Model Confusion

All models are wrong, but some are useful

- George Box

But not today

- Wasatch Weather Weenies

Numerical weather prediction models form the backbone of modern weather forecasting and will play an increasingly important role in the future.

Most of the time they are extremely helpful, but there are times when they seem to make the crystal ball even cloudier, and this is one such situation.  We've already seen situations this week where the NAM and GFS have produced dramatically different forecasts (e.g., Which Model Will Win), and we have another one tonight.  

In particular, compare the 6-hour accumulated precipitation in the 0600 UTC GFS forecast with that from the 1200 UTC NAM (sorry about the different initialization times, which is an artifact of availability as I write this, but this is still a reasonable comparison).  

0600 8 March 2013 GFS forecast of sea level pressure,
 surface wind, and 6-h accumulated precipitation valid
0600 UTC 9 March 2013. 

0600 8 March 2013 GFS forecast of sea level pressure,
 surface wind, and 6-h accumulated precipitation valid
0600 UTC 9 March 2013

In particular, look at northern Utah.  The NAM goes for a precipitation maximum that runs up the Wasatch Plateau and Wasatch Mountains to the area around the Cottonwoods, and then it tapers off further north.  The West Desert is dry.  The GFS goes for a gap in precipitation around the Cottonwoods, puts a precipitation maximum over the northern Wasatch, and goes berserk over the West Desert.  

As a forecaster, especially if you are trying to predict what will happen in the central Wasatch, this is sort of like having Clint Eastwood point his 44 Magnum at you and ask you if you feel lucky.  

Diversity in model forecasts can be an indication of forecast uncertainty, which is something we have discussed in previous posts and is important to consider for any forecast.  However, the precipitation these models are producing seems to reflect their physics (i.e., their formulation) more than that driving the real-world precipitation.  Further, the GFS has been overdoing precipitation bands the past couple of days, while the NAM overdid precipitation over the Wasatch Mountains during a similar pattern in early February.  As a result, it's unclear if these models are really all that useful for generating odds of snowfall amounts of various thresholds in this instance.  Sometimes I feel like I can throw the model precipitation out the window and come up with something better based on experience, but this is a weird pattern.  In other words, I'm largely clueless.

So, I'm going to stick with what I said yesterday.  Hope for a few inches today and tonight and remember that it is better to have low expectations and be surprised than to have high expectations and be disappointed.