La Nina & Winter:
When we first put together a seasonal forecast for the upcoming winter, La Nina was at the front of the conversation. Everybody was on board that we were heading towards another La Nina year, but the strength and length was yet to be determined.
In an average La Nina year we anticipate drier and warmer conditions through the winter months here in the southeast. December hit those conditions perfectly. We didn't have to use the heater too much and to a lot of smiles, we left the umbrellas/shovels perched against the wall.
That being said, watch the video and you will see that last year's December was anything but "average". Is it the strength of the La Nina that is making the difference? Certainly something to consider. Unfortunately, our atmosphere is so complex that it is very difficult to pinpoint one variable (La Nina) and directly correlate it with future outlooks (January forecast). But some similarities between years do show up in the climate data.
At this juncture, the jet stream isn't set-up to push winter storms our way. Sure we've had our share of cold air outbreaks this season, but recall how long they have lasted...a couple days at most? What this means is that the jet stream is moving too quickly for cold air to settle into our area. Keep in mind I'm writing this from the prospective of those wanting snow and well, cold air is very important.
As snowcasters, we want a jet stream set-up the keeps the cold air socked in for a while. A set-up that will "block" the atmosphere from transitioning from warm to cold too quickly. As the name would imply, we are looking for indications of a blocking pattern in the atmosphere. This would guarantee one of the most important ingratiates in producing snow...cold air.
If you've ever wanted to locate a trough or ridge on a weather map here is a website that will help.
A large U shape over the united states is a dip in the jet stream. Another name for this U is a trough. During our winter season, these U's (troughs) have been moving too fast through the United States to lay down any significant snow. Flip the U upside down and you have what's called a ridge <--- Topic for a future World of Weather.
As it stands, a small shift in the pattern could be coming in February. Some long range models indicate a wetter than average patter for the Midwest. This would give eastern Carolina the moisture upstream and in turn all we would be watching for is an opening in Canada's Arctic gates.
All that aside, if this winter holds onto a La Nina pattern see at the start of this winter then our snow totals we will finish much different than last year.
What Does History Say?
Of the top 10 Highest daily snowfalls in Greenville:
6 have come in February
3 have come in March
1 has come in January