Wednesday, September 10, 2014

Multiple Arctic Blasts Projected to Hit North America During Coming Weeks

The remainder of September into October looks to feature a series of Arctic cold shots, potentially record-breaking at times.

Tropical Tidbits
Click to enlarge
The first cold threat looks to arrive only in a matter of days after our ongoing cold shock. Shown above is the GFS ensemble forecast over the Western Pacific, showing expected 500mb geopotential height anomalies. Reds indicate above normal height anomalies, consequentially warmer and quiet weather. Blues depict negative height anomalies, which often result in cold and stormy weather. 

In the image above, valid September 11th, we see a swath of negative height anomalies overtaking Japan as a rather strong upper level low scrapes the nation to the north. Seems pretty mundane, sure, but the consequences here at home are far more than mundane. As has been discussed consistently for the past couple of years, the weather in East Asia can have a significant impact on weather here in the United States. Utilizing the East Asian correlation of ridging over Japan equals ridging in the US 6-10 days later, and the same situation with negative height anomalies, we can foresee long range weather patterns weeks out at a time. This mechanism is referred to as the Typhoon Rule, and states that weather patterns found at the 500mb level can replicate themselves over North America 6-10 days later after they appear over Japan. 

 If we use this rule on the model image above, a rather strong upper level low with the accompanying threat of some cold Arctic air may begin affecting the US around September 17-21st, within the six-to-ten day timeframe discussed above.

Tropical Tidbits
The image above once again shows forecasted 500mb height anomalies over the West Pacific, now valid on September 18th. Again, we see another swath of negative height anomalies over Japan, apparently due to another strong upper level low pressing south into the northern part of the country.

Using the aforementioned Typhoon Rule, it appears our second cold blast may impact the country around a September 24-28th timeframe. This one may be more significant than the last, as ensembles are already highlighting well below normal height anomalies this far out, and the upper level low appears deeper and more pronounced than the one in our first Arctic air threat.

Plymouth State
Our third threat at Arctic air follows closely behind our second threat. The image above, from Plymouth State University, shows the ECMWF model's forecast of 500mb heights (shaded colors and white contour values) and mean sea level pressure (MSLP) values (black lines). In this image, valid September 13th, we see a strong upper level low placed over the eastern Bering Sea. Just as we saw in Japan, this appears rather mundane, but actually plays a significant role in our upcoming weather.

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. In this case, with the forecast above valid on September 13th, we may expect a rather significant cold blast over the United States for the end of September and into October, in a general September 30th - October 4th time period.

Plymouth State
It's this last Arctic air threat that has me concerned. The image above once again shows the ECMWF's outlook of 500mb heights and MSLP values, now valid on September 17th. In this image, we see decidedly below-normal heights spread across nearly the entire Bering Sea, with the core of this messy trough centered just north of the Aleutian Islands.

This scenario worries me a bit. Even though this is definitely a long range forecast (and thus shouldn't be seen as high-certainty), the premise is concerning. The ECMWF's proposition of a cold air mass across the entire Bering Sea would suggest more of a long-term cold weather period, across a big chunk of the nation. Extrapolating the forecast time period of September 17th, we might expect some cold weather around an October 4th - October 8th period, possibly for longer.

A short-term warm-up can be expected in the last week or two of September before the cold continues.

Stay tuned for continuing updates on these threats.