February 7-9 Northeast Winter Storm

A winter storm will impact portions of the Northeast beginning Friday and continuing through Saturday Night.  While confidence is high that this winter storm will form, differences in model tracks and timing have left some questions.  Below are the 12Z and 18Z GFS and NAM model runs depicting snowfall through Saturday evening.

The main difference here is the GFS starts the formation of the low pressure off the coast of the Carolinas about 6 hours sooner than the NAM model does.  This time difference results in the precipitation shield reaching the Northeast sooner and more of the precipitation falling overnight.  The nighttime temperatures will be more conducive for snow or sleet to occur at the surface and thus accumulations could occur.

The other difference is in the track and strength of the low pressure.  The 18Z GFS model is depicted to the right and the 18Z NAM model is depicted below that.  The NAM model takes longer to mature the low and keeps it farther off the coast than the GFS model.  The result is a more impressive look to the GFS but the NAM is still able to achieve similar snowfall amounts in most areas due to its higher QPF (Quantitative Precipitation Forecast) totals.  This is likely due to its better understanding of processes related to the deformation zone as well as stronger PVA (Positive Vorticity Advection).

Another major difference has to do with the phasing of the energy between the shortwave cutting through Michigan and Upstate New York and the Coastal Low.  The NAM model appears to hold off on phasing these energies until after the Coastal Low is north of New York City.  Meanwhile, the GFS phases the energies much sooner, in the vicinity of New York City, about at the point when the low begins to push more east, rather than north.  This earlier phasing helps to strengthen the low and produce heavier precipitation amounts across the New York City area instead of out to sea and along coastal Maine as the NAM does.

Keeping in mind these model differences, a major snowstorm will impact portions of the Northeast beginning Thursday night.  Given what the current American models are thinking, I would lean closer to the GFS solution as it seems to have a slightly better handle on how the energy will evolve.  Additionally, it has been the more accurate model thus far for this event, predicting better what would happen today over Texas with a portion of the energy that will help create this Coastal Low.  The Arctic Oscillation is also forecast to be slightly negative, which could be further evidence for favoring the GFS solution.  Another way to verify this thinking is to consult the ECMWF model.

The ECMWF model has a track of the low that is very similar to the GFS model.  One reason for the similarities in these models is they are both global models, which means they are able to take into account information from the whole globe and produce an output for the globe.  The NAM model on the other hand only takes into account information from around North America and therefore only has an output in that same area.  One major similarity between the ECMWF and GFS then is that they both forecast a strong area of high pressure (ridging) that results in the stalling of the Coastal Low off the coast of New England (as shown in the image with the black line showing the blocking ridge).  This results in a longer period of precipitation as the low slowly moves and may have been why the GFS phased the energies better as previously mentioned.  The NAM model does not depict as strong of a high pressure due to its limited domain.  Therefore, it continues the Coastal Low on a northern track instead of stalling it.  Now it is likely that as the event draws nearer that the NAM model will understand the effects of the high pressure more, but for now this is a major influencer of the Coastal Low track that the NAM is missing.  The GFS and ECMWF both forecast the ridge to begin breaking down over the Atlantic Ocean late Saturday night.  This would allow the Coastal Low to begin moving out to sea as opposed to northward as the NAM model has depicted.  This will result in major snowfall forecast differences in the northern portions of Maine, New Hampishere, and Vermont.

So what am I thinking in the way of snowfall amounts then?

Well, given the overall better solution of the track via the GFS and ECMWF but better feedback via the deformation zone by the NAM and ECMWF, I will go with a blend of the best qualities of all of these models.  Below is my snowfall totals for the event across the Northeast.  This is subject to change but as of right now this is my best estimate.

The snow will be combined with strong, gusty winds in many areas, especially along the coast too.  This will result in blowing and drifting snow, making travel almost impossible at times Friday and Saturday.  Stay tuned to this evolving situation.