Monday, January 9, 2012

Forecasting a Pattern Change

Over the past few weeks, I've spent more time looking at long-range (7 days or more) forecasts than I care to admit.  I'm as desperate as most other Utah skiers and keep looking for the pattern change that we need to open up the Wasatch to a battering by Pacific storms.

For reasons we have discussed, there is a good reason to recognize the limited ability to predict weather at longer lead times.  That being said, there are several modeling systems (and their ensembles) that can be consulted when producing a long-range forecast including Global Forecast System (GFS), Global Ensemble Forecast System (GEFS, based on the GFS model), NOGAPS (Navy Operational Global Atmospheric Prediction System), GEM (Global Environmental Multiscale model run by Environment Canada), and the ECMWF (European Center for Medium Range Weather Forecasting) global model.

When you consider all these models and their ensembles there's a fire hose of data to examine, but I have a tried and true approach for dealing with such problems, as University of Utah Atmospheric Sciences students can attest.

Actually, I follow the Keep It Simple Stupid (KISS) approach and just quickly peruse the upper-level (500-mb) forecasts from the models and their ensembles.  This is perhaps a form of what author Malcolm Gladwell calls thin slicing.  I use spaghetti diagrams and postage stamp plots to do this, as discussed previously.  If I see a pattern change that exhibits some consistency between the models and their ensemble members, I'll dig in some more and evaluate if there is a good reason to expect such a change and if the processes driving it offer some level of predictability at longer lead times.

Over the past few weeks, I haven't seen much to get excited about for the Wasatch.  Every now and then a couple of ensemble members would hint at something exciting happening in the 7-10 day time frame, but I'd look at the full spectrum of model forecasts and find all sorts of other solutions, most of which would keep us mainly dry.  This continues today.  For example, in the 10 day forecast from the 0000 UTC GEFS, there are a wide range of solutions, but most keep us under a ridge or on the anticyclonic (southern) side of the jet, which isn't great for precipitation.

Source: Penn State E-wall.  Click to enlarge.
If I hunt through the various models and their ensembles, I can find a couple solutions to get excited about, but that's about it.  I interpret this to mean that we're still looking at a low likelihood of transitioning to a stormy pattern in the next 10 days.

Note that this doesn't mean I rule out a pattern shift, but merely that I need more evidence to emerge, such as the development of greater consistency between the models and ensemble members as the forecast lead time decreases, before getting excited.


  1. Not what we want to hear.

  2. Low probability doesn't mean no probability. Further, the models aren't perfect. Then again, one needs to stay realistic...

  3. I know. Been watching the GFS model runs which occasionally look like something good will happen and then take it away. My trip to utah is not until Jan. 27, but most runs of late seem to show a bit of a pattern change, but one that if anything is even drier. Arrrgh!There's still time for things to change of course, but I've been saying that to myself for the last 6 weeks. Tough year. Thanks for your blog which at least gives us non-metereologists a better idea of what's going on and the possibiliites.Steve

  4. I am a little more optimistic about the long term. Almost all models and ensembles are showing an upper level ridge develop over the Bering Sea, something we haven't seen in a long time. Physically you would expect this to push the Pacific jet further south, which some of the GEFS members and ECMWF show 8-10 days out. The Climate Prediction Center even puts northern Utah in an area with 33% chance of above average precipitation 8-14 days out! (
    In general I would say the pattern is changing, but subtle shifts in the location of the Pacific Jet will determine if Utah gets snow or not and these shifts cannot be forecasted 10 days out. Will be fun to follow

  5. I'm old school and cutoff looking at 10 days. 33% of above average is essentially a forecast of climatology (or just above climatology), but we'd take climo over what we've had lately.

  6. Climo would definitely work. I believe average snowfall for Alta for January is close to 100 inches. I'd be happy for half that at this point

  7. (jdm83) I was looking at this too, in fact the GFS has been fairly consistent with it for about a week now. Have noticed in the past that getting a large cut-off ridge in that area can give us good odds of some heavy precipitation, and at that latitude those types of features can be very slow to move and so might signal a longer-term pattern change. But this one seems a bit too far to the north/west and if anything these tend to retrograde with time, so I am afraid now that we might still end up stuck with a ridge. I hope for everyone's sake that my analysis is wrong!