Numerical Weather Prediction teleconnection index forecasts and deterministic model guidance are forecasting a turn to below-normal temperatures into the Southeast US before mid October with at least a chance of early frost if not freeze for parts of Georgia and adjacent states outside of central Florida between the 9th and 18th give or take a few days. The NAEFS shows a 60-70% chance of below-normal temperatures and that probability has been trending up. It’s possible freeze or frost will come a month or so early and for the northern Plains and upper Midwest there can be some snow with a weather pattern more akin to Thanksgiving week for a good chunk of the country. But most of that will be North of Georgia and West of Georgia.
It looks like October and November will average a degree or so cooler than normal for Georgia with rainfall near-normal to above-normal. If it verifies that would be good news since the drought continues unabated to date.
A +PNA west coast ridge (REX block type with upper low underneath) and a Midwest to east coast jet stream trough is forecast to bring a sharp chill especially to areas West and North of Georgia but we will get our share of that air mass too.
Computer projections continue to show a weak Modoki type El Nino (central Pacific) condition fading late winter to next summer. The name “ El Niño Modoki ” was first coined by Prof. Yamagata. Researchers of the Climate Variations Research Program of Frontier Research Center for Global Change (FRCGC) lead by Prof. Yamagata documented this phenomenon. Current Pacific Ocean surface temperatures are more neutral but below the surface water is warmer. This is a trend away from the previous state of a double La Nina.
The Atlantic Ocean remains in a warm biased tripole configuration. Meanwhile solar output remains muted despite being near the sunspot cycle max. This will be the second El Nino since the PDO went negative. The Pacific-Atlantic patterns are similar to the 1950s.
Looking ahead to winter I expect a split-jet stream pattern with solid input from a southern stream making for a more active weather pattern for precipitation and big swings in temperature. ENSO should stay neutral to weakly positive. The PDO should be predominantly negative and the AMO positive for the winter months. I expect the EPO to be somewhat negative on average. I expect the PNA to average positive with the QBO in a west mode for the most part by late winter. It is currently in an easterly phase. I will also factor in solar cycle, summer matches, high latitude volcanic activity, North and Central Pacific warm and cold pools, and indices like SOI and MEI. We also watch the October and November weather patterns as a clue to how the long-wave Rossby Jet Stream will cycle, this pattern often becomes established in the mid to late autumn.
The AO/NAO, perhaps the most important predictand for the winter forecast is the most difficult to predict far in advance. Last year’s warm winter was in large measure a result of the lack of blocking of the northern jet stream as indicated by a positive to neutral NAO. The Atlantic Ocean temperature pattern and expectations for the QBO favor a more negative AO/NAO this winter which should close the door to another warm winter like the last one. So a repeat is not expected for the winter of 2012-2013. However, there are some climate models that show a mild winter, and the warm water pool NW of Hawaii is also a signal for more warm, dry ridging in the SE so it would be foolish at this point to expect a brutal winter like 2009-2010 or 2010-11. The NAO varies from week to week and month to month which results in more volatility. The NAO is also difficult to predict more than a few weeks in advance, so knowing how it will behave during the winter is an unknown quantity. In making a winter forecast the AO/NAO is something all we can do is make an educated guess about.
The analog set will come from trying to match the above factors to past years that were similar to make a winter forecast. As always similar does not mean the same but it at least gives us something to hang our hat on as the saying goes. The QBO and NAO/AO are critical and if they end of being different than my expectations when I make the final winter outlook the outlook would likely fail even if the other expectations are largely on the mark. This happened last winter when the ENSO La Nina etc. were as forecast but the NAO/AO ended up opposite and that was the whole ball game. This is the curse of weather forecasting you can get a lot right about patterns bust still be wrong about sensible weather which is all we care about.
The only other thing I can say at this point as I continue the research is that there are conflicting indicators as to whether the winter will start off fast or not get really winter like until late. I am on vacation until October 8th.