More colors this weekend than last in Big Cottonwood Canyon, but many trees remain green.
It's a somewhat surprisingly complicated weather forecast for the weekend, but not that atypical for September as we transition from the so-called monsoonal flow pattern to the cool-season westerly storm track.
We'll begin with a look at the GFS forecast valid 0600 UTC 18 September (0000 MDT Saturday). At this time, a deep upper-level trough (black contours) and frontal system are pushing onto the northwest coast. Ahead of this system is a much weaker trough over California. It doesn't look like much, but it contributes to a surge of monsoon moisture (color contours are precipitable water – a measure of the total atmospheric water vapor) into Utah (red arrow).
As the deep upper level trough and cold front push into the Pacific Northwest, the weak trough slowly moves northeastward and the monsoon moisture streams into northern Utah. By 1800 UTC 18 September (1200 MDT Saturday) the GFS is generating some scattered precipitation in northern Utah, mainly in a narrow region coincident with the monsoon surge.
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| Source: NWS, downloaded 8:35 AM MDT 17 Sep 2021 |
Sharing below this week's COVID-19 update by Dr. Michael Good, CEO of University of Utah Heath.
The good news is that COVID-19 cases on the University of Utah campus have been quite low, with an average rate so far of 9.1 per week. We now have 83% of University employees and 70% of students fully vaccinated. Another 6% of students are partially vaccinated. Let's push these numbers higher and continue to mask. I thank the students enrolled in my class for their consistent masking. They have made the semester easier and less stressful for me and I suspect their fellow classmates.
The bad news is that the coronavirus continues to rage through the unvaccinated population in Utah. Hospitalizations and ICU levels are now almost as high as they were during the peak of the pandemic last fall and winter. Cases amongst children (1-14 years old) are much higher this fall than last. In contrast, cases and hospitalizations amongst the vaccinated are much lower and the former has even begun to go down the past couple of weeks, perhaps due to adjustments in activities and behavior or greater masking amongst the vaccinated.
I have blogged previously about the threats of digital misinformation to society (see Challenges of Science Communication in the 21st Century, posted just before coronavirus exploded in the U.S.). We are seeing this playing out now.
Over the past several days, the extended range models have been suggesting that we will see a switch to cooler weather later this weekend or early next week. The latest (0600 UTC) GFS forecast for 1200 UTC (0600 MDT) Monday morning is unlike anything we've seen in some time with a deep upper-level trough centered over the interior west and Utah.
700-mb temperatures on Monday are as low as about -4˚C, certainly cold enough for snow down to about 7000 or 7500 feet.
If you must ask, here's a peak at our downscaled NAEFS product. Most members generate 0-3" of snow, but there are a few that are a bit more enthusiastic.
Today was an absolute peach of a day with smokeless air, blue skies, and hints of fall.
We've had few days like this over the past several weeks and I'm glad I was able to take advantage of it.
I hiked up Gobblers Knob via Butler Fork and Mill A Basin. If you're curious about the leaves, things are just starting to get going with a few trees and bushes changing as illustrated in the pictures above and below.
The aspen groves of Mill A Basin and upper Big Cottonwood are still green, although if you look carefully, you can see one patch of yellow along the Park City ridgeline in the 2nd photo below.
No snow yet, but it is acceptable to click on the photo below and start dreaming.
The fall equinox is only ten days away.
I'm not much of a fan of long-term outlooks for snowfall in northern Utah's mountains. There is a tendency in the western United States for the southwest to be dry when the northwest is wet and vice versa, so northern Utah sits near the transition zone between these two regimes. That tendency is sometimes referred to as a precipitation dipole, and it is related to the see-saw between El Niño and La Niña in the tropical Pacific Ocean, with the former favoring a wetter southwest and the latter a drier southwest, although such correlations aren't perfect.
For this winter, it is looking increasingly likely that La Niña conditions will predominate. This means cooler than average sea-surface temperatures in the tropical Pacific east of the dateline and warmer than average temperatures in the tropical Pacific west of the dateline. This affects the location of tropical thunderstorms and related precipitation complexes and, in turn, the position of the jet stream in midlatitudes.
The National Weather Service Climate Prediction Center (CPC) currently expects La Niña to develop in the next couple of months, with a 70-80% chance of La Niña during this winter (Dec-Feb). For more, see the diagnostic discussion they issued this morning.
Frequently seen characteristics during La Nina winters include a colder northwest North America, a wetter US Pacific Northwest, and a drier southern United States, as depicted below.
There is, however, variability from event to event, so I think of such patterns as a loading of the dice. And, because northern Utah is in the transition zone between the wet northwest and dry southwest, there's not much useful correlation between La Niña (or El Niño) and snowfall at Alta. This is illustrated by the graph below, which shows the 3-month accumulation of snowfall at Alta Guard compared to an index of La Niña/El Niño strength (more discussion of this at https://wasatchweatherweenies.blogspot.com/2015/08/wasatch-weather-weenies-survival-guide.html).
In my view, the future of seasonal forecasting lies with numerical modeling. Such forecasts are, however, in their infancy and perhaps where numerical weather prediction was in the 1970s. Seasonal forecasting requires the ability to simulate not just the atmosphere, but the oceans, the biosphere (plants and their response), and the cyrosphere (ice and snow coverage) and their interactions. We are making great progress in these areas, but we need higher resolution modeling, better "coupling" between these Earth system components, and much larger ensembles.
That being said, we can take a look at what these modeling systems are producing for this winter (December-February). Forecasts of precipitation anomalies (departures from average) from several modeling systems are below. One can see the influence of La Niña in these forecasts with most systems producing a drier southwest and southern United States and a wet Pacific Northwest. One outlier is the GEM_NEMO modeling system, which is keeping the entire west coast dry.
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| Source: CPC, https://www.cpc.ncep.noaa.gov/products/NMME/current/usprate_Seas3.html |
These models do not resolve the fine-scale terrain effects of the western United States. This is just one of many shortcomings. Some call for northern Utah to be wetter than average (e.g., NCEP_CFSv2). If you are looking for hope, none have the region around the central Wasatch below average, although I don't put much stock in that.
We can also look at the temperature anomalies, and here we see most models are calling for a warmer than average winter across most of the continental US, although there is one exception, the GEM_NEMO again which goes for a colder than average western US.
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| Source: CPC, https://www.cpc.ncep.noaa.gov/products/NMME/current/ustmp2m_Seas3.html |
Putting all of this together, I'm concerned about this winter extending the drought in the southwest US. Although not a guarantee, the dice seem to be loaded for below average precipitation and above average temperatures. It's tough to say what will happen in the northern Wasatch given that we're in the transition zone. Ski it if it's white.
