An Interesting and active weather period is unfolding for the next several days. Model differences promise to make the forecast more challenging.
Tonight’s NAM has snow developing early Sunday evening and extending into early Monday morning. QPF values have increased over the day and are now at 0.40 inches water. If the NAM is correct, 4-5 inches of snow will accumulate by early Monday morning in and around the immediate Philadelphia area.
The same event is depicted differently by the GFS, which has high pressure blocking the moisture’s northward movement. QPF values tonight are about 0.10 inches water, even less just north of Philadephia.
I’m leaning towards the NAM, which has done the best for the last two wintery weather events. So the current predicted amount is 3, possibly 4 inches of snow Sunday night, ending early Monday morning.
Another larger system will affect us late Monday afternoon through Tuesday. Let’s get through the first one before focusing on details, but the trend is for snow, several inches, changing to sleet and freezing rain Tuesday morning. An extended period of freezing rain is possible even in Philadelphia. An eventual change to all rain later Tuesday morning.
Further complications include the possibility of a secondary low formation.
I’ve been looking at the latest GFS and NAM models from this morning. Sunday night into Tuesday promises some interesting (at least for this season) winter weather.
Two systems will affect us, one Sunday evening and the other starts Monday evening. The first approaches Sunday with light snow developing sometime during the evening and ends early morning Monday.
There are differences in the models with the QPF. The NAM is showing 0.23 inches water, the GFS less. With the prior two winter weather events, the NAM out-performed the GFS. So it appears that 2, possibly 3 inches of snow will have accumulated when it ends Monday morning.
Monday will be mostly cloudy as another more intense storm moves to our west. Cold air in place at the surface with warm, moist air over-running the the cold air will result in snow initially.
The GFS has the snow starting early on Monday evening. Critical temperatures in the upper atmosphere warm by 3-5 AM Tuesday morning, with the snow changing to sleet and freezing rain at that time. It’s possible that 3-4 inches of snow will have accumulated before the changeover.
Surface temperatures may remain at or below freezing until 7 AM Tuesday, so things will be icy and messy Tuesday morning.
North and west of the city, the freezing rain and sleet may be prolonged into late morning.
By noon on Tuesday everything should have transitioned to all rain. This looks messy and the temperature profiles and QPF predictions will likely change in the next few days. Stay tuned.