NCHurricane2009's Blog

SURPRISE SURPRISE SURPRISE!!

Posted by: NCHurricane2009, 6:29 AM GMT on February 13, 2010

Some rare snow was forecast for the deep south. For days, the storm was forecast to be so far south that it would stay even south of much of North Carolina. For me in Raleigh, that meant that we would miss yet another snow storm. The past week's pair of historic blizzards in the New England States just missed us to the north (I hope everyone up there is warm and safe by the way), and now this one was supposed to pass us to the south (things were getting lucky for snow haters in NC).

That took a complete 180 during the day of February 12. Forecasters suddenly saw the storm taking a more northerly track, putting us in Central NC (for whom 24 hours ago forecasts said we would barely get a few flakes) right in the path for some snow. By sunset, it was imminent that the snow was coming, and boy did it explode on us "like gangbusters" when it started. I was driving home from work around 8 PM EDT when the snow started as a few small flakes only visible by streetlights.

The snow was a surprise, my sister had gone to a birthday party that evening not knowing it would snow. We had to pick her up. When we drove out around 9 PM to do so, well let the picture speak...

WHOAAA (Intersection of Kildaire Farm Rd. and Cary Pkwy in Cary, North Carolina at 9 PM EDT)

Snow showers continued through the evening. After coming back home, here are some pictures of the snow piling up on the backyard deck (left: 1", right: 1-1/2")

Why this Happened
How did we go from forecasting zero inches of snow 24 hours before the storm to getting hit with more than 1" of snow (and by peering outside as I write this it is reaching 3")? When forecasting weather, we rely on computer model outputs, which run equations that obey the laws of atmospheric physics. Problem is that the equations require a dense network of observations as their input in order to make accurate calculations into the future. Thus, computer models may not be seeing what's happening between weather stations when they are spaced several miles apart as they are today. The models for days kept repeating a solution that had the surface low that is driving the storm in central Florida. As February 12 progressed, the models suddenly saw that the track would be in northern Florida, pushing the snow shield further north so now it would affect us too.

An extreme case for us was in January 2000 when computer models failed to see that a coastal storm similar to this setup would dump 22" in Raleigh (the models only saw flurries). This is similar to that case, but not as extreme.

Surface map during the evening of February 12, 2010. The east-northeastward tracking surface low in northern Florida (which computer models for days prior insisted it would end up in central Florida) was driving this entire event. Central NC (where I am at) was on the NW side of the storm. Because of the counter-clockwise flow around surface lows in the northern hemisphere, that meant winds for us were blowing from the north, bringing in cold air. Plus, there was good surface convergence as moist Atlantic Ocean air spiraled into its center, allowing the air to rise and condense to snow clouds.

200 mb upper air analysis, February 12, 2010 evening. The surface low in northern Florida is supported by the upper air divergence on the east side of a strong upper trough located over the eastern United States. The upper air divergence over NC was really high at the time. From western to eastern NC, you can see shading change from light blue (slower winds) to purple (faster winds), which means air is accelerating and spreading apart aloft (thus, upper air divergence).

Summary: Surface convergence on the NW side of the surface low at Florida plus upper air divergence from an upper trough over the eastern United States creates forced lifting for the formation of clouds and precipitation. Because winds were blowing from the north on the NW side of the surface low, that brought in cold air for the precip to be snow. Computer models erred by thinking these dynamics would pass just to our south, but the surface low ended up further north so that these dynamics came over us.

Another Surprise?!
Watching the radar through the night into the early morning hours of February 13, it seemed the snow would finish by 3 AM.

By 2:30 AM (noticed the circled time), the snow showers were finishing...

Again notice the circled time. A snow band from NOWHERE explodes on the radar just to the west in only 1 hour! That prolonged snow showers for a few more hours, helping to top off accumulations to 3" at my house (remember forecasters said it would not snow 24 hours ago).

Snow Leaving as fast as it Came
After waking up late next morning (been storm watching during the evening), the sun was beaming and the snow was melting quickly. Time to take some last photos of it:

Shot from screened window into the backyard. Looks like I can't take a ruler to it to prove the total was 3" because it melted so fast. Well, I'll wait for the NWS totals map to see.

This is odd. There was a patch of leftover snow from the January 29 event on the backyard deck next to the house. That patch was melting from the bottom first, making a mini-snow blanket a small critter could tuck itself under (that is if any animal likes to sleep under the snow). Because of last night's snow, the snow blanket built up itself even more.

The Magic Ice Wave
This is the second day after the February 12 evening snowstorm (valentines day). The thaw was leaving a magical mark on the backyard deck. Remember that snow patch melting from the bottom first (shown above)? As things continued to melt, it became a neat little ice wave arcing toward the house: