Updated Weekend Snowstorm Forecast

The subtropical jet stream energy that will supply the moisture for this weekends eastern US snowstorm is producing moderate precipitation over the central Appellations, and the northern stream energy that will combine with this area of precipitation and allow for a strong low pressure to form off of the Mid Atlantic coast is beginning to dig farther south over the Plains. Things will really take off when the strong jet streak over the Dakotas rounds the trough tonight and begins interacting with the southern stream energy.

The 12z global models are in general agreement on a low track from off the lower Mid Atlantic coast to south of Long Island, near the benchmark and then getting a slight nudge east (possibly due to the lack of a strong –NAO block) and likely missing Cape Cod to the east.

The focus of this forecast will be to look closer at where heavy banding may setup and where the rain/snow line will be, and fine tune yesterdays map. The synoptics behind the storm discussed yesterday have not changed.

The storm will begin taking off Saturday morning as the jet streak rounds the bottom of the shortwave over the Ohio Valley, and allows the storm to take on a more neutral to negative tilt. Ahead of the energy will be an area of favorable upper level divergence and positive vorticity advection. Tomorrow morning this area will be over the lower Mid Atlantic, where heavy snows will likely be ongoing over the mountains of VA, WV, western Maryland and beginning to spread into southern PA.

By late afternoon as the storm takes on a negative tilt and intensifies, the upper level left will continue to strengthen. Note how the 500mb height lines (black lines above) spread farther apart over the northern Mid Atlantic and New England, indicated good upper level divergence and lift. In addition, note the very strong mid level vorticity spreading over much of the eastern Mid Atlantic. The area immediately ahead of this strong vorticity will see additional left because of the strong vorticity advecting into the area.

This very strong lift will spread into much of Southern New England during the evening, and in fact overnight the upper level system will close off over New England:

This may aid in the storm slowing down some off the New England coast overnight, maintaining heavier bands of precipitation through much of the night, especially over eastern New England.

In addition to good upper level forcing, as the mid level low tracks up along the immediate coastline, a stark mid level temperature differential will develop along the coast and just inland. This will result in strong mid level frontogenesis, which will contribute to strong low level convergence along the coast and just inland, which will assist in making bands of heavy precipitation possible.

Note how the NAM is depicting very strong upward motion at both the 700mb and 500mb levels (respectively) tomorrow evening over the eastern Mid Atlantic/New England:

This strong upward motion suggests convective bands may be possible, and that heavy precipitation is likely. Both the 12z GFS, NAM, and ECM suggest this strong vertical motion will move well into southern/eastern New England overnight.

In addition, as warm air on a NNE flow off the water collides with colder, denser air moving on a NNW flow over eastern New England a coastal front will form. East of this front a heavy, cold rain is likely. However immediately to the west, the front will aid in focusing heavy snow bands.

Essentially, the lower, mid and upper levels of the atmosphere will support an 8-12+ hour period of moderate to heavy precipitation from the lower Mid Atlantic, north through much of the Mid Atlantic including eastern upstate NY and a good amount of PA into New England. In fact the 12z NAM, GFS, GEFS and ECM all agree on a large swath of 1”+ liquid equivalent over much of the Mid Atlantic and New England.


The next question is precipitation type…

As discussed last night, the mid level low track is expected to be from near DC through the Delmarva and southern NJ and then off the Mid Atlantic/New England coasts, expect possibly close to Cape Cod. This means temperatures aloft will generally be cold enough to support snow…HOWEVER, from central NJ points south most of the storm will occur during the day (the sun angle isn’t high, but is enough given the warm ground to keep the boundary layer warm) and it is October, meaning it will take some time to work out the warm air in the boundary layer.

In areas with any elevation (above 750’), snow should stick even during the day fairly well, especially given the heavy, wet nature of the snow. In areas below 500’, I expect mainly rain until late Saturday afternoon, when the setting sun and continued deepening of the storm will allow the lower levels of the atmosphere to try to cool.

I expect DC and Baltimore may end as a period of wet snow, but I’m not expecting accumulations. Philly may see a longer period of snow, but the best snows will be in the higher elevations away from the city. However, over the northern half of NJ and the lower elevations of New England, including Boston and NYC, the situation is not as clear cut…here, I still expect the precip to start as rain…however, more of the storm will occur at night. In addition, as shown above, it appears very strong vertical motion will occur over these areas beginning late Saturday afternoon over NJ and into the evening over New England. This will change rain over to snow, given enough time, even in the low elevations relatively close to the ocean. Thus I believe areas immediately along the coast stay mainly rain/mixed with relatively little accumulation, while areas not very far inland (or high up, for that matter) potentially see a few hours of moderate to heavy snow.

I will reflect this in the forecast, with a sharp gradient of little snow to decent accumulations from the mountains in VA up through DC/Balt and central NJ including Philly, up right through NYC and then extreme southeastern New England.

To the north/west of this line, it appears major accumulations are possible. Given the duration (some areas 12+ hours), moisture involved (waters off the coast still generally 60+, with the Gulf stream still well into the 70s) and very strong dynamics which will ring this moisture out very efficiently that the model forecasts for over 1” liquid equivalent will verify fairly well. This yields a potential foot of snow (even with low ratios) where the precipitation stays snow the whole storm.

Will paint the highest accums over the higher elevations of the southern Mid Atlantic (daytime storm, storm not as dynamic as it will be farther north) and through the higher elevations farther north. However, as the storm rapidly deepens and as night falls will gradually paint in moderate accumulations even in the lower elevations. Will use a blend of the models for the western edge of the snows, as the models at this time are relatively similar with the low track.

Final forecast:

Snowfall by city:

Boston: 1″ (coast)-4″ (western sections)

NYC: 1″ (coast)-4″ (western sections)

Philly: 1-3″

Balt: T-2″

DC: T-1″

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