Long-Range Update

Now for the Long-range update that I have been promising.  First let me explain what's happening.  Right now, in the Central Pacific, a raging La Nina is underway, with the Nino 3.4 region experiencing values near -1.1 degrees Celsius (note: these values are anomalies and not represent actual temperatures.  Actual temperatures in this region are near 70 degrees Fahrenheit.).  Over the last few weeks, this area of below normal sea surface temperatures has spread westward, causing a basin-wide La Nina.  This is what we had last winter, which means we should start seeing the weather pattern become closer to what happened last year, especially by the end of January.  One thing that has surprised many meteorologists is the fact that the SOI values have really dropped, even though the La Nina has in fact strengthened.  As explained in previous posts, SOI stands for Southern Oscillation Index.  When the index is positive, a La Nina is underway, because the pressure difference between Tahiti and Darwin, Australia is positive.  When the index is negative, an El Nino is indicated.  The daily values for yesterday and the day before have been sharply negative, which is not a good sign for the rest of the winter.  If these negative values continue for a lot longer time, expect the La Nina to weaken significantly, and in fact reverse to an El Nino by the middle of March or April.  This would cause a lot less precipitation over the area, which is really bad news for snow lovers in the Ohio Valley.  Right now, this scenario does not look at all likely, mainly because these negative values may be because of the westward expansion of colder than normal waters in the equatorial Pacific.  In fact the most likely scenario right now is that the La Nina may be actually strengthen a bit before the end of January, before beginning a gradual weakening trend.  The CFS climate model is forecasting the continued strengthening of a moderate La Nina to a strong La Nina, with the La Nina becoming stronger after July.  This would mean, a very snowy end to winter, even if temperatures are above normal (not taking into account the pattern change).  On the other hand, the CFSv2 model (an enhanced version of the CFS model) is forecasting the La Nina to remain steady or slightly strengthen before the end of January, and then it it is forecasting a rapid weakening of the La Nina, becoming neutral by May and becoming an El Nino by July or August.  The most likely forecast is a combination of these models' forecasts, with a moderate strengthening of the La Nina through February or March, with a rapid weakening beginning in March, becoming neutral by July and possibly becoming El Nino by the beginning of next fall.  This will cause continued above normal precipitation and snowfall.  Another factor that I am taking into account is the teleconnections, which affect the position of the jet stream.  The NAO, which stands for North Atlantic Oscillation, is an index measuring the difference between observed sea level pressure between the Azores and Iceland.  When it is positive, a strong low pressure center is positioned over Iceland, when it is negative, high pressure is located over Iceland.  The positive phase tends to bring warmer than normal conditions to North America and Europe, while the negative phase tends to bring widespread cold and snowy conditions to North America and Europe.  Right now, ensemble forecasts have the NAO becoming negative especially after January 16th.  This will finally allow winter to return to the United States.  The next teleconnection, which is very closely related to the NAO, is the Arctic Oscillation or AO.  When it is in a positive phase, stratospheric temperatures become very cold over the Arctic, while in the negative phase stratospheric temperatures become much above normal.  When it is in its negative phase, cold, Arctic air is pushed southward out of the Arctic, causing brutal winters over the United States and Europe.  When it is positive, warm air dominates both continents, as a strong low pressure system dominates the Arctic, keeping the arctic air "locked up".  Ensemble forecasts show general agreement that the AO will go strongly negative by about January 16th, with the lowest values since December 2010 showing up.  This will finally allow winter to return to the United States, with several snowstorms possibly in our area after the mid month.  In fact, we could see some hefty snowstorms by February.  But for now, expect to be in a transition phase, with a more turbulent pattern before this happens.  One sign that shows this might actually be happening is the fact that a Significant Stratospheric Warming event is occuring, which is the tell tale sign of the weakening of the Polar Vortex, a major factor that has kept cold air out of the area throughout most of the winter.  In the lower stratosphere, over the past few weeks, temperatures have warmed from much below normal back in mid December, to well above normal in only a few day's time.  When significant warming in the stratosphere occurs above the Arctic, a disruption of the powerful polar vortex occurs.  The polar vortex is just the meteorological term for the very center of the brutal arctic air.  If this vortex weakens, that means that the cold, arctic can finally come rushing out of Canada, and stay in the United States.  Even though significant warming has occurred, the atmosphere usually takes a couple of weeks to respond to this burst of warm air.  Therefore, we should be seeing the longer outbreaks of Arctic air into the air by the middle or end of January, and continuing shots of arctic air into February and early March.  This will be something I will continue to monitor, so that an accurate forecast of arctic outbreaks can be made.  As an example, next week, a powerful cold front (just about as powerful as last Sunday's) will push through the area, dropping temperatures from the lower 50's to the lower and mid 20's by Friday of this week, with heavy lake effect snow returning.  This cold air will likely retreat after a several days, but there will not be as much warming as with the last time.  In fact, the Climate Prediction Center is forecasting that a large trough will dominate the country for the rest of the month, leading to frequent arctic outbreaks and rather stormy weather to persist across the Ohio Valley.  This may bring our first real snowstorms by the end of January.