The storm will start to develop over NC/VA Friday night as a strong 500mb vort. max dives down from the northern Plains and phases with a short-wave traveling across the southern U.S. As this energy moves closer to the East Coast, a surface low will form off the NC coastline and rapidly deepen as it heads NNE along the East Coast through early Sunday. It will draw in some cool air from the north as it develops, but there is not much cold air available, so the surface temperatures will be marginal for snowfall accumulation in many areas. In the southern edge of the snowfall accumulation, surface temperatures are expected to remain too warm for accumulation of an inch or more. Some higher elevations (above 800 feet) just south of the one inch cut off could scrape out a dusting to an inch of accumulation. Weak warm air advection N/NW of the surface low is expected to keep the lower levels warm enough to prevent most of the accumulation in southern and central NJ and northeastern Delmarva.
What we’re looking at here is a fairly rare event… a solid coastal storm in the Mid-Atlantic in October during a La Niña. Statistically speaking, this storm shouldn’t happen, but it is and here’s the forecast:
I decided to go with a 50/50 split between the latest (12z) GFS and ECMWF operational models. The NAM seems to be out to lunch with this system, as most other models agree with what the GFS and ECMWF put on the table.
I’ll probably do an update tomorrow with some minor adjustments… still trying to figure out the southern extent of the one inch accumulation area, with some uncertainty in the total accumulation in central PA and in the WV/VA mountains.