Wednesday, April 30, 2014

Climate Change for the Mountain Accord

If you love the central Wasatch Mountains, you might want to start following or get involved with the Mountain Accord, an initiative that is currently underway to develop a vision for an area spanning from the Wasatch Front to the Wasatch Back.  For more details see or their Get Involved page.

Knowledge of current climate and projections for the future are an important part of the Mountain Accord process. Court Strong (University of Utah), Tim Bardsley (Western Water Assessment), and I have been providing guidance and analyses to the Mountain Accord team to assist them in this effort, and I thought I would share some of the key graphs and figures below, as I thought they would be of interest to many readers.  

One key point with regards to climate model projections for the Central Wasatch Range.  It's difficult to imagine a situation in which our climate does not warm, barring some dramatic change in volcanic activity, decrease in the output of the sun, or calamity to civilization.  Indeed, the models consistently project a warmer future, with the rate of warming varying depending on future greenhouse gas emissions and the model.  Precipitation (liquid precipitation equivalent), however, is more uncertain and the models produce a wide range of future outcomes, some drier, some wetter, and some near current averages.

Snowfall Sensitivity

Percentage of precipitation (liquid equivalent) that currently falls as snow that would instead fall as rain per ºC of warming.  Based on Jones (2010).  Source: Steenburgh, J., 2014: Secrets of the Greatest Snow on Earth. Utah State University Press, in press.
Monthly Runoff Sensitivity

Average monthly flow sensitivities as percentage decrease in flow per 1°F for City Creek, Parleys Creek, Big Cottonwood Creek, Little Cottonwood Creek, and the Four Creeks combined.  Source: Bardsley, T., A. Wood, M. Hobbins, T. Kirkham, L. Briefer, J. Niermeyer, and S. Burian, 2013: Planning for an uncertain future: Climate change sensitivity assessment toward adaptation planning for public water supply. Earth Interactions, 17, 1-26.
Annual Runoff Sensitivity

 Average annual volume decrease under various projected temperature changes for City Creek (CK), Parleys Creek (PC), Big Cottonwood Creek (BCW), Little Cottonwood Creek (LCW), Provo River (Provo), Weber River (Weber) and Duchesne River (Duches).  Source: Bardsley, T., A. Wood, M. Hobbins, T. Kirkham, L. Briefer, J. Niermeyer, and S. Burian, 2013: Planning for an uncertain future: Climate change sensitivity assessment toward adaptation planning for public water supply. Earth Interactions, 17, 1-26.

Greenhouse Gas Scenarios for the Future

Greenhouse gas concentration trajectories (CO2) equivalent adopted by the IPCC for its fifth assessment report, 2013 (AR5).  Source: RCP Database version 2.0.5,
Projected Seasonal and Annual Temperature Change for the Central Wasatch Region

Projected seasonal and annual near-surface air temperature 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:
Projected Precipitation Trends

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:
Mean Trends in Snow Related Variables

Mean trends (%/year) in snow-related variables in the “Wasatch Mountain Region” (orange in left figure) derived using downscaled climate models for the RCP4.5 (center) and the RCP8.5 (right) climate scenarios.  SWE/P = Fraction of precipitation from 1 Oct – 31 Mar that remains in the snowpack on 1 April.  SWE = Amount of water contained in the snowpack on 1 Apr.  SFE/P = Fraction of total water in the precipitation from 1 Oct – 31 Mar that has fallen as snow. 95% confidence interval indicated by pink shading, green stippling, and vertical bars, respectively.  Source: Pierce, D. W., and D. R. Cayan, 2013: The uneven response of different snow measures to human-induced climate warming.  J. Climate, 26, 4148–4167. 

Tuesday, April 29, 2014

A Big Honkin' Ridge

The atmosphere is transitioning into outlier mode this week with the development of a remarkably high amplitude wave pattern at upper levels.  The impacts of this anomalous pattern are already being felt in the western United States.

The loop below shows the development of the high amplitude pattern over the past couple of days.  Note the amplifying upper-level ridge along the US West coast (black 500-mb height contours) and the strong high pressure system forming at the surface and eventually centered near the ID–MT boarder (color sea level pressure contours).

This pattern is already having a major impact on southern California with the resulting pressure gradient driving strong Santa Anna winds.

The strongest winds I've been able to find thus far are at Sill Hill at 3556 ft east of San Diego with a peak gust of just over 80 mph.

Given the drought conditions present over California, let's hope that they escape this event without any fires.

The ridge is expected to amplify through tomorrow afternoon [1800 MDT 30 Apr (0000 UTC 1 May)], with the overall pattern forming an omega-block like pattern over the eastern Pacific and North America (note the omega shape below).  The strength of the ridge over the Pacific Northwest and southwest Canada is quite unusual for this time of year with 500-mb heights of this magnitude unobserved over the past 30 years during the 3-week period from 20 April to 11 May. 

This will result in unusually warm spring weather for the Pacific Northwest (more on this "heat wave" at the Cliff Mass blog).  In fact, maximum temperatures in the lowlands west of the Cascades will approach or exceed 80 at many locations.  How about these forecasts from the National Weather Service for Portland and Seattle (I've truncated the stuff past Thursday). 

The ridge axis will eventually move over Utah (in somewhat weakened form), so we will see temperatures on the rise throughout the rest of the week. 

Monday, April 28, 2014

Lake Band Eye Candy

I love it when a band comes together.  

Lake-band panorama taken looking south (left) to northwest (right).  Click to enlarge.
The view from space, showing the narrow, isolated nature of the band
Source: College of DuPage
The loop below shows the development.  Note in particular the surge of northerly or northeasterly flow along the northern Wasatch Front and into the North Salt Lake City/Bountiful Area as the band forms.  What is chicken and what is egg is unclear, but we have seen similar surges in other events, but don't understand their cause or their role in lake-effect events. 

Click to enlarge

Never Give Up on the Ski Season

I wrote the previous post Korn Killer yesterday afternoon thinking it would be a good segue into a week in which we would see gradually rising temperatures and the eventual emergence of dust on the snowpack.  That's going to happen eventually, but it's going to take a bit longer thanks to last night's storm.

Let me start this post by saying this morning could provide some of the best powder turns of the year.  Whether you are at Snowbird or in the backcountry, the snow this morning will probably be sublime at upper elevations and you probably have the whole mountain (or mountain range) to yourself.  All I can say is curses to you and the skins you climbed in on.  Yeah, I'm jealous.  Powder is fun, but late season powder is such a treat.

Here's what's going on.  Alta-Collins got 11 inches of snow overnight.  Then, a lake band exploded shortly after 8 am this morning.  Check out the evolution in the radar loop below.

The transition from a broader area of weaker orographic precipitation to a more intense band shows up quite nicely in this video from our mountain meteorology lab on upper campus.  The perspective is looking south toward the central Wasatch.  The lake band develops on the right side of the loop near the end.

Ah, frosting on the cake.  Pity I will miss it.

Addendum: This post was edited to remove the video loop, which was causing problems with some browsers.  It can be accessed by a link contained in the text above.  

Sunday, April 27, 2014

Korn Killer

Last week we experienced a couple of dust events that are going to have a significant impact on the melt of our snowpack this spring.  

The first occurred late Tuesday when strong south winds produced a plume that extended from the Sevier Lake Bed to the Salt Lake Valley. 
Satellite image source: College of DuPage
Dust in the eastern Salt Lake Valley and Wasatch Mountains was thickest just ahead of, during, and immediately following the cold frontal passage.  The post frontal dust may have originated to the west.  In any event, the visibility at the Salt Lake airport dropped to 1/4 mile and PM2.5 concentrations reached 149 ug/m3, higher than achieved during wintertime inversions the past two years.  Nasty stuff.  The dust was followed by a relatively small storm on Tuesday night, but with a bit of wind to blow the snow around, plenty of dust-covered chocolate snow remained unburied at Alta on Wednesday.

Photo courtesy Will Farr
Then we had a weaker dust event on Friday.  One could see the degraded visibility in the afternoon before the precipitation picked up.

Dust in the air on Friday afternoon
The weak Friday event was followed by a more substantial storm Friday night though Saturday night, with more than an inch of SWE at Alta Collins.  

As a result, the dust layers formed by the Tuesday and Friday events are buried, but one needs only whip out the snow shovel to find them lurking 8 inches below the surface.  

What will happen as the snow starts to melt is it will percolate through the snowpack, passing through the dust layer.  Eventually, the dust will reach and remain on the snow surface, forming snirt (part snow, part dirt). Dust is darker than snow and, as a result, it absorbs more energy from the sun than pristine snow.  This leads to a more rapid snowmelt and an earlier loss of snowcover in the spring or early summer (depending on elevation).  The bottom line is that dust is a "Korn Killer."  Spring skiing would be better without it. 

Saturday, April 26, 2014

Spring Dumpage

It's looking Cascadian in the upper elevations of the Wasatch today.  Through 9 am, Alta-Collins has a storm total of 7 inches with 0.98 inches of water. Yeah, that's 14%, but it is fresh and it is late April.  

With a trough passage this afternoon shifting winds to northwesterly and bringing in cooler weather, one might find some decent turns tomorrow morning.  Get up early and get high (I'm referring to altitude—this is Utah, not Colorado).

Friday, April 25, 2014

Pretty Good Forecast

Following up on this morning's post, I thought I'd show that the HRRR is doing quite well for this event.  Below is the "analysis" for 2035 UTC (0235 PM MDT).  I've added a blue line showing a somewhat convoluted leading edge to the southerlies (note that I've ignored mountaintop obs for this analysis).  The southerlies have entered the southern portion of the Dugway Proving Grounds, have met the Great Salt Lake Breeze in the northern Tooele Valley, and have penetrated into the western Salt Lake Valley.  Note that the flow in the eastern Valley is either light southerly, easterly, or northwesterly.

The HRRR image I have from this morning's forecast is technically for 2130 UTC (55 min later), but still is a remarkably good forecast, especially with regards to the strong southerlies pushing northward into the Tooele Valley and western Salt Lake Valley.

The one quibble one might mike is that the surge of southerlies might have been a bit slow compared to observed (note that the observed southerlies have penetrated a bit farther north than forecast for about an hour from now).  Nevertheless, this is a good harbinger for the future of numerical weather prediction.

A Glimpse at the Future

Yesterday's post discussed some of the uncertainty in the forecast for today (Friday) and Saturday.  At issue for today was the position of the trough that was expected to develop over the Great Basin and extend downstream from the Sierra Nevada into northwest Utah.

Currently, that trough is located to the south of the Salt Lake Valley with northerly flow evident across much of northern Utah.  It is difficult to nail down the precise position during the early morning hours when shallow cold pools and drainage flows in the valley obscure the large-scale wind direction, but if I had to give my best guess I would probably put it somewhere in Utah county.

That's a bit south of where the models had it yesterday, but as I said, this was an event where one needed to recognize there was uncertainty in the forecast trough position.

Now I want to share with you some amazing images from a new forecast model called the High Resolution Rapid Refresh, known as the HRRR (pronounced "her").  Developed at the NOAA Earth Systems Research Laboratory (ESRL), the HRRR has 3-km grid spacing, radar data assimilation, and all sorts of other goodies that should allow it to become a game changer when it is finally implemented as an operational model at the National Centers for Environmental Prediction (NCEP).  Unfortunately, there have been numerous delays in the acquisition of a new computer at NCEP (see this post by Cliff Mass for why), but one can access the experimental version run at ESRL.

Those of you with a lot of bandwidth can access a monsterous loop of HRRR forecast surface winds for today by clicking here (it's worth a look).  Otherwise, just have a look at the two images below, which are wind forecasts for 2130 UTC (330 PM MDT) and 0100 UTC (700 PM MDT).  At 2130 UTC, note the dramatic transition between the strong southerly flow and the lighter flow to the north, with the boundary extending just into the southwest Salt Lake Valley.  Other tidbits here include downslope flow to the north of the Uintas (encircled by ligher flow) and numerous shear boundaries related to flow interaction with the topography.

By 0100 UTC, the boundary has pushed just a shade further north (quite a bit further north over Dugway Proving Ground), but remains just south of the Great Salt Lake.  Meanwhile, a front is pushing into western Utah from Nevada.

We will have to see how well the HRRR does on this event, but it has great potential to revolutionize short-range forecasting over Utah, as well as other parts of the continental U.S.  I'm hoping to integrate it into blog posts coming in the future - especially those related to snow!

Thursday, April 24, 2014

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.

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!