March Comes in like a Lion

March 1st marks the beginning of meterological Spring, though obviously the atmosphere is in full winter mode, moreso than it has all season long for some areas, in fact! For snowlovers in the eastern U.S, this has been quite a storm. While in some spots it was slow to really get cranking last night, many, especially in Virginia up to the D.C. / Baltimore areas, woke up this morning to more snow than they probably would have expected upon going to bed. But if you were tracking the upper level features of the storm, this should have been expected. I have a map of the 700 mb level (about 10,000 feet above sea level) below (first map), and have drawn in a red "L" to show where the upper low was located at this level with the storm this morning. The black arrow shows the approximate direction the low was tracking. The green shading shows the moisture content (specifically the relative humidity at that level). Notice the moisture along and the west of the track. It is in this zone where "surprises" can occur given a strong enough system. A common rule of thumb is that you want to be near or just west of the 700 mb low track, which worked well with this storm given how intense it was.


The second map below map is at the 500 mb level (approximately 18,000 feet above sea level). The colors here show vorticity, which is a measure of spin in the atmosphere. Ahead of strong areas of vorticity, called "positive vorticity advection", you have stronger rising motion. Behind them you have sinking motion, or "negative vorticity advection". With sufficient moisture in place, the lift from an incoming vorticity maximum (vort max for short) causes clouds and precipitation. I have circled the main vort max with this storm. The track of this feature was similar to that of the 700 mb low shown in the first map. So if you look at this map, you see the areas of greatest moisture content, which coupled with the strong vort max, led to bands of very heavy snowfall just to the west of the track of these features today. These bands typically can produce several inches of snow in one area, and very little not too far away, which is what I tried to show with the sharp cutoff in snowfall on the forecast map.

So hopefully you had a chance to not only enjoy the storm, but learn from it as well. The idea here is that even if a storm like this is a slow starter, don't ever give up on it if you are just west of the forecast track of the 700 mb low with a strong upper level vort max in play. You can often get a pleasant surprise in such a setup. Once the storm is totally done, I'll look at verifications versus the forecast issued yesterday.