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Climate patterns for 2007 - 10.21.06
The upcoming year promises to be full of surprises. The present early cold on the continent and unseasonable warmth on the West Coast are typical of the negative PNA pattern. Find out why in Doc Weather.
Fig.1
negative PNA pattern
The PNA (Pacific North America) is the dominant climate pattern in the eastern Pacific. The PNA has two phases. The negative phase ( - PNA ) consists of a strong and extensive Hawaii high and a weak and very local Aleutian low. (figure 1)The Hawaii high tends to be strong in the spring and summer when the Aleutian low is weak due to cold water in the Gulf of Alaska. This seasonal pattern can also occur as a fluctuation in climate pattens from year to year. In the negative PNA a persistent ridge builds against the mountains. This sends the storm jet into the Pacific Northwest for record rains. The ridge keeps the weather south of Mt. Shasta dry and warm. The ridge off of the coast pushes up into western Canada. On the eastern side of the ridge a trough forms over the High Plains and dominates the weather patterns on the continent. The resulting weather from the negative PNA gives record snows and cold in the High Plains and the Midwest. This is the negative PNA pattern. This is the present situation with the weather in the fall of 2006. Early snows have come to Minnesota and the Dakotas. Cold has traveled far into the central states and the Corn Belt. California is still balmy. Notice that the ridge is linked to the disturbance diamond. This is an area to watch in the next few months when the Hawaii High shrinks as part of the seasonal cycles. A shift here in the Hawaii High has great potential for establishing the opposite pattern, the positive PNA. The flip flop may be sudden.
Fig.2
positive PNA pattern
In the positive PNA pattern a trough forms over the eastern Pacific (figure 2 ). The polar jet is forced to the south in the eastern Pacific and a ridge forms over the higher latitudes of the Great Basin from the Dakotas northward. This placement allows for a trough to form over the east coast of the US bringing unusual snowfalls to the Mississippi valley and the eastern seaboard. The High Plains are dry and warm and the West Coast is wet and cold. The positive phase of the PNA pattern is often the dominant climatic pattern during El Nino years. Excessive warmth in the eastern Pacific from an el Nino allows for persistent storm formation along the West Coast to occur. In effect the Aleutian low migrates to the south and the storm track across the Pacific migrates with it. During the positive phase there is often a strong persistent high which forms over the Bering Sea at a high latitude. This high pushes the polar jet down to the south in the central Gulf of Alaska. Talso part of this patterns is a variation where high pressure over the West Coast migrates north along the coast to settle over British Columbia and the Canadian Rockies. As a result of this finger like blocking high, storms coming in from Hawaii tend to slip to the south to slide under the high latitude high pressure. This variation is the typical El Nino wet to the south and dry to the north pattern. During the positive (El Nino ) PNA phase the mean polar jet in the eastern Pacific is through the Sierras and not the Cascades as it is in the negative ( La Nina) phase. In essence the warm water in the eastern Pacific shifts the blocking ridge on the West Coast on to the continent instead of the ridge being out over the Pacific ocean. The more southerly jet brings record rains to California and drought conditions to the Pacific Northwest.
With the disturbance diamond (yellow) placed strategically off of the West Coast this year. The two phases of this pattern could undergo rapid shifts. The El Nino presently forming off the coast of South America is showing signs of fading into a very moderate event. It should be noted that the very moderate El Nino events are often the ones that create the most fluctutation in climate patterns in a given year.