Winter Will Take a Holiday, but “Ensembles” Show It’s Not Done.

January 4th, 2013 at 11:52 am by under 4 Warn Weather

You often hear meteorologists talk about computer models, blog newcomers. But over the last decade a new set of important tools are being used by professionals who have been keeping up with this aspect of the science. A few words about “Ensembles”: because we cannot know the precise state of the atmosphere at any given moment (far too many variables, far too much unavailable or missing information) supercomputers with enormous crunch power run models with 20 or more different-but-reasonable-variations on initial conditions at the start of model runs. That is, an NWS model may haveĀ  over 20 different runs, each a member of an ensemble. When these 20 ensemble members show only relatively small differences between one another, confidence in the use of that model and its ensembles increases. But if the spread between those ensemble members is large, and the ensemble shows a huge range of outcomes, then confidence shrinks. The NWS global model, called the GFS (Global Forecast System) has a counterpart from Environment Canada, and from the European Centre for Medium Range Weather Forecasting, in England. The latter, the European model and ensembles, is generally thought of as the most reliable global model. But NO model or ensemble can be counted on to automatically be the best on a daily basis (some models run 4x per 24 hours), so we need to look at all of them. One might assume that the Mean of the many ensemble members is most likely to be the most accurate, but it doesn’t always work out that way. Sometimes members close to being “outliers” turn out to do best, but it’s very difficult to know when that’s going to be the case. The best we can do is to combine basic meteorology (looking at surface and upper air observations, satellite imagery, buoy data, doppler radar products, etc) and combine that current reality with the performance of models and ensembles in the near term. If a model is already way off the mark early in its forecast, the odds favor it being even more way off further out in time. Ensemble forecasting is the wave of the present and the future. Meteorologists who fail to jump in to this new and complex statistical set of tools do so at their own peril. The NWS and academia have led the way in putting these tools into everyday use, and those of us in the private sector have caught on as well. Ensembles, particularly those from the European model, are what made the forecasting of Superstorm Sandy so incredibly accurate before the disaster unfolded.

As for what those ensembles are showing now…our snowpack will take a significant hit during the week of January 7th, with airmasses of Pacific origin replacing the arctic air. Temperatures will likely move well above average by midweek and late week, and to make matters worse for ski resorts and snow mobile trails, some rain is likely to hasten the thaw around Thursday and possibly again by Saturday (European ensemble). However, it had been looking until yesterday (the 3rd) as if that mild trend would persist most of the time over the next 16 days. Today’s ensembles are now showing seasonably cold air (not brutally cold) which had been slated to stay over the northern plains and Canadian prairies will be able to make it back to the eastern Great Lakes by around the 17th-19th…possibly a bit sooner, future runs will determine that. Right now there are no signs of truly harsh, bitter cold getting to us, but the temperature scheme I’m seeing in these ensembles will be returning closer to average or a bit below average and, in any case, it will be chilly enough for rain events to be replaced by snow events.

437 Responses to “Winter Will Take a Holiday, but “Ensembles” Show It’s Not Done.”

  1. Thinksnow13 says:

    Sunday night has a better chance for a minor snow accum and then it will definitely stick around. There are some websites looking to the end of the month for a more active storm track so we may have to bide our time as they say.

  2. Juju the cat says:

    Really Dave? You would know better than I but I thought a more WNWrly flow hammered Rochester with the frictional/convergence components coming more into play, but it would definitely make sense that a WNW flow would bring less of the lake in play and keep any bands/cells north of you

  3. Juju the cat says:

    Just one run of the gfs, which is usually colder than it’s ensemble runs, but hopefully this starts to become a trend and it keeps up. It shows at least 8 days under 25 for highs and a few lows below zero!

  4. Don Paul says:

    Juju…that is NOT going to happen.

  5. Dave from Roc says:

    This was a fun one that I remember from last February…one of the more ideal setups for Rochester:

  6. Juju the cat says:

    Don, yeah I figured. I couldn’t help myself though haha.

  7. Pete J says:

    Does a negative PNA mean more active storm tracks? Just checked the indices on the CPC today and it looks like a lot of them are going in the wrong direction by Feb., but also got to thinking about it based on what the websites are saying that ThinkSnow posted about.

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