The little follow up wave most of our models sent harmlessly out to sea for most of the last 7 days is now beginning to get a bit more interesting. Enough interaction occurs between 2 branches of the jet stream that this is going to be one to watch.
The American models have been pretty gung ho on the storm while the foreign models (Canadian, European, UKMet) has been very conservative. It is important to note that the American models caught on first and the foreign suite as made a significant jump northwest following the American lead. Here’s an example of the difference valid at 1 p.m. Tuesday.
The GFS model on top produces much more precipitation (in this case snow) than the European model does. The GFS drops nearly 0.5″ of liquid south and east of I-84 while the Euro struggles past 0.2″. That’s a big difference! The main reason for the difference is subtle interactions between both branches of the jet stream. This impulse responsible for the Tuesday excitement is screaming south out of Saskatchewan. A much weaker disturbance is ejecting out of the 4 Corners region. These will phase – the question is how early! The GFS is an earlier phase which results in a more amplified system (that gets drawn north and west) while the Euro is several hours later which results in a flatter and more strung out system for southern New England.
One item of note is that the models that are bullish with the storm also show some parameters that are very favorable for heavy snow. Strong frontogenesis, a bit of instability, a deep and saturated column, and strong vertical motion right in the middle of an unusually deep snow growth zone (around -15C).
This is an odd storm. If the NAM or GFS verify a band of 6″-9″ of snow is possible somewhere within our borders – more likely, however, a more modest storm will occur. For now I’m leaving my forecast at 1″-3″ with the potential for higher amounts. This is a low confidence forecast and there is some significant upside bust potential!