Wednesday, September 3, 2014

Model Guidance Foreshadowing Cold, Stormy September Ahead

Latest model guidance has continued hinting at the idea of a cold and stormy period in September, now possibly spanning the remainder of the month.

Tropical Tidbits
The image above shows a five-day average of 500mb geopotential height anomalies over the Western Pacific. In this graphic, warm colors correspond to positive height anomalies, which generally bring about quiet and warm weather. On the other end, blues indicate negative height anomalies, which are acknowledged by the presence of cooler and stormy weather. This five-day average is valid for the period from September 8th to September 13th.

By observing the weather occurring over East Asia, we can actually determine the weather for the United States a handful of days down the road. For example, if there is a storm system over Japan, we can generally expect a storm system affecting the United States around 6-10 days later. The same rule applies for high pressure and warm weather, with the 6-10 day lag period.

If we extrapolate the forecasted cold and stormy weather over Japan around September 8-13, we can expect similarly cool and stormy weather here in the United States in the September 19-23 period. Considering this forecasted cold/stormy period lasts longer than the five-day average in the image above, this sort of pattern may last longer than September 19-23.

Tropical Tidbits
In the image above, we once again see a five-day averaged 500mb geopotential height forecast from the GFS Ensembles, valid for the same September 8th-13th timeframe as the image at the top of this post. Similarly, warm colors correspond to ridging (warm and quiet weather), while cold colors indicate troughing (cooler and stormy weather). This time, instead of looking at the outlook over the Western Pacific, we are looking in on Alaska.

Once again, we can use weather occurring in the North Pacific to predict weather in the United States. By observing trends in the Bering Sea, weather conditions in the US can be predicted 17 to 21 days in advance. For instance, if the Bering Sea experiences a deep storm system, cold weather may be expected 2.5 to 3 weeks later. The same goes for high pressure in the Bering Sea.

Per the graphic above, a steadfast trough looks to position itself in the eastern Bering Sea, with ridging to the west. This tells us that the long range may feature a period of below normal temperatures, possibly with storm systems, before brief ridging returns. Very long range ensembles then have troughing return to the area with force, but that'll have to be examined in coming days. For this image, however, we might expect this cold weather in the final days of September and into October.

To summarize, colder weather looks to make a comeback in the last few weeks of September, possibly into November.