I started talking about the potential on Monday and throughout this week the confidence has been growing for a winter storm the middle of next week. There are still many variables at play, but my confidence is around 30-40% right now. Primarily the track of the low pressure system is the biggest variable which has yet to move into the western US.
Also at play is the amount of cold air that will be in place for the storm. Those 2 variables alone make for a tough forecast 4-6 days out. So here’s my thinking right now.
The Track of the Low:
There are 3 distinct possibilities with the track of the storm. All 3 are laid out below. The trends all week has been for the #1 track, and a few runs of our guidance have been similar to track #2. I still can’t discount Track #3, but it appears as of right now some combination of track 1 or 2 is most likely at this point.
What does each track mean?
The simple answer is a WHOLE bunch! The farther north the track the warmer the air is. The farther south the colder the air is. I should note as well, the strength of the storm is important the coastal low track would be the strongest. Any time the low forms over the relatively warmer waters of the Gulf of Mexico or the Gulf Stream it will have a lower pressure and more moisture to work with.
Track #1 is primarily an ICE storm but mixing in of sleet, rain and even a bit of snow.
Track #2 is primarily all snow but some mixing in of sleet and freezing rain likely as well.
Track #3 is primarily a cold rain with some sleet and freezing rain. Ice storm likely in the mountains.
Cold Air Source:
One thing I am watching closely along with the track of the low is the cold air. High pressure to the north will provide the cold air. Depending on its location the cold air could be shallow or deep. The depth of the cold air is what determines what kind of wintry precipitation you receive. The deeper that cold air the higher the risk for snow or sleet. Shallow cold air means ice and maybe lots of it.
You can see below how each forms and if you look at the cold air in blue you can see how depth is important. Also note it’s sedge shape which is where that term “Cold Air Wedge” comes from. This is also why if you are slightly north you end up in colder air versus south with shallower cold air.
The model guidance is notorious for under estimating how cold the shallow arctic air is. Especially when there is a deep and large snow pack under the high pressure. Most models only model the atmosphere and don’t take into consideration what’s on the ground. So a large snow pack can keep things colder and shallower than the guidance will display.
Notice how much snow is on the ground over the U.S. especially to our north and west.
I know it seems like a cope out to say “stay tuned” but in this case there are just so many variables that there is a larger than average potential for a busted forecast. So right now it’s just a heads up to be ready for a large storm and to pay attention to the weather this weekend into early next week. I don’t want this thing to sneak up on anyone.
Also, there will be lots of rumors and hype about 1 foot of snow, or a crippling ice storm etc… I can tell you just today one model had 13″ of snow over Charlotte, but the likely hood of 0″ is just as likely. So don’t believe the hype of mega storm or the doubters of no storm. The most likely outcome is always somewhere in between.