Monday, January 8, 2018

"The Elephant in the Room" - Friday Snow Potential

Questions are starting, and since we're about four days out, I thought I would take a few minutes to address this...

It's the Big Snowy Elephant in the Room! (If the room were large enough to hold a mountain!)

Forecasts you see from multiple sources are starting to include the potential for snow in the forecast Friday and Friday night. Some of you may have even been lucky enough to see the snow map from the "vastly-superior" European model this morning. Well, I'm here to bring some reality and level-headedness to the situation. And that doesn't necessarily mean "no snow." In fact, before you go any further, check my official MWN Forecast. There are a few things that need explaining though...

1. Any map that shows snow totals in the south that are multiple times larger than the average annual snowfall beyond three days in advance of the event need to cause your "Fake News" meter to spring to life. That doesn't mean that solution is wrong, it just means that there is still a good deal of uncertainty and there are likely a number of other solutions that would discount it.

Enter this morning's European model, which believe it or not is sometimes incorrect! I'm not showing the map, but I will tell you it showed snow falling Friday night that was closer to a foot than a toe. What people haven't shared is the American version of the medium-range model, which brings the cold air in much quicker and produces a treacherous ice event Friday morning!

Do I believe one or the other is absolutely correct? Nope! Could they be? Perhaps. But both will change their minds multiple times over the next 72 hours and likely will end up somewhere removed from either end of the spectrum.

2. Ensembles rule the day in these scenarios. What are ensembles (don't answer, music friends!)? Most models are run multiple times with slightly different parameters. These parameter tweaks can produce different results from the same model. The ensembles provide the forecaster a level of confidence in that model. For example, if the main model output is in line with all of the various ensemble members, confidence is high in the solution.

Let's look at that European model from this morning. Here is the ensemble output for snowfall:

European model ensemble output for accumulated snowfall from this morning's model run. Description below. (WxBell)
The European model has a 51-member ensemble (it's run 51 times with various slight parameter tweaks). The top graph shows the snowfall from each of the 51 members in a separate row, with time going from left to right. You'll notice that most rows have some color in them in the red box that represents Friday into Friday night. Some of them turn blue in that box, meaning 2" or more. Many are gray, which is less than 2".

In the bottom graph, the snow map that some of you have seen is the "deterministic" run of the model and you can see that it peaks at 8" in the red box (yikes!). You'll also notice that the "ensemble mean" (or the average of the other 51 members) is about 1". So the snow map referenced earlier is showing about EIGHT TIMES the total that the average of the ensemble members are showing. And only 1 of those 51 members produces an amount similar to the deterministic run. In statistics, we call that a 2% chance of occurrence.

What the European ensembles tell me is that there is a pretty high probability of seeing some snow, but that totals will almost certainly be closer to 1" than 8". Let's looks at the same thing for the American GFS model:

GFS model ensemble output for accumulated snowfall from this morning's model run. Description below. (WxBell)
The GFS model only has 20 ensemble members, not 51, but the graph shows the same thing. During the period in question, many of the members produce snow. A couple show totals of 9-12", but the ensemble mean (bottom row in this graph) is about 1". Another ensemble that says an inch. Hmmm...

Ensembles bring the reality back to outrageous snow maps, which is the responsible way to forecast. We attempt to excel in that area. :-)  

(BTW - the Tuesday areas that are highlighted in the boxes above - disregard those for now. Too far out to give any credence to. But it'll definitely be very cold between Friday and Tuesday, so we'll address that as we get closer.)

3. Consider all valid sources. Here's another one - a new product from a branch of the National Weather Service that (on an experimental basis) predicts the probability of "impactful" winter weather in the 4-7 day range. The forecast is a probability of 0.25" of liquid equivalent snow or sleet. What does that mean? Assume that snowfall generally melts down to about 0.10" of liquid for every 1" of snow (a 10:1 ratio, which is very close to average). So 0.25" of liquid is roughly 2.5" of snow. The forecast below, issued today, says that we have about a 10% (or a little higher) chance of reaching that 0.25" threshold. That means it's not likely - at this point given the available data - that we'll see 2-3" of snow. But the fact that we're in the area of interest indicates that we should prepare for the possibility that we could get something.

So there is your lesson on snow forecasting in the south! Beware the share - if it seems to good to be true, it probably is and shouldn't be shared on your timeline. Besides, you hate fake news anyway, so throw crazy snow maps in that bucket.

What do I really think? 

After reaching the 60s Wednesday and Thursday, I believe we have a decent shot at seeing some winter precipitation on Friday and/or Friday night. We could see some ice mix in as rain transitions to snow, but I don't know when that would occur just yet. I do NOT know how it will affect schools - depends on the timing of the transition and individual school district decisions. And if we DO get more than an inch or so, it'll likely happen Friday night. I'll get more into the setup in a couple days, but for now, just know the answer is...


Erik Proseus
MWN Meteorologist

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DanTheCigarMan said...

Thanks Erik. Of all the sources in the area, I always fall back on your explanations which very often seem to be spot on. Great work as always!

Barfolomew said...

As you said, at least there is a chance (albeit very small) of a big snow. Well, at least we can dream.