Deep low pressure will intensify over the Great Lakes as it moves into Canada over the weekend.
Moisture moving over cold air during the morning on Saturday will initially result in snow and sleet around Philadelphia before turning to rain by noon. Light snow may begin as early as daybreak on Saturday, which is earlier than previously predicted.
Cold air at the surface will be slow to leave, making Saturday a cold and wet day. High will be near 40. Rain will be moderate to heavy Saturday afternoon and night.
An associated warm front will move through late Saturday night, but rain is likely to continue Sunday morning before tapering mid day Sunday. A cold front moves through late afternoon Sunday also accompanied by rain. High on Sunday mid 60s. Cold weather returns later Sunday night.
As predicted a few weeks ago, the warm phase we’ve been in over the month of December would likely end by the beginning of January and it seems that’s the case. Whether it’s due to the next phase of the -Madden-Julian Oscillation is anyone’s guess.
The models continue to have trouble with details in prediction more than 3 days in advance. Perhaps it’s because much of the energy is coming off of the Pacific where there are few observation stations.
The next uncertainty will affect us this weekend. After very cold weather for New Years Day, a low pressure system will move in from the southwest. The models have settled on a rainstorm for us Saturday night into Sunday, but there is very low confidence that this storm will really track over the Great Lakes. Development of a secondary coastal low or change in track will vastly affect our weather. So stay tuned.
Either way, if temperatures moderate over the weekend, the moderation will be brief, as a cold cyclonic jet flow will develop for next week.
A southwesterly flow of mild air will continue through early Sunday. For Saturday, a bit of clouds early morning, then mostly sunny skies and temperatures in the mid 50s. Beautiful for December in Philly.
A cold front will move through during the day on Sunday. Sunday will be cloudy with showers likely during the afternoon and evening. Temperatures still mild with highs near 49.
A broad trough will bring colder temperatures for upcoming week. As mentioned in an earlier post, the cold front moving through on Sunday will stall not all that far to our south. Disturbances may develop along this boundary. The GFS suggests some light precipitation early Tuesday, otherwise dry.