Some of you may be wondering, "What in the world is going on?". Well, I've got to tell you, I'm wondering the same thing. Over the past 20 days, most days have seen above normal temperatures. Truth be told, we have seen some pretty chilly days in between these warm periods. Even this week, most computer models are forecasting a major ridge to form over a large portion of North America for the holidays. This would cause a relatively warm and humid air mass to surge northward into a large chunk of the United States. Along with this warm air mass, sunny conditions will prevail for Thursday and possibly Friday. As the polar jet stream is very strong this year, the ridge will shoved out just as fast as it came in. By Friday Night, clouds will be on the increase, ahead of the next strong storm system. This storm system will develop quite rapidly, as a major trough develops in the Plains and surges eastward. According to the GFS model, heavy rain and quite possibly thunderstorms will likely develop by Saturday and into Sunday. During these days, temperatures will rise into the mid fifties, much above normal levels. The storm system won't stop there. After the surface system leaves, a rather large upper level low will develop and remain for several days, bringing colder air and possibly areas of snow, bringing the first accumulating snow of the season to many areas. After this it is unclear what will happen, but the GFS is forecasting that the AO will plummet to negative values for the first time in several weeks. This may cause a major trough to form for the first and second weeks of December, bringing widespread snow and cold to much of the Central Plains and East. With all of that cold air in Canada, I'm sure it will spill southward into the US. This would make the weather pattern eerily similar to last year. In fact, according to the latest run of the GFS model, towards the beginning of December, that upper level low mentioned earlier, will likely cause multiple storm systems to form and swing around it, also drawing in very cold arctic air to start the season off. With low upper level height anomalies, combined with surface cold air, and plenty of moisture to work with, I'm sure that we will see quite a bit snow for the first two weeks of December. As for the rest of December, if the AO and NAO remain negative, and the PNA remaining positive, expect cold temperatures and frequent bouts of snow to continue even through Christmas. Unfortunately, I don't believe any major storms will likely form this period, as most of the storms will traipse up the East Coast. I will keep you updated as December gets underway.
There is one thing I do want to touch up on is dealing with the current La Nina. The values I want to speak about are the SOI values, Southern Oscillation Index. The Southern Oscillation Index is determined by taking the difference of sea level pressure readings from Tahiti and Darwin, Australia. If this difference (Tahiti-Darwin) is negative, usually this correlates to either an El Nino or weak La Nina. If this difference is positive, usually this means that either a weakening El Nino exists or a rather established La Nina event is underway. If values are either very high or extremely negative, this indicates a solid ENSO event is underway. Today, the Climate Prediction Center released their weekly analysis of the ENSO. They found that SOI values are only about 10.1, which is pretty modest for an expected strengthening La Nina. Some meteorologists are pointing out that they may need to revise their winter forecast, due to the SOI values being much lower than last year's. I really don't agree with this, mainly because first of all every ENSO event is different so SOI values will range differently. Second of all, generally second year La Nina's are usually weaker than the previous La Nina, with generally weaker SOI values. Also, during east and central La Nina events, SOI values will be weaker than normal because of the fact that the most dominant weather influence will be located farther east. When I look at how strong a La Nina is, my first index will not be SOI values. I will first look at sea surface temperature anomalies across the equatorial Pacific. Currently, anomalies are between one and three degrees Celsius below normal, indicating a moderate La Nina is already underway. The second factor I look at is 850 millbar and 200 millibar wind anomalies. If the 850 millibar mean wind over the equatorial Pacific is stronger than normal from the east, this means the trade winds are stronger than normal, indicative of a persistent La Nina farther east. Right now, 850 millibar winds are stronger than normal from the east over the Equator. If 200 millibar winds are stronger than normal from the west, this is also indicative of a La Nina because higher pressure aloft is located over Indonesia during a La Nina, and lower pressure aloft is located over the equatorial Pacific, meaning the winds will blow strongly from Indonesia to Peru. This is what I have been observing over the Pacific Ocean. I believe with these factors in play, plus a favorable MJO (I will explain MJO in another blog post.) in December, has lead me to believe in the strengthening in the La Nina and SOI values during the month of December and peaking in February and March going along with the CFSv2 computer model's forecast. Other things such as NAO, AO, and PNA, have made me shaky with this winter's pattern, as they have not been consistent with many forecasts in the month of November. We will have to see if the GFS and ECMWF are right in their forecast for a change to a negative NAO/AO along with a positive PNA. If these factors do not turn as they are forecasted, expect a winter that is not as cold as was originally expected, with most of the cold air bottled up over Alaska, Canada and the Northern United States. The good thing is that if these factors do not come into play, if it is not as cold this winter, expect heavier precipitation, and a greater chance for a very stormy and snowy winter, just like the Farmer's almanac says. According to extremely long range forecasts, this may be our last hurrah winter, as next winter there is expected to be an El Nino in the Pacific, making it very unfavorable for snow around here. In fact, next winter, we may say a return in Nor'easters and East Coast snowstorms, while the Midwest and Northern Plains is snow starved. With the forecast through December 2011, I will keep you updated if something changes. Other than that, this is the official forecast. Stay tuned.
There is one thing I do want to touch up on is dealing with the current La Nina. The values I want to speak about are the SOI values, Southern Oscillation Index. The Southern Oscillation Index is determined by taking the difference of sea level pressure readings from Tahiti and Darwin, Australia. If this difference (Tahiti-Darwin) is negative, usually this correlates to either an El Nino or weak La Nina. If this difference is positive, usually this means that either a weakening El Nino exists or a rather established La Nina event is underway. If values are either very high or extremely negative, this indicates a solid ENSO event is underway. Today, the Climate Prediction Center released their weekly analysis of the ENSO. They found that SOI values are only about 10.1, which is pretty modest for an expected strengthening La Nina. Some meteorologists are pointing out that they may need to revise their winter forecast, due to the SOI values being much lower than last year's. I really don't agree with this, mainly because first of all every ENSO event is different so SOI values will range differently. Second of all, generally second year La Nina's are usually weaker than the previous La Nina, with generally weaker SOI values. Also, during east and central La Nina events, SOI values will be weaker than normal because of the fact that the most dominant weather influence will be located farther east. When I look at how strong a La Nina is, my first index will not be SOI values. I will first look at sea surface temperature anomalies across the equatorial Pacific. Currently, anomalies are between one and three degrees Celsius below normal, indicating a moderate La Nina is already underway. The second factor I look at is 850 millbar and 200 millibar wind anomalies. If the 850 millibar mean wind over the equatorial Pacific is stronger than normal from the east, this means the trade winds are stronger than normal, indicative of a persistent La Nina farther east. Right now, 850 millibar winds are stronger than normal from the east over the Equator. If 200 millibar winds are stronger than normal from the west, this is also indicative of a La Nina because higher pressure aloft is located over Indonesia during a La Nina, and lower pressure aloft is located over the equatorial Pacific, meaning the winds will blow strongly from Indonesia to Peru. This is what I have been observing over the Pacific Ocean. I believe with these factors in play, plus a favorable MJO (I will explain MJO in another blog post.) in December, has lead me to believe in the strengthening in the La Nina and SOI values during the month of December and peaking in February and March going along with the CFSv2 computer model's forecast. Other things such as NAO, AO, and PNA, have made me shaky with this winter's pattern, as they have not been consistent with many forecasts in the month of November. We will have to see if the GFS and ECMWF are right in their forecast for a change to a negative NAO/AO along with a positive PNA. If these factors do not turn as they are forecasted, expect a winter that is not as cold as was originally expected, with most of the cold air bottled up over Alaska, Canada and the Northern United States. The good thing is that if these factors do not come into play, if it is not as cold this winter, expect heavier precipitation, and a greater chance for a very stormy and snowy winter, just like the Farmer's almanac says. According to extremely long range forecasts, this may be our last hurrah winter, as next winter there is expected to be an El Nino in the Pacific, making it very unfavorable for snow around here. In fact, next winter, we may say a return in Nor'easters and East Coast snowstorms, while the Midwest and Northern Plains is snow starved. With the forecast through December 2011, I will keep you updated if something changes. Other than that, this is the official forecast. Stay tuned.
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