Rain & Chilly Temps on the Way
by Cody Matz
October 23, 2012 8:04 PM
Stuck with the clouds and fog across many areas on Tuesday leaving many of us far cooler then it could have been. But the cooler temperatures aren’t anything like what are expected toward the end of the week. In fact, highs Tuesday were still at least a couple degrees above average. Now record breaking temperatures are not in the forecast, however highs will be a solid 10 to 20 degrees below average and lows will be back well below freezing, and in some cases, to the teens. This will just be a taste of what we can expect over the coming months as temperatures fall into the winter season. So why the big change? A large trough is digging into the Pacific Northwest and will be moving eastward through the rest of the week and the weekend. A trough describes the state of the jet stream, or that river of air at the top levels of the atmosphere, which you can see in the picture on your left. When it dives south, it allows cold Canadian air to travel south it with often giving way to below normal temperatures. This is the scenario we are expecting to impact the area and stick around through the weekend.
Much like the last storm system, the good news with this trough will be the potential rainfall. Rain showers are expected Wednesday and Thursday through the area with up to a half-inch of rain possible. This would continue our trend of adding much needed moisture to the ground before it freezes sometime next month. Right now many locations are below average on the month so we need all the rain we can get. Check out the current October rainfall totals and the departure from average shown below.
Take notice that the northeast is actually at or above average for rain in the month of October. This same area has been far wetter over the last several months then the rest of the area leading to a far less significant drought. So for the sake of spreading the wealth, the hope is that the heaviest rainfall will fall in other locations in our area.
Unfortunately though, things just aren’t that simple anymore. Im talking about precipitation type…. AKA, snow. Temperatures will be falling far enough where snow will become a possibility for some areas Wednesday night and early Thursday. Remember though that the entire column of the atmosphere has to be below freezing, which for this time of year takes more energy then you might think. Even if it is cold enough to snow, the ground is still quite warm with soil temperatures generally in the 40’s and low 50’s. This means that the snow is not going to accumulate on roads or most surfaces, with a slushy dusting possible on some grassy surfaces, but that would be the extent of it.
So how do we know that the atmosphere will be cold enough? Well we have a few ways to check with a few different products. But, the first sign can be simple computer forecasting model output showing snow accumulation. Here is what 2 different models show for snow accumulation through the end of the week.
Notice that both show some small accumulation in the south and the east. This gives us the first sign that snow will be a possibility. But we have to go into further analysis to find out for sure. Remember that these graphics show model output that doesn’t factor in many factors for accumulation with the biggest one being an unfrozen ground. So we aren’t expecting accumulation at the moment.
Just how cold will it get?? Likely the coldest air of the season moving southward into our area. Compare the temperatures 5000 feet above the surface from Tuesday afternoon to Friday afternoon.
See the big difference?? Temperatures at this level will drop some 30 to 40 degrees between Tuesday and Friday. This will lead to a somewhat similar drop in temperatures at the surface putting afternoon highs some 20 degrees below average Friday and Saturday. Stay warm!!!