How did the forecast change so quickly? - Dallas News |

How did the forecast change so quickly?

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Heavy snow started late this morning and shows no signs of quitting for some of us. The heaviest amounts extended from Baltimore through Wilmington, Philly and Camden. Parts of South Jersey have already seen well over 6."

Snow gradually tapers off this evening. The second wave of precipitation later tonight will mainly be a rain snow mix changing gradually to all rain.

Many of our viewers have expressed concern about how quickly the forecast changed. We agree. Philadelphia and Delaware counties weren't even placed under winter weather advisories until 11:25 AM late this morning. Prior to that, most of the accumulating snow was expected to fall north and west.

So we wanted to answer all of your questions. How did this happen? How did we go from a forecast of a small amount of snow to such huge numbers in a short period of time? Here is the science of that answer:

At around 12:40pm Philadelphia, south and east, were placed under winter storm warnings as a band of heavy snow set up with snowfall rates of 2-3" per hour. The intense band exhibited a training feature where it moved over the same locations for a prolonged time dumping heavy snow.

This band of heavy snow created an environment that supported colder air in several layers of the atmosphere and kept the snow accumulating and temperatures down below freezing. The cold air got trapped into the south parts of the Delaware Valley and kept the highest totals In South Jersey and Delaware.

Slippery travel will develop on untreated roadways. Bridges and other elevated surfaces tend to ice up first, and visibilities could drop below a half mile at times. Temperatures are in the upper 20s to around 30 degrees

Accumulating snow tapers and the atmosphere warms with a mix of light flurries, sleet, and rain as more precipitation moves in.

The low temperature in the city Sunday will be 32, resulting in an icy mix before temperatures slowly rise overnight and the changeover to rain occurs.

Then, we should see the rain on Monday, another possible snow event Tuesday before we're just plain old cold towards the end of next week.

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