This will be yet another quite technical article on behalf of my discussion about what will happen during this winter season. While it is quite uncertain about what will happen.. There are several factors that can be used to determine a winter's temperature and precipitation relative to average.
WARNING: SKIP THIS PART IF YOU DO NOT WANT THE TECHNICAL DETAILS
In this article, I will specifically be discussing atmospheric blocking and the observed jet stream wind patterns. One of the defining factors of last winter's weather was the observed lack of high latitude atmospheric blocking across Greenland and the Arctic. This was caused by the fact that a Sudden Stratospheric Warming event never occurred until the latter part of the winter, by which time it was well too late for any significant change in the winter pattern. The first variable I will be discussing is the North Atlantic Oscillation.. I won't go into detail about this variable, as I have explained quite thoroughly in other blog posts from last winter. Last year, its tendency was mainly positive. When the NAO is positive, below normal upper level heights (or below normal upper level pressure) exist over Greenland, and above normal upper level heights exist over the Central Atlantic, usually near the Azores extending eastward to Bermuda. This causes a stronger than normal height gradient (and thus a higher pressure gradient), which causes a stronger jet stream. In this type of pattern, the weather across the U.S. is normally variable (though some meteorologists falsely claim that a positive NAO is associated with warmer weather across the United States, which is not always the case...), but favors a warmer, and stormier weather pattern across the Central Plains and Midwest. When it is negative, below normal heights occur over the Central North Atlantic Ocean, while above normal heights occur over Greenland and Northeastern Canada. This causes a more meridonal jet stream pattern, and more stagnant weather pattern. The strong, looping jet stream will force cold air out of the Arctic and into the Eastern and Southern United States, as a negative NAO favors a trough over the U.S. Over most of the past several months, the NAO has trended negative.. While this gives no indication as to what it will be during the winter months, its trends during the fall can be rather significant to the winter forecast. During the late summer and early Fall of 2011, the NAO had been trending slightly negative, but abruptly went into the positive territory... Many meteorologists were going on the boat that the index would fall again by November and December. THIS DID NOT HAPPEN. The reason was because of the Arctic sea ice extent across the Arctic Ocean. Arctic sea ice extent became the highest in the last 12 years in October of last year, right after it had hit record lows. When Arctic sea ice extent is higher, heat transfer to the atmosphere is rather minimal, and easily favors the development of upper level troughs and thus the reason for the "Polar lows". When Arctic sea ice extent is below normal, heat transfer into the atmosphere is above normal, and favors the development of upper level ridges.. And when this happens, the NAO plummets, as well as another variable, known as the Arctic Oscillation, which is a very similar index. Just last week, it was recorded that Arctic Sea ice extent was at its lowest on record. Now, whether that will mean colder weather during the winter, no one knows.
This is a map of Sea surface temperatures relative to normal as measured by satellites and buoys. According to the NCDC, the far North Atlantic and Arctic Ocean SST anomalies have increased since last year. This is another indication that atmospheric blocking may set up during the upcoming winter.
Other variables that are looked at include the PDO, PNA, and many others. In this post, I would like to focus on the PNA.. Basically, when the PNA is positive, A ridge is usually evident in the Western U.S... There is normally a stronger Aleutian low, and below normal heights across the Southeast United States. When it is negative, trough is located in the western United States, a ridge in the Gulf of Alaska and normally a southwest jet stream pattern over a good chunk of the United States (good for storminess in the Ohio Valley). A positive PNA favors very cold weather and a stagnant weather pattern across the East (unless of course conditions are ripe for coastal storms). A negative PNA favors stormy weather, especially in the West, Midwest and Central Plains. This could go either way, but, according to ESRL/PSD data, the PDO (Pacific Decadal Oscillation) tends to correlate well with the PNA... Normally, when the PDO is negative, the PNA also averages neutral to negative. Of course, since the PNA varies on weekly timescales and the PDO varies on yearly time scales, it's very hard to tell any kind of pattern.
This is a chart of how the PNA index has varied over the past fifty years. So far, no pattern has been recorded by this particular index, unlike other atmospheric variables.
This is a chart showing both the variations of PDO and the PNA in comparison to each other. There is somewhat of a correlation between them, the actual calculated value of correlation is 0.250 for the past 10 years, which is quite a bit better than most variables correlations.
STOP!
In conclusion, I believe that all of this will influence the winter's weather patterns. In fact, I have come to believe that the PNA will average near neutral for the winter, which would favor a jet stream pattern through the lower Ohio Valley (depending on the development of the El Nino of course). Now, this is not my official forecast. This is only one of the many possibilities. I believe that with the recent development of record breaking low ice extent, a tendency for the NAO to trend negative over the past few months, the PDO index dropping, and SST anomalies, that the winter will feature near to slightly above normal precipitation in the Ohio Valley, in fact, mainly areas east of the Mississippi. For right now, with so much uncertainty, temperatures will be a "wild card"... Even though I believe that we have a better chance of seeing below normal temperatures across the area, historical data suggests otherwise. Only a few analogues that match up with the current year actually show below normal temperatures during the winter months. The ironic thing is that each of those respective winters featured particularly brutal cold outbreaks, mainly the winter of 1993-1994. My first official forecast will be released over the weekend, so expect it then. I will reveal my preliminary thoughts on storminess, temperatures, precipitation, and other things.
WARNING: SKIP THIS PART IF YOU DO NOT WANT THE TECHNICAL DETAILS
In this article, I will specifically be discussing atmospheric blocking and the observed jet stream wind patterns. One of the defining factors of last winter's weather was the observed lack of high latitude atmospheric blocking across Greenland and the Arctic. This was caused by the fact that a Sudden Stratospheric Warming event never occurred until the latter part of the winter, by which time it was well too late for any significant change in the winter pattern. The first variable I will be discussing is the North Atlantic Oscillation.. I won't go into detail about this variable, as I have explained quite thoroughly in other blog posts from last winter. Last year, its tendency was mainly positive. When the NAO is positive, below normal upper level heights (or below normal upper level pressure) exist over Greenland, and above normal upper level heights exist over the Central Atlantic, usually near the Azores extending eastward to Bermuda. This causes a stronger than normal height gradient (and thus a higher pressure gradient), which causes a stronger jet stream. In this type of pattern, the weather across the U.S. is normally variable (though some meteorologists falsely claim that a positive NAO is associated with warmer weather across the United States, which is not always the case...), but favors a warmer, and stormier weather pattern across the Central Plains and Midwest. When it is negative, below normal heights occur over the Central North Atlantic Ocean, while above normal heights occur over Greenland and Northeastern Canada. This causes a more meridonal jet stream pattern, and more stagnant weather pattern. The strong, looping jet stream will force cold air out of the Arctic and into the Eastern and Southern United States, as a negative NAO favors a trough over the U.S. Over most of the past several months, the NAO has trended negative.. While this gives no indication as to what it will be during the winter months, its trends during the fall can be rather significant to the winter forecast. During the late summer and early Fall of 2011, the NAO had been trending slightly negative, but abruptly went into the positive territory... Many meteorologists were going on the boat that the index would fall again by November and December. THIS DID NOT HAPPEN. The reason was because of the Arctic sea ice extent across the Arctic Ocean. Arctic sea ice extent became the highest in the last 12 years in October of last year, right after it had hit record lows. When Arctic sea ice extent is higher, heat transfer to the atmosphere is rather minimal, and easily favors the development of upper level troughs and thus the reason for the "Polar lows". When Arctic sea ice extent is below normal, heat transfer into the atmosphere is above normal, and favors the development of upper level ridges.. And when this happens, the NAO plummets, as well as another variable, known as the Arctic Oscillation, which is a very similar index. Just last week, it was recorded that Arctic Sea ice extent was at its lowest on record. Now, whether that will mean colder weather during the winter, no one knows.
This is a map of Sea surface temperatures relative to normal as measured by satellites and buoys. According to the NCDC, the far North Atlantic and Arctic Ocean SST anomalies have increased since last year. This is another indication that atmospheric blocking may set up during the upcoming winter.
Other variables that are looked at include the PDO, PNA, and many others. In this post, I would like to focus on the PNA.. Basically, when the PNA is positive, A ridge is usually evident in the Western U.S... There is normally a stronger Aleutian low, and below normal heights across the Southeast United States. When it is negative, trough is located in the western United States, a ridge in the Gulf of Alaska and normally a southwest jet stream pattern over a good chunk of the United States (good for storminess in the Ohio Valley). A positive PNA favors very cold weather and a stagnant weather pattern across the East (unless of course conditions are ripe for coastal storms). A negative PNA favors stormy weather, especially in the West, Midwest and Central Plains. This could go either way, but, according to ESRL/PSD data, the PDO (Pacific Decadal Oscillation) tends to correlate well with the PNA... Normally, when the PDO is negative, the PNA also averages neutral to negative. Of course, since the PNA varies on weekly timescales and the PDO varies on yearly time scales, it's very hard to tell any kind of pattern.
STOP!
In conclusion, I believe that all of this will influence the winter's weather patterns. In fact, I have come to believe that the PNA will average near neutral for the winter, which would favor a jet stream pattern through the lower Ohio Valley (depending on the development of the El Nino of course). Now, this is not my official forecast. This is only one of the many possibilities. I believe that with the recent development of record breaking low ice extent, a tendency for the NAO to trend negative over the past few months, the PDO index dropping, and SST anomalies, that the winter will feature near to slightly above normal precipitation in the Ohio Valley, in fact, mainly areas east of the Mississippi. For right now, with so much uncertainty, temperatures will be a "wild card"... Even though I believe that we have a better chance of seeing below normal temperatures across the area, historical data suggests otherwise. Only a few analogues that match up with the current year actually show below normal temperatures during the winter months. The ironic thing is that each of those respective winters featured particularly brutal cold outbreaks, mainly the winter of 1993-1994. My first official forecast will be released over the weekend, so expect it then. I will reveal my preliminary thoughts on storminess, temperatures, precipitation, and other things.
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