A tale of two outputs

A quick moving, potentially hard hitting system is moving through the Mid-Atlantic this afternoon and evening is proving somewhat enigmatic for the model outputs. Let’s take a look at what I mean by that, and just look at where the NAM and GFS differ so wildly.

Look at the precipitation outputs for the 06z time frame, or about midnight tonight, when the storm will be battering Long Island and Southern New England. The NAM, with robust precipitation totals is on top, the GFS, which is more docile, is on the bottom.

That’s a pretty significant difference in the amount of available moisture for this system, and this lack of clarity is reflected in the confidence forecasts from the NWS on Long Island. Here is the max and min snowfall possibilities for the forecast area.

That’s an 8 inch range in New York City, which is obviously an untenable situation for the Nation’s most populous region. This forecast is nearly 10 hours old, however, so maybe we can hone it a little bit. The best way to start is to see how the models have initialized. Here is the radar for the Ohio Valley, followed by the models in the same order, so you can see how the performance looks to start.

The radar is a bit further advanced than the model period noted above, but the trend is still easily discerned. The NAM is doing better with the voluminous precipitation, however the NAM is a full state too far south with the placement, with the heaviest rain and snow in the Ohio Valley, rather than over Tennessee. If this trend continues, that means that the precipitation as it arrives on the coast will also be further to the north, or as the case may be on the coast, further inland.

One thing that we can see from the observations is that the rain-snow line, the position of the low and the temperature profile are pretty much in line for both the NAM and the GFS. I’ve drawn the rain-snow line on the map below, while the center of low pressure appears to be near Jackson, Kentucky.

This bodes poorly for a “light snow” event, though with so much moisture on the north side of the feature, it seems possible that the accumulations will be blunted by warmer temperatures at the middle layers.

So yes, there will be more snow than the GFS expects, but not as much as the maximum accumulation that the Weather Service fears. I would guess most of the region, excluding the typically warm Montauk and Hamptons will be in the 5-8″ range for accumulation. The big winners will likely be the Berkshires and places in land. Half a foot of snow can sure tie things up, but at least it isn’t 10 inches, and at least it is coming on a holiday weekend.