Wednesday, December 19, 2012

Prospects for a White Christmas

Many of you are wondering if we will have a White Christmas in Salt Lake City.  That's a question that is difficult to answer.

First the easy part.  Warm southwesterly flow will spread over the region for tomorrow and Friday.

With little snow on the ground now, we'll be heading into the weekend snow free.

After that, some models call for a weak system to move late Saturday night and Sunday, but right now valley accumulations appear to be nil to little.

Heading into the holiday, the ECMWF forecast ensemble calls for a trough to move across the western US.

Similarly, the GFS also brings a trough through and gives us a period of unstable northwesterly flow that would probably bring snow to the valley.

These are still medium range forecasts.  Let's see how things evolve over the next couple of days and hope we see something like the above verify.  Beyond the valley, a forecast like that would be great for Christmas freshies in the Wasatch.


  1. Thank you! I'm going to keep my fingers crossed for a Christmas snow miracle.

  2. Thats weird cause the NWS is showing a major event for the 23/24. Been leaning that way for three or four days now but nobody else is hinting at that, not even there own precipitation forecasts agree with the 6-10 day climate forecast probabilty of above normal precip. The climate prediciton center has been showing a 70-80% chance of above normal precip for the 23-24 (In the past that has lead to a BIG weather event, especially since it has hinted at that possibility consistently for 4-5 days now...) Can you please explain why this would be, are the NWS people not checking each others work to have some consistency here?, just messing with us amateurs?....confused on this one.

    Even the temp probability 6-10 days out is not in agreement, 6-10 shows normal or colder than normal...were as most all other forecasts show above normal for the same time period.

    1. The CPC and the local NWS office forecasts are different forecasts produced by different groups. The 6-10 day forecast issued by the CPC is for a four day period, whereas the NWS forecasts issued from Salt Lake are more detailed.

      The latest NWS forecast for the 23rd calls for "chance of snow". Ditto for the 24th. The latest GFS forecast does bring a major storm in, but not until Sunday night (late on the 23rd). My use of the term "holiday" in the post was meant to encompass Christmas Eve and Christmas. Sorry for the lack of clarity.

      That being said, I tend to be cautious with forecasts that are out this far. Looking at the GFS, I'd say the odds are good for a White Christmas, but I'm not going all in on that yet, in part because there are some members of the GFS ensemble (a suite of model forecasts) that take the storm further south.

    2. Thanks for the clarity, I'm just used to watching the CPC forecasts religiously just to get an idea of what the expect for the near future, something just beyond the 7 day forecasts everyone produces. Either way your blog is the coolest!! Thanks for your insight.

      Deven Serr

  3. Here's a question, weenie - A friend in vermont (where winter is missing) posted a link to a made-for-vermonters forecast discussion calling for a rex block that would swing moisture down south, then up the east coast to vermont where it would produce big storms. The forecaster said the models were consistently showing it, and he was excited about this rex block, fondly recalling January of 2007.
    Now, I was remember January of 2007 quite well. It sucked for Utah, to put it mildly. I also know enough about blocking patterns to be concerned anytime anyone mentions it. The rex block sounds like some sort of chess move, check mate on our ski season, if you will (though I looked it up and found out it is named after a guy named rex, and is not some sort of king blocking pattern).
    But I don't see that in any of the models I am looking at. Do you? Was this just an example of a vermont-skiers forecast latching on to something that sounded good for his audience? I don't mean to bring up the sore subject of blocking patterns and depress you during good times, but maybe by discussing them, we will ward them off.
    On a broader note, is it easier to predict blocking patterns that active snowfall patterns? It sure seems that way. It seems that, when we get one, you can look at the models and confirm it is not going to snow again for 384 hours. But, when models look active, it seems forecasting is less relliable.

  4. I don't see much evidence for a bonafide Rex block it in the latest GEFS ensemble forecasts.

    Blocking patterns can be active snowfall patterns - everything depends on where you are relative to the block (there are winners and losers with every block). Often, the far west is a "loser" from a snow standpoint simply because there is a tendency for Rex Blocks and their associate split flow to set up near the west coast.

    The presence of a Rex Block implies a slowly varying large-scale pattern. I suspect such patterns would be more predictable than one with a more progressive westerly flow, especially for day-to-day weather variations.

    1. More on this-

      You can always compare the current pattern to Jan 1997:

      Blocks are persistent patterns. I don't see anything block like about the flow over the west the next couple of weeks.

      Many factors contribute to east coast storms. The presence of a Rex Block is neither a necessary nor sufficient condition for a storm on the east coast.