Friday, May 24, 2013

Memorial Day Morning - Upper Midwest

A meteorological setting conducive to MCSs and locally heavy rains is forecast to develop over the upper Midwest during the next three days. Since it is a Holiday weekend, I thought that this situation was worth a closer look. The graphic above is the GFS forecast for 500 mb valid 12 UTC on Memorial Day morning. The central US is basically beneath a large ridge; however, several weak vorticity maxima are forecast to be embedded within the heart of the ridge. Such a setting is often associated with intense thunderstorms, heavy rains, and nocturnal MCSs.

The forecast of 12-hour accumulated rainfall valid at 12 UTC Memorial Day, from the GFS, is shown above. The GFS forecasts significant, overnight rainfall from north of St. Louis to Des Moines to Omaha and northward., as well as a secondary maximum over northeast Kansas. The NAM model forecast for the same  valid time and period is shown below. This higher-resolution model forecasts a large area of heavy rains over southern Wisconsin and a second maximum south of Omaha. Both models indicate a substantial area having the potential for locally heavy rainfalls, but with substantially different geographical foci. For example Madison, Wisconsin, has no rainfall in one model forecast but is at the core of the heavy action in the other forecast. The area with potential for heavy rains and nocturnal MCSs lies within a region of both substantial, low-level warm advection and high moisture content.

I looked at the forecasts and forecast discussions that cover Sunday night for the NWS Offices at Madison, Wisconsin, Des Moines, Iowa, and Omaha, Nebraska. The discussion from Madison covered the developing situation well and mentioned various possibilities for Memorial Day. The discussion from Des Moines was excellent and included a briefing highlighting the possibility of serious storms over the weekend. One of the slides from that briefing is shown below.

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