Thursday, January 18, 2018

Frontal Snowfall Event On Tap for Late Tomorrow and Tomorrow Night

After another 10-day or so stretch with limited to no accumulations, our next storm will be served up late tomorrow and tomorrow night. 

Most of the precipitation for the central Wasatch looks to be primarily frontally forced.  The large scale setup is shown below and features an upper level trough that is initially tilted from southwest to northeast (referred to as "positively" tilted by meteorologists) that closes off and becomes more north-south oriented as it moves inland across the western US.  




This has both pluses and minuses for snowfall prospects in the central Wasatch.  The plus is that the front may slow as it drags through northern Utah, extending the period of frontal snowfall, as depicted below in the 1200 UTC NAM forecast.  At 000 UTC 20 January (5 PM MST Friday), the surface front is over Utah County with precipitation over the northern Wasatch.  


Frontal precipitation fills in, however, as the front phases with moisture sneaking around the southern end of the Sierra Nevada over the next 3 hours. 


That precipitation continues for another 3 hours as the front makes slow progress into southern and eastern Utah. 


By 0900 UTC 20 March (2 AM MST Saturday) the main frontal band is just downstream of the central Wasatch, with some post-frontal snow showers persisting. 


The minus for snowfall prospects is with the low closing off, the post-frontal winds shift very quickly to northerly, when we would prefer a period of northwesterly flow for better orographic forcing.  Note in the Salt Lake City time height section below that the post frontal flow is predominantly northerly and deepens gradually from about 0Z Saturday through 6Z Sunday.  



Actual numbers derived from the 12Z NAM show the wet bulb zero dropping during the day Friday (snow level is usually about 1000 ft below this level), with values low enough that most of the precipitation produced during this event should fall as snow in the mountain valleys.  Perhaps Mountain Dell might see a bit of rain to start, but then turn over to snow.  Total water equivalent at Alta is 0.64" through Saturday at 8 AM, with snow densities decreasing during the storm for a right-side up snowfall.  



Looking more broadly at the ensembles shows that the NAM is roughly in the upper half of the SREF plume for Alta.  Through 18Z 20 January (11 AM) the SREF members put out anywhere from 0.3 to 0.8 inches of water, the former being a slightly better than dust on crust event adding up to perhaps 4 inches of snow, the latter representing a lower end deep powder day with perhaps 10-12 inches of snow.  

That spread represents variations in the strength and speed of the front.  Increases in precipitation after 18Z 20 January occur in some model runs that are more bullish on the post-frontal precipitation.  

I continue to keep expectations low and hope for the best.  

14 comments:

  1. This comment has been removed by the author.

    ReplyDelete
  2. Your students are forecasting much higher amounts over on the Utah Ski Weather blog. However, the below was written yesterday, and your post wherein you put forth 4-10/12" was published today. Did something change overnight, or is this just a case of youthful optimisim? :)

    http://utahskiweather.com/storm-coming/

    Taking all model output into consideration here are my expected totals through midday Sunday:

    North, central, and southern Wasatch: 10-20 inches
    Areas favor in NW flow (ie, Cottonwoods): 14-24 inches

    ReplyDelete
    Replies
    1. My comments and numbers above focus on the period through Saturday 8 AM in the case of the NAM and 18Z (11 AM) in the case of the SREF. I did not consider anything beyond that.

      Delete
    2. Ok, that makes sense. It sounds like they are looking for the NW flow to add another 6-12" of snow during the day on Saturday.

      Delete
    3. It's also worth remembering that the Wasatch Weather Weenies is a blog. Our topics are wide ranging, and we like to use everyones interest in future dumps as a gateway to education. A true forecast, however, is spatially and temporally specific, quantified, and verifiable. My discussions often are not. As such, this is the Wasatch Weather Weenies, not the Wasatch Weather Forecasts.

      Question #1 of the Day: Is Saturday really a classic NWerly flow day as presently forecast by the NAM and the GFS? Look in particular at profiles from 12-0Z.

      Delete
    4. Point taken. Thanks as always for taking the time to share your thoughts and respond to questions!

      Delete
  3. How does Powder Mountain due in north winds given that it sits at the pointy southern end of the cache valley? Any evidence to suggest that a shift to due north is a good thing for we PM skiers?

    ReplyDelete
  4. Just to verify understanding... If winds are forecast to shift to Northerly, that cuts off the moisture supply, correct?

    ReplyDelete
    Replies
    1. By my reading of Jim's book, orographic lift is more important than the lake. But does a North wind have inherently less moisture than a NW? My assumption is that anytime you have moisture and a long fetch before an abrupt change in topography, you get enhancement.

      Delete
    2. It's complicated....

      The orographic forcing is stronger for NW flow than N flow, mainly because there's nothing immediately upstream when the flow is from the NW and you can get deep ascent. As the flow shifts to northerly, you are losing some of the depth of the lifting and the orographic forcing is not as strong.

      Orographic forcing isn't everything, however, and northerly flow isn't necessarily dry. It depends on the larger-scale context.

      For during the day on Saturday, the models are varied on how moist the airmass will be. The NAM, for example, has had the RH going up and down from run to run like a yo-yo. However, they have consistently been showing crest level flow from the NNE-NNW or thereabouts, not necessarily *optimal* for orographic forcing.

      That being said, that doesn't mean there couldn't be another forcing agent for the snowfall or that we get enough orographic lift to give us some snow. The models are producing snow during the day Saturday (most of them), but amounts vary, perhaps because much depends on the structural details of this period.

      So, my comments are really just to point out that this isn't a classic NW flow event. That being said, one of the reasons I didn't talk about the day on Saturday is that I don't feel there's a lot of useful predictability at the 48+ hour lead time (based on when I wrote my post) in a pattern like this other than to say "chance of snow" during the day on Saturday. This is very apparent if you look at the ensemble plumes.

      Jim

      Delete
  5. Thx for the detailed reply. I'll start paying more attention to N winds and snow reports at PM. I would hypothesize that it's more favorable for snow than the cottonwoods just based on terrain. Wish there was a snotel station there.....

    ReplyDelete
    Replies
    1. Oh oh, my bad. The orographic comments here are relevant for the cottonwoods. I read Lisa's comment and didn't realize the connection with PM.

      I haven't paid much attention to PM over the years. Maybe I need to change that!

      Delete
    2. As of 1/20 8am, NWS is forecasting 2 to 3 feet for the high Uintas, looks to be above 10k, and 12 to 18 for the Cottonwoods. Is this just better orographics w higher altitude, or is there something else going on. Seems like on average, Alta does a lot better than Kings Peak looking at season totals. Just curious if there are special events where higher is better.

      Delete
    3. The short answer is that the way the low is closing off and evolving, the Uintas will be in the moisture longer and the wind directions are very good for enhancement along a N-S oriented mountain range.

      Note that in this case the wind directions and flow trajectories are not uniform over the region. There is especially true Saturday night when the models right now have N-NE flow impinging on the north slope of the Uintas and NNW flow impinging on the Wasatch.

      There's never any reason to be jealous of the High Uintas. Most of the snow there is blown to another state.

      Jim

      Delete