Reviewing the major models, at this time, it appears that the balance of December won’t have a pattern for snow in Philadelphia.

Short duration dips in the jet stream will bring cold periods, but moisture flow from the southwest will make these colder periods of moderate duration, allowing wet and mild weather to return along with deep storms whose tracks will be moving to our west instead of along the coast.

Today’s (Sunday) GFS and FV3-GFS differ significantly in the very long range with these similar models having very different forecasts by New Years Eve.

For this week— A storm for Friday is a now posing to be a significant rain storm and the storm potential mentioned for Sunday into Monday also appears to be wet instead of white.

The chance of seeing a white Christmas here have faded, at least for now.

The global models show very frigid air building in the polar region, Siberia and Greenland during the last week in December. A shift to much colder weather and stormer conditions (snow) is expected by the end of December into the first week of January.

[su_box title=”Wednesday Evening Update” box_color=”#defcdc” title_color=”#000000″]Tonight’s NAM has joined the GFS in predicting rain for much of Saturday and Saturday night. The GFS continues with some showers continuing on Sunday.
The still unofficial FV3-GFS is more optimistic for Saturday evening and Sunday, as it keeps this precipitation to our south. Too early to tell.[/su_box]

The [relative] warm-up mentioned in my posting last week is on track to arrive late Friday.   Unfortunately, the milder temperatures will be accompanied by low pressure and rain.

The low pressure system arriving late Friday will be interacting with a cut-off air upper low in the southeast.  Much of the heavier rain will be Friday night and it may taper on Saturday.

There’s much uncertainty with the specifics and timing of this rain, because interactions with upper closed lows are difficult to model.

There’s some suggestion that the upper low may spawn another low pressure system for part of Sunday.  Too soon to tell.

Milder temperatures will persist into next week, with a bit of a roller coaster effect as cold air dips in several times.  It will be milder, then a day or so of cold, then milder again.

Real changes back to winter are forecast to occur around Christmas Eve with a large storm and deep cold returning.    

The coming weekend is going to be unseasonably cold and dry. The coastal storm’s track has consistently been forecast to pass south of the Philadelphia area late Sunday into Monday.

[su_note note_color=”#d9f2da”]Despite my winter long range climate forecast for a cold winter in Philadelphia posted last weekend, the latest climate models show the balance of December to be somewhat milder than average. This change is predicted to occur later next week. We’ll see if this happens.  [/su_note]