Tuesday Evening Update

A Winter Storm Watch is in effect for the following counties in Central Iowa from 12AM Thursday to 12PM Thursday: Story, Marshall, Tama, Hamilton, Wright, Franklin, Cerro Gordo, and Grundy. This watch was issued Tuesday afternoon around 2:30pm or so, but additional updates and tweaks to the watch area are likely in the next 12-24 hours.

As I continue to monitor the evolution of tomorrow night's storm, it is pretty much on track with what most of the model guidance has been indicating as far as orientation and location with the European model modeling this system the best. Still no major changes to the overall synoptic setup that was discussed in an earlier post. Cold air is going to be the overall crutch to this storm, but with dynamic cooling and melting processes, that should cool the column enough to support rain transitioning to snow after midnight Thursday. Low level temperatures will likely hover around the 32-33 degree mark which will likely lead to some melting, but accumulations are still expected due to the potential for very heavy snowfall rates. Based on some of the evening guidance, I've adjusted some of the snowfall contours a bit farther south and east than earlier--primarily due to a slight southeastward shift into the total QPF. With QPF likely to range from less than 0.1" to locally over 1.2" across the central third of the state, SLRs in the 7:1-8:1 range, and temperatures that will likely struggle to reach freezing, I've opted to go in the more conservative range of snow accumulations. 2-4" appears likely in the Des Moines metro north toward Ames, with a sharp increase to 4-6" farther east along I-80, US-30 and HWY 163. Significantly lighter amounts are expected farther west and north away from the precipitation shield.

I still anticipate a localized band of very heavy snow to set up east of Des Moines, likely between Des Moines and Cedar Rapids. This is where a combination of a trough of warm air aloft will enhance frontogenesis leading to bands of heavy snow. Rapid accumulation will occur wherever this band sets up, possibly leading to some zones with exceeding 8".

Temperatures will moderate during the day on Thursday into the upper 30s, and with some solar insolation, this will allow any slush to primarily turn to water on roads.

A second period of light to moderate snow appears very likely between sunset and midnight Thursday as a second disturbance and cold front swing in from the northwest. This time, cold air advection will be stronger and temperatures will drop below freezing, so there will be a better potential for some light accumulation on all surfaces that fail to get as well covered from the morning round.