Jeremy’s Quick Snow ThoughtsJanuary 17th, 2013 at 7:16 pm by Jeremy Wheeler under Weather
Ok. Lots of folks are chiming-in about the 18Z NAM. So I thought I’d try and address it. It does look impressive at face value. I looked at the NAM’s FOUS data (numerical output), and if you look at it then it spits out something like 11 inches of snow with a 10-1 ratio. However, you have to look at the time increments. The NAM has a value at every 6 hours. So the temperature in the lowest layers drops from the upper 30s in one increment to the lower 30s in another. That is during the time of the heaviest precipitation. BUT… the window for that to happen will likely be 2-3…maybe 4 hours. In order to get the maximum snow out of that it would have to snow 2-4 inches per hour, and there would be no mix at all. Well it will likely be 1-2 inches per hour at the most and there will be a mix in that 6 hour window. Plus, there will definitely be melting for at least the beginning of all snow in the region. Richmond is in the low/mid 30s as I write this, but everywhere else is in the upper 30s to low 40s. Richmond is seeing a mix already. The snow will change over sooner between there and Williamsburg over to Gloucester and up to the Northern Neck. So I wouldn’t be surprised if we have to increase the amounts there. Maybe even 6 inches or more near Richmond. We’ll see. But Hampton Roads is still expecting a longer time with rain and mix.
Jeff Edmondson wrote an earlier blog that described the situation pretty well. I have seen heavy snow overcome a warm ground around here several times. But the heavy snow would have to come down right at the beginning for it to really stick and maximize the snowfall. Plus there is a lot of standing water out there right now along with the warm ground. I know the NAM has been looking very ominous (overall) for the past couple of days, but again I just don’t think the 6 hour increments can capture what is going to happen between midnight and 4 am very well. Also, I spoke before about the thicknesses, and that the best ones won’t come through here until towards the end of the precip. However, the temps at 850 mb (millibars) will be cold enough. So the question is: Will the snow be heavy enough and last long enough to drag the cold air down to the surface and also overcome the warm ground? The NAM is promising, but the hi-res version isn’t as impressive, and our Future Trak has also been sticking to its guns with mostly a mix in Hampton Roads. Difficult forecast??? You bet!! But it’s not so much because of the meteorology as it is the hyper-local weather and the climatology.
Here is the latest National Weather Service forecast from their model. It is running fairly close to Don Slater’s forecast tonight.
He also highlighted that a lot of melting will occur in the metro area. At least on the roads.
So I would love to write another 2-3 paragraphs here and do more analysis, but I have to get to bed in a little while. I am going to be in at 4am along with Jeff. I appreciate everyone’s input. There are some very smart people out there. I am actually very curious to see how tomorrow plays out. Have a good night.
Meteorologist: Jeremy Wheeler