MEMPHIS, TN (WMC) - Determining what weather systems will do days in advance is hard by itself, but when you add snow into the equation, it makes forecasting even more difficult.
Snow reaching the surface requires the right combination and those factors can be little details that make a big difference. Forecast models can have a tough time if one small detail is off. Forecast models predict weather using current atmospheric conditions. They are unable to consider the whole atmosphere, which we know is enormous. If they could, we would have perfect forecast.
It’s not just temperatures at the surface that matter, but also temperatures all the way up to 20-30,000 feet. The levels in between are important too. That’s a lot to cover and just like everyday weather, the temperature column can change in one area and not the other or vice versa. If there are any warm areas in between, it may fall as a rain/snow mix, or if it is too warm in the mid-levels, it could turn out to be just plain old rain. A small change in the temperature can affect accumulation too.
On top of that the track of a storm system can bring about change. Usually, we need a low pressure system to track along the gulf with cold air already in place. If the low is too far inland, we get all rain. If it’s too far south, we get nothing. But if it’s in the middle right along the coast, we can see a decent snow.
Sometimes an “upper low” will move over the Mid-South. The rain that falls from that low pulls cold air aloft down to the surface directly beneath the low. Temperatures can drop 5-10° when this happens. If the air is already cold, then the rain changes to snow and occasionally be heavy. This doesn’t happen with every upper level low, that’s why they are so tricky. A subtle shift in either direction can also change what we see. And that’s the situation Wednesday night. The track and strength of the low will determine who sees a cold rain or who could see a quick snow.
This is just one example of that rain snow transition line, which is outlined in pink and is nearby. A shift west in that line would change the forecast, from snow to rain depending on where it sets up or to a mix. This is an example of what can make it hard. Weather systems can also speed up or slow down and then the system would have to provide moisture during the time when temperatures were at or below freezing. There is just so much to consider!