Wednesday, 28 November 2012

Pacific Changes On The Way

Recent runs of the OP GFS and to a lesser extent the GFS ensembles are showing the pattern change really take hold by the 10th of december or so. Now remember that the GFS was showing a major pattern change by the 5th and i said that it will be a step down process. This appears to be the case this time but it may take some more time for the pattern to really transition. Right now the jet is shifting further south and maturing as we get closer to winter. right now a persistent -pna and +epo pattern is allowing for warm pacific air to flood north america much like last winter although we are seeing blocking present which should not allow the warmth to last for too long. We will see an impressive warmup for the first week of december but the warmth will be centred west over the american midwest and plains states. As we go on in time the effects of the modiki el nino should allow for more troughiness to form in the aleutian islands due to the enhanced convection around the dateline. This will allow for the persistent aleutian ridge/ alaskan trough pattern to break down which will allow for more cold air to move into eastern and central canada while warmth and dryness returns to the west. This coupled with the negative AO pattern caused by the PV splitting into tow pieces one of which is on our side of the globe. So the pattern we are going in supports front side warmth followed by a pattern change with a step down process to take place in mid to late december. This will allow for a cold january and february pattern to set up. Some support for a trigger to flip the epo phase is the GAM going negative which will allow a ridge to build in the GOA i n the longer range.   Right now we are undergoing some ground work but the rewards will be felt in the heart of the winter! 

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