**** RELEASED 11-22-2017 ****
December is almost here and that means winter is just about upon us. It’s time for the final winter forecast.
There are a few changes in this final forecast when compared to the Preliminary one released on 9-29-2017. The main thing I will discuss in this post are the changes made and some of the why behind those changes. If you want a very detailed explanation, be sure to watch the video at the end of this post. I dive in-depth into the reasoning behind the forecast for you winter lovers.
First off, let’s look at the current ocean temperatures. There have been some noticeable changes in the past two months. The most obvious thing is that La Niña has now taken a firm hold. Back in September, we were under a La Niña watch. We are now under a La Niña Advisory. La Niña conditions should persist through the winter before easing up in the spring. But carefully note from the graphic below that we have seen a large expansion of the colder waters off the coast of South America. This is what we call an East Based La Niña and it has some major implications for the winter forecast. East Based Niñas behave much differently than basin-wide events. If you want a more in-depth discussion of this, watch the video at the end.
The other thing that you will note is that we may be starting to see the infamous warm blob reappear. This was the primary driver in our brutally cold winters a few years ago. The warm blob is a pool of very warm waters in the Gulf of Alaska. This helps to promote ridging there in the Jet Stream, which in turn promotes a trough over the Eastern USA. Those warm waters were non-existent last year. There was a lot of cold water in its place. You can see that from the graphic below. Of course, this leaves some big questions. Will that warm water stick around long term through the winter? Will La Niña override its effects? Lots of questions and speculation, but we’ll have to wait and see. But it is one more piece of the equation to take into consideration that could give us some colder risks in the forecast.
As a result of the changes in the ocean temperatures, I’ve selected a new set of analogs to help us see how the winter might turn out with an East Based La Niña in place. The analog years I’ve chosen that I feel most closely match our ocean state right now give this for a Jet Stream pattern. Notice the amount of high latitude blocking going on over Greenland and near Alaska. That helps to buckle the Jet Stream and bring some colder air into the USA. Notice also that the Southeast Ridge has been pushed farther west because of this East Based La Niña.
The implications of a pattern like this would mean a much cooler look overall for winter. We are already seeing that. This high latitude blocking has already set up shop here in November and that is why we are noticeably colder than we saw in October. The record warmth is gone. We aren’t in the deep freeze and we’ve still seen some occasional warmer days (and even some severe storms), but we’ve also seen quite a few colder days this month. Could this be our winter pattern taking shape? It’s possible. The analogs suggest this blocking sticks around. Unfortunately, that can’t be accurately predicted by the modeling more than a couple weeks out. So time will tell.
What would a pattern like this do to temperatures here? Here is the December through February temperature scheme with these analogs. Blue shades are cooler than normal. Warmer colors are warmer than normal. White is near normal temps.
Compare this with the analogs from two months ago before this became an East Based La Niña. Notice the shift west in the warmer anomalies. That’s because in an East Based La Niña, the usual Southeast Ridge a La Niña produces gets shifted further west. If you like cold and snow here in Indiana, that’s good for us because it pushes the warmest temps a bit farther away from us and helps to angle the Jet Stream in a more favorable fashion for winter mischief.
Let’s look at the updated snow totals for these new analog years. Like the previous analogs, the totals run the table with a wide range of values. Normal total snowfall for Evansville is 12.1 inches for the season. Note here that three of the analog years featured above normal snowfall and three featured below normal snowfall. So it would appear that there are risks on both the above and below normal sides. The years may have changed, but this is close to what the other analogs in September showed. For that reason, it would appear we have about equal chances this year of being above or below normal in snowfall this year. But the potential is there for some large totals, so snow lovers take heart. This is some potential, at least.
MY UPDATED FORECAST
Here are my final thoughts on this upcoming winter. First let’s look at temps. You’ll note some changes from the previous forecast.
Here is what I did have in the Preliminary Forecast back in September:
And this is the final forecast. With the change in orientation due to La Niña, I do think the warmest anomalies are south and west of us. That said, I think the major, consistent cold stays to our north. I’ve placed us in equal chances for above or below normal temperatures this winter. I know many of you cold and snow lovers would love to see me place us below normal, but I’m not confident in that and so equal chances are as low as I can go and still not be wishcasting. It is a colder look than the previous outlook, at least. I will say this. I would not be surprised for us to see some big cold snaps this winter. Do I think it lasts all winter? Doubtful. I think we’ll be a winter battleground around here like we usually are. We always seem to have to fight for the cold air here and I see this year as no different.
Precipitation has not changed much. Here is the old map from September.
And here is the new forecast. The position of the well above normal precipitation is all that changed for us. I still think we’ll be above normal in rain, with an active storm track through the Ohio Valley.
Snowfall also changed. Here is from September:
And here is my new forecast for snowfall. I’ve taken us out of the below normal category and placed us in equal chances. Notice also that the above normal areas have come closer to us. I’m not confident enough in the forecast to place us above normal in snow this year, but I will say this, it would not shock me if we were. Equal chances means exactly what it says. It could go either way.
Here are some highlights to consider for this winter.
- Temperatures will be much colder than last winter. Don’t expect record warmth again this winter. That said, I’m not saying they will be brutally cold again, either. We’ll face times of above and below normal this winter. When it’s all averaged out over the three month period of December through February, we have equal chances of being above or below normal. It appears we may have a seasonable winter for once.
- Rainfall does look above normal to me. Snowfall is much more questionable. As I noted above, it would not shock me for us to be above normal. I’ve placed us in equal chances for snow. I do think we’ll see some snow this year. Last year, most of us saw less than an inch. It was a terrible year for snow lovers! I think that changes this year. I do think we’ll see some snow. I’m going with an educated guess of 75% to 125% of normal totals for this season.
- One more word of caution. La Niña years are notorious for bringing ice storms around here. I’m not saying we will for sure see an ice storm, but don’t be surprised if we do. I think the odds are slightly higher this year.
So what could go wrong with this forecast? As always, several things.
- Could the models be too warm? Could the analogs be too warm? I think that’s a real possibility. I’m very conservative in my forecast here. I do think there could be colder risks, but I’m not confident enough to pull the trigger on a colder look yet.
- That said, what if that southeast ridge decides to flare up? Maybe the East Based La Niña fades and goes back to a basin-wide event. If so, warmer risks would be introduced. That’s also possible. Less likely than the other risk I mentioned, but still possible.
- The other thing I’d say is that some pattern drivers just can’t be accurately forecasted more than a couple weeks out. So there are always lots of unknowns with a winter forecast. The blocking we’ve been seeing in November may not stick around. Then again it might. Modeling can’t show us that more than two weeks away.
Well, there you have it. The final winter forecast for 2017-18. If you want more of an in-depth analysis, watch the video below. Look for a mid-winter update later in January where I’ll analyze how we are doing so far and what the remainder of winter looks like.