Teleconnections EPO, NAO, AND PNA (2006) All you snow lovers should know that the most favorable pattern for a snowstorm on the east coast (between the EPO, NAO, and PNA) is a - EPO, -NAO, and +PNA. Rapid changes from negative to positive of the NAO can cause major east coast snowstorms. Recently, we have been seeing pretty much the opposite of all of those, hence the warm conditions. A -EPO favors cold conditions across the east, while a +EPO does the opposite. The recent EPO trend has shown a decline which would probably favor a -EPO to be put into effect by the end of November. The NAO was almost record negative in October. It was only the third time in the last 58 years that the monthly NAO went below -2.0, the only two other years to accomplish this are 2002 and 1968, with 2002 only being away from the in October Both of those years also saw the NAO rise in November and then eventually fall again in December, which is probably what could happen this year.

Teleconnections and Favorable/Unfavorable Snow Patterns I do expect to see some big storms out of this pattern and in fact one can't get any better setup for the mid- Atlantic and Northeast. Consider... unbelievably warm water offshore, a negative EPO, southern stream coming along, cold, and yes we can debate how cold, coming into the picture. But as I was showing Elliot and Chris Shabbot this morning, the Jan. 20, 1978, storm up the East coast which was Boston's greatest storm ever until the blizzard of 1978 a couple of weeks later, had a strong positive NAO, a positive EPO and AO and a subtropical cyclone northeast of Puerto Rico that was not named. Some you get the bear, sometime the bear gets you. That was the anti-pattern for a big storm with teleconnections but low-level cold air from the cross polar flow got involved and built high pressure and the rest is history, rather than mystery. I have seen patterns work out the way I envisioned, but the weather did not. -JB (Jan 2007)- Note: Not snowstorms for the I-95 corridor in an unfavorable pattern are rare. As DT has stated they rarely stay all snow and to get mostly or all snow the cold air has to be formidable and holding its own. -Hurm-

Teleconnections and Pattern Change (January 2007) WHY has the last two months of this winter-- all of NOV nearly all of DEC and the first 10 days of Jan --been so warm? You could argue it was the moderate El Niño. I sytated earlier that when the El Niño was weak we were cold and September October... when it reached moderate intensity the pattern turned mild. You could argue it was the developmental large vortex over the Bering Sea flooded western Canada with a fairly strong Pacific Jet that prevented any sort of amplification in the northern branch of the Jet stream. Of course such a vortex in THAT location is common during moderate El Niño events. You could argue that the Pacific Ocean -- the PDO -- was in the Negative Phase which means a lot of cold water over the eastern Pacific / West coast which supports a trough of the West Coast and therefore Ridge in the East. You could argue that one reason was the lack of any High latitude blocking over eastern Canada Greenland because of the very bad unfavorable sea surface temperature configurations in the Northwest Atlantic. Continued…

Teleconnections and Pattern Change (January 2007) You could argue that since the Polar Vortex was on the other side of hemisphere ---that is to say over Asia it has been impossible to get any sort sustained cold pattern in North America since the heart of the cold air is on the other side of the world. You could argue that is because the EPO has been a positive phase for most of November December and the first 10 days of January there cannot be any sustained Ridge on the West coast. Pick your reason. You may pick or select several of them. I would. Some of the above stated reasons are interrelated. SO.... If you a forecaster that is still arguing that the pattern shift is not really a big deal.... and it's just a cold weather interval... then you have a serious problem. If your reasons for issuing a warm forecast and a continuation of the more pattern over North America to all November all the semblance first 10 days of January... how do you STILL make that forecast since almost ALL of those item / facts I listed above have changed? A+B+C+D= forecast -DT (January 2007)-

Teleconnections and Favorable/Unfavorable Snow Patterns (January 5, 2007) El Nino is a naturally occurring cycle of warming in the Pacific that has profound impacts on North America’s weather patterns. The El Nino is marked by above normal seas surface temperatures over the equatorial Pacific from off the coast of South America westward to the International Dateline. This particular El Nino event has been forecast since last summer and we accurately forecast a warmer than normal November and December throughout the Northeast – primarily as a result of this El Nino. A typical El Nino is known to provide above normal temperatures over much of North America – especially Central and Western Canada (where our cold air typically comes from) along with much of the northern portion of the United States. This particular El Nino event has been characterized as “moderate”. This, coupled with a natural “warm climate” cycle has combined to provide consistently above normal temperatures throughout much of the country so far this winter season. -Ross Murley-

Teleconnections and Favorable/Unfavorable Snow Patterns (January 5, 2007) The El Nino is showing signs of weakening – and more importantly, we are seeing a significant change in the “MJO” or “Madden Julian Oscillation” – which has been intensifying in the past few weeks. The “MJO” is another index that gages fluctuations in pressure, winds, temperature and rainfall over the tropical Pacific. It has been shown that the MJO can enhance or suppress the El Nino and have marked impacts on the weather and general circulation over North America. Additionally, one other intra-seasonal oscillation we keep an eye on, and perhaps the most important one is the Eastern Pacific Oscillation. When this index is positive (as it has been so far this winter), the influence of the Pacific tends to send a strong zonal - or west to east jet stream across the country. This in essence, keeps all the polar and Arctic air masses blocked up into Canada. This is perhaps one of the strongest contributing influences to the extraordinary warmth thus far this winter. This EPO is now forecast to turn negative – which will weaken the strong zonal (west to east) jet stream across the lower 48, allow more of an “amplified” jet and help to introduce progressively colder air masses. Cumulatively, the strengthening MJO, an EPO going “negative” and a weakening El Nino are all strong factors that point to a MUCH different pattern for the remainder of the winter.