GFS RUN TO RUN

One thing that is nice to see when fine-tuning the timing of a forecast is run to run consistency of model information. The closer the solutions are to each other on a consistent basis implies that the model has a good handle on the solution. The two maps (12Z and 18Z runs) represent the position of a healthy Low that will pass through the central part of the country this week and although they appear identical, on close inspection, subtleties can be picked out. Notice the precipitation (green) that dips into NE OK and NW AR on both maps (feature shows great consitency). This precipitation is coupled with thickness values less than 540 decameters; one of the "critical thicknesses" looked at to decide where it will rain or snow. Thicknesses are the vertical distance between two isobaric surfaces (pressure levels) and when they are below a particular critical value for a layer of atmosphere, the average temperature within that layer is below freezing. In regards to the precip. in our area, although light, it would be snow! There are many "critical thicknesses" to consider though. Check out the table below.

Critical Thickness Rain/Snow Line
1000-500 mb / 5400m
1000-700 mb / 2840m
1000-850 mb / 1300m
850-700 mb / 1540m
850-500 mb / 4100m
700-500 mb / 2560m

The map below shows the thicknesses for 6:00 AM on Thursday morning. In this case nearly all critical thickness values are just south of parts of NW AR and NE OK. "If", and it's a BIG IF... the present model solutions hold true... we will see some Flurry activity late Wednesday night and into Thursday morning as the Low pulls out. A lot could and will likely change between now and then, but there's nothing wrong with a little wish-casting.

Let it snow, Let it snow, Let it snow!

Posted by Ted Zarras at 11:59 PM