More Storm Fine Tuning

Probably “messy” will turn out to be the best way to describe the developing storm. The various computer models are beginning to come around to the idea that warm air(+3 to +6 degrees celsius))will be drawn into the storm at the 850 mb level(several thousand feet above the surface)over the Baltimore area. This is expected to happen late in the day on Tuesday, right when the precipitation rate starts to increase. What was snow at that altitude becomes rain. Temperatures at the surface are expected to remain below freezing and the rain drops that fall into this cold layer will either become sleet or freeze on contact with cold surfaces.

The change from snow to a “wintry mix” will cut into potential snow accumulations and makes it very difficult to come up with a snow depth forecast. A guess at this early stage might be that 4 to 8 inches of snow could fall around the Baltimore area northward to the Maryland line with over a foot of snow to the northwest in Pennsylvania. A lot more rain and sleet will likely be in the mix south and east of Baltimore and snow accumulation potential rapidly drops off. My confidence in these numbers is quite low but I guess it is a starting point.

I have included two charts from the 18z Sunday NAM model run. The charts display data for the pre dawn hours on Wednesday and show the 6 hour precipitation totals/sea level pressure and the 850mb heights/temperatures/winds.

I hope this discussion gives you an inside look at the complex nature of trying to forecast a storm like this. The public responds to snow forecasts in this region with quite a bit of anxiety and storms like this one can be frustrating. Forecasters tend to hold off on giving precise numbers in these cases until confidence levels are reasonably high. We’re not quite there yet. Tom, Neal and Domenica will have a lot more information to work with as the storm matures so stay tuned.

John Collins
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