The predictions from different weather models are generally similar, but there are sometimes significant differences in the exact timing and location of specific features. An example is the forecast for Saturday's minimum temperature at Philadelphia. The two main U.S. models, the GFS and NAM, agree that the high temperature will be in the 80's, about 20 F above average. But the minimum temperature forecasts are more than 15 F apart. The current NWS forecast discussion says: "There is model disagreement regarding the timing of a backdoor cold front late Saturday." If you look at a map of temperature anomaly at the end of the day Saturday, you can see the strong temperature gradient very close to Philadelphia, so the difference in position of the front between the two models makes it difficult to forecast what the temperature will be at the very end of the day (2 am EDT). So let's predict Tmin for Philadelphia and see what happens.
Question: What is a "backdoor cold front"?
Saturday was a complete victory for the NAM over the GFS. All of the NAM's predictions for Saturday's low were in the 40's, all of the GFS predictions were in the 60's, and the observed low was 47 F. The temperature dropped 25 degrees from 9 pm until 2 am as the backdoor cold front moved through.
Answer: A backdoor cold front is just one that comes from an unusual direction. In North America, most cold fronts come from the north or west, on the back (western) side of a low pressure center. But in this case the cold air behind the low was well to the west, and northerly flow brought colder air from the north and east into Philadelphia.
|Tmin, deg F (error)