A very dynamic and complicated low pressure system is developing in the Mid Mississippi River Valley, Tennessee River Valley, and Southeast this morning. This storm is already producing periods of snow to the south of Chicago and strong to severe thunderstorms over the Southeast and eastern Gulf Coast.
Here are some important observations I’m picking up this morning.
1. There will be no strong high pressure to the north of this storm. In fact, the high pressure system is weakening and eroding over the Great Lakes. This means that there will be no mechanism to draw low level cold air from Canada or New England. As such, cold air must be drawn from aloft, specifically 850 MB via strong mesoscale lifting (dynamics).
2. The strong to severe thunderstorms over the Southeast tells me the NAM QPF output is going to have a touch time verifying but the ECMWF seems a bit extreme as well. Taking an average of the GFS, ECWMF, and CMC and going for precipitation totals of around 1.00″ to 1.50″.
The convection is an important clue as the moisture transport is limited due to the interference of the moisture pull of the thunderstorms. The NAM doesn’t recognize this influence while the ECMWF over plays the hand on this impact.
3. Boundary layer temperatures are marginal even with relatively clear skies early this morning and light winds. This tells me again that this air mass at the boundary layer (although cold for this time of year), will need significant enhancement from mesoscale dynamics to support accumulating snowfall.
4. The disturbance over the Mississippi River Valley is very impressive. However, there appears to be a clear separation between the moisture transport from the Gulf of Mexico and that disturbance. Even more interesting is the speed of this disturbance. This storm isn’t going to slow down much and will likely truck right along tomorrow leading to this storm being a rather progressive storm with precipitation entering and exiting from west to east at a steady pace.
The forecast remains generally unchanged from the past two days.
The best potential for accumulating snowfall is away from the coast, specifically over eastern Pennsylvania (northwest of Philadelphia metro), northwestern New Jersey, and the Hudson Valley. Most locations can expect to have the low end of these snowfall accumulation forecasts. However, given the potential for frontogenesis or strong mesoscale snow banding, there is a threat for as much as 8″ of snow with this storm. One key point to remember is that snow growth at the mid levels will be hampered by the strong east-northeasterly wind off the Atlantic Ocean and the March sun will have a significant impact as well. Locations over 500 feet in elevation are likely to have a better shot of snow accumulating on pavement.
Further north towards extreme northeastern Pennsylvania through northern/central Connecticut, the snow shield will be more broken and weaker, thus only expecting snowfall amounts of a trace to 2″.
The far more tricky forecast is along the coast. There is a high bust potential here for more snow than forecasted. As I stated before, if this was January or February, I’d likely go 6 to 12 inches of snow here just on 850 MB temperatures alone. However, boundary layer dynamics matter along with the time of year and time of heaviest precipitation, which will be in the afternoon. As such, I think the best potential for snowfall accumulation will be late tonight into tomorrow morning with some very light snow. The closer you are to the coast, the more likely you’ll just have a trace due to influences from the Atlantic and the March sun influences as well. Further inland towards the Delaware River, the boundary layer will be slightly colder allowing for faster snowfall accumulation. However, accumulation on the roadways will still remain difficult. I have the potential of the snow pushing 3″ to account for strong banding seen on the NAM guidance.
In order for more snow to fall on the coastal plain, intense mesoscale banding would be needed and temperatures will have to crash from the upper 30′s into the 30 to 32 degree range, which is currently not forecasted by any guidance, even the NAM or WRF. However, the threat will have to be watched given how dynamic that upper level low is to the west.
I expect snowfall to develop after 1 AM throughout the region but will remain light until 11 AM tomorrow. The heaviest precipitation will fall between 11 AM and 6 PM then taper off from west to east.
This storm will have to be NOW-Casted to watch for any unexpected twists and I will of course have updates through today and tomorrow on this storm.