Sunday, November 25, 2012

Wild, Wild December Ahead For East Coast

A wild month of December could be ahead for the East Coast, including the Northeast, Mid-Atlantic and New England, thanks to several factors, but could also be hampered by one big player. Let's jump right in.

This is the forecast for the North Atlantic Oscillation (NAO) in the next several days from the Climate Prediction Center. The NAO involves the presence or lack of a semi-permanent storm system or ridge over Greenland. If that ridge of high pressure is in place over Greenland, this is a negative NAO, which then creates a favorable pattern for cold and storms in the Northeast. The ensemble forecasts from the CPC call for a big dip in the NAO by the start of December. From then onward the NAO becomes a bit more difficult to forecast, as the ensemble members begin to spread out. However, the general idea is for a negative NAO to continue.

This is the forecast for the Arctic Oscillation (AO) in the next couple of weeks from the same ensemble forecasts from the Climate Prediction Center (CPC). The AO's forecast has more of a consensus than the NAO's forecast, which means higher confidence. The ensemble members predict a plummeting AO through the end of November, but also keeps the AO negative through the rest of the forecast period. When you have the AO in the negative phase, the cold air in the Arctic is released south, and temperatures trend on the chillier side for much of the Lower 48.

This is the Pacific North American index (PNA) forecast from the same CPC ensemble members, for the same time period. Note how the PNA has been negative for the last few weeks, and this is thanks to a persistent stormy pattern over the Northeast Pacific, and I will explain more about this next. The PNA forecast ensemble members have the PNA staying negative, but trending more positive in the future. A negative PNA has a warmer pattern over the East, while a positive PNA displays cold and stormy conditions in the East US.

Here is the problem we need to watch. There is something ongoing in the Bering Sea called an 'Omega Block' pattern, where a ridge of high pressure forms in the Bering Sea, with storms on either side. This creates the Greek Omega sign, as I outlined in the image above (and which was the first to note). Now, this Omega block has encouraged a persistent stormy pattern to develop to the east of the high pressure, which is the base of the negative PNA. So this is a pretty darn stubborn pattern, which has been holding for quite a bit. If we don't see this Omega block get going soon, December could be in trouble.

Let's divert our attention away from the Bering Sea for the moment and look in the upper left hand corner of this image. That area is East Asia, and this forecast for 3 days away shows a strong storm hitting the region. The GFS Ensemble averages (as seen above) have a stormy pattern persisting in the East Asia region for at least a week. Those of you who have followed me for a while know that there is a correlation between East Asia storms and East Coast storms, with a gap between the two of 6-10 days. So, if we see that strong storm in East Asia valid for November 28th, we could see another stormy pattern in the East US from December 4th- December 8th, and possibly beyond if a stormy pattern persists in the East Asian region for a longer time.

Another factor supporting an intense December is the Madden-Julian Oscillation (MJO). The MJO involves where a stormy pattern is in certain parts of the Bay of Bengal, Pacific Ocean and Atlantic Ocean. These different areas where the stormy pattern could be make up the 8 phases of the MJO. This forecast for the MJO has Phases 8-1 coming on, possibly into Phase 2 if this forecast verifies. The average conditions for temperatures (bottom right images) and precipitation anomalies (top right images) for Phases 1 and 2 are shown, and these two phases generally bring cooler than normal and wetter than normal conditions to the East US. Not so much Phase 2 as Phases 8-1.

So we have a favorable NAO, AO and MJO coming in for the next week or two. I would really like to see that Omega block get out of the Bering Sea as soon as possible. If that block holds steady for the next few weeks, precipitation could be affected, especially for the Ohio Valley, Plains and other similar regions. However, I feel that the negative NAO/AO will be able to squeeze in good shots of cold air in between ridges set up by the Omega Block.

Something interesting that has been found is a negative correlation (Factor 'A' goes positive, so Factor 'B' goes negative) between the 30 day sunspot cycle and the PNA index. So, we are seeing the 30 day sunspot cycle begin to go back down to its negative (as shown in the image above), so it is plausible that we could see this Omega Block start to back off, leading to a slightly better pattern for a positive PNA to develop.



wiiluigi1998 said...

If we have this "Wild December" I'll be i heaven.

Alicia Smith said...

Wen will you be releasing the personal winter forecasts?