If you’ve listened to any weather forecasts recently, you’ve likely heard that [several] oubreaks of cold air will be affecting us over the next week.
The models are in reasonable agreement that a deep upper air trough will develop over the eastern US, probably in three different stages. The first influx of cold air will enter Thursday afternoon. Another will move in Saturday night. A third will likely affect us next Tuesday into Wednesday.
What is less clear is the amount snow (if any) that will accompany each of the clipper distubances that herald in the cold outbreaks. Right now, the amount of snow expected in PHL for each of these disturbances looks to be less and less with each model run. Thursday will likely just have some snow showers. Saturday afternoon’s clipper also appears to be trending towards less snow here. Additionally, the sharpness of the trough appears to be somewhat less, meaning less cold air in Florida.
The only real threat is next Tuesday, where a low pressure system from the southwest (not a clipper) may interact with the re-developing upper trough. Of course, much can change in a week, but these are the current trends.
Over the past day, the NAM model has done well with the temperatures and the precipitation predictions. The freezing drizzle and very light rain should continue on and off this afternoon. Just a few snow flakes this evening. The NAM only cranks out 0.01 inches of water every 6 hours, ending late evening.
During the daylight hours, solar insolation through clouds may keep some dark roadway surfaces just wet, but towards evening, more icey conditions are possible.
The afternoon NAM model continues to show colder surface temperatures around PHL. Chances of freezing rain or drizzle appear more likely for later tonight and Monday closer to PHL.
I don’t think the models are capturing this low pressure system well.
A small cyclonic surface wind circulation is developing off the NJ coast. Could an unexpected secondary low be developing? Stay tuned.
A difficult forecast period for late Sunday night through Monday, as the recent models have significantly changed the track of a developing low pressure system and associated frontal boundary. There is low confidence with the weather forecast for Monday. (Indeed, there has been more sunshine both Saturday and today, so even in the short range, the models have not been capturing this setup very well.)
Previously, the models had the low tracking across PA and we would be in the warm sector on Monday. However, the new track of the developing low will be to our south, allowing significant cold air to mix in at some levels of the atmosphere. While snow is not expected for much of the day in the immediate PHL area, the precipitation type is looking to be more in the sleet and possible freezing rain variety. This mixed precip will likely change to snow before ending. QPF values remain light, so only 1 or 2 inches of snow is expected by late Monday night. However, the real hazard may be the sleet and freezing rain.
I should point out that there is uncertainty about the development of this storm as it reaches the coast late Monday night. Right now, it’s expected not to affect us, but there is low confidence with that scenario.