Wednesday, April 23, 2014

Everything but Bench Snow

Yesterday's Intermountain cold front delivered in spades.

Lower-elevation wind gusts of 78 mph in Eureka, 74 in Sherwood Hills, 74 at Diddle Knoll, 73 at Fish Springs, 73 in Lehi, 72 in Tremonton, 72 in Draper, 72 in Wendover (I-80 Milepost 1), and 71 near Magna.  The 72 in Tremonton toppled a couple of semis and occurred just ahead of the cold front.

Source: MesoWest
The 74 at Diddle Knoll in the West Desert occurred just after frontal passage, although this site had gusts to 70+ both ahead of and following frontal passage.

Source: MesoWest.  Note data outage overnight. 
I haven't heard of any reports of semis blown over on I-80.  This event, however, never reached the magnitude of the 25 March 2006 event that featured post frontal wind gusts of just over 80 mph.  

The previous post illustrates the plume that extended from the Sevier Dry Lake Bed over the Salt Lake Valley.  Online data from the Utah Division of Air Quality shows a remarkable spike in PM2.5 concentrations to almost 150 ug/m3, very near the time of frontal passage.  If that value is legit, it is much higher than anything observed during inversion events the past two winters in the Salt Lake Valley.

People have tried to argue that these dust storms are "natural" events, but they aren't.  The strong winds are natural, but dust emissions from the Sevier Dry Lake Bed and other sources clearly have a strong anthropogenic component.  Diversions from the Sevier River limit water flow to the lake bed and human disturbance of the land surface is a major factor in dust emissions.

Unfortunately, the deposition of dust onto the mountain snowpack will likely screw up the corn skiing later this spring.  Whether or not we see a chocolate-brown snowpack emerge with the spring melt as we did after the 2002 Tax Day Storm remains to be seen, but I won't be surprised if we do.

For now, that dust is buried under and probably a bit within the mountain snow that fell overnight.  Looks like 4 inches at Alta-Collins, get them back over the 100 inch snow depth again.  Unfortunately, precipitation did not hang on until the later hours of the night when it would have been cold enough to perhaps get a skiff of snow down to bench level.  Instead, accumulating snow only made it down to about the upper reaches of Olympus Cove.  You can't have everything!

Tuesday, April 22, 2014

Sevier Lake Bed Dust

Great view of the dust plume that is impacting the Salt Lake Valley ibelow.  The source is the Sevier Dry Lake Bed.  The plume has migrated eastward across the Salt Lake Valley in the past hour as the flow has veered just a bit from S to SSWerly.
Satellite image source: College of DuPage
Expect to see a spike in PM2.5 concentrations once the plume is over the DAQ sensor at Hawthorne Elementary.

At 4:45 PM, the front is over the Great Salt Lake and will be reaching the Salt Lake Valley in the near future.

What a Front!

A remarkably strong Intermountain cold front developed rapidly over Nevada today.  Check out in the loop below how the lines of constant temperature (called isotherms by meteorologists) at 850 mb (about 1500 m in elevation - near valley level) become highly concentrated over eastern Nevada.

If zoom in and add surface observations, we can see a temperature drop from 79ºF just west of the Great Salt Lake to 39ºF in Elko.  Unbelievable given that there was very little temperature contrast upstream over western Nevada and northern California.

This is a classic example of what I call Intermountain frontogenesis.  Frontogenesis means the birth or intensification of a front.  These fronts tend to develop or intensify over western Nevada as that where confluence between southwesterly flow that wraps round the southern "high Sierra" meets westerly flow that moves across the lower northern Sierra and southern Cascades.  The boundary between these airstreams tends to remain fixed while colder air approaches from the west.  If you add what we have today - a dry slot just ahead of the front and precipitation to cool the airmass behind it (see above), you get rapid frontal sharpening along the airstream boundary and a whopper of a front.

It's going to be interesting to see this front move across northern Utah.  Wendover had a wind gust of just over 60 mph shortly after frontal passage.

Source: MesoWest
Given the tendency for winds to increase over the very smooth salt flats, we could see gusts along I-80 near 80 mph.  I've been watching the UDOT traffic cameras out on the salt flats and noticed that there are still some semi drivers tempting fate.  A couple of those might just get toppled.  There's also plenty of blowing dust out there.  The Oquirrhs are nearly fully obscured now from my office.

 It's just going to get more interesting from here.  Enjoy the wild weather, but be smart.

The Warm and then the Storm

Last night was remarkably warm with a minimum temperature of 63ºF at the Salt Lake Airport occurring at about 11 PM.  That's the average high for today. Given that we will be cooler than that this evening, it won't stand as the official minimum for the calendar day.  

This morning's sounding shows a surface temperature of 68ºF and about the weakest nocturnal stable layer possible.  Conditions above that stable layer are what meteorologists call "well mixed" to about 500 mb.  As soon as we mix out that shallow stable layer, we're going to see wind gusts picking up in strength.  

Source: NWS/Storm Prediction Center
Expect a warm, windy day today in advance of an Intermountain cold front that will be intensifying as it approaches northern Utah.  The NAM forecast below shows the front moving into northwest Utah at 3 PM, near ideal timing for maximum frontal intensity.  

Winds both ahead of and immediately behind the front should be quite strong.  The NWS is calling for gusts in excess of 60 mph across much of the western half of the state, including the Wasatch Front.  It will be interesting to see how much dust this kicks up and whether or not we get into a plume. 

After a high in the early afternoon today near 75ºF, the bottom falls out with and following the cold frontal passage.  NAM 700-mb temperatures drop from about +6ºC at noon to -7ºC by midnight, and then even a bit colder. If the forecast below verifies, we will see flakes on the benches with a coating possible on cold surfaces and the grass by morning.  Ah, spring!

Yesterday, the GFS was quite a bit warmer than the NAM following frontal passage, but appears to have shifted toward the colder NAM solution.  No complaints here as I like my cold fronts to be as strong as possible.  

Monday, April 21, 2014

I'm So Excited!

I've been waiting all spring for a nasty Intermountain cold frontal passage and it looks like we will get one tomorrow.

The GFS shows a whopper of a front moving across Nevada at 1800 UTC (noon) tomorrow.  The Salt Lake Valley is in the strong southerly flow ahead of the front.

This looks like a good setup for the usual pre-frontal spring mayhem, including gusty south winds and blowing dust.  The NWS has already issued a high wind watch for the Salt Lake Valley and most of western Utah.

The front is expected to sweep across Utah late in the day tomorrow.  The GFS has it just on our door step at 6 PM.  That usually ensures a very healthy drop in temperature.

The front will bring rain in it's immediate wake - possibly with some thunder.  Temperatures behind the front drop enough that we could see some bench snow overnight.

The GFS is warm enough we'd probably see mostly rain before things let up, whereas the NAM is quite a bit colder.  We'll have to see how things come together, but it's nice to see some real weather again!

Sunday, April 20, 2014

The Deception of City Creek Canyon

City Creek Canyon has to be one of the most remarkably accessible and spectacular canyons in the United States.  It's mouth is just a couple of hundred yards (if that) from downtown Salt Lake City, with the upper basin reaching altitudes of over 9000 feet.  It is also an important source of water for Salt Lake City.

Looking up the canyon from the Avenues foothills this afternoon makes you wonder where all that water comes from.  It was a poor snow season in the lower elevations and, remarkably, there's already very little snow visible in the canyon.

Indeed, the Louis Meadow SNOTEL at 6700 ft in City Creek Canyon bottomed out and went snow free today.  However, City Creek is a very deceptive canyon.  It is very hard to see City Creek's upper basin (purple area below) between Grandview Peak (also pictured above) and Lookout Peak (blocked in the photo above by Black Mountain) from either downtown Salt Lake City or the Avenues foothills (approx location and view of photo indicated by triangle).

If one looks at the Lookout Peak Snotel, just east of City Creek's upper basin at at an elevation of 8200 feet, one finds that the snowpack actually peaked a bit above average and is currently near average for this date.  A hint of this can be seen in the photo where you can just see some of the snow on Grandview.  Altitude makes a big difference this year.

Source: CBRFC
Given the pathetic low-elevation snowpack, the total runoff volume in City Creek will probably be below average this year, but the high elevation snowpack should keep some natural flow going through the spring.