Thursday, December 6, 2012

Looking At The Pattern for Mid December.

Many are wondering where is the winter weather, and will this winter be a repeat of 2011-2012. As for where is winter, The real cold air has been locked up across the interior of Alaska and northwest Canada since the middle of November. Parts of Alaska have had  -50 degree temps for over a week, with yesterday being the coldest day yet, –56. As for will this be a repeat of last year? The Short answer is NO. If you’ve read my 2012-2013 winter outlook; you know, I said the first third to half of this winter season would be warmer than average, with the second part of winter coming in colder than average. I still feel this will be the case. Here is a link to the winter outlook. The pattern over the next week or so,will be very unfavorable for sustained periods of cold and snow along the East Coast. But This will change.

As I said, the cold air is bottled up in Alaska and Canada. Why is this happening? The negative PNA and Positive NAO has high pressure high pressure to our south forcing warm air to move in from the southwest. What is the reason for this. The giant vortex up in Alaska is the reason it's been so warm here in the Northeast. This vortex will start to break down soon. I will once again go to the teleconnections, to show you how all of this should workout.  

The Teleconnections:  

I will be going into four indices, three of them I've talked about quite a few times in the past. However, there is one that I have not...I will talk about that after go into the NAO,AO, and the PNA.

   The NAO:

As you can see on the chart the NAO is currently sitting around neutral. If you follow my post on this blog and fan page, then you know, that when the NAO is positive our weather in the Northeast is generally warmer and drier. This is because of the lack of any real block slowing the storms. Looking ahead especially around the 10th-16th and on, the forecast shows the NAO going strongly negative. When this happens we will see a trough develop and the colder air to our north coming down to visit.

The AO:

In order to get real cold air out of Canada, the NAO needs a helping hand. This is where the AO comes in. When the AO is negative it allows the cold air in Canada to drop south into the United States. Now some of you may be wondering the AO is negative. If you look at the AO forecast chart you will see the AO has been negative since the 20th of November; So why is the cold air still locked up in Canada. To answer that we will have to look at two other indexes. The first I've mentioned before, that one is the PNA. However there is one other that I haven't talked about before, that one is the EPO, both of these are playing a role as well.

The PNA:

The PNA does the same thing for the West Coast, that the NAO does for the East Coast. By this I mean, when it's positive the West Coast is warmer and drier; when it negative the exact opposite is true. If you look at the PNA forecast chart, you will see that it has been weakly negative. So the west has been cold....but the cold can't push farther east. just as the NAO being positive has prevented the negative AO from supplying us with colder air. Looking at the PNA forecast chart again, you can see the PNA looks to stay negative going into mid month. The forecast had been calling for it to be neutral to positive, this is a fly in the ointment. So it will have to be watched to see if it trends back more to the positive side. But, the PNA has a variable effect on the East Coast, I will have more on that in a bit. Looking at the NAO and AO forecast charts you can see they will be in a negative phase. Even with a negative PNA I still think we will see colder air infiltrate the Northeast after mid month. How do I know this? For that we have to look at another teleconnection, the EPO.

The EPO:

12zecmwfensepoeuroEPO Now while the PNA has an impact on northeastern pressure patterns, as I said it is variable. But, the EPO whether directly or indirectly has even more influence over the East Coast and the GOM.

The EPO (Eastern Pacific Oscillation). The EPO is, at its simplest, the alternating pattern of high and low atmospheric pressure, located four to seven miles above the Pacific Ocean. The EPO helps determine the pattern of the jet stream moving from the Pacific across North America. This means it's what controls troughs and ridges around Alaska; when it's negative, a trough will be in place near Alaska. So a positive EPO will produce a ridge in the same place. If you look at the EPO forecast chart, you will see that by mid month the EPO looks to be positive.

OK, why the warm weather?

I'm sure all of this negative / positive teleconnection talk is giving some of you a headache. So I will try to explain it another way. Most of us know that generally in the northern hemisphere weather systems moves from west to east. So in order see what the weather will be like down the road here on the East Coast, one has to look to the west. Since the EPO is influencing the weather in the Pacific Northwest, we must look there first.   

OK since the EPO is negative, we see a trough over Alaska and a ridge off the West Coast. This trough in the west lends support for a ridge in the east. Now since the AO will send the cold where the trough is, which in this case is out west. Add in the positive NAO and ta-dah, we have warm weather in the Northeast.

So it makes sense that when the EPO goes positive along with the negative NAO and AO that's being forecast around mid month, that this all points to colder air moving into the eastern part of the United States. The negative NAO should create a Greenland block which will in-turn force coastal storms to form and move into the northern Mid Atlantic and Northeast. How much staying power the block has will depend on the vortex over the Atlantic. ... But all and all, the way the indices are trending is a very good thing if you're a snow lover.

The GFS showing the 17th of December  .


  A look at the GFS 12Z model run showing the 20th.


And here is the GFS showing the 21st.

If you look at the model runs above, you can see the results of what I was talking about. The EPO is no longer negative. and with the high pressure that's been hanging off the East Coast moving out of the way, cold air is moving into the Northeast. The pattern still looks to become more active than we've seen the last 10 days. The NAO does look to become east based. So if we get any storms this will help steer them our way. As I said in my winter outlook, this winter will be a roller coaster as far as cold and warm air outbreaks. As you can see, the GFS does show a lot of cold air in place just before Christmas. The models also show we have a good chance for a storm around that same timeframe. With the Greenland blocking to the north, the storms after mid month will finally have the cold air to play with.    But the bottom line is, we should have plenty of chances for minor to moderate snow events after the 10th of December.

When I looked at past snowy Decembers, many of them had the same general pattern. We are looking fairly far out for any kind of real forecast. But, I feel our chances are good in seeing more in the way of snow in 10 to 14 days.   

Well that’s it for now, I will have more on all this as we go on…just check back here or follow the sidebar gadget to my Facebook fan page.     


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