Changes in the weather…

Posted Monday, February 26th, 2018 by Mike Wessler

An important component of forecasting is keeping track of how the model forecasts evolve over time. These trends can be imperative to an accurate forecast, and can also help explain a bust. In the case of the significant mid-week storm we were expecting, models have backed off significantly. Largely, this is due to observed conditions not stacking up to modeled conditions, and the models use the observations to adjust and inform the next forecast.

We’re still likely to get some snow, but totals have been cut back quite a bit as the best dynamics are now much further north than the central Wasatch. If there was one figure to sum up my thinking right now, it’s the following:

ECMWF Salt Lake City QPF dProg/dt
ECMWF QPF dProg/dt for Salt Lake City

To explain what we’re looking at – from the top down, the forecast becomes more recent with the bottom row initialized this morning and the top row initialized 120 hours prior. From left to right is 30 hours ago (verification) to 240 hours into the future. The numbers are the precip totals up to that hour, with green < .5″ water, blues < 1″ water, and yellow/orange < 1.5″ water.

What’s important: the midweek storm has been trending drier, from once ~.5″ water to now < 0.1″. Keeping in mind this is a global model that often under-forecasts orographic precip, we are still expecting some snow up high tonight/tomorrow but 1-4″ seems about it.

Next, the weekend storm: what once looked to be a potent system producing over 1″ of water in the valley has weakened quite a bit in the forecasts, and we see this reflected in the QPF trends. There has been a steady decline over the recent ECMWF runs which seems to be leveling out around ~0.5″. This is still far out enough that we could see these numbers vary significantly in the next few runs, so not all hope is lost yet. For the sake of completeness, I’ll include the GFS forecast, which began much lower than the ECMWF and has been trending upward towards the ~0.6″ mark.

GFS QPF dProg/dt for Salt Lake City
GFS QPF dProg/dt for Salt Lake City

Either way, tomorrow will be snowy with a little fluff on top for a refresh, and I have high hopes for another stellar weekend of skiing. Either way, it seems appreciable snow totals are likely for the mountains by Sunday morning. We’ll keep you up to date as the models keep rolling in!


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