A slug of moisture associated with the northern branch of the jet stream is beginning to impact the northwestern US. It is this energy that will carve out a trough over the central US and eventually Great Lakes and New England, and potentially cause a storm system to develop over the eastern Lakes into New England and southeast Canada.

This afternoon’s runs of the GFS and European (ECMWF) models continue to show drastically different solutions with the potential Friday storm, but a trend since yesterday afternoon’s models has been clear.

This afternoon’s (12z) run of the GFS model shows the cut-off energy associated with the sub-tropical jet stream hanging back and NOT PHASING with the trough carved out over the north-central US. This is due to the trough over the northern US being a bit flatter and faster, not allowing it to quite reach the sub-tropical jet stream energy.

The 12z GFS ensembles for the most part agree with the operational GFS. Note how the majority of the members shown on the above image show the trough over the northern US (denoted by blue shading) bi-passing the cut off energy over northern Mexico. This is a strong trend against an extremely deep and phased cyclone.

The 12z ECMWF, shown above, still phases the northern trough with the southern cut off low. Note how the cut off energy has been drawn east and is about to be absorbed by the trough in the above image. The model does still wind up a decently strong low pressure, in the 980MB range in the St. Lawrence Valley, by Saturday morning. However, it should be noted that the ECMWF, when compared to yesterday’s run featured in my previous post, is slower with the phase and shows a low that is about 20mb weaker in the end! Although the European model remains in the “phased storm” camp, it has trended weaker with the phase and eventual storm over the eastern Lakes into northern New England and southeast Canada.

The large scale pattern, IMO, continues to argue against enough amplification to see a full phase with the subtropical cut-off energy. While ridging is building off the west coast, there is still a clearly seen fast flow off the Pacific on the above image as the northern trough tries to dig down into the Plain States. Note the tightly packed height lines along the west coast of the US, representing a fast flow. It is hard to get a fast flow to buckle.

Also, note the lack of true lacking over the Arctic or over the northwest Atlantic. This insures a fast flow in the polar jet all across the US, and will make it hard for the jet to buckle and dig down into the Gulf Coast states and tap the sub-tropical jet-stream moisture/energy.

Given this, the trend away from a wound up storm since yesterday is a logical one. And there has been a clear trend.

Yesterday evening I posted that both the 18z GFS and its ensembles trended away from the wound up and phased solution that the 12z GFS suite seemed to favor. Since then, last night’s 0z GFS suite, 6z suite, and now 12z suite have for the most part shied away from a more phased and wound up solution.

Even the European model, which had led the way with the phased and very deep solution, shows a later phase and weaker storm in its most recent run.

In my post last evening, I said there were a few things to watch for regarding this upcoming storm threat:

1. Does the subtropical jet energy trend weaker or stronger? A weaker piece of energy may mean a weaker storm.

The energy has not trended weaker or stronger.

2. Does the subtropical jet energy trend faster or slower? A slower solution might get left behind by the polar branch shortwave, while a faster one may mean phasing does not occur until the system is off the east coast.

The energy has trended slower, meaning it is increasingly likely that the northern trough bypasses it to the north/east.

3. Does the polar jet trend more zonal? This makes a phased solution less likely. A more amplified polar jet may make the amped up solution more likely.

The polar jet/northern trough has trended more zonal, which is represented well by the lack of phasing by the GFS suite.

So, while it is still to early to completely discount the Euro’s more phased solution, I believe the unphased solution should be more favored than yesterday, and has the best chance of winning out.

This may result in a light to moderate snow event from the lower Great Lakes points east if the northern trough can dig enough for a wave of low pressure to develop along the cold front diving southeast with the trough.