September 14-18 Potential Wintry Storm System

A storm system looks to affect the United States around the September 14-18 period, and may have some wintry implications.
* Note: This is not expected to be a significant winter storm, and any wintry implications should be minimal. Read below for additional information.

Tropical Tidbits

The image above shows 500mb geopotential height anomalies over the Western Pacific. Oranges and reds indicate the presence of ridging, typically resulting in warm, quiet weather. Blues tell the presence of below normal height anomalies, normally seen by stormy and cold weather.

On the forecast image above, valid on September 8th, slightly below normal height anomalies are observed just to the east of Japan. 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 extrapolate the storm skirting just east of Japan on September 8th, we can estimate a storm affecting the US around a September 14-18 period.

But that's not all... long range models are hinting at some winter weather.

COD
Long range caveats apply

The image above shows the GFS model projected 850mb temperatures over the United States on September 15th, in the middle of our September 14-18 storm system timeframe. The model indicates a swath of cold air will surge south, dropping temperatures below the freezing mark only 5,000 feet above the surface! It is expected that this cold air would drop south on the eastern side of a ridge in the Rockies, intercepting... you guessed it... a storm system.

COD
Long range caveats apply

The image above gives us a view of precipitation accumulations over a 12-hour period, mean sea level pressure (MSLP) values in the blue contours, and the 1000-500mb thickness lines in yellow. This graphic is from the GFS model, valid at the same time as the 850mb temperature one above. On this picture, we see our storm system traversing the East US, likely the storm system in the September 14-18 timeframe we outlined above.

Take a look at the Upper Midwest. Weather enthusiasts analyzing this chart may recognize a familiar reading on the yellow 1000-500mb thickness line- the 5400m demarcation. This value is commonly used around the weather community as a rain-snow line. Areas north of the 5400m line are presumed to see snow, while rain is favored south of the 5400m line. As this model is showing, the 5400m line is forecasted to stretch down to Illinois, encompassing a swath of precipitation in Wisconsin and Minnesota in the process.

Just for kicks, if we use a heavier-than-average snow ratio of 8 inches of snow :1 inch of water (average is 10:1), the GFS lays down up to 6 INCHES of SNOW across portions of northern Wisconsin and northern Minnesota.

However, don't get too excited just yet. The GFS is notorious for forecasting snow in situations when snow is not climatologically likely, and this appears to be one of those situations. While a reservoir of cold air is expected to be dangerously close to the US, I'm not expecting anyone to see any major accumulation from this system, which is still in quite a bit of uncertainty. If any wintry weather is to occur, which is a significant uncertainty in its own right, it would likely be in the form of a sleet-like mixture to some non-accumulating snowflakes.

This is all subject to potentially drastic change, given the long range nature of this event.

Andrew