A big winter storm will move up the East Coast this weekend. With cold air in place, this will be primarily a snow event along the I-95 while the Outer Cape may see some rain/sleet mixed in.
The medium range forecast models agree that a storm will develop, but differ slightly in timing and in track. The US GFS model is ~ 6 hours faster than the EURO ECMWF model bringing the storm up the coast. The GFS brings snow to DC Friday morning and to Cape Cod Saturday morning with the EC 6 hours later. The EC storm track is slightly east of the GFS track. This means all snow but lower amounts than the GFS and less snow spilling west of the I-95 Corridor.
The GFS forecasts heaviest snowfall in MA and Cape Cod from 0700 EST Saturday to 0700 EST Sunday. The EC is more limited in duration with heaviest snow from midnight Saturday night to early afternoon Sunday.
Winds will shift NE early Saturday (again the GFS is 6 hours faster here too) and blow hard from early Saturday afternoon until early Sunday afternoon (2 tides). The EC has weaker winds than the GFS which has gale force winds Saturday and Sunday. The NE fetch will be limited in time but the intensity of the storm may make up for it. A chance of coastal erosion is very possible as we will have to sit through 2 tides.
The difference is the forecast models stems from differences in their handling of the two disturbances that will merge (“phase”) over the Southwest. The slide below from my class this morning shows the current location of the two disturbances – in this slide color contours are a measure of spin (vorticity) – more spin, more storm. The “southern stream” disturbance well east of CA doesn’t look like much but remember it comes from El Nino Land and is therefore laden with moisture. The vigorous “northern stream” disturbance packs the wallop that will unlock the energy held as moisture in the southern system. In any event, both disturbances are well offshore and so not very well observed. As a result, the forecast models have to fill in the gaps and they differ ever so slightly in the spin up of the storm thus affecting its track and strength. As the disturbances move onshore tomorrow, and the observations improve particularly from weather balloons, we will get a better read on the forecast.
“Those who can make you believe absurdities, can make you commit atrocities.” Voltaire