Tuesday, November 24, 2015

How Much Should You Trust High Resolution Models?

The forecast for tomorrow morning provides a good example of one of the problems we have as meteorologists using higher resolution models.

For the 3-hour period ending at 1800 UTC (1100 MST) tomorrow, the NAM is generating just a bit of light precipitation over the northern Wasatch.

In contrast, the high-resolution 4-km NAM goes for considerably more precipitation, amounting to 0.16" of water in three hours at Alta-Collins.  Those aren't huge numbers, but they add up.

I'm seeing similar behavior in the NCAR ensemble for tomorrow.  The largest precipitation rates are being produced in the southerly flow ahead of the front at Alta-Collins.

High resolution forecasts like these are alluring as we know the central Wasatch can have strong effects on precipitation.  The problem, however, is that these models produce heavy mountain precipitation far more frequently than Mother Nature does, so the issue in a situation like this is how much to believe such a forecast?  If you bite on a forecast like this every time, you're going to have a lot of false alarms, and nothing annoys a forecast consumer more than false alarms.  Hitting a home run every now and then, but striking out frequently is not a path to success in the forecast business.

It will be interesting to see how things verify tomorrow.  Odds are that the precipitation rates being advertised for the central Wasatch by the high-resolution models are too high during the morning.

In the afternoon and evening, the front is expected to traverse northern Utah and bring snow to all elevations.  Although accumulations will probably be in the 1-4" range at most valley and bench locations, keep an eye on the forecast and adjust travel plans as needed as the currently advertised timing couldn't be worse.

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