WINTER STORM SOON TO ARRIVE, WHAT TO EXPECT...
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THE LATEST THINKING
The winter storm we've been tracking for a week is set to begin influencing the are this afternoon. It is complex in that transitions are involved and that makes snowfall forecasts (which are already challenging) even more difficult. Where the heaviest snow is anticipated, winter storm warnings are in effect. Further south down to roughly HWY 34 winter weather advisories are posted.
The official forecast NWS snowfall forecast looks like this.
That may or may not work out due to the uncertainty of precip type. Again, near and south of I-80 rain or perhaps a mix will be the dominant types to start early this afternoon. This will continue into the evening before a transition to snow mid to late evening in all but the far south.
North of I-80 to just past HWY 30, freezing rain, perhaps mixed with sleet, rain, or even a bit of snow will kick in mid-afternoon. In the early evening this will turn to snow as colder air aloft works its way from north to south.
North of HWY 30, probably closer to HWY 20 snow should be the dominant form of precipitation but even here a mix with freezing rain or sleet at times is possible until nightfall when it stays all snow.
I have all the models in now and I will show you what the raw data is indicating and then make a summary of what to expect in terms of snowfall.
The 12K NAM
The 3k NAM
Just looking at all the new models there seems to be good consistency that most areas north of a line from Marshalltown to Iowa City, Muscatine and on to near Galesburg should see 4 or more inches. There is a tendency on the new HRRR for a heavier band to set up from about Cedar Rapids and Iowa City east along I-80 to the Quad Cities and points east. Here, some enhanced forcing may generate amounts greater than 6 inches, especially closer to the Quad Cities and on to Princeton where amounts of up to 8, even 9 inches are shown. That data just came in and this heavier trend is not reflected in other models but needs to be monitored. The HRRR is usually in the ball park and it's served me well over the years. Not always, but much of the time in short term trends! Here it is.
Really, the big caveat is the transition from mixed to snow. If the timing holds my thinking should be close. If its sooner or later that could add or subtract an inch maybe 2. That's impossible to call at this point and we must rely on the accuracy of models until we see the change taking place. We'll just have to watch it.
Of course there is the issue with freezing rain especially in my central counties (particularly in Iowa). Amounts are not expected to by big (maybe 1/10" of an inch but it does not take much of that to cause dangerous conditions so be aware north of I-80 that's on the table until early evening.
That where we stand as precipitation gets set to move in. Stand by as the party is about to begin. Roll weather...TS