Weather Ramblings: September 24, 2013

An interesting weather pattern will be evolving in the next few weeks, as we transition from summer weather to a fall-like pattern, especially in the medium and long range.

Model guidance for the 7 day forecast period shows deep troughing across the Gulf of Alaska, which helps to provoke some minimal high pressure development across the Central and Eastern United States. This high pressure is very weak when compared to typical high pressure anomalies when such deep negative height anomalies appear in the Gulf of Alaska. Such a pattern resembles a negative phase of the Pacific Decadal Oscillation, which Eric Webb of Weather Advance illustrated. The image below shows correlations of 500mb heights to the Pacific Decadal Oscillation. The image shows positive and negative colors- positive values mean the phase of the PDO would coincide with height anomalies in that area, while negative values mean the phase of the PDO would be opposite of height anomalies in any given area.

This image shows that the negative PDO produces a positive correlation with 500mb height anomalies in the Gulf Alaska- in this case, a negative PDO would provoke negative height anomalies in the Gulf of Alaska, which we see in the forecast above. That said, deep negative values are shown in the Central and East US, meaning a negative PDO would produce positive height anomalies in the aforementioned areas into Canada. The model forecast at the top of the page somewhat resembles this negative PDO pattern in the sense of deep troughing in the Gulf of Alaska, but we see no significant ridging in the east US and east Canada. The main cause of this seems to be positive height anomalies across the Arctic Circle and troughing across Europe and Asia- those two factors go against the typical -PDO pattern illustrated in the image above.

By the 264 hour mark, model guidance projects general high pressure formation across the Pacific Northwest and southwest Canada as troughing in the Gulf of Alaska shifts west to the waters south of the Bering Sea. This high pressure formation in the west coast of North America provokes the positive phase of the Pacific North American index, which helps cooler temperatures enter the central and east US. Ridging west of Greenland provides a basis for a colder weather pattern in the East US and Central US too, and thus it is no surprise that at the 360 hour mark of model guidance, deeper negative height anomalies (and colder temperatures) arise in the land east of the Rockies.

Teleconnection guidance confirms this idea of a turnaround to a positive PNA and persistent negative NAO throughout the forecast period. The bottom two panels show the West Pacific Oscillation forecast on the left and the East Pacific Oscillation on the right. Without going too in-depth with these two indexes, the negative phase helps cold weather prospects in the US, and the positive phase enhances warm weather across North America. With both oscillations projected to fall by the end of the forecast period, we could see some actual cold weather arrive in time for the second week of October.

A convection-related index called the Madden-Julian Oscillation will be heading into Phase 6 (it has 8 total phases), so let's take a look at a 500mb composite chart for worldwide longitudes zoomed in on the 70-50 north latitude.

As you can see on this chart provided by Mike Ventrice, colors on here show anomalies for the 500mb level. In this case, cold colors indicate negative height anomalies (low pressure), while warm colors show positive height anomalies (high pressure). On the left side of the image, numbers in negative and positive values are shown. Basically, the numbers indicate how many days before (negative numbers) and after (positive numbers) the MJO enters Phase 6. Taking a look at this chart, there is a large signal of strong ridging over the 90W to 30W longitudes. When looking at the 70N-55N latitudes and 90W-30W longitudes, we find Canada and Greenland in that location. So, going by the image, it is plausible to expect strong ridging of high pressure from Central Canada (90W longitude line) to eastern Greenland (30W longitude line) approximately 15 to 30+ days after the MJO enters Phase 6. With model guidance predicting the MJO entering Phase 6 in just a few days. This means I am highlighting October 8 to October 23 (and possibly beyond) for increasing chances for cold weather in portions of the US due to this ridging over eastern Canada and Greenland.

Andrew