Friday, January 15, 2016

The Sticky Situation with Snow Levels

Another wintry day in Reno with snow levels down to valley floors, which means it was likely a slower and slicker commute for most this morning. Even with my all wheel drive vehicle I lost traction a couple of times. As the traffic signs were suggesting...

Anyways, back to the snow level forecast. Snow levels are one of those forecast parameters that can be very tricky to pin down, and yet can have a significant impact to the overall weather conditions! Snow levels will play a key role in the forecast through the weekend, especially since it looks like northern California and western Nevada will be caught up in a very wet pattern. Check out the details in the latest forecast discussion here

What is the snow level exactly? Think of it as the rain-snow line. We define it as the level at which precipitation is turning into snow, so there may be some rain mixed in there as well, and it isn't where snow starts to accumulate either. So when we are trying to determine where precipitation is transitioning into snow, we can take a look at a few different parameters. Here at NWS Reno we can look at temperatures in a layer (e.g. within 2000 feet of the surface) and temperatures at the Sierra ridge top. We tie these two together by looking at forecast soundings, vertical samplings of temperature throughout the atmosphere. Without getting to deep into the details, the parameters that we use depend on the trajectory of the incoming storm system and also on previous forecaster experience. When looking at the forecast models there is always a bit of a drawback since they can struggle with boundary layer interactions and dynamics. This can lead to some discrepancies and/or variability in the accuracy of snow levels. That being said, we as forecasters can look at a number of forecast models and each one could have a very different solution for what that particular model produces as the actual snow level. We definitely have our jobs cut out for us! 

Let's take a look at this from another angle. Even if we have fairly high confidence in snow levels, let's say within a 500 foot range (which is pretty good!) that still leaves quite a bit of variability on what we have in the forecast. How much is 500 feet anyways? Seems like a lot, but when you compare it to a football field (360 feet) it really isn't that much! Then you talk about nailing down snow level forecasts every 6 hours for 7 days in the future? It's a tough challenge, but we certainly do our best. 

Let's take a look at an early January example. The first graphic shows a most likely scenario for snow amounts with a 70% confidence for a January storm. Notice we have snow amounts for much of western Nevada. Now let's take a look at a warmer scenario, where snow levels would be a bit higher. This would result in no snow for much of the Reno area. You can see where just a difference in a few hundred feet would result in a potentially significant difference in impacts to traffic and travel for this scenario. So what is the best way for you to know what the snow level is going to be? On a day-to-day basis your best bet is to read through the area forecast discussion, where we as forecasters typically discuss our confidence in the snow levels and the impacts that could result if the snow levels are higher or lower than forecast. This will especially come in handy over the next week as we enter a fairly active and wet weather pattern for the west coast. A good rule of thumb is to always apply a +/- 500 foot error bar to any snow level forecast you see, just to be better prepared. Until then, be safe on the slick roads this morning and maybe wait until the snow melts to run errands today.