Will it Snow? Will it Rain?

2016-11-27 14:25:59.000 – Adam Gill, Weather Observer/ IT Specialist

This upcoming week is looking very interesting weather wise. A big storm is going to be forming in the lee of the Rocky Mountains and form into a large storm in the upper Midwest. Right now, it looks like it is going to stall over the North Dakota and Minnesota border and bring a bunch of snow and wind to western ND and northwestern SD. This storm will help move a bunch of energy up from the Gulf of Mexico toward the North East. The warm front associated with this storm looks to reach the region on Tuesday with precipitation beginning over New Hampshire by the evening.

There are two major models that I usually use to look at large storms more than 4 days out and those are the GFS (Global forecast system) and the ECMWF (European Center for Medium range Weather Forecasting). If these two models are not in agreement I will look at one or two more like the GDPS (Global Deterministic Prediction System) from Canada.

This storm system does not have much disagreement in terms of precipitation in the North East but have very large differences in the snowfall. This storm will have some very cold air moving in just behind it; it is all about the timing of the arrival of the cold air. That is quite tricky to nail down, and a slight change in the storm tract could mean a massive change in the amount of snow that may fall. In the picture below is the GFS’s most recent run depicting its forecasted snow accumulations for the next 180 hours. This is based off 10 inches of water to 1 inch of liquid equivalent and snow can have a wide range of snow to liquid ratios so this is definitely not representing what may actually fall but a good way to estimate. The model does not have the bulk of the cold air arriving till after the storm has moved off into Canada.

 

On the other hand, the ECMWF has the cold air arriving at about the same time as the beginning of the heavy precipitation on Thursday. This means that much of the precipitation is going to fall as snow over the higher terrain if this model run were to vary at all. In the image below, the ECMWF has over 18 inches of snow for the Whites! Though this is also based off of 10 to 1 liquid equivalent and with how much moisture this storm has, that ratio will likely be closer to 7 or 8 to one. Still with those lower snow water equivalent values, we could still see over a foot of snow if the ECMWF were to verify.

 

The reason that there is such a large spread in the models and why there could still be a lot to change is the fact that the storm is just now moving into the west coast. The energy associated with the storm coming on the west coast will have to traverse the intermountain west and it will get torn up and disorganized. Once the energy gets to the lee side of the Rocky Mountains, then a new area of low pressure will develop. There are so many mesoscale and microscale mechanisms affecting the energy as it passes over all the mountains that models cant compute because it is at a smaller scale than the grid points in the model. This can cause error, the storm will vary every model run until the low reforms in the Midwest, and then most of the models will start to converge on a storm path and strength.

This far out neither model will be exactly right and the storm will likely have rain and transition to snow at some point. The timing of this transition will determine the total snow amount that will fall in the White Mountains. Definitely something to keep an eye on over the next couple of days to see how it will play out! I personally am hoping for it to be all snow!

Adam Gill, Weather Observer/ IT Specialist