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Last month the folks in the Northeast were getting bulletins about a severe weather condition, brush fires. One short month later and now the New Englanders are getting evacuated from their homes as rivers all over the Northeast are reaching flood stages.
Conversely, a month ago the folks in California were shivering under soggy skies as one of the wettest springs in recent years kept crops about a month behind seasonal norms. Now as New England receives unusual rains the skies over California are cloudless as the temperature reaches up into the high 90's in the Central Valley. The adverse weather conditions in these two sections of the country are related to the shifting of a blocking ridge that produces what is known to climatologists as the Pacific North America (PNA) pattern.
The PNA occurs in two phases. The first image shows the negative PNA that was the weather maker for March and April. In this image there is a ridge over the Aleutians, a trough over the West Coast and another ridge over the Great Lakes and Northeast. The jet stream (green arrow) is pushed up to the north by the Aleutian ridge. The Aleutian ridge is supported by the two 72° jet curves(blue) from the two Atlantic points. These were aspected to high pressure by the lunar node for most of March and April while the node was on station to these two points. This created an anomalously strong high pressure pattern over the Aleutians. Simultaneously, on the continent a ridge formed over the two 45° jet curves (blue) that were situated over the central Plains and the Mississippi Valley. Between these two well established ridges, kept in place by the lack of motion of the lunar node between the eclipse points, a trough kept forming that tracked the 72° jet curve from the Pacific points (red curve). This trough was influenced by Jupiter that was also on station. This influence persistently kept a low pressure jet curve just off of the West Coast that fell significantly between the Aleutian ridge and the Mississippi ridge. This created strong cold and wet conditions for California and strong dry and warm conditions for the Northeast.
On May 3rd a significant shift of the values on all of the points took place that introduces a low pressure field over the Midwest and over the Aleutians. With this shift the ridge broke down in the Midwest and the pattern of high pressure shifted to the West Coast. With that shift a positive PNA pattern developed for the West Coast. This is depicted in figure 2.
With the shift of the ridge to the West Coast the door was open to low pressure across the continent and the prevailing track from the third to the 9th of May was across the southern sections of the US. On May 9th the node shifted in its position for the first time in two months. The pattern once again shifted in the Eastern Pacific. This time Jupiter had shifted to high pressure values on both of the Pacific points. There was no West Coast trough to set up the negative PNA pattern like there was in March and April. The jet pushed up in the west and found no opportunity to drop to the south over the western mountains or the High Plains. The only place where a trough could develop was now over the eastern states. In place of the previous PNA pattern a significant rain pattern, known as the North Atlantic Oscillation (NAO), went into its negative phase. Figure 3 depicts the negative NAO that presently is flooding parts of Massachusetts. All of the grid lines are dominated by high-pressure (blue). The jet stream (green arrow) vaults over the West Coast, the inter-mountain west, the High Plains and the western portions of the Midwest. Over the Northeast the jet finally turns south under the influence of a high over Greenland. This is known as a Greenland block. It is fairly rare in the late spring since it is primarily a winter pattern. However, the shift to low pressure on the 3rd allowed the Atlantic jet to re-position itself after many weeks of stagnant placement. This shift on the 3rd allowed for a western transposition of a high latitude high pressure area that was locked over the North Sea to shift west to Iceland. When the high pressure once again established on the 9th, the high latitude high kept drifting to the west in the circumpolar flow (easterlies) and ended up over the Maritime Provinces. That motion depressed the jet into New England and the shift from drought to flood was complete.