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Where is the cold coming from? - 02.05.07
The current cold snap on the continent is part of an unusual climate pattern. Doc Weather compares the NAO and the PNA patterns in this article.
The present cold wave in the Midwest and Northeast is a long awaited event for Midwesterners and residents of New England. In the Midwest, the way that cold penetrates to the south is a result of either one of two different climate patterns the Pacific North America (PNA) and the North Atlantic Oscillation (NAO). In New England it is almost the NAO that is the source of the cold. This year the cold is taking an unusual path to get to the Northeast. The cold in the Northeast is unusual in that it is part of a very extended (PNA) pattern rather than a more typical (NAO) pattern. To appreciate the difference the following charts are provided.
Fig.1
NAO pattern
In the chart the typical NAO pattern that brings cold into the Northeast and Midwest is shown. This phase of the NAO is known as the Greenland Block. It is called that because in a typical block sequence a high that is centered over Iceland in the eastern Atlantic moves westward in the polar easterly winds. The polar winds blow from east to west at high latitudes. By convention, the designation easterly or westerly regarding wind direction always describes the direction from which the wind is coming. The polar winds blowing from east to west would then be called polar easterlies (dark blue arrow).
When the Iceland ridge moves to the west in the stream of the easterlies it comes to rest over Greenland or the Maritime Provinces in northeastern Canada. The block then pushes the jet stream from the west into a more southerly position as it comes across the continent towards the East Coast. If the block is a bit more towards Hudson Bay then both the Northeast and the Midwest get the cold polar fronts. The Midwest position is shown in the chart as the light blue arrow. With the block further east near Greenland, the jet would be more displaced to the east and provide the most typical winter source of cold for the Northeast. (darker arrow across New England)
Fig.2
In the second chart the most typical source of winter cold for the Midwest is shown. This pattern is the PNA. The block is against the West Coast in this instance. The jet stream is forced high into the Polar Regions by the ridge against the West Coast and the resulting eastern side of the ridge brings strong cold into the Midwest. In that pattern the cold exits the continent in the Middle Atlantic States or brings ice storms to the Gulf Coast and exits the southeast without influencing the Northeast. This year the extreme northern reach of the ridge on the West Coast has extended all the way to the North Pole. This has given the descending cold a trajectory into the area between Minnesota and Pennsylvania. As the cold drifts to the east the area east of the Great Lakes is becoming engulfed in the cold snap.
Fig.3
It is interesting to look at the planetary dynamics of this eastern blocking ridge this season in the context of the jet curves used in Doc Weather. The last set of eclipses put the disturbance diamond (yellow area) in the central Gulf of Alaska. The 72�° jet curves from the western points are colored blue to signify high pressure aspects. Since September there have only been two weeks that these two curves have been under the influence of low pressure values. In both of those two weeks the low pressure has been the result of a Jupiter motion in arc event. Those two weeks were the only incidences of rains on the West Coast since September. As a result of this rhythm of Jupiter there has been persistent high pressure influences on these two 72�° jet curves. The two curves are just to the east of Hawaii, the prime source of high pressure in the eastern Pacific. Since September, the enhanced Hawaii High has been blocking the jet from dropping south along the coast creating drought conditions on the West Coast.
Fig.4
In late January however, a significant evolution took place. Mars, Venus and Mercury, planets that had been crossing the coast at a very southerly declination most of the fall and early winter had moved east out of the eastern Pacific into the Atlantic. By the first week of February they had begun to move into a more northerly declination pattern as well. The active southern branch of the polar jet on the West Coast that had been tracking across Mexico for most of their transit across the eastern Pacific began to fade in the daily jet stream charts. The northern jet began to wobble. The stage was set for some changes.
Starting on January 23, 2007 and continuing for the next four days four planets moved to high pressure values on jet curves around the disturbance diamond. With each new aspect the persistent horizontal ridge from Hawaii first formed and enclosed high over California and then each day slowly moved up into the PNW as each planet shifted the jet curves to another high pressure aspect. While moving north the jet stream began to push up into Canada on the West Coast. A ridge in this kind of surge situation often pinches off to form an enclosed high at high latitude over Alaska. When that happens the remainder of the jet stream drops south and often brings rain. That kind of pattern is a typical El Nino signature in which a high latitude high in the Gulf of Alaska pushes the eastern Pacific jet to the south bringing El Nino rains to California. However, that is not what happened.
Fig.5
Instead of pinching off to form an enclosed high, the contours of the ridge stayed together and the jet stream was forced first to the north in the west and then to the south on the eastern leg of the ridge. This began a stream of cold air down into the inter Mountain areas and a current of warmer air up the coast from the south. This shift broke the cold spell on the West Coast that had created a citrus freeze This motion was the first real migration of this block in many weeks.
By the beginning of February the high that resulted from the northward surge had formed what climatologists call an omega pattern. In the omega the high latitude high pressure area doesn't pinch off but stays at a high latitude. The jet stream travels up the western side of the high to an extreme north latitude and then descends down the eastern side of the high to bring deep cold into the continent. On the first of February, Jupiter once again, moved to a high pressure aspect to the western eclipse points. The high latitude omega surged northward on that day, maintaining its contours while reaching the North Pole without pinching off. The resulting downside cold front spread out like a devastating fan shaped wave that covered the area between Minneapolis and the Great Lakes. In the succeeding days the broad swath of cold has engulfed the Northeast as well as the Midwest. The persistence of the omega into the polar regions without pinching off at a high latitude is just another instance of the remarkable tenacity of blocking highs associated with Jupiter when that planet is locked into a series of consecutive high pressure values. This kind of remarkably tenacious blocking pattern has been the signature of the influence of this planet since September 2006.
Coming up however is a time period when Jupiter must give way to changing its stubborn attempt to control the whole northern hemisphere from its seat near Hawaii. That will take place on February 9 when Saturn shifts the values on the two eastern 72�° jet curves changing them to low pressure for the first time in months. On the 14th Jupiter will reestablish high pressure over the eastern Pacific. Then on February 21 Jupiter and Saturn both move to a low pressure aspect to the western eclipse points on the same day. That should shift the jet stream on the eastern Pacific into a more zonal pattern and provide rains to the west and more temperate conditions to the continent. That trend should continue into the end of the month.