## Monday, November 20, 2017

### Precipitation Overprediction Problems with the NAM Conus Nest

High resolution forecast models are not necessarily better forecast models and precipitation forecasts produced by the NAM Conus Nest (hereafter the NAM-3km) are a prime example of this.

The NAM-3km covers the continental US at a grid spacing of 3 km, four times the resolution of the 12-km NAM in which it is nested.  With such high resolution, you would think the NAM-3km would be especially useful for precipitation forecasting over the complex terrain of the western US, but it isn't, because it has a major overprediction problem.

Tom Gowan, a graduate student in my research group, recently led a study examining the performance of several forecast models at mountain locations across the western U.S. during last winter.  I have been holding off on publicly sharing these results broadly since the paper describing this work is still in review, but the results are too pertinent to forecast needs right now not to share at this juncture.  In the case of the NAM-3km, we used pre-operational test runs from last winter that were kindly provided by NCEP.

The plot below shows the ratio of mean-daily precipitation produced by the NAM-3km to that observed at SNOTEL stations.  Overprediction is evident at the majority of sites, with on average the NAM-3km producing 1.3 times as much precipitation as observed.
 Source: Gowan et al., in review.
A major reason for this is that the NAM-3km produces far more major precipitation events than observed, especially over the interior western US.  In the plot below, the frequency bias is the ratio of the number of forecast events to the number of observed events in each event size bin.  The NAM-3km has by far the largest overprediction problem.  Note that the NCAR ensemble also produces too many large events, although the magnitude of the problem is not as acute.

 Source: Gowan et al. in review.
I bring up these issues today because the NAM-3km is going batsh-t crazy for the storm later today and tonight.  For Alta-Collins, the 12-km NAM is producing .08" of precipitation through 10 AM tomorrow.  In contrast, the 3-km NAM is producing 2.04"!

The loop below shows steady, drippy precipitation over the Wasatch and nearby ranges during the overnight period.

Now, it is always dangerous to say a model is wrong before the forecast verifies, but I'm going to say it anyway.  This forecast is wrong.  There's little evidence to support such huge precipitation totals.  Even in the NCAR ensemble, 7 of the 10 members produce less than 0.2" of precipitation, and the wettest goes for about .57".

This issue plagued the NAM Conus Nest when it was run at 4-km grid spacing and it appears to carry over to the higher resolution upgrade.

The bottom line is this.  If you want great deep powder skiing, consider using the NAM-3km for your holodeck experience.  However, if you live in the real world, avoid using the NAM-3km precipitation forecasts unless you want to be severely disappointed on a regular basis.