This afternoon’s models have greatly diminished the chances of any accumulating snow Tuesday night. In fact, they have backed away from the high rain amounts, with the heaviest rain now tapering in the afternoon.
This is a very dynamic system and I expect further changes in the forecast.
The models are coming together in predicting the development of a coastal storm that will be a slow mover, affecting us directly from Monday night into Wednesday morning, then indirectly into Thursday.
The models have become somewhat consistent in predicting very heavy rain for our area on Tuesday. The NAM has almost 2.5 inches of rain falling! Temperatures appear too warm for snow to form during much of the storm, but it looks like a possibility that wrap around cold air and moisture will bring us some wet snow, especially in the NW suburbs Tuesday night into early Wednesday morning.
Temperatures at the surface will be just about or above freezing, so accumulations will be limited. But this wrap-around moisture will be occurring at nightime Tuesday, perhaps allowing 2, maybe 3, wet inches to accumulate on grassy surfaces by Wednesday morning.
So, no accumulations on Tuesday, just heavy rain and windy, then possibly 2 inches or so wet accumulation later Tuesday evening into Wednesday morning.
Not much cold air around after this storm and a significant warm up may occur next week.
For the past week, the European model, the ECMWF, has suggested the formation of a coastal low pressure system in the Sunday to Tuesday time frame. The GFS has only gradually come onboard with this possible scenario, but last night’s GFS is developing a slow moving coastal low, bringing heavy rain and and changeover to snow late Tuesday to Wednesday.
For most of last week, the GFS advertised a warm up with cold air sequestered in Canada and the low pressure development out at sea.
The GFS is now advertising much more of an amplified jet stream and a deepening hug-the-coast storm. In fact, the jet stream becomes negatively tilted with the storm track blocked, resulting in a slow-mover (read, high precipitation)
So, the GFS has finally caught up and the NWS extended range forecasters have been on-target in anticipating this coastal low development.
Many unknowns with this as-yet undeveloped storm- the degree of amplification of the jet stream, the degree of negative tilt and the degree of blocking. What this means is that we don’t know if it will be heavy rain or heavy snow or whether this will be a slow mover.
Right now, it looks like mostly heavy rain for NJ and SE Pennsylvania, but a changeover to snow is very likely late Tuesday afternoon and Tuesday night. A couple of inches is possible and the wrap-around snow may linger into Wednesday morning. Doesn’t look like a major “stormstorm” for our area at this time but a different story for interior New England.
With this storm in the hypothesized, unformed stage, we won’t know specifics about rain-snow line and snow amounts until late Monday.
There are the usual differences between the GFS and NAM models – the NAM is colder and wetter- the GFS has much less precipitation and is a bit warmer in the upper atmosphere.
In recent years, the NAM has often over-predicted the amounts of snow with model runs more than a day in advance. So while the NAM is predicting several inches of snow in PHL, I think we can ignore it and go with the GFS.
The GFS model has temperatures falling in the upper atmosphere to support snow just as much of the precipitation winds down. It maintains surface temperatures that are above freezing until the precipitation shuts down. So conditions for snow and snow accumulation are very marginal with the GFS model. While a changeover to snow looks to occur about 2-3 PM, conditions for accumulation will not be optimal because the precip will be winding down. With warm and wet surface conditions, expect snow accumulations to be about 1 inch or so in the immediate Philadelphia area.
Traveling conditions will not be good late afternoon, but the precipitation ends about 7 PM.
Again, much of this will be RAIN, changing to snow 2-3 PM and not looking to accumulate very much before ending by 7PM.