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.  

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.  

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.  

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.

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.

The Tropics Are Coming to the Southwest

Three days ago we discussed the possibility of a major monsoon surge into the Great Basin (see Intricacies of Medium Range Monsoon Forecasts).  The large-scale pattern producing that surge now looks as if it will deliver in spades to the Southwest US and Northwest Mexico over the next several days in the form of not only the surge, but also the tracking of Hurricane Hilary along Baja California and then through SoCal after it has decayed into a tropical storm or depression.  

The GFS forecast shows the situation for 1800 UTC (1200 MDT) Sunday.  Hilary is off the Baja California Coast.  Tropical moisture in the form of a Monsoon Surge streams up the Gulf of California and into the Great Basin between an upper level trough along the Pacific Coast and a beastly ridge over the central US.  The latter has maximum 500-mb heights over 6000-m, which is quite high.  

This pattern persists through 1500 UTC (0900 MDT) Monday, with Hilary weakening but moving northward and affecting southern California, the monsoon surge persisting in the Great Basin, and monsoon moisture moving around the upper-level ridge into southern Canada.  

Thus, the impacts of this event stretch from Mexico to Canada.  

The GFS forecast for the Hilary storm center is on the west side of the potential track area forecast from the National Hurricane Center.  Their cone that illustrates the probable path of the storm center covers all of SoCal and the SoCal bight before extending up into northern California or Nevada.  Hilary is expected to be a hurricane through 12 AM Sunday, weaken to a tropical storm by 12 AM Monday, and then a depression as it continues northward.  

For northern Utah, much will depend on the location of the monsoon surge and the track of Hilary and her remnants.  I'll be keeping an eye on this and am interested to see what happens across the southwest US.