Dual Atmospheric Rivers

Tuesday, December 6, 2016 at 7:30 am

Summary:

A very cold system will bring snow showers to the region later today into tonight with mostly light accumulation.  Models have been keying in on the possibility for two sets of atmospheric rivers to bring the region tremendous amounts of precipitation over the next 10 days.

Details:

Before I begin, it’s important to note that atmospheric rivers are very volatile and if the jet buckles or points slightly to our north or south, that the forecast amounts could change dramatically.  So none of this is guaranteed, but it is worth exploring the possibilities.

Today we will see a very cold follow-up system drop through the region.  It is currently 15F at my house on the bench and almost all mountain locations are in the single digits.  These temps will drop even colder for tomorrow morning.  BE PREPARED!  As for snow, generally it looks like 2-5″ of snow in the mountains today and tonight as snow showers become more widespread this afternoon and evening.  The Cottonwoods could get the benefit of some lake enhancement and could see slightly more.  The NAM is calling for up to 8″ for the Upper Cottonwoods by tomorrow morning:

1206_nam

Not sure I buy it, but is definitely possible.  Tomorrow could be a bit of a sleeper powder day if this comes to fruition.

We should have a break Wednesday into early Thursday with temperatures warming significantly on Thursday ahead of the next system.

AR #1:

The first AR event will push into the region starting on Thursday night.  We should see snow levels rise with this system as mild air moves in from the Pacific.  Starting with such a cold airmass means that there could be pockets of low elevation snowfall or even freezing rain if we can’t fully mix out the valleys before precip starts.  Snow levels should rise up to between 6-7k feet.  For Park City, it looks to remain snow in town at this time, but snow levels are always tricky so no guarantee.

The mountain snowfall should continue off and on thru Friday, then perhaps a break late Friday into Friday night.  Then a second wave pushes in for Saturday and brings more heavy snowfall to the area.  We should start to clear out by Sunday.  Total snowfall with this first wave should be significant for the high elevations.  Currently, the NAEFS plumes looks like this:

1206_plumes

Univ of Utah

If we ignore the light blue lines (which represent the canadian ensembles) as they seem to be erratic, we find that the mean snowfall for the Upper Cottonwoods thru Sunday AM is about 35″.  Usually this is overdone.  The Euro is showing about 2″ of liquid for this same time period.  All things considered, I think 10-30″ is possible for the mountains of Northern Utah with this system.  The highest elevations that are favored by a westerly flow could easily see higher amounts.

The screaming message is that it seems likely that we should get a good shot of base building snowfall.

AR #2:

After taking a bit of a break Sunday and perhaps Monday of next week.  It looks like we have a good chance for a second, perhaps even stronger AR event.  Models have been very consistent in the last few days with handling this system.  Too early to know what snow levels will look like, but precipitation amounts through the end of next week look off the charts.  When you couple this second AR with the first one, you get a total GFS forecasted snowfall that looks like this:

1206_gfs

A huge swath of the mountains is “off the charts” snowfall.  This map actually portrays a 10-day maximum snowfall of 148″.  Now, before you go running around telling people that we are going to get 150 inches of snow, it’s important to point out that this is almost certainly overdone and I expect this model to come back to earth.  As mentioned earlier, these atmospheric rivers are hard to forecast and there’s a solid chance we get missed completely.  The Euro is much more conservative and is portraying plenty of precipitation for us, but it’s certainly nothing mind-boggling like we are seeing from the GFS.

I think getting excited is ok, especially for this first AR event as it’s much closer and confidence is higher but it’s important to note that none of this is guaranteed and the forecast could change rapidly.  I’ll try to update often if anything changes over the next 10 days.  Who needs sleep anyway?

I’ll leave you with this 10-day loop showing total QPF.  Notice the Sierras getting slammed…

1206_total_precip_loop

WSF





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  • Matt Kasner

    Time to take some PTO from work!!!

  • Florida Sam

    WSF

    Every year we plan a trip from Florida, and we are doing that again this year. BUT, this year in addition to a week trip, we are going to try to thread the needle for a 2 – 3 day powder chase…. from Florida…. hahahaha Obviously, that’s not easy considering it’s a 5 hour or longer flight there.

    I know the none of the resorts have the entire mt open yet, but do you think the first event would be enough to open most mt’s up and the second event could just be an amazing time?

    or am I way ahead of myself, and it’s a bit early for a chase trip. The chase trip goal is getting some deep and hopefully refills.

    Should I just look at this as events that get the Mts open and move on….

    Thanks!

    • Peter Donner

      Sam, don’t want to steal Evan’s thunder, and he has committed to update frequently, but you can get an updated picture on your own by going to weather.utah.edu, and looking at the “plumes”, which are automatically updated. Evan has the Alta plume in this post.

      The Park City base plume is at
      http://weather.utah.edu/index.php?runcode=2016120600&t=naefs&d=PL&r=PCB

      Right now it’s got a minimum of 13 inches by Dec 13, and a mean of almost 20.

      Based on my experience w these plumes living in Salt Lake skiing every storm I agree w Evan the mean is probably overdone. So you can check this once or twice a day and see how it evolves then pull the chord to buy a plane ticket.

      Or you can wait a few hours or a day and Evan will give you an excellent summary

      • bcbuse

        From weather.utah.edu how exactly do I pull up the Alta “plume”?

        • Peter Donner

          It’s not obvious. Took me a bit to figure out myself–that’s why we love Evan.

          On the left under the “U” there is a bunch of “stuff”

          GFS-0.5 deg
          NAM-12km
          etc.
          NAEFS-Downscaled
          more etc.

          Click on NAEFS Downscaled, it opens to a list
          Click on Plumes

          Click on the plume you are interested in

    • Chad Niel

      I’m not sure what you mean by most of the mountain, but the word from the snowbird Lifties we talked to yesterday was the cirque should open with another 12 inches of good base snow which this stuff is.

      • Florida Sam

        That’s sorta what I was talking about. Cirque isn’t open (yet), at Brighton Milly isn’t open, Powder Country at PowMow isn’t open… I know there is a lot terrain open, but I don’t think anyone is at full operation yet. Sounds like AR1 might be the key. Thanks for the info from the lifties.

        • Peter Donner

          Yeah, that’s where the Park City base plume comes in. I’m not sure, but I think it is 7000 feet, if you drop the lat and long into google earth you can get an (almost) exact elevation. Anyway, if you monitor this particular plume you get an idea of snowfall at the base of the resorts. The Alta Collins plume is upper mountain at the snowiest resort. Combine the two you get a good sense of how much terrain will open. As a sidenote, the minimum of the Powder Mountain plume is 20 inches by Dec 13, mean is 35 inches.

          If something between the min and the mean on these plumes verifies, winter is on and most terrain will be open.

          If you don’t want to do the work yourself, Evan will serve it up for you at least once a day. I like to monitor the plumes on my own and listen to what Evan has to say. In addition to being very good at meteorology, he is also entertaining.

  • Mike R

    All I can say is, “Whoa.”

  • Db

    i hope this puts down the base at PC/Canyons. It needs it with the lower snowfall numbers. Theres a chance the whole mountain will be open minus a few small runs… heres to hoping