Colder Weather Ahead! Maybe Even A Little Snow!

We’re in for a cool down here in West Central Ohio over the next week beginning with a cold front moving through the region tonight. Temperatures will drop down into the 50’s this upcoming weekend, and with it being the middle of October, I can’t help but look for the first signs of snow!

By Wednesday and Thursday of next week forecast models are hinting at even colder conditions, conditions possibly cold enough to support some light snow! Our first snow last year was at the very end of October in Lima, so if this ends up panning out, there wouldn’t be much difference from last year.

When looking at forecast models like the GFS or the ECMWF, first and foremost, the meteorologist’s eye is drawn to what are called “thickness” lines on a map when looking for conditions cool enough to support snow. Both of these widely used forecast models hint that by next Wednesday or Thursday it could be cold enough for snow here in West Central Ohio. The 540 Thickness line, which I have highlighted in both of the following pictures is a standard threshold for a possible rain/snow line when forecasting the potential for winter weather. Basically, wherever this line falls is where to look for a possible mix of precipitation. Generally, the further you go North of the 540 line, the better chance for less rain mixed in because of the colder temperatures.

The European (ECMWF) model shows the 540 thickness line South of Ohio by Thursday. The HIGHER the thickness number, the warmer the temperature. And the LOWER the thickness number, the cooler the temperature.

The European (ECMWF) model shows the 540 thickness line South of Ohio by Wednesday. The HIGHER the thickness number, the warmer the temperature. And the LOWER the thickness number, the cooler the temperature.

The GFS model shows basically the same thing as the ECMWF but a day later, by the beginning of Thursday

The GFS model shows basically the same thing as the ECMWF but a day later, by the beginning of Thursday

So now that I see that temperatures will potentially be cold enough to support snow, I look at a precipitation forecast. If the models show possible precipitation over the region during the Wednesday and Thursday timeframe, based on looking at the thickness values above, I can say with some confidence that is possible that it could fall as SNOW and not rain. Sure enough, the GFS shows precipitation over the region mid-week.

GFS PRECIP

So you may be asking yourself, what the heck is thickness anyway? Thickness is the HEIGHT of the atmosphere in dekameters from the 1000 millibar (mb) level (roughly sea level) to the 500 mb level. Depending on the time of year, the distance between these two levels fluctuates. During the Summer-time the distance between the two is GREATER, or taller if you’re looking at it in 3D. During the Winter-time the distance between the two is LESS, or shorter if you think of it as a 3D column.

Thickness is both a function of the AVERAGE temperature of the air AND the moisture content 1000 to 500 mb layer. So when the air temperatures drop heading into the Winter-time, the thickness levels will also drop!

Meteorologists not only look at the heights between the 1000 mb and 500 mb level, which is the most common. We also analyze other thickness levels like 1000 to 700 mb level for signs of what type of precipitation may end up falling to the ground during the Winter!

While the forecast today is certainly hinting at some possible light snow by Wednesday or Thursday of next week, it is still over a week out, models change! It’s the job of a meteorologist to watch these trends and changes over time that will ultimately lead to more confidence (or less) in the forecast!

Just wanted to share that tidbit of Winter weather forecasting, it really is challenging and fun at the same time!

-Kyle

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