The first storm today over-performed, so what does that mean for the second storm of the week? Sadly, for many it likely means that its impacts will be more severe as well. This blog post is going to break down the initial forecast for the storm on Wednesday and finish briefly analyzing the storm today, including what went right in the forecast and what went wrong. In many ways, the storm today was a large win for short range model guidance, which I will be using very closely with the upcoming storm as well (and it will be even more important then). I’ll start first by breaking down timing and accumulations though.
This is a very complex setup, often called a Miller-B, where there are going to be two low pressure centers interacting with each other. The primary low pressure center is going to move up into West Virginia, where it is then going to transfer most of its energy to a low pressure center just to the south of Long Island. As is standard with low pressure cents, when you are to their east they are pumping warm air into the region, but when you are to their north or east there tends to be colder air pulled in. What’s happening with this storm is at first there will be warm air advection from the primary low pressure center before the secondary low pressure comes in and pulls in colder air again. The exact timing of the pressure falls along the coast will determine what surface and atmospheric temperatures are, and in such a dynamic scenario one or two degrees centigrade could male all the difference between snow, sleet, freezing rain, and rain. Obviously, this forecast is far from easy.
Running the latest data shows there are a few key differences in weather models that are yet to be reconciled. The main difference is the existence of a “warm layer” in some weather models that is not present in most other ones. Basically, what happens here is that with the primary area of low pressure pulling up warmer air, the warmer air moves at different speeds at different places in the atmosphere. Imagine a single column of atmosphere over your house, rising from the surface all the way up to the stratosphere. On a normal day you gradually get colder as you go higher up due to the lapse rate of the thermosphere, but when dealing with warm air advection and vast pressure differentials, like we will here, sometimes the warm air is able to flood in quicker higher up in the atmosphere where there is less friction. One model in particular, the NAM, has shown this, and what this would do is result in a lot of sleet and freezing rain with the storm, as around 7,000 or 8,000 feet in the atmosphere there would be a layer with temperatures above freezing that would melt any snow. By 5,000 or 4,000 feet temperatures would drop back below freezing, so any rain would either refreeze into sleet pellets or would fall to the surface as supercooled droplets and freeze on anything it fell on that was below freezing at the surface (freezing rain). Sleet and freezing rain are the two most dangerous types of precipitation for travel, which is specifically why this storm has grabbed my attention.
So is this storm a perfect recipe for freezing rain and sleet? Not exactly. The snowpack laid down today will help to cool the surface, meaning that I do not expect much plain rain. But the impacts of additional reflected sunlight can often be felt at more regions of the atmosphere than just the immediate surface. With better data sampling coming in for weather models, they have trended colder, and I expect this to be a trend that could also continue into the day tomorrow. This is reflected in my forecast, as it has been a trend that has continued all winter, including today. Storms tend to be colder than initially forecasted. This means that while I do think there could be some sleet and freezing rain, especially at the coast, I expect that most of the storm ends up being snow (and inland areas by I-84 and north could be all snow the entire storm). Now let’s break down some of the impacts.
Precipitation looks to start between 2 and 3 AM Wednesday morning. It will start as all snow for all of SWCT, and within a few hours of starting the snow will be heavy. With temperatures below freezing at the start of the storm, the snow should have no problem sticking on all surfaces, including roads, as it falls quite intensely. Confidence is very high that snow will continue straight through 6 or 7 AM, and at that point there should be anywhere from 2-5 inches of snow on the ground across the area. Over the next 3 hours precipitation will continue to be heavy but we could be dealing with sleet or freezing rain mixing in along the coast. This is where things get complicated, because the NAM weather model has sleet by 6 AM or so, but most weather models show that the storm could be all snow through 10 AM, which is when precipitation lightens up and freezing rain/sleet could mix in further south. As I’ve said, I’m erring on the side of the colder weather models as that has been the trend and we have the fresh snowpack, meaning that while I do believe there is a period of sleet and freezing rain by the coast, I don’t believe it will be until after 10 or 11 AM. I then believe that period of sleet and freezing rain, where a section of the atmosphere from 800-850mb rises above freezing to melt snow, will last from 11 AM or so through the rest of the storm, with a “wintry mix” of snow, sleet, and freezing rain ensuing. With the current snow pack, I do not expect a chance to plain rain for any part of SWCT, meaning the entire day on Wednesday we will be dealing with extremely slick roads. That is why I see almost no way for schools to open on Wednesday, and I expect a very high snow day percentage to be issued in my first call tomorrow morning.
As for total accumulations, in coastal areas I am looking at a general 4-8 inches, with 5-10 inches expected inland. The highest amounts will likely be closest to I-84 and further north, where the cold air aloft will win out longer. Someone up there could see 10+ inches if there never is a change to any other type of precipitation. I feel quite confident in this forecast, as I believe that the entire region will see 4 or so inches of snow by 6/7 AM when the first possible changeover to sleet is, and then another 4 or so inches of snow if there is not a chance to sleet at all. If it appears the coast will stay all snow for the entire event, then I will bump up coastal areas to 5-10 inches of snow, but I think the immediate coast will see amounts cut by 2-4 inches of those further inland due to the freezing rain and sleet mixing in. At the coast I see around 1-2 tenths of an inch of ice being an issue, while further inland (anywhere a few miles north of the Merritt or further north) I expect they will see a tenth of an inch of ice or left. Still, any amount of ice from freezing rain can make roads EXTREMELY treacherous, which is why I am now advising that you make plans to stay home and not travel on Wednesday. Travel Wednesday will be significantly more dangerous than it even was today, and that is saying something as the area saw numerous accidents today from those that decided to be out and about.
I am extremely busy analyzing data from the storm today, going over forecasts for the next storm, and also looking ahead to the storm now Sunday into Sunday night (it is a guarantee that it is coming, at this point it is basically a matter of how much snow), so I am going to try and post another blog tonight but there is no guarantee, and it may be out very late. I figure that is the crucial information that I feel confident enough to give right now for Wednesday, and I am also fairly confident in the total snowfall amounts that were given as well. Of course, if there are any major differences in short range models I will be sure to factor that in because the RAP/HRRR performed so well with this last storm, but those will not get into range until tomorrow. I expect to have continuous coverage tomorrow afternoon and evening, and there is a good chance that districts actually close the night before as it becomes clear that Wednesday morning will be extremely icy and dangerous. If I have another blog post tonight I will try and outline the Sunday threat as well as review the storm today, but sadly I’m also in college taking classes and do not have the time that I did over break. I’ll do my best to get you all the info you need to stay ahead of the storm and know exactly what is coming next, so make sure to keep it here for the latest.