THE ICEMAKER COMETH
Just off the west coast, far, far away in a state called California, an upper air disturbance, (a consolidation of energy) exists with an eye on the Midwest. Its arrival Saturday will bring the first widespread chance of significant precipitation in more than 3 weeks. In fact, if nothing falls through midnight Friday in Cedar Rapids (a near certainty), that will make it 25 consecutive days with no measurable precipitation there.
Other areas in the south have seen some minor amounts, but even there, totals have generally been under 1/10th of an inch during that period. One such "light" event occurred Thursday, and it brought the season's first inch or more of snow to my counties in far SE Iowa and WC Illinois. (I’m still waiting for my first 1-incher up here in Dubuque)! With the cold air in place, snow ratios were quite high (20:1 or more), producing a powdery, fluffy Christmas type snow even though moisture content in most areas was no more than .05-.09 inches.
The 1-2" band, which generally occurred in a 20-30 mile wide swath, intensified even further as it progressed into Illinois with 2.5 to 3.5" from Peoria to Champagne, and on to Danville.
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THE ICE MAKER COMETH.
The next system, hot on the heels of Thursday's, looks to be a complex one involving multiple precipitation types. The GOES Satellite image, taken Thursday night, shows moisture increasing ahead of the system sweeping towards the central Rockies and Plains.
Before we get any deeper into the dynamics, take a look at the lows found around my area Thursday morning. The first zero degree temperatures of the winter were observed in Waterloo and Dubuque.
Cold air of that magnitude is dense, and will be somewhat tough to scour out at the surface. Even though temperatures a mile up at 850mb are progged to rise several degrees above freezing by Saturday evening, surface temperatures will lag by several hours, even longer in the north. When it's warmer aloft than at the surface, that creates an atmospheric condition known as an inversion, a classic set-up for freezing rain.
So essentially, warm, moist air will be drawn northward ahead of the advancing storm. It's forced to ride over the colder dense are at the surface. The upglide eventually results in liquid precipitation aloft, where it's warmer. However, at ground level where temperatures are at or below 32, the rain freezes on contact and travel gets nasty. The question in this case is how long does it take for surface readings to rise above freezing to end any freezing rain and convert it to rain? The transitional process should occur fairly quickly after the onset of precipitation in the south, but in the north, especially closer to HWY 20, the process could take 5 or 6 hours, allowing ice accretion of up to .25 inch. It's also possible in the far north that some snow could precede the freezing rain, although little if any accumulation is expected initially. Where things could get a bit more interesting is that some of the hi-resolution CAMS are showing a small punch of cold air entering the back end of the system later Saturday evening. That could allow any rain or mixed precipitation to change back over to snow north of HWY 30, especially closer to HWY 20 before it ends. Needless to say, this is a true mystery to untangle, and a degree or two could radically change a forecast that much thought has been put into.
One thing is for sure, models are slowing the system down and in most areas mixed precipitation may not even start until Saturday morning in the southwest and the afternoon in the northeast. Additionally, precipitation amounts continue to lower, especially east of the Mississippi. Models are also settling in on the track of a weak surface low through SE Iowa. Such a path would clearly favor the north as opposed to the south for most of the wintry weather issues due to colder thermal profiles.
For this event, the solution that fits my thinking most is tied to the hi-res 3k NAM and 10k GEM. At least for now, I'm leaning toward their ideas. Here's what the 3k shows for total precip.
Freezing rain on the 3k NAM looks like this. I have concerns that it is too high on ice accretions in SE Iowa and WC Illinois. Furthermore, I also think it's ice accumulations could be a bit too high in the I-380 corridor from Waterloo to Iowa City.
I prefer the GFS solution, which is noticeably lower on ice accretion. For what it's worth, the GFS shows this for icing, which is more reasonable but perhaps a bit far east of the Mississippi.
As for snow, this part of the forecast is very low confidence and based on recent trends, that could still change later Friday. The 3k NAM has this for snow.
The EURO ensembles show this.
The GFS indicates this for amounts.
Last but not least, the 10K GEM has this.
Because of the potential for travel issues due to mixed precipitation, including ice and perhaps some snow, the NWS has issued a broad winter weather advisory from Midnight Friday night until noon Saturday for most of my area. A small part of EC Iowa centered on Cedar Rapids, Iowa City, and points west was upgraded to an ice storm warning. Based on the slower movement of the system, I suspect these could be adjusted for timing by daybreak, and some counties could be added and some dropped. That's out of my jurisdiction.
After whatever precipitation ends Saturday night, dry conditions are expected to close out the weekend. Highs eventually should reach 33 north to 38 south on Saturday, and readings should remain fairly steady Saturday night. Sunday should be a mild day, with highs potentially reaching 40-45, assuming there is not much in the way of snow cover in the NW.
Hopefully, the evolution of this icing event will become somewhat clearer later Friday. However, if you plan to travel in the region this weekend, especially Saturday, pay attention to the latest forecasts. Meantime, I wait for additional data. Happy Friday and roll weather...TS
