A very dynamic weather pattern is developing off the west coast of N. American, with a ridge off the coast causing a very active polar (blue) jet stream to ride over the ridge and dive southeast into the Rockies. As one can see above, there are multiple mid to upper level storms embedded within the polar jet stream, and these will move into the US over the coming days. Farther south, a feed of moisture extends from the tropics into Mexico and the Gulf Coast States.

As the ridging off the west coast continues to build in the coming days, storms will gradually move farther south, and may eventually tap the moisture feed and produce widespread areas of heavier precipitation.

As we look ahead to Thursday, it is evident that ridging off the west coast of the US extends north into Alaska, with energy dropping into the Rockies and trying to spread east. In addition, a moist piece of subtropical jet stream energy, seen on the above image by the blue shades over south Texas and northern Mexico, is advancing to the east-northeast, and is coming dangerously close to interacting with the trough developing over the Plain states. In the arctic, some weak ridging has developed over extreme northwestern Canada, indicating a neutral AO, however an upper level low is expected to remain in place over Greenland, keeping the NAO positive. This supports southeast ridging, and will make it hard for the pattern to slow down and amplify.

As for what happens next, we have many solutions being shown…the 12z European model (ECMWF) and to some extent its ensembles show a clean phase between the moist subtropical energy and the polar jet energy diving in from the northwest Thursday into Friday, resulting in a deep low pressure developing over the Ohio Valley and riding into the eastern Great Lakes by Friday morning:

The Euro continues to bring a deep storm into eastern Canada, and actually shows what would be close to a blizzard over a good portion of southeastern Canada Friday into Saturday:

Today’s run of the ECMWF is actually much deeper and farther west with this potential Thursday-Saturday storm than yesterday’s run, which I declared to be unlikely in my blog post last night.

The GFS and its ensembles continue to play a game of cat and mouse…yesterday afternoon’s (12z) run of the GFS and its ensembles almost unanimously disagreed with the amped up ECM solution for this storm…then the 0z run came around, and the GFS ensembles had several members jump to the amplified European model solution:

Several members showed a very wound up low tracking through the eastern Great Lakes or New England/Southeast Canada with an area of accumulating snows and high winds to the west of the low track.

Then, this afternoon’s 12z runs of the GFS ensembles had several members, at least half, develop a very strong area of low pressure with heavy snow and wind somewhere between the eastern Great Lakes and the Northeastern Seaboard.

This evening’s 18z run of the GFS ensembles, shown above, backed off the deep low cutting well west of the east coast idea some, but several members still have the deep storm tracking well west of the east coast.

So, what exactly determines if we see a deep storm develop Thursday into Friday and where it tracks?

Essentially, it has to do with if the trough developing over the central US can phase with the piece of moist subtropical jet-stream energy moving across the Deep South before the energy slides off the east coast.

The 12z Euro (which showed a deep storm bombing over the eastern Lakes) when compared to the 18z GFS (which did not show a deep storm bombing over the eastern lakes) valid at 12z Thursday, about one day before any deep storm would really bomb out, shows differences already. The GFS is a bit flatter with the polar jet across the US and is a bit faster with the polar jet energy. In addition, the Euro is bringing the cutoff energy east a bit faster than the GFS.

When rolled forward 24 hours, the Euro shows a well timed phase between a polar branch shortwave that digs down into the deep south and a piece of cutoff energy that moves east at just the right time, while the GFS is more zonal with the polar branch of the jet stream and is now showing the subtropical jet shortwave too far southeast to fully phase west of the Apps, resulting in a much weaker storm system.

The question becomes, which solution is more likely, phased or unphased? The trend this season has certainly been for the unphased solution to win out more times than not, and given the lack of true blocking in the current pattern, a well timed phase would need to occur for the more amped up solution to play out. Given this, I will continue to slightly favor the more weak/fast solution to play out, however model trends over the past 24 hours have been strong and the Euro has led the way thus far, so chances for a snowstorm from the eastern Lakes east in Upstate NY, New England, southern Ontario and southern/eastern Quebec have increased for Thursday night into Friday and Saturday.

What to watch with relation to this storm threat:

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

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.

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.

As we head into next week, the active pattern will continue across much of the US. More details on that in the coming days.

Tuesday is statistically the least favorite day of the week among Americans, but I’m sure you’ll pull through!

Jim.