As quite a few meteorologists have, I have come to the conclusion that I need to make few corrections to my long range forecast. In my original final forecast made back at the end of October and early November, I stated that the second half of November would be very cold and somewhat dry around. As we all know this didn't happen. In fact, quite the opposite happened, with a somewhat warm and very wet pattern making it seem as if winter would never come. In this article, I will try to explain my revisions to the forecast as well as what went wrong.
For the final days of November (where we are now), I expect very cool temperatures after the last storm system comes through. On Wednesday, November 30, I expect temperatures only in the mid to upper 30's, under partly to mostly cloudy skies. These temperatures are only slightly below normal and not so bad for this time of year. During this time period, I expect that the teleconnections will finally become favorable for winter weather across the U.S., especially as we head into the first and second weeks of December. Currently, (as of Sunday, November 27) the Arctic Oscillation Index, a teleconnection that indicates the strength of the polar vortex, is still strongly positive. The Arctic Oscillation (AO), is a teleconnection (mode or phase of the atmosphere) that changes due to changes in temperatures in the lower stratosphere (layer of the atmosphere just above the troposphere). When these temperatures are above normal above the Arctic, surface temperatures warm, and create an atmospheric phenomena called the Greenland Block. The Greenland Block is a ridge of high pressure above the Greenland land mass that causes a metaphorical "road block" in the jet stream. Instead of the jet stream traveling west to east through the Atlantic, it takes a large northward swing into the Arctic. This sets up a major trough in the upper atmosphere over much of North America, sending very cold air southward during its NEGATIVE PHASE. Usually, when the AO is negative cold and wintry weather is prevalent over the United States during the winter months. During its positive phase, just the opposite is true. The positive phase causes a zonal or more west-east flow in the jet stream, allowing for warm air to penetrate farther north into the United States. Another index called the NAO, usually correlates strongly with the AO. When these two are both in their negative phase, and there is very cold air in Canada and Alaska, a major arctic outbreak is likely in the Lower 48 states. Over the last month or so, ever since that big Nor'easter hit the East Coast, both the AO and the NAO have been mainly positive, suggesting a warmer pattern for much of the United States. This is exactly what happened. Over the next several weeks, I expect both teleconnections to go negative, but it take longer than originally thought. It seems the computer models are all over the place with where these indices will go over the next few weeks. More than likely, I think that they will go negative especially by about December 15. After this I expect, the winter forecast of a cold and snowy winter to hold prominent. During the transitional phase from December 1 through December 14, I expect a few ups and downs in the pattern, with cooler than normal temperatures dominating throughout the period, while drier than normal conditions take hold in the beginning.
Another index that influences are weather quite a bit is the PNA. PNA stands for Pacific North American index. This teleconnection also has a positive and negative phase to it. In its negative phase, positive height anomalies (higher than normal pressure) exist over the Central Pacific, while a trough (lower than normal heights or pressure), exists in the Western United States and a large ridge is present in the Southeast. This, coupled with a positive AO/NAO will generally lead to very warm air spreading into the United States. In the positive phase of the PNA, negative height anomalies exist over the Central Pacific and above normal height anomalies exist over the Western United States, while the in the East, a trough dominates. When this is coupled with a negative AO/NAO, very cold air will likely dive deep into the United States, which explains the very cold weather during last winter. Over most of November, the PNA index ranged from neutral to strongly negative, leading to the development of a southeast ridge, allowing for warm and humid air to spread from the Gulf of Mexico into the East. Right now, computer models are in fairly good agreement on what this index will likely do. It looks like it will become positive, allowing for a trough to develop in the East. Because it is not coupled with a negative NAO/AO for the time being, only below normal temperatures will dominate for short periods of time, with some fluctuation in between. All in all, until all of the teleconnections are in agreement, I expect only slightly below normal temperatures to continue from December 1-14. After the fifteenth, I believe all of these teleconnections will align themselves in such a way that winter will finally hit hard, just like I said in the forecast.
For a brief summary of the long range forecast, to begin December, I expect temperatures to be slightly below normal, with highs in the mid to upper 30's. After this Friday, a clipper system will move through, bringing some colder air and temperatures farther below normal in possibly the low to mid 30's by next week. Next week, I expect temperatures to fluctuate a bit from below normal and some snow in the beginning of next week to temperatures near normal and some rain with any storm systems. After next week, the week of the 12th, I expect this to be the week of change, with temperatures starting the week possibly slightly above normal, with temperatures in the lower 40's, and ending the week well below normal, with highs even in the mid twenties. This will also be the time when I expect the storm systems to start coming back into the region as forecasts look like we will average below normal precipitation from December 1 through the 14, with an uptick in precipitation after the big cool down, likely in the form of snow. After the changeover, I expect the rest of December to also be cold and snowy, heavier than normal lake effect snows and a couple of moderate snowstorms. I would even hope for a snowstorm or blizzard during the week of Christmas! For the end of December, unlike last year, I expect to end cold and dry. By January, the same pattern may continue, but as of this time, I am really not sure if the pattern will reverberate back to what happened in November. This is the official forecast. If there are any updates I will revise my forecast again.
For the final days of November (where we are now), I expect very cool temperatures after the last storm system comes through. On Wednesday, November 30, I expect temperatures only in the mid to upper 30's, under partly to mostly cloudy skies. These temperatures are only slightly below normal and not so bad for this time of year. During this time period, I expect that the teleconnections will finally become favorable for winter weather across the U.S., especially as we head into the first and second weeks of December. Currently, (as of Sunday, November 27) the Arctic Oscillation Index, a teleconnection that indicates the strength of the polar vortex, is still strongly positive. The Arctic Oscillation (AO), is a teleconnection (mode or phase of the atmosphere) that changes due to changes in temperatures in the lower stratosphere (layer of the atmosphere just above the troposphere). When these temperatures are above normal above the Arctic, surface temperatures warm, and create an atmospheric phenomena called the Greenland Block. The Greenland Block is a ridge of high pressure above the Greenland land mass that causes a metaphorical "road block" in the jet stream. Instead of the jet stream traveling west to east through the Atlantic, it takes a large northward swing into the Arctic. This sets up a major trough in the upper atmosphere over much of North America, sending very cold air southward during its NEGATIVE PHASE. Usually, when the AO is negative cold and wintry weather is prevalent over the United States during the winter months. During its positive phase, just the opposite is true. The positive phase causes a zonal or more west-east flow in the jet stream, allowing for warm air to penetrate farther north into the United States. Another index called the NAO, usually correlates strongly with the AO. When these two are both in their negative phase, and there is very cold air in Canada and Alaska, a major arctic outbreak is likely in the Lower 48 states. Over the last month or so, ever since that big Nor'easter hit the East Coast, both the AO and the NAO have been mainly positive, suggesting a warmer pattern for much of the United States. This is exactly what happened. Over the next several weeks, I expect both teleconnections to go negative, but it take longer than originally thought. It seems the computer models are all over the place with where these indices will go over the next few weeks. More than likely, I think that they will go negative especially by about December 15. After this I expect, the winter forecast of a cold and snowy winter to hold prominent. During the transitional phase from December 1 through December 14, I expect a few ups and downs in the pattern, with cooler than normal temperatures dominating throughout the period, while drier than normal conditions take hold in the beginning.
Another index that influences are weather quite a bit is the PNA. PNA stands for Pacific North American index. This teleconnection also has a positive and negative phase to it. In its negative phase, positive height anomalies (higher than normal pressure) exist over the Central Pacific, while a trough (lower than normal heights or pressure), exists in the Western United States and a large ridge is present in the Southeast. This, coupled with a positive AO/NAO will generally lead to very warm air spreading into the United States. In the positive phase of the PNA, negative height anomalies exist over the Central Pacific and above normal height anomalies exist over the Western United States, while the in the East, a trough dominates. When this is coupled with a negative AO/NAO, very cold air will likely dive deep into the United States, which explains the very cold weather during last winter. Over most of November, the PNA index ranged from neutral to strongly negative, leading to the development of a southeast ridge, allowing for warm and humid air to spread from the Gulf of Mexico into the East. Right now, computer models are in fairly good agreement on what this index will likely do. It looks like it will become positive, allowing for a trough to develop in the East. Because it is not coupled with a negative NAO/AO for the time being, only below normal temperatures will dominate for short periods of time, with some fluctuation in between. All in all, until all of the teleconnections are in agreement, I expect only slightly below normal temperatures to continue from December 1-14. After the fifteenth, I believe all of these teleconnections will align themselves in such a way that winter will finally hit hard, just like I said in the forecast.
For a brief summary of the long range forecast, to begin December, I expect temperatures to be slightly below normal, with highs in the mid to upper 30's. After this Friday, a clipper system will move through, bringing some colder air and temperatures farther below normal in possibly the low to mid 30's by next week. Next week, I expect temperatures to fluctuate a bit from below normal and some snow in the beginning of next week to temperatures near normal and some rain with any storm systems. After next week, the week of the 12th, I expect this to be the week of change, with temperatures starting the week possibly slightly above normal, with temperatures in the lower 40's, and ending the week well below normal, with highs even in the mid twenties. This will also be the time when I expect the storm systems to start coming back into the region as forecasts look like we will average below normal precipitation from December 1 through the 14, with an uptick in precipitation after the big cool down, likely in the form of snow. After the changeover, I expect the rest of December to also be cold and snowy, heavier than normal lake effect snows and a couple of moderate snowstorms. I would even hope for a snowstorm or blizzard during the week of Christmas! For the end of December, unlike last year, I expect to end cold and dry. By January, the same pattern may continue, but as of this time, I am really not sure if the pattern will reverberate back to what happened in November. This is the official forecast. If there are any updates I will revise my forecast again.
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