The 12Z NAM has finally come on board with respect to the threat this coastal storm poses for the region between Philly and NYC (from NYC and points northeast of there it looks like an all-out blizzard).  Prior runs did generate massive snowfall amounts for much of New England (in fact bordering on insane amounts), but the latest run produces quite a snowfall for as far south as southeastern Pennsylvania.  The upper-level feature that we’ve focused on in recent days is again an important player here with respect to the latest changes in the NAM.  Take a look at the latest forecast map for the 500 mb pattern at 1am tomorrow night (first map below) and compare it to last night’s 00Z model forecast map (second map below).  The upper-level short-wave is a little deeper, a little slower and a little farther south, and these “little” differences mean a lot with respect to snow chances in the “deformation zone” (aka comma head) in the region between Philly and NYC.  In fact, the wrap around snow is much more impressive with this run in the region with snow lasting until early Saturday.  The surface maps at the same time shown below result in significant differences between the latest 12Z NAM model run and last night’s 00Z run with a deeper low that is closer to the coast.  In general, this run (as well as the 12Z GFS run) is also colder than the prior run (storm draws cold air in towards the center).  Bottom line, latest NAM is certainly more bullish for snow in the corridor between Philly and NYC - for specific amounts see Rob’s blog.  Video discussion available at thesiweather.com

Paul Dorian/Meteorologist

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