Sunday, January 20, 2008

Disappointing Snow Totals...

Saturday was a disappointing day for snow lovers in North Carolina. The above snowman picture sent in by a News 14 Carolina viewer in Hope Mills was about the best snowman anyone could build with this weekend's snow. Most accumulation reports ranged from a dusting to about a half inch.

So what happened to that several inch snowfall that was expected? Part of it can be blamed on the temperature
. We expected Saturday morning temperatures to start in the upper 30s and then drop in the afternoon. Temperatures did drop in the afternoon as anticipated, but it started just a little warmer at near 40 to the now 40s. Usually a temperature forecast that is 3 degrees off is considered pretty good. Forecast a high of 70 and the temperature actually hits 73, most people don't give it a second thought. However, as I have mentioned in the blog in the past, just a couple degree difference in temperature can make the world of difference in a winter weather forecast.

If the rain had indeed changed to snow around midday and had the snow stuck when it started snowing, some folks would have
seen close to 2". Instead, the colder air did not arrive until early afternoon and the changeover to snow did not come until around 2:30pm. Since temperatures had been at near 40 or the low 40s most of the morning, the snow did not initially stick. Temperatures did drop to the mid 30s when the snow started, but that is still above freezing and cold enough for for much accumulation except for a dusting on grassy surfaces. That meant a lot of the snow that fell melted when it hit the ground. The melting process actually takes heat from the ground and causesthe ground temperature to cool down. A snowfall that is coming down at a good intensity can actually cause the ground to cool pretty fast and cause the snow to start accumulating faster.

Just from Saturday's example you can see a lot has to be considered when it comes to winter weather forecasting. Not only does a forecaster
have to forecast how much precipitation will fall but he or she has to consider ground conditions and when an accumulation would begin. Snowfall intensity, ground temperature, temperatures above the surface, the temperature from previous days, timing of a rain to snow changeover, and amount of precipitation are all factors. Miss any one of those factors by just a little and a forecast can change from 2-4" to just a dusting.

Now we settle in for very cold weather into Monday. As I write this blog Sunday afternoon just after 1, the temperature has yet to reach 30. We may hit the low 30s in the next hour in some spots, but highs in the low 30s under full sunshine is rare for Raleigh, North Carolina. The very cold air mass should give us lows in the teens early
Monday morning.

Lows in the teens are cold, but if you really want to see cold, watch the Giants-Packers playoff game tonight from Green Bay. This cold weather is affecting much of the eastern half of the country. In Green Bay, today the high is struggling to get above zero. Game time temperatures will be just below zero with wind chills near -15! Now that's cold!

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