2011-2012 Winter Outlook Update

[weatherwiz]

Now that we have entered the first few days of meteorological winter it's time to take a much closer look at how the overall weather pattern has fared over the past several weeks and take a look at how this pattern is expected to evolve through the next several weeks as the winter season is just about officially upon us. Below is a link to my winter outlook which I completed on Monday, October 24th, 2011. Since we are now into the first week of December and into the beginning of meteorological winter it will be time to issue an update to my winter outlook.

https://www.facebook...135560349878094

Below I will discuss the latest trends regarding ENSO as well as some of the key global teleconnection indices including any influences from solar activity as well as tropical forcing with the MJO.

ENSO:

Back in October conditions in the equatorial Pacific were consistent with that of a La Nina episode as sea-surface temperature anomalies (SSTA's) in the equatorial Pacific were running below-normal. Here is an image of the SSTA's from October 24th, 2011.

Below is the same exact image only from December 1st, 2011. Notice there have been some drastic changes across the Pacific Basin. These changes aren't so much regulated to the strength of the ENSO episode but rather the coverage of the ENSO episode. Waters across the coast of Central American and South America have cooled significantly as have the waters from about 20°N to 40°N and 150°W to 120°W. While according to the Oceanic Nino Index (ONI) definition we are not in a La Nina, remember for an ENSO event to be classified a La Nina or an EL Nino you need to have five consecutive trimonthly periods with the ONI reading of +/- 0.5°C, it is extremely likely to guaranteed when all said and done this winter will be officially classified as a La Nina winter.

During the October issuance of the winter outlook we took a look at the ENSO weekly readings which gave you the average SSTA readings throughout each ENSO region; ENSO 1+2, ENSO 3, ENSO 3.4, and ENSO 4. While last winter was a La Nina winter by the spring/summer of 2011 that Nina had faded away and we were in neutral ENSO territory for a while, however, Nina conditions were still present albeit the signal was weak. By mid to late July ocean temperatures across the entire Pacific Basin slowly began to cool and from about the first week of August through the first two weeks of September we saw rapid cooling take affect. From this point to the time the original winter forecast was issued the cooling across the equatorial Pacific and in the ENSO regions remained rather steady with slight variations between very weak cooling or ever so slight warming. Since this time we have seen another phase of cooling occur and it does appear that this La Nina will indeed become a high end weak La Nina to a low end moderate La Nina and long-range computer model guidance continues to suggest this will be the case.

When further assessing ENSO to get a better indication of where this La Nina may be heading we will first take a look at the trade winds. The trade winds are the pattern of easterly winds which are found in the tropics. In the northern hemisphere these easterlies blow from a northeasterly direction, in the southern hemisphere they blow from a more southeasterly definition. With the trade winds blowing from a more northeasterly direction if the trade winds are stronger than normal this increases upwelling off the coast of Central/South America which allows for colder waters below the surface to rise up to the surface and then move through the Pacific...this leads to colder than normal waters and to the development of a La Nina.

We will now take a look at the progression of the trade winds since the beginning of September. The image on the top left is a composite of the 850mb zonal winds for the month of September, the top right is for the month of October, the bottom left is for the month of November and the bottom right is for the final week of November.

During the last week of November we saw yet again another impressive burst of trade winds across the western tropical Pacific Ocean. This will help to keep the Nina going and will perhaps continue lead to slight strengthening of the La Nina through much of December. How strong the Nina gets in the end will really depend on how much more the Nina strengthens in response to this recent burst of winds.

PDO:

The PDO continues to be negative, in fact quite negative at times and will likely remain so through the remainder of the winter thanks in part to the La Nina conditions in the Pacific and due to the fact that the PDO has entered it's negative phase cycle.

The combination of a rather strongly negative PDO and La Nina conditions could have a significant impact on the overall weather pattern at times, especially during any periods where the NAO/AO/EPO are also all positive. When the PDO signal is strong this usually correlates well with above-average temperatures across much of the south-central and eastern US for the month of December. As we move through January and February the above-average warmth usually resides across the southeastern US. Below is a composite of some of the most negative PDO December's on record and temperature anomalies as well as a composite of the most negative January's and February's:

NAO:

Since about mid to late summer the NAO has been predominately positive and since about mid-November the NAO has been extremely positive. Computer model ensemble forecasts indicate that over the next week this strongly positive NAO will begin to relax a bit as the NAO almost more neutral, however, it appears as if we will see yet another major spike within the NAO as we near mid-month. With in the previously issued winter outlook I stated I believe we would see a positive NAO for the month of December I mentioned due to the configuration of the SSTA's around Greenland that we at some point should see the NAO become more negative in nature. Looking at how strong 500mb zonal winds have been during the month of November in the highlighted area below and how even stronger they have been this past week any hope for a negative NAO or a prolonged lasting negative NAO may be rather slim

Positive NAO's, especially when the signal is very strong correlates to much above-average warmth across much of the US during the winter months. Especially when you factor that with a Positive AO and a Positive EPO. Below is a composite of some of the most positive NAO Decembers/January's/February's and temperature anomalies:

AO:

Since last August the AO has been pretty much positive and like the NAO it has been extremely positive since about mid-November and especially over the course of the last 7-10 days. Computer ensemble guidance is suggesting that this extremely positive AO will relax for a bit in the upcoming week, however, forecasts are for the AO to spike once again as we near mid-month. This combined with an expected rise in the NAO will really help to keep the overall weather pattern above-normal to much above-normal at times.

One major factor which has lead to the highly positive AO is the very cold stratosphere around the North Pole region. This has really helped to intensify the Polar Vortex and keep it quite strong at times resulting in a stronger pressure gradient between the Polar Vortex and an area of low pressure around 45°N. This increased gradient has lead to the stronger 500mb zonal winds which we looked at above. Below is an image of stratosphere temperatures since the beginning of 2010:

EPO:

While since late summer the EPO has been negative at times the stay has been rather brief as the EPO has been predominately positive since this time, in fact there have been periods where it has been quite positive. The latest period, which was from mid-November through the end of the month saw an impressive positive EPO period and when you combine this with the positive NAO/AO we had in place that lead to that period of extreme warmth across much of the US. Currently the EPO has been negative and it appears as if it will remain negative for at least the next week. This has allowed for much cooler weather to finally enter the northern tier of the US, however, with the positive NAO/AO in place these cool shots quickly weaken as they try and advance to the east coast. There are some signals within medium and long-range computer models that the EPO will once again head back into the positive phase and possibly be quite strong once again. Positive EPO's are correlated quite well with warmth across much of the US. Below is a composite of some of the most positive EPO December's/January's/February's and temperature anomalies:

MJO:

Currently the MJO signal is not that strong so it is having very little influence on the overall weather pattern, however when the signal has been stronger it has played a factor, this goes especially true during that period of extreme warmth. Currently the MJO is between phases 2-3 and with the Nina conditions present in the tropical Pacific with some continued strengthening going on the MJO signal may not become quite strong. There does remain a possibility that we could see some more pulses of MJO activity as SSTA's in the Indian are fairly warm.

All in all...

[weatherwiz]

Given everything said above I feel the winter overall will be slightly warmer than normal for a large portion of the US with the warmest anomalies occurring across the southeast and deep south and the coolest anomalies occurring across the west coast thanks in part of the negative PDO.

Below is my precipitation forecast:

Finally here is the snowfall forecast for the Northeast:

I know the precipitation/snowfall maps aren't very detailed but forecasting precipitation can be a bit more of a crapshoot than temperature forecasts and I'm not really sure how to develop a more detailed outlook yet on these two .

[Tropopause_Fold]

Wiz, nice work. Looks like you put a lot of time into this.

[weatherwiz]

Wiz, nice work. Looks like you put a lot of time into this.

Thanks Phil!

Yes, I can say I've good a great deal of time and effort into this. I've been working extremely hard lately and really trying to understand long-range forecasting more and everything that goes into it. While I've always had an interest in long-range forecasting I never really paid much attention to global indices and how the pattern evolves year round but this is something I'm going to start and do more of instead of just waiting until early fall to start looking at everything.

I can actually say I think I've learned a quite of bit over these past few months.

[CoastalWx]

Paul, nice job. You're picking up some good things from this board.

[weatherwiz]

Paul, nice job. You're picking up some good things from this board.

Thanks Scott!

Yes, I've been picking up quite a bit lately.

[weatherwiz]

Well now that December is over can take a look back and see how things matched up. Overall pretty awful with both the original and updated forecast. Did have some "ideas" right with the warmth in the east (although still too cold) and cold in the west.