September 10th Potentially Strong Severe Weather Event

September 10th is being monitored for the emergence of a strong severe weather event.

SPC
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The Storm Prediction Center has outlined an area from Chicago to western New York for potential severe weather on Wednesday, September 10th. The outlined region includes states such as Michigan, Indiana, Illinois, Wisconsin, Ohio, Pennsylvania, and New York.

Instant Weather Maps
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On the morning of September 10th, a surface low is expected to track northeast into the Upper Midwest, rapidly strengthening as it does so. This sort of rapid cyclogenesis was discussed by the SPC in their outlook for this day's severe weather, and it does appear to be a valid concern. The wind field around this surface low is expected to tighten and intensify, as the close proximity of isobars on the map above shows. With cooperative surface winds, severe storms become quite a bit more likely (assuming all other components for severe weather are in place, of course).

Instant Weather Maps
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Another item mentioned in the SPC outlook was the presence of a moisture-rich environment in the outlined area. A look at precipitable water values confirms this idea. Precipitable water values are determined by theoretically taking a single column of air and collecting all the water from it. For instance, PWAT values of 1" suggest if you were to compress that column of air and squeeze all the water out of it, you would see 1" of water. Typically, values above 1.5" tend to be humid and uncomfortable for many, while values above 2" can feel 'soupy'. All of the moisture projected to be in the air on September 10th, per the image above, should allow for any strong storms to possess the potential for torrential rainfall, with flash flooding a possibility.

Additional updates will be posted in the near future.

Andrew

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