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TO SNOW OR NOT TO SNOW....


I'm continuing to track the development of a fast moving disturbance that will bring inclement weather to start to weekend. This is challenging forecast because it involves the potential change over of rain to snow for a 2-4 hour period Saturday morning.

As with all systems that have a wintry side there are the usual issues of track and intensity. A slight shift north or south can make the difference between flurries or shoveling 4" of snow. Timing a changeover is even worse. One or two degrees one way or the other can mean no snow or a bunch...or something in between. Usually different models show different timing scenarios and that adds another layer of doubt.

An even bigger headache is when some models show snow and others nothing but rain. This thermal issue is the factor I'm faced with late Friday night and Saturday morning.

I've seen a whole host of solutions Thursday and all I can say is confidence at this point is low to medium at best. For sure some light rain or drizzle will develop Friday night as warm air advection within an inverted trough crosses the area. Overall this precipitation window produces very light amounts and it's all rain.

Late Friday night a deepening surface low will cross northern Missouri in its way to Champaign, Illinois. By daybreak my area is within the left exit region of the upper jet shifting east. This allows another period of lift that is stronger than the first. With some nice frontogenetic forcing and a coupled jet a band of precipitation will blossom out ahead of the 500mb trough. Despite the fact surface temperatures will be a few degrees above freezing, dynamic cooling is shown on several models changing rain over to snow (or at least mixing with snow) for a 1-4 hour period Saturday morning.

The 3k NAM has the coldest thermal profiles showing a sharp crash in thickness levels around daybreak Saturday. The EURO and GFS have trended colder but only in a marginal way. Even if snow develops with fairly warm ground and marginal surface temperatures slightly above 32 degrees, it will be tough to get more than slushy accumulation on elevated surfaces or grass. It is possible a localized narrow band of light to moderate accumulating snow could occur. We may not know if or where that happens right up to the very end. It`s something that needs to be fine tuned in later model runs.

One thing that does seem certain is that the system will be a wind producer. Saturday morning gusts of 25-35 mph look likely, perhaps as high as 40 mph. A wind advisory is a possibility. Throw that into the rain and snow with temperatures in the 30s and you have yourself a lousy day.

Here's the surface low Saturday at 6am near Springfield on the GFS at 993mb

At noon it has moved to NE Indiana and deepened to 986mb!

While the GFS and EURO do show a mix or brief transition to snow, neither model indicates and accumulation. The latest run of the 3k NAM (the model that indicates accumulating snow) showed this for accumulations.

The previous run was further south and showed this.

The previous run to that was further north and involved more of my area.

As you can see there remains inconsistency with the placement of the heavier snow band as well as whether or not there will even be accumulations. This one is up in the air for now. If it does snow, Saturday morning is the time it happens in my area. Stay tuned for the latest on this difficult forecast. Roll weather...TS

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