Another Spring Storm, But With Greater Uncertainty

Another spring storm will impact the Salt Lake Valley on Friday and Saturday, but unlike the one we "enjoyed" on Tuesday and Tuesday night, I'm less certain how this one will play out.

The Tuesday/Tuesday night storm featured an upper-level trough preceded by flow from the south-southwest to southwest (lower left panel below).  The interaction of that flow with the high Sierra results in a trough that extends from the Sierra to northeast Nevada, well to the west of Salt Lake City and environs (lower right).

As a result, we could be confident that we would be in the strong southerly flow ahead of the storm on Tuesday and that there would be a good chance that we'd see blowing dust, as noted prior to the event on Monday.  In addition, the upper-level trough was moving quickly to the east, so that precipitation would wind down fairly quickly after the cooler air moved into northern Utah.

In contrast, the storm for for Friday and Saturday features a broader upper-level trough preceded by flow from the west-southwest (lower left below).  That subtle change in wind direction is significant because the west-southwest flow generates a surface trough that extends downstream from the Sierra into northwest Utah (lower right).

As a result, the trough and associated surface front will be developing right over northwest Utah tomorrow.  Some of the issues at play for anticipating the weather tomorrow are suggested by the latest NAM forecast for 6 am, noon and 6 pm MDT (1200, 1800, and 0000 UTC) tomorrow, shown successively below.  Northern Utah and the Salt Lake Valley sit near a somewhat disorganized surface trough in the morning that intensifies during the day.  Note that Salt Lake City is very near the transition to southwesterly flow to this period.  A slight shift of the trough position to the north or south will have a big impact on the weather we see tomorrow.

The GFS has a slightly different solution, perhaps because it is smoother, putting the trough just north of Salt Lake at noon before pushing it through during the afternoon.

It will be interesting to see how this plays out tomorrow.

In addition, the upper-level trough amplifies and slows down over our area, so there is greater potential (emphasis on potential) for periods of precipitation Friday night and Saturday.  The models are all over the place on this one with variations in the coverage and intensity of precipitation reflecting the complex nature of the event and the fickle nature of spring showers.  To illustrate this, I'm simply going to show a graph showing the forecast three-hour precipitation accumulations from the 22 members of the NCEP Short Range Ensemble Forecast system (SREF).  Note that the forecasts are best summarized with the phrase "all over the place" with contrasts in both the timing and intensity of precipitation.

Source: SPC

The total accumulated precipitation through 6 pm Saturday varies from 0.15 inches to 0.96 inches, with an average of 0.45 inches.  That's a big spread.  If we end up on the high side of these forecasts, there could be some decent skiing early Sunday (for late April), but no promises.

Everything but Bench Snow

Yesterday's Intermountain cold front delivered in spades.

Wind
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.  

Dust
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.

Snow
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!

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.  

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!