|Fig 1 -- NWS watches and warnings as of 327Z, Dec. 11, 2010. From the NWS website.|
Well, here's the actual surface analysis from 21Z this afternoon--the time we were looking at in the ensemble forecasts in my last blog post:
|Fig 2 -- Surface analysis from 2100Z, Dec. 11, 2010. From the HOOT website.|
A quick look at the 700mb map from 00Z tonight--
|Fig 3 -- 700mb analysis from 00Z, Dec. 12, 2010. From the HOOT website.|
- The upper air pattern (and, it seems, the surface pattern as well) is moving more slowly than the models had been indicating. This means adverse conditions may linger longer over areas that didn't expect to see them for so long.
- The upper air pattern seems to be lagging more than the surface pattern. Remember from my previous posts how a stacked low--one where the upper-air trough is directly over the surface low--represents a cyclone that is no longer strengthening. When there is a separation, particularly a westward tilt in the low with height, then the cyclone is still continuing to strengthen. In our case here, the more the 700 mb and the rest of the upper air pattern are lagging behind the surface low, the deeper and stronger the surface low will get. Rapid cyclogenesis (strengthening of the surface low) can mean very, very strong winds and lots of vertical motion--two ingredients that can make a bad snowstorm even worse.
So how much snow has fallen so far? It's difficult to tell in the middle of the event. The Iowa Environmental Mesonet has compiled a map of snow reports so far in that state:
|Fig 4 -- Objective analysis of snowfall amounts from local storm reports as of 320Z, Dec. 12, 2010. From the Iowa Environmental Mesonet.|
|Fig 5 -- Regional base reflectivity radar mosaic from 0348Z, Dec. 12, 2010. From the NWS website.|