Well, the snowstorm that wasn't ended up turning into the snowstorm that was. After the low pressure system stayed to our south and didn't give our area much precipitation, except for the shore points, it moved offshore and drifted northeastward, only to stall. It was due to this stalling that snow bands were able to wrap around the low and come into New Jersey and Philadelphia from the northeast. This was actually modeled to happen, except to a lesser extent as accumulations ended up at least doubling or tripling predictions in a lot of places. Here are snow accumulation totals from southeast Pennsylvania and South/Central New Jersey from the National Weather Service...note that a lot of these are from the morning before the snow ended, as the times of the measurements are listed next to each town:
Snow Total Prediction
Before I get to a very nice weekend forecast for the area, I want to explain how I come up with predicted snow totals and why snow accumulation can be so hard to predict. Models like the EURO, GFS, and NAM all have runs where precipitation amounts are outputs that are displayed on a map using different colors for different ranges of values. This output of precipitation values is called QPF, or quantitative precipitation forecast, which is the amount of liquid water precipitation that an area is projected to receive.
Once you get the QPF, you use a snow conversion table where liquid water amount, in inches, can be converted to snow amount, in inches. However, this is dependent on air temperature. The higher the temperature, the denser the snow, and the more it will compact together on the surface, causing a smaller snow accumulation than at colder temperatures. Conversely, the colder the temperature, the fluffier the snow, and the accumulation on the ground will be greater because the snow pack on the ground is less dense and compacted. Heavy, wet snow is good for snowballs, and fluffy snow is easier to shovel but not good for snowball-making. Here is a liquid water to snow ratio chart:
This is not the end of predicting snow accumulation, though. Several factors go into predicting totals. First, you need to know the QPF and liquid to snow ratio, along with the air temperature during snowfall. Second, you need to know where the rain/snow line will set up and move during the storm, as rain, sleet, and freezing rain dampen snow accumulations quickly. Third, you must know the ground temperatures, because if they are above-freezing, then it will take heavy snowfall or take time for cold air to cool the surface and get snow to stick to the ground. Fourth, the track of the storm is quite an obvious one that you must know. Then, finally, the set up of heavy snow bands within the snowstorm can cause 'jackpot' areas of localized, heavier accumulations relative to surrounding areas. These are main factors in predicting snow accumulations, and some of these caused problems with forecasting Wednesday's storm.
5-Day Forecast
Finally, we get to the forecast for the next week...After a messy and cold couple of days, we'll be rewarded with a sunny weekend for Saturday and Sunday, with highs in the mid to upper-50's. On Monday, clouds will move in, accompanied by a high of around 60. Showers on Tuesday, modeled to bring around a quarter of an inch of rain, with highs in the mid-50's. Then, it will clear out for Wednesday, leaving behind a nice day with highs in the lower-50's.
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