Monday, October 2, 2023

October Snow Is Here

October is coming in like a lion with thunderstorms late yesterday and more action overnight.  Dawn broke this morning with a light coating of snow at Alta.  I'm surprised nobody has put a skinner in yet. 

Source: Alta Ski Area

Looks like trace amounts at the base, but perhaps 2-3" up high, which may include a bit of snow from Saturday night.  It's unclear if the Collins snow-depth sensor is working yet, but the gauge came in with 0.38" of water equivalent in the last 24 hours. 

I mentioned in the previous post the possibility of snow, but was leaning toward a few inches and was inclined to have low expectations for the event.  The models, however, have been trending toward a wetter event.  The GFS, for example, is putting a low moving precipitation band directly over the central Wasatch today.  

It then brings a cold, moist secondary trough trough Utah tomorrow.  

Telescoping down to Little Cottonwood, the GFS generates a remarkable 1.75" of water at Alta-Collins from midnight last night through this afternoon, followed by dribs and drabs tomorrow with the trough passage.  It's a warm event, with the wet-bulb zero between 8000 and 9000 feet, but that would still mean snow levels around 8000 feet.   

The HRRR is coming in with somewhat similar numbers, but has a bit less precipitation for today.  

Snow totals for the HRRR are around 12" and for the GFS around 20" for Alta-Collins, which is at 9700 feet.  I suspect the snow-to-liquid ratios being produced by our algorithm are going to be too high for this event since it's not really designed for early season events with a warm ground and frequent excursions during dry periods to temperatures above 32°F.  

I'm inclined to lean toward 4-8" of wet snow from 8 AM to 6 PM for today at and above Alta–Collins.  The GFS is putting out some big numbers, but I think it may be generating a bit too much precipitation based on what I'm seeing on the radar this morning.  Things should quiet down tonight and then we will have to see what happens with the trailing cold trough.  

Friday, September 29, 2023

Fall Weather for the Weekend

I had a meeting yesterday at Snowbird and it didn't disappoint with lovely fall colors in Little Cottonwood Canyon.  

The concept of "peak color" in Utah has always been an odd one for me.  Unlike the northeast US where the elevation contrasts are smaller and the trees and shrubs seem to have a more consistent timing for the fall transition, there are fairly large variations with elevation and species here.  That said, I thought it was quite pretty throughout the canyon and especially enjoyed what I think were gamble oak that were on fire in places in the lower to mid canyon.  

The forecast for this weekend is also very fall like.  A deep upper-level trough is digging and amplifying just to our west today and tomorrow.  Saturday we will be ahead of the system in the warm southerly flow with temps near 80°F in the valley.  Expect some breezy conditions in the valley and in the mountains and high clouds at times.  

Then the wheels begin to fall as the trough progresses slowly eastward and colder, moist air bleeds in on Saturday night bringing some showers and possibly thunderstorms.  Sunday will be more than 20°F cooler with showers and possibly some thunderstorms at times.  

Just how much rain we get on Sunday will depend on factors I can't reliably anticipate at this time.  If we can get into the so-called "dry slot" associated with the low, it might not be too bad of a day.  On the other hand, we could also see considerable shower activity.  The downscaled SREF has some members with fairly light precipitation (less than 0.1" total, whereas others are wetter.  Of the latter, some are wetter Saturday night, others during the day Sunday.  

I'm going to do my leaf peeping on Saturday and then adjust plans depending on evolving forecasts and weather on Sunday.  

I'd be remiss if I didn't mention the possibility of some mountain snow Saturday night and Sunday and then through early next week as the coldest part of the trough moves through.  The latest GFS-derived forecast for Little Cottonwood shows the wet-bulb zero level dropping to as low as 8000 feet Sunday morning and then lingering around 9000-8000 feet through Wednesday (upper right panel below).  

The snow level is usually 500-1000 feet below this.  For Alta Collins, which is at 9700 feet or so, the GFS is putting out a total of almost 0.7" of water through Tuesday, which equates to perhaps 5 or 6 inches of snow.  The sometimes overoptimistic NAEFS ensemble has a few members eclipsing 15" but most are in the 5 to 12 inch range.

I'm inclined to knock those numbers down a bit, with a few inches on the highest peaks the most likely scenario.  If you are hoping for skiable snow, it would probably take the right flow and some lake-effect magic to kick in early next week as the trough slides eastward.  For example, the Euro puts us in cold northwesterly flow at 0000 UTC 4 October (6 PM MDT Tuesday).  

That's nice, but a lot would need to come together to give us a big dumpage.  I'm inclined to have low expectations for this event.  At this time, the odds we get enough snow for marginal ski touring in grassy areas are fairly low.  However, if you are desperate for hope, they are non zero. 

I'll also add that this appears to be a one-off trough with a return to dry fall in its wake.  We'd be better off if it didn't snow at all (yes, I know that is heresy). 

Wednesday, September 27, 2023

Another Year of Limited Bus Service

It was pretty disappointing to see yesterday that UTA will only be offering limited ski bus service again this season.  

Last year, even with the frequent closures of Little Cottonwood, Utah reported a record of 7.1 million skier visits.  On many days, parking lots were full.  Parking at trailheads was also tough to find.  What few buses were run were packed.  

We are still two years away from the so-called "phase 1" enhanced bus service for the Cottonwoods planned for fall 2025.  At that point, we are told that the roads will be tolled and there will be a mobility hub at the gravel pit, winter roadside parking restrictions, etc.  That will provide some much needed infrastructure upgrades, but one has to wonder if there is real commitment to providing serious bus service.  

In the interim, maybe it is time for slugging, or ad hoc casual car pooling, as is done in Washington DC, where drivers can pick up passengers (in this case skiers or snowboarders) at specified locations.  Call it ski slugging if you want.  I've always been surprised that we haven't seen more of it here.  

Thursday, September 21, 2023

Is It Really Going to Snow?


At upper elevations.

But not a lot.

Don't plan on skiing.  

The instigator is a cold upper-level trough that is dropping into eastern Oregon and Nevada today.  By 0000 UTC 22 September (6 PM MDT Thursday afternoon), the low center and the coldest air over southeast Oregon.  

During the day today, we will be seeing some periods of rain as the weak front ahead of this system pushes through.  The temperature this morning (0700 MDT) is 34°F, with a wet-bulb temperature of 31°F, so that's cold enough for snow on the highest peaks of the Wasatch.  Snow levels will lower later this morning and probably get to around 9000 feet this afternoon.  

The GFS is putting out about 0.35 inches of water for Alta Collins through Friday night, which given relatively low snow-to-liquid ratios equates to about 3.5 inches of snow.  Those snow-to-liquid ratios are probably higher than they should be as this time of year the ground is warm and there will be melting between snow periods as temperatures rebound when there isn't precipitation.  

The HRRR is less excited, and putting out 0.17" of water and 1.4" of snow.  Yawn.  

If you want more, you'll have to hope that the strong showers (there may even be some thunderstorms) setup in the right place to provide a strong and prolonged period or two of snow.  That's not impossible, but it's more likely that we see 1-4" of wet snow above 9000 feet, with the higher accumulations happening only if we can sustain snowfall rates for a while.  

It would be better to save this until early November, but Mother Nature does what she wants.  

Monday, September 18, 2023

Congrats to Sepp Kuss


Source: @lavuelta

A few years ago a friend and I were discussing the state of American cyclists on the world tour.  He said I should keep an eye on Sepp Kuss.  Kuss then proceeded to destroy the field in the Tour of Utah.  Perhaps he was on to something.  

Fastforward to today and Kuss is now the winner of the Vuelta a EspaƱa.

It was great to see the famed superdomestique and exceedingly nice Kuss win a grand tour.  Many of speculated for some time if he could do it.  It certainly wasn't according to the script going into the race (he was riding in support) and some good fortune and a great team helped of course, but that's how these things are won.  

Of course Kuss is also a Nordic skier, so that's a good thing too.  

Congrats to Sepp.

Friday, September 15, 2023

Glacier Change on Mt. Rainier

Anyone who has lived in Seattle, as I had the good fortune to do from 1989 to 1995, knows it as "The Mountain."  On clear days, the snow-and-ice-covered mass that is Mt. Rainier looms over the city and serves as a beacon for mountain adventures.  I spent a great deal of time hiking on its lower flanks and even summited once.  Amongst many gems in the National Park System, Mt. Rainier National Park is one of the best.  

But as discussed in a New York Times article published earlier this week, the glaciers of Mt. Rainier are suffering (that link is not paywalled, but I'm not sure if there is a limit on how many people will be able to use it).  The article is based on a recently released National Park Service report available at at   

As detailed in that report, glacier area on Mount Rainier declined 44% from 1896 to 2021, a net loss of just over 55 square kilometers of glacier area. 


Not surprisingly volumes are also declining.  Quoting the report:
"All 28 glaciers at Mount Rainier are in retreat, losing an average of -0.430 km2 per year (-0.166 mi2 × yr-1) parkwide during the last 125 years. Since 2015, this rate has increased to -0.544 km2 per year (-0.210 mi2 × yr-1) parkwide. When looking at volumetric data, the Park has lost a total of -1.057 km3 (-0.254 mi3) of glacial mass since 1970, at a rate of about -0.021 km3 per year (-0.005 mi2 × yr-1)."

One glacier, the Stevens, has been removed from the glacier inventory and is now considered a perennial snowfield.  Two others, Pyramid and Van Trump, are on life support.  Glacier retreat is most pronounced on the south side of Mt. Rainier, but is also occurring on the north side.  

For the lovers of ice, these are sad times.  

Sunday, September 10, 2023

Views from the Crest Trail

It has been a remarkably long time since I last rode the Wasatch Crest trail.  It was great to get out on it today and enjoy the lingering wildflowers at upper elevations.  

They seem to be hanging on unusually late this year, but that's just an anecdotal impression. 

More than 25 years ago, when I first moved to Utah, we used to ride from what is now known as Canyons Village, up what was then the Park West Ski Resort, to the top of Mill Creek.  We would then drop all the way down Mill Creek and back to campus or the Avenues.  Using the cantilever breaks of the time, one would get to the bottom of Mill Creek and not be able to get your hands out of a claw.  

Modern disk breaks are simply amazing and they handle such a descent with no problem.  Plus the recently completed upper pipeline means its single track all the way to Rattlesnake Gulch.  And, instead of descending down the heinous Rattlesnake gulch, one can "snake" down the Rattlesnake section of the BST, pictured below.  

You youngsters don't know how good you have it. 

Kudos to the bike manufacturers and trail builders.  

Friday, September 8, 2023

The Great Salt Lake Is Still on Life Support

The situation report for the Great Salt Lake is still quite concerning.  Although it is true that the south arm of the Great Salt Lake rose just over 5 feet during the spring runoff, the reality is that this was accomplished due to a remarkably anomalous heavy snow season combined with plumbing the rock causeway that divides the lake to keep most of the inflow in the south arm.  

Below is the landsat satellite image for the 29th of August 2022 (i.e., last summer) when the lake elevation was about 4189.3 feet and approaching its record low that fall.  Considerable lake bed is exposed all around the lake, including along the east side in the Bear River and Farmington Bays. 


Shift to this year and there is some greater coverage in the Bear River Bay and near Antelope Island, but extensive exposed playa remains and there has been little discernible change in the coverage of the North Arm, at least with a quick glance at these images (more on this in a minute). 

Lake elevation measured at Saltair on the south shoreline shows a substantial increase from the historical minimum just below 4189 feet last fall to about 4194 feet in June.  Since then, the lake level has declined about a foot and a half to 4192.5 feet today.  


A fall minimum just below 4192 feet seems likely.  Keep in mind that until this latest low stand, the historical record low elevation was 4191.35 feet in 1963.  Basically, we are now just a bit over a foot above that.  

The Great Salt Lake is divided in half by a rock-fill causeway that limits the flow of water between the two halves.  Due to efforts to restrict flow between the two halves and raise water levels in the south arm, the spring recovery there was more limited.  In fact, water levels there peaked out about a foot below the prior year, although the decline this year has been less abrupt and it currently sits at 4189 feet, just a bit above last summer.  


So, if you need another reason to root for another big snow season, this is it.  The Great Salt Lake is still on life support.  

Monday, September 4, 2023

This Is All Pretty Incredible

What a deluge we are having today.  The precip totals across portions of the southwest US this August and early September are truly remarkable.  The irony is that in other parts of the so-called monsoon region, precipitation has been well below average.  

Let's have a look at some analyses and numbers. The National Weather Service produces a gridded precipitation analysis for the continental US that one can use to examine percentages relative to a long-term average.  The analysis has some serious warts in places (i.e., there are areas with poor radar coverage and no observations that cause artifacts), but for the most part, it is quite helpful in illustrating what has been going on over the past 30 days.  It shows well below average precipitation in south-central and southeast Arizona and most of New Mexico.  These are areas typically see the most monsoon precipitation.  

Then there's southern Califonia, Nevada, southern Idaho and western Utah.  Well above average including > 600% of average in some areas.  Now these are areas that see little precipitation, but still therte are widespread areas with > 2" and lots of local areas with > 5". Some of this is due to the remnants of Hurricane Hilary, but there have been other potent monsoon surges in this region. 

How about we look at some numbers from 1 August through 4 September at a few sites.  

For Salt Lake City, we've received 2.77" of rain through yesterday (3 September) and about a half an inch so far today.  That pushes us up to about 3.25" with more possible.  That will put us at least in the top 3 for the period with #2 being 1945 with 3.29" and 1968 #1 with 3.66".  Basically, this is easily one of the wettest monsoon periods on record.  

St. George has had so much rain it's ridiculous.  Not including today, they've had 5.40".  The previous record was 3.07" in 1925.  That said, there is a bit of an asterisk since the records for St. George are not complete and there are quite a few periods with a large number of missing days.  Still, that's a hell of a lot of rain.  

Las Vegas has more complete records back to 1937.  Not including today (I haven't looked to see if they have or are getting anything), they've had 2.35", good for #2 behind 1957 (2.59").  There are two other periods with more than 2", 1979 and 2012.  


What to have a laugh.  Check out these numbers for Death Valley.  The site has complete records back to 1965, but there are some gaps prior to that.  Still, they are at 2.43", compared to the prior record of 1.89" set just last year).  Most of this year's precip was from Hilary.


I could go on, but you get the point.  This is a dry region.  Portions of California really got bombarded by Hilary.  One event did the damage.  Multiple monsoon surges have affected western Utah.  Critical in nearly all of these events has been the interplay of monsoon moisture, often originating from near the Gulf of California but also potentially including moisture from the Gulf of Mexico, with troughs in the mid-latitude westerlies.  

Just as an example, below is the GFS analysis for 1800 UTC 1 September (1200 MDT Friday), which was the precursor for this current event.  At 700-mb (lower left) flow originating over the Gulf of Mexico is confluent (i.e., merges with) flow moving over Baja California and the Gulf of California over the Lower Colorado River Valley.  A corridor of strong vapor transport (lower right) extends through the Lower Colorado River Valley into Utah.  All of this ahead of a closed low over northern California and Oregon.  

The interplay between the monsoon moisture and the troughs in the midlatitude westerlies are often critical for precipitation in northern Utah as one has to get the monsoon moisture into our area.  Additionally, the flow associated with the midlatitude trough can help with thunderstorm dynamics, although for the current storm, we almost have a winterlike system with monsoon moisture.  The latest radar loop shows lots of precipitation features in northwesterly flow like we would see in the winter or spring.  Hopefully it comes out.  Blogger has been pretty cranky lately with movies.  

All of this rain makes me very happy.  I'll take wet sweater weather over 100°F nuclear summer any day.   

Friday, September 1, 2023

Summer Is Over!

September has arrived and meteorological summer (June, July, and August) are now thankfully in the rear-view mirror. 

Eddie Vedder wrote Rearviewmirror about leaving his stepfather, so it's about more serious things, but the song seems appropriate today.  Turn it up to 11.

Summer 2023 comes in as the 8th warmest in Salt Lake City with an average temperature of 78.6.  


If it didn't seem very hot, that's a case of recency bias as this summer was 2.3°F cooler than 2001 and 2.9°F cooler than 2022.  

It was also a fairly wet summer with a total of 3.2" of precipitation, rating 36th out of 150 summers with records. Most of that (2.75") fell in August, which rated as the 5th wettest on record and the wettest since 1968 (1983 was close with 2.64".  


There was one other record set this summer...most pageviews of this blog.  Sadly, that's not due to human traffic, but bot traffic from Singapore.  It appears to have ended after a couple of weeks.  

Thursday, August 31, 2023

Labor Day Monsoon Surge

It looks like an active Labor Day weekend (not to mention tomorrow) in the weather department.  The models are advertising a potent monsoon surge similar to the two previous big ones we've had so far this summer.  

The GFS forecast below for 0000 UTC 2 September (6 PM MDT Friday) shows an upper-level trough over northern California and an upper-level ridge over Kansas. Between these two systems is southerly flow and northward vapor transport at all levels with moisture streaming into Utah from the subtropics and Gulf of California.  

Bottom line is expect showers and thunderstorms beginning tomorrow and through the Labor Day weekend.  I'll add that the models are also forecasting that the west-coast trough will become mobile over the weekend and move through Utah late Sunday and Sunday night, which could yield a frontal passage and an especially active period of thunderstorms and rain.  

It's been a wet water year (October – September), so it seems appropriate that we have wet Labor Day weekend as we enter the last month.  

My plan is to stay home and take advantage of the breaks.  If you are heading out, consult National Weather Service forecasts, monitor watches and warnings, and be aware of flash flood and lightning risk.  

Monday, August 28, 2023

Expectations for the Coming Ski Season

You may have seen some guesses about what will happen this coming ski season.  Such predictions are often issued with great fanfare and overconfidence.  I'll pick on AccuWeather here because they are an easy target.  The outlook they issued in late September 2022 included the following.

"Unfortunately, we have bad news as far as the drought goes in parts of California, Nevada and the Southwest," Pastelok said. "The main storm track will be even farther north than it was the first half of the winter season last year including the late fall."

Then there was the accompanying snow outlook. 

What a spectacular crash and burn!

The reality is that these seasonal forecasts, based largely on the loading of the dice due to the anticipated presence of El Nino or La Nina, have somewhat limited practical utility for most applications.  If you didn't have any knowledge of meteorology, you might expect there to be a 33% chance of below average, 33% chance of near average, and 33% of above average precipitation in any given season.  At best, we can shift these odds just a bit.  For temperature, we can shift them a bit more simply because of the growing influence of global warming.  

So let's look at expectations for this season.  El Nino conditions are currently in place across the equatorial Pacific Ocean with the most anomalously warm sea surface temperatures (SSTs) off the South American coast, but anomalously warm sea surface temperatures extending across the entire equatorial Pacific.  

Source: NOAA Climate Prediction Center

El Nino is expected to persist through this winter (the Climate Prediction Center gives a > 95% chance it will survive through at least February).  This affects thunderstorm characteristics in the tropics, which in turn can affect the midlatitude jet stream.  Correlations between El Nino characteristics and temperatures and precipitation over the western US weigh heavily into seasonal forecasts, with recent long-term warming trends also considered. 

For Nov-Dec-Jan this leads to the dice being loaded for above average temperatures across the western Continental US.  In this case for Utah, there is a 40–50% chance of above average, or what the Climate Prediction Center calls "leaning above" (see scale at lower left).  

Source: NOAA Climate Prediction Center

For precipitation during Nov-Dec-Jan, they are loading the dice slighly for below average in the Pacific Northwest, but indicate equal chances across the rest of the western continental US.  Basically, they don't have any confidence in it going one way or the other.  

Source: NOAA Climate Prediction Center

Below are the projections for Feb-Mar-Apr.  

Source: NOAA Climate Prediction Center

Source: NOAA Climate Prediction Center

I'm a little surprised that they aren't leaning above for precipitation in portions of SoCal with El Nino being expected.  This may be due to the fact that this was issued in mid August and perhaps we'll see that appear if confidence in the El Nino grows over the next month or two.  

Bottom line here is that I see no reason to change the seasonal snowfall forecast that I have issued for the past several seasons in the Wasatch Range.

I have no idea what is going to happen. 

I guess I might add on caveat and that is that I would lean toward above average temperatures for the cool season and a vulnerable low-elevation snowpack.  As we saw last year, a favorable jet stream can still give us an enormous low-elevation snow season, and we will need probably need an active storm track with some colder storms to overcome recent warming trends and what are currently remarkably high global temperatures.  

Friday, August 25, 2023

Snowfall in the Deer Valley Expansion Area

Yesterday Deer Valley dropped one of the biggest stories in Utah ski history, announcing plans to add 16 new chairlifts, 3700 acres of terrain, and 135 ski runs in an area including, around, and above the nascent Mayflower resort.  This includes terrain south of the current Jordanelle Gondola and below the Sultan and Mayflower lifts, above the developing Mayflower resort (labeled "new base area" below), and the ridge that extends southward from Bald Mountain toward Heber Valley, including Park Benchmark Peak (9384 ft).  

That is an enormous expansion, but this is a blog about mountain meteorology and snow snobbery, so lets take a look at the meteorology.  

For my book Secrets of the Greatest Snow on Earth, I produced an analysis of mean annual snowfall in the Wasatch Range.  It illustrates what, in the 1930s, meteorologist S. D. Green called the natural advantages of the Cottonwood Canyons.  Snowfall in the high terrain around the Cottonwoods, especially Little Cottonwood, is much greater than it is on and east of the Park city Ridgeline and the Deer Valley area.  

Source: Secrets of the Greatest Snow on Earth, 2nd Edition

In fact, near the bases of Park City Mountain Resort and Deer Valley, snowfall is about 40 to 50 percent lower than found at comparable elevations in Little Cottonwood Canyon.  This is largely a consequence of being on the climatological leeward side of the central Wasatch.  

I've identified the area of the Deer Valley expansion in yellow in the figure above.  Snowfall at the base of the Mayflower area is probably around 150 inches.  The terrain between Jordanelle Gondola and Mayflower and below the base of the Sultan chair probably averages 150 to 200 inches of snow a year, with the highest amounts near the base of the Sultan Chair at just over 7500 feet.  Near the summit of Park Benchmark Peak the averages will probably be a bit over 300 inches.  

The area will be favored in southerly to southeasterly flow storms that favor heavy snowfall in the current Deer Valley Resort.  Probably the best terrain for powder skiing, will be the northeast facing shots off the ridgeline that extends southward and southeastward from Park Benchmark Peak.  Depending on how far south they are going, there could be some northeast and north facing options off of other ridges a bit farther south.  

Map source: CalTopo

Due to the low elevations of much of the terrain, it will be more vulnerable to climate change.  In a warming climate, a greater fraction of precipitation that previously fell as snow will fall at rain at 7000 feet compared to 10,000 feet.  More mid-winter snow-loss events will occur at those lower elevations as well.  Even in the current climate, snowmaking for much of this new terrain is essential, and will become even more critical in the future.  

Monday, August 21, 2023

Hilary Storm Totals

As anticipated, Hilary brought quite the deluge to SoCal and Southern Nevada.  Below are non-quality-controlled gauge reports to MesoWest for the 3-day period ending this morning (there are a few erroneous observations plotted if you look).  Widespread reports of more than 2 inches in the LA Basin, 5 inches in portions of the San Gabriel Mountains, more than 5 inches in the southeastern end of the Sierra Nevada, 3 inches in the Amargosa Valley, and 5 inches in the Spring Mountains.  

A few reports that caught my eye include:

Mt. San Jacinto with 11.75".  What an unbelievable forecast by the HRRR model discussed in an earlier post.

Palm Springs Regional Airport with 3.23". The mean annual is about 5.5 and comparison with Mt. San Jacinto to the immediate west illustrates remarkable orographic enhnancement. 

Hunter Mountain with 5.89".  This site is in the mountain forming the northern end of the Panamint Valley just west of Death Valley.  Despite elevation (6880 ft), it is remarkably dry.  

Furnace Creek (Death Valley) with 1.70".  Well over half their mean annual precipitation of 2.2" and 17 times their average for August (0.10").

Spring Mountains Deluge.  I'm not sure how much faith to put in the highest reports from the Spring Mountains of Nevada, but lower sites in Kyle and Lee Canyons reported just over 5" and upper canyon sites over 7".  The Lee Canyon SNOTEL site reported 8.8" and the Bristlecone Trail site a bit higher 10.80". 

Quite an event.

Saturday, August 19, 2023

Hilary Update

It's just after noon on Saturday with a very eventful 60 hours or so on tap for SoCal, Nevada, and possibly portions of adjoining states due to one of the more significant tropical cyclone events in recent memory for the southwest US.  

According to the latest (12 PM MDT Saturday August 19), Hurricane Hilary is currently a category 3 hurricane with 115 mph winds.  Satellite imagery showed the circulation center of Hilary just west of the southern tip of Baja California with high clouds extending northward across Baja, SoCal, Nevada, and even northern Utah.  

Source: College of DuPage

Beneath those high clouds is a predecessor monsoon surge that is working its way up the lower Colorado River Valley and environs.  Radar imagery at 1200 MDT (1800 UTC) Saturday showed showers and thunderstorms in southwest Arizona and southeast California. The heaviest were near and within an area with precipitable water values at or above 50 mm (2 inches).  

Precipitable water is a measure of the total integrated water vapor content of the atmosphere with height, expressed as a depth if the vapor were condense.  Values in excess of 60 mm (2.35 inches) are very close to Yuma where upper-air sounding records extend back to 1955 and the highest observed is 2.33 inches.  

This is a juicy airmass and it is going to get juicier!

The National Hurricane Center calls for Hilary to track northward across northern Baja, SoCal, and central Nevada.  Winds are expected to weaken to tropical storm strength when it crosses into SoCal and then further as it moves into Nevada, but most of the impacts will be due to precipitation and flooding.

Source: NHC, Issued 12 PM MDT Aug 19, 2023

Thus, don't be fooled by language like Hilary being downgraded or weakening.  That pertains solely to the strength of the maximum sustained winds.  The potential for serious flooding and even some wind damage in some areas remains.  

To the former, check out the GFS total accumulated precipitation forecast from 1200 UTC 19 August (this morning) through 0000 UTC 22 August (6 PM MDT Sunday).  More than 2 inches with locally greater accumulations across portions of SoCal and south-central Nevada and local accumulations around 5 inches. 

Source: TropicalTidbits

And these are remarkably dry areas.  The GFS is putting out 4" for Death Valley, for example, which averages 2.24" a year.

The HRRR isn't quite as excited, but is still generating widespread 1-2.5+" for much of SoCal into south-central Nevada. A few high elevation areas are well over 5".

I plucked out the HRRR forecast for near Furnace Creek and it's just over 2.5".

And for Mount San Jacinto west of Palm Springs its 12.5+ inches, I think.  I say I think because my code wasn't design to handle these outrageous totals so things are being plotted completely off scale.

The NWS forecast is for 5-7" in the Mount San Jacinto area, but also enormous totals of 4-5" in Palm Springs which is approaching their average annual precipitation of about 5.5".  

Source: NWS; Issued 4:33 AM PDT 19 August

This storm will come hard and fast.  I don't really have an analog in my mind for it.  It's probably good the storm is moving rapidly so that the intense precipitation doesn't linger for another day or two.