Saturday, October 26, 2013

Halloween Potential Major Storm System

There is the potential for a major storm system in the Plains and Great Lakes during the Halloween timeframe.

There are currently two different model projections, so we'll start with the ECMWF model. The sea level pressure forecast for the morning of Halloween shows deep pressures across northern Minnesota and Wisconsin, with a tight pressure gradient across the eastern Midwest and Great Lakes. This tight gradient is suggestive of a high wind field in those regions, and the ECMWF confirms this with 5000' wind speeds peaking just over 50 knots in Michigan, Wisconsin, Illinois and Missouri on Halloween. This translates to some awfully gusty winds for the aforementioned states, and windy conditions across the north central states in general. As far as precipitation, the ECMWF hints at non-accumulating snowfall across the upper Plains, and overall precipitation isn't horribly more impressive. This would be more of a big wind event rather than a big winter storm.

The GFS model keeps this storm system further south, with the lowest pressure centered over southern Wisconsin and northwestern Illinois/northeast Iowa. The GFS actually follows the ECMWF with low to practically non-existent snowfall over the upper Plains, but differs from the model with a rather high amount of precipitation falling across the southern Plains and parts of the Midwest.

I'm thinking that a compromise between the two models may be appropriate, with wind being a pretty good portion of the storm, but some wet conditions also arising in the Midwest and southern Plains. With the Pacific North American (PNA) index rising to a weak-moderate positive during this storm, but the North Atlantic Oscillation (NAO) in a strong positive phase, expect the storm to go through the upper Midwest.