(10-28-11) Winter 2011-2012 Forecast

As you know, 2011 has been a very volatile year weather-wise for the United States. Dubois County alone has received nine confirmed tornado touchdowns this year. We’ve went from a wet spring to drought conditions this summer to huge temperature swings with a fair amount of moisture this fall. And I’m sure that has everyone wondering what kind of winter we can expect?

My goal is to give you a general idea of what I’m seeing in our weather future this winter. But before I get into the nuts and bolts of the forecast, let me talk about a few of the factors that will have an impact on our situation this winter.

La Nina

The most obvious one that everyone seems to be talking about is La Nina. La Nina is basically the cooling of surface temperatures in the Central and Eastern Pacific Ocean waters. See the graphic below.



El Nino is the exact opposite of this. El Nino is the warming of these same waters, as seen in this graphic.



Right now, NOAA says that we are in a La Nina pattern, and will continue to be. Listen to what the folks at NOAA are saying, "Currently, La Niña is not as strong as it was in September 2010. Roughly one- half of the models predict La Niña to strengthen during the Northern Hemisphere fall and winter. Of these models, the majority predict a weak La Niña (3-month average in the Nino-3.4 region less than -0.9°C). In addition, a weaker second La Niña winter has occurred in three of the five multi-year La Niñas in the historical SST record since 1950. However, the NCEP Climate Forecast System (CFS.v1) predicts a moderate-strength La Niña this winter (between -1.0°C to -1.4°C) and CFS.v2 predicts a strong La Niña (less than -1.5°C), which rivals last year's peak strength. For CFS forecasts made at this time of year, the average error for December-February is roughly ±0.5°C, so there is uncertainty as to whether this amplitude will be achieved. Thus, at this time, a weak or moderate strength La Niña is most likely during the Northern Hemisphere winter."

Bear in mind, though, that we had a strong La Nina last year, but as you can tell from the graphics I provided above, our weather didn’t exactly match up with a classic La Nina situation. Why? Last year we had one other key factor play a larger role that dominated our weather – the NAO (see below). So even with La Nina forecasted to continue through our winter, it may not mean all that much if the NAO steals the show again, so to speak.

The North Atlantic Oscillation (NAO)

NOAA defines the NAO as “the climatic phenomenon in the North Atlantic Ocean of fluctuations in the difference of atmospheric pressure at sea level between the Icelandic low and the Azores high. Through east-west oscillation motions of the Icelandic low and the Azores high, it controls the strength and direction of westerly winds and storm tracks across the North Atlantic.” So in real terms, basically you have a high pressure zone and a low pressure zone that are wedged in between the Jet Stream. How strong or how weak these pressure zones are controls the path of the Jet Stream. See the graphic below.



In the Positive Phase of the NAO, the low pressure area near Greenland is strong and the high pressure area off the coast of Africa is strong. This pushes the Jet Stream farther north and thus keeps the snow storms more to our north because storm systems usually follow the Jet Stream.

In the Negative Phase of the NAO, the low pressure and the high pressure areas are both weak and that allows the Jet Stream to move farther to the south. That takes storm tracks in the winter time right into our area, increasing our potential for snow. And as it would turn out, the NAO is once again forecast to go negative.



In the chart above, the dashed line in the middle is neutral or the zero line. Right now you can see that the NAO is positive barely, but the red lines are the forecast ensembles which are trending the NAO more negative. In fact if the graphic were to go out further you would see that the models are actually taking the NAO more negative than the above graphic shows.

If this holds true, I think we will continue to see “blocking” becoming a major determining factor in the Jet Stream’s path. When the NAO goes negative, this weakened area of generally lower pressure allows a strong high pressure to form just off the coast of Canada. This serves as a blocking mechanism that shoots the Jet Stream south over our area, then back north up the Atlantic Coast. This is sometimes called a Greenland Block, as you can see in the illustration below. This Greenland Block was very prevalent last winter with the NAO being negative and I think it’s going to come into play in a major way once again this winter.



The Bottom Line

So how will all of this affect us? I think a strong Greenland Block will set up multiple times this year and will cause a large swath of cold, arctic air to be forced over the Eastern USA. The Jet Stream will be set up to drop snow makers into our area, but the temperatures will be the biggest concern. I think we’ll see below normal temperatures the first part of the winter for sure. The real question will be where will the freezing line be most of the winter during these precip events? I can’t rule out the possibility that we’ll see significant ice accumulation, but I’m expecting the ice to be more of an issue in Southern Kentucky, Tennessee and even into Virginia/West Virginia. I do think we will see above normal accumulations of snow.

So just for the heck of it, here are three “just for fun” predictions for the winter ahead. We’ll see how I do once the season is over.

Coldest Temperature: -3F
First Snowfall: Week of November 23rd
Total Snow Accumulation: 26”

So, what do you think? Let me know in the comments!

Potential New Year's Eve Storm Update

Just for fun, thought I'd show a graphic of the latest on the New Year's Eve/Day storm. Models continue to show a monster coming into our area. Take a look at this.



You can visually see how strong the cold front it and you can also see the major low pressure centered near Chicago. Ahead of the front in the green you see 12C temps (about 54F) and right behind the front in the blue you see -4C (about 25F)! That's a sharp front! I would expect to see severe storms in a squall line with temps like this and I'd think snow showers would be most likely behind the front, with blizzard conditions to our north. This will definitely be one to watch closely in upcoming days.

(12-24-10) White Christmas Almost Here!

Models continue to come into agreement that main surface low will move into the Gulf of Mexico. However, a nice dip in the jet stream in the mid layers of the atmosphere will cause enough spin in the atmosphere to produce upward motion needed to produce snow. The timeline still looks like the main brunt of the snow will beging late this evening. At 8am this morning, snow is already moving into southern Illinois, but I think that since the air is not saturated yet, it will take a while for this to reach the ground. It still looks like it will be late afternoon into the evening hours before we start to see much in the way of snow here in Dubois County.

To that effect, the National Weather Service in Louisville has placed us under a Winter Weather Advisory beginning at 6pm tonight and lasting through Noon on Christmas. While this won't be a major blizzard by any means, a few inches are expected to fall. Bufkit sounds place snow ratios in the 13:1 to 17:1 range with a QPF (quantitative precipitation forcast - liquid precip in other words) at about a quarter of an inch. That would be just over 3 inches of snow at the 13:1 ratio and about 4 inches at the 17:1 ratio. All in all, I think a solid 2-4 inches is a pretty good estimate of what to expect. Some areas of Southern Indiana may get less and I hope we're one of them!

As you can see in the graphic below, the HPC has placed us at the edge of a moderate risk (40%) for having snowfall at least 4 inches. There is some pretty strong agreement among all models and forecasters now that we'll see a decent snowfall.






Again, travel conditions tonight could get very rough, so pay close attention to the road, slow down to a reasonable speed and don't go out unless you have to once the snow begins. Have a very Merry Christmas and I'll return to blogging in a couple of days.

(12-22-10) New Year's Eve Nightmare?

Just wanted to make a quick post about the potential storm heading in over the New Year's Eve weekend. It's way too early to see how this will truly pan out. The models are all over the place at this time, but almost every model run is showing a massive storm...somewhere! The exact placement is to be determined, but someone is going to get hit hard. Look at this graphic:




If this pans out, we'd have temperatures in the 50s ahead of the front with severe thunderstorms somewhere along that frontal boundry! Then it would turn sharply colder into the 20s behind the front and give us a quick inch or two of snow in the wrap-around moisture!

Yuk! Almost gives you a headache just thinking about all of this! The models have been consistent in that a major storm is brewing...it's only a matter of where it seems like. I think someone could get slammed with some strong storms and someone else could get a major blizzard out of this. We'll see how it pans out. I'll post more on this in coming days as we get past this Christmas system and as we get closer to the actual event with better model guidance.

(12-22-10) Diminishing Snow Chances!

All kinds of chatter out there as to this winter storm right now and all kinds of opinions about it. At least the idea of what is headed our way is becoming clearer...a bit anyway.

The leading edge of this system is just now pushing onto shore as you can see from the enhanced infrared satellite image below. I've placed a low where the system is currently located. You can see the counter-clockwise circulation quite well.

The last of the models that were hanging onto a more norther track have now went south. In fact, the trend is to push this low into the Gulf of Mexico by Christmas. I think over the next couple of model runs overnight and into early tomorrow morning, we'll get a much better feel of this system - simply because of the increased data points to be entered into the equations the model relies on to produce the maps. There will be much more data available on this system now that it is on shore.

As I mentioned yesterday, the models are in disagreement as to how the northern and southern tracks of the jet stream will go and when they will phase together. I've looked at a lot of model runs over the past two days and it seems that the trend is to take the low into the deep south and phase the two jet streams over the Atlantic Ocean before the storm moves up the east coast and turns into a noreaster for the New England area.

You'll recall from yesterday's post that some of the older model runs were taking the low into Tenneesee and Alabama and some forecasters are still holding out to that option. But I disagree. I think that's too far north for a number of reasons. For starters, there is a blocking high situated in Canada at the 850mb level. That is forcing everything to the south. Secondly, the low coming on shore right now is just too far south to make it much farther north and overcome that blocking high. It would really need to be moving on shore in mid-California to overcome this and take a Tenneessee/Alabama track. Thirdly, based on the latest model guidance, I don't think the two waves of the jet stream will phase together until late Christmas day over the Atlantic Ocean. Amd there will not be enough energy and momentum to shift it north until that happens. Over the ocean is too late to take it north into our area!

Here's a visual idea of what I'm talking about. The model run on the left is the GFS 0Z run (7pm last night). Notice that it takes the low right over New Oreleans. To the right is the GFS 12Z run (7am today). Notice that it takes the low even further south over the Gulf of Mexico and slows down the timing quite a bit.

I think the low will eventually turn out to travel somewhere in the deep south near the Gulf. And frankly, I don't think it will have much of an effect on our weather. The mositure that you see in our area on these two models is actually from a dip in the northern section of the jet stream in our area and there just isn't enough precipitable water associated with that stream to be able to produce much in our area. Consider the latest snowfall map from the GFS 12Z run:

This seems more on track with what I'd expect from just a small upper-level distrubance from the jet stream in our area. This takes us at about 2 inches of snow by Saturday. With the low largly out of the picture for our weather, I really think we'll see less than 3 inches total with this storm and I think that the 1-2 inch range is a much better bet. I do think we will see something and I do think it will make travel rough for a while. I still expect it to start late Friday afternoon or evening. If you are planning to travel on Friday, take caution and stay tuned as the days get closer. I'll blog more about this tomorrow as more model solutions come out.

(12-21-10) I'm Dreaming of a White Christmas...

Well, some people are anyway! If you're a snow lover, this could be the storm for you. Sorry to be so slow in updating this. I've had a pretty busy week and all my family including me have colds, but plan to blog more over the next few days as the storm approaches.

It's looking like we'll almost certainly have a white Christmas this year. The models are in disagreement on the exact timing of the system, but almost all models are now agreeing on a southern track for this low moving in. Some have it near Tenneesee and other have it near Alabama. Either way, it appears we will be all snow. Just how much snow we get will depend on the exact track, with more with a Tenneesee track and less the further south it goes.

Right now the system is just moving onto the west coast and as it moves on over the next day, we'll be able to get more data points into the models. There aren't many data points over the ocean! With more data on this system, the models will come into much more agreement and we'll have a better feel for what to expect.

One thing worthy of note is that over the last few model runs, the forecasted snow has continued to increase. Consider the graphic below. The graphic on the left is the snow accumulation predictions from the GFS model at 7pm last night (0Z). The one on the right is the same model at 1pm today (18Z).

In just a few short model runs, it has went from about 1 inch in our area to 5 inches! Yikes! Again, this is just preliminary and may well change, but it serves as a good warning that we need to take caution. This will likely be one of the busiest travel weekends of the year and this storm could well have a major effect on travel plans for many people. I suspect that the National Weather Service will be issuing winter storm advisories and/or warnings for our area in the next day or so as we get closer to this event.

Much of what we get will depend on how the jet stream acts out for us. Right now there is a strong high pressure system in Canada that will be acting as a blocking mechanism for us, keeping this winter storm on a southerly track. How strong this blocking mechanism is will depend on how far south this storm is pushed. Also, the southern track and northern track of the jet streams are predicted to phase together, increasing the strength of this winter storm. If that phasing happens over the plains, expect a stornger system with more snow for us. If that phasing happens over the deep south, expect a weaker system for us with less snow. I think the models will get a better handle on this over the next day or so as the system moves onshore.

Let's talk about a timeline for this event. Right now I'm thinking that most of the snow will not begin until Friday afternoon. The low will be moving to our south during this time frame and will pull in some significant moisture for us. How much is yet to be determined, but I do think it will be signifcant enough to cause travel problems Friday evening and night. Snow will get heavy at times and last overnight into Christmas morning. As the low moves off to the east and out to the Atlantic Ocean on Christmas day, expect snow to taper off and light snow showers to continue into Sunday as wrap-around moisture comes into play. Overall I'd say 3-4 inches or more is not out of question if these current model solutions verify.

Next week looks to be quite cold! As this system moves out to sea, it could turn into quite a noreaster for the New England area. That will turn our winds around to the north and push bitter cold temperatures back into our area. Also taking into consideration the snow pack from this system that will still be on the ground, we won't get out of the 20s for the first half of next week. The long range model also have a winter storm moving in over the New Year's Eve timeframe. It's too soon to predict anything definite, but it could be quite a system if it verifies out.

For now, here's your updated 7 day forecast.

(12-17-10) Mon-Tue Wintry Weather

The latest model guidance is trending warmer for next week, which is a good thing. For the past three days, the model output statistics (MOS - a numerical prediction of high temps) have trended warmer for our area. Monday's MOS has went from 35 to 37 all the way up to 39 on this last run. I stil don't think it's accounting for the snow that's still on the ground in our area, but I do like the trend, though I doubt we'll see 39 on Monday. I'd say 35 is more likely.

These latest models have also shown a dramatic shift in the tracking of the surface low coming into our area next week. Yesterdays model was sort of apocalyptic in some ways, with the low tracking to our north and 5 or so inches of snow in our area. This latest model tracks the low south of us and puts us with a light snow event to start and then changing over to plain 'ol rain by the end of it. So which will it be? I'm hoping for the latter, as I'm sick of snow! I'll wait for the model runs to come out again later tonight and tomorrow. If it keeps trending south of us, I'll be able to fine tune the forecast more. South of us would also send us warmer air, lending more toward rain.

To get an idea of what I'm talking about, here's a small part of these past two GFSX 0Z Models from yesterday on the left and today on the right. You can see how the low was predicted to be in Illinois at 7am on Tuesday. The latest model takes it into Arkansas at 7am on Tuesday. This would be a dramatic shift in what we would get with this system! You can also notice that the low was forecasted to be at 1006mb and the new one takes it to 1014mb. This means it would be weaker, with less fuel to work with and much less wind for us. All in all the new set up is much nicer!

Now for what everyone is really thinking about - Christmas! What will it be like? Below is the current model run for Christmas day. You can see it's showing a bit of snow activity on a broad level in our area. This would be light snow showers/flurries. No major system around us, just a small dip in the jet stream over our area that might spark up a few things. Cold NW winds will push highs only into the 20s on Christmas.