WINTER STORM UPDATE, A FINE LINE TO CUT BRINGS LAST MINUTE CHANGES...
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A FINE LINE TO CUT...
A couple days ago I would not have expected todays storm to have struggles with precipitation type, particularly in eastern Iowa with a track through central Missouri and 5-12" of snow on the ground. However, as the past 24 hours have unfolded a shallow layer of warm air at 5,000 feet has fought its way into my SW counties and that makes for a challenging snowfall forecast with rain changing to snow and lower snow ratios chipping into accumulations there.
In this animation, you can see how the freezing line necessary for snow, (0 on the graphic) gets north of I-80 at the onset of precip. Wednesday afternoon. That allows it in those areas to start as rain, freezing rain, or perhaps even a mix of snow and sleet thrown in. Eventually, the entire column cools in the evening, especially when evaporative cooling gets involved changing the mix to snow. By then, potentially a significant amount of what could have been snow has been lost. The question is how much and precisely where? Without a doubt it's my southern counties, near and south of HWY 34 that get little snow but a hefty rain. There are places in the south where over an inch of liquid precipitation falls but only an inch comes down as snow. Had that been all snow at the standard 10:1 ratio it would equate to a solid 10"+ snowfall. What a shame to waste what could have easily been a big snow over a couple of degrees. The point is, the mild wedge has chipped away on potential snow totals in the central and southern parts of my region.
The other issue I'm seeing late Friday night is models across the board have backed off on what we call QPF (the quantitative precipitation forecast), especially north of I-80. In other words the amount of measurable liquid precipitation had decreased, in some cases by 3/10ths of an inch. I would not have anticipated such a sharp drop less than 24 hours before the event begins. For example the QPF in Cedar Rapids went from .94" to .60" in two runs. Here's the latest.
That has resulted in a snowfall forecast on the EURO that now looks like this. It's still a decent snow but even in western Illinois amounts have gone down. Now the general range is 1-6 inches across the region with the heavier amounts 4-6" near and north of a line from Marshalltown to Muscatine and southeast to near Galesburg. South of there an inch or so of snow may fall south of HWY 34. Even where the heaviest snow occurs it may only meet winter storm warning criteria in a a handful of counties.
This is what the HRRR shows for snowfall. It's a bit higher, especially closer to the Mississippi and points east.
The official NWS snowfall forecast from the NWS looks like this.
Here are the winter storm warnings and advisories in effect as I make the post. An important change is that counties near and SE of I-80 including places like Iowa City, the Quad Cities, Muscatine, Geneseo, Kewanee, and points south the winter storm warnings that previously were in effect have been dropped and replaced with winter weather advisories due to the aforementioned issues.
Needless to say, there is still a fair amount of uncertainty for a storm that is knocking on the door. Much of that is due to the uncertainty regarding thermal profiles and transitions. Additional changes are still possible with higher or lower snow totals as models struggle to get a grip on this issue. I think the prudent approach at this time is to note that we still have a disruptive snow event on the way Saturday night north of I-80 but one that looks more manageable than 24 hours ago. I will be monitoring the morning data and will have fresh thoughts on where things stand around mid-day shortly before the event begins. Nothing like a few last minute curveballs. Time to regroup. Until then, Roll weather...TS