.  Here's a remarkable video of the straight-line winds from Antelope Island:

Colleagues here in our mountain meteorology group installed a sensor on the playa just east of Antelope Island earlier this summer that collects data every minute.  The passage of the storm's gust front was accompanied by a nearly instantaneous drop in temperature of more than 20ºF, wind shift from SSE to W, and a wind gust of 75 mph.  

3. Ogden tornado. The storm also spawned a tornado south of Ogden, which was captured in a video posted by Neil Essig on YouTube.

3. Tornado Damage Scale.  I have been asked by a few people how strong the tornado was.  Tornadoes today are classified using the Enhanced Fujita (EF) Scale, which is an updated version of the Fujita Scale originally developed by Ted Fujita, a meteorological pioneer in many areas, including forensic investigations of tornadoes and severe storms.

Source: NWS
Other than to say it was probably an EF-2 or lower tornado, I hesitate to speculate on the tornadoes EF rating based on videos and photos, especially given the wide-spread straight-line winds observed in the area.  Careful site surveys are needed to ascertain the characteristics of the damage path, the degree of damage, and to separate damage from the tornado and the straight-line winds.

5. Try to avoid saying it was "only" and EF-0, EF-1, or EF-2 tornado.  I used to tell people that I really wanted to see a tornado, and then I saw the August 11, 1999 tornado move through the Avenues.  I was completely naive about the damage that a "small" tornado could do.  Like a surgical knife, it cut through the neighborhood, doing considerable damage along a path one or two houses wide, with nearby homes mostly unscathed.  For those impacted, "only" and EF-0, EF-1, or EF-2 makes little sense.  Their homes and lives have been upended.

6. Overnight snow.   Yup, the white stuff has arrived in the upper elevations of the Wasatch Range.

Source: Alta
The Alta-Collins automated sensor shows a total snow depth of 7", but don't get too excited about that as it was at 4" when there was no snow on the ground prior to the storm.   You can see this in the 5-day time series below.  Accumulations so far at that elevation (9700 feet) probably are around 3 inches.  Not quite enough to break out the new skis...

7. Future snow.  The circulation center for the system is currently moving over us and precipitation is occurring mainly to the north and west of the Salt Lake Valley.

If you feel disappointed, don't despair, we will see more snow today and tonight.  I'm going to go for another 5-10 inches through 6 am tomorrow morning at Alta-Collins.  There's a chance for more, and indeed there were 2 members of last night's NCAR ensemble that pumped out more than 1.5" of water today and tonight for that area (which would probably give us something like 12-18" given the high densities we are dealing with), but 5-10" is the most likely range.  

Thursday, September 22, 2016

Today's Thunderstorm Potential

Editors Note: Post has been updated to include correct SPC categorization of thunderstorm risk, which was erroneously called "severe" in the original.

Yesterday evening, the northern portion of the state won the thunderstorm lottery with strong storms developing over the West Desert and Great Salt Lake and moving across the Northern Wasatch Front, Box Elder County, and Cache County.

Strong wind gusts were reported at a remote observing site east of Snowville (78 mph) and the Logan airport (61 mph).  Lightning data shows numerous cloud-to-ground strikes across that area, but none south of Ogden or over the Salt Lake Valley.  

The National Weather Service watches, warnings, and advisories map is lit up statewide today.

Source: NWS
Issues of concern for today include flooding and flash flooding, high winds, and severe thunderstorms.  The Storm Prediction Center has upped our severe thunderstorm risk from severe marginal to slight.

Source: SPC
The weather situation this morning shows that we are currently sandwiched between two precipitation bands, one to our west that is near a surface trough (not shown) and the other to our east.

During the course of the day today, the surface trough is expected to progress slowly into northwest Utah, while the precipitation band to our east remains roughly between Salt Lake and Vernal.  The big question mark for our weather is whether or not something will bubble up in the intermediate dry slot.  The HRRR shows this as a possibility.  Note the strong simulated reflectivity cells between the two precipitation bands early this afternoon.  

However, these types of storms are very chaotic and we're just going to have to see how this plays out.  

This evening the surface trough will approach Salt Lake, and should be accompanied by showers and thunderstorms.  It's not the usual rapidly moving cold front.  It sort of stalls over northern Utah as the upper-level trough moves through and eventually the so-called "wrap around precipitation" moves across the state.  Tonight through tomorrow night looks quite unsettled and the mountains will get snow.  I need to run to class, however, so discussion of that will need to wait until tomorrow.