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). 

1 comment:

  1. I assume you went to the cloud seeding event at Snowbird? North America Weather Consultants is claiming a 4-13% increase in water falling as snow. These are crazy claims! Did they provide any proof?