COMPLEX WINTER STORM HEADS FOR THE MIDWEST...
After a quiet couple of weeks the weather around the Midwest is expected to take a turn for the worse later Thursday and Friday as a strong winter storm takes shape. Most data indicates the worst of the system will remain just northwest of my area but we won't get off the hook entirely. Winter Weather Advisories have been posted for the area along and north of HWY 30 for the late morning and afternoon hours Thursday. A mix of freezing rain and snow here will change to snow. Up to an inch of snow accumulation is possible. Further south amounts will be less and relatively mild surface conditions with above freezing temperatures should mitigate the impacts of any snow or ice. Even in the north marginal temperatures could minimize travel impacts. This will need to be monitored as conditions unfold.
This is just the first phase of a slow moving storm that will impact the region with additional light snow or drizzle later Thursday night and from time to time into Friday. This second wave of the storm is going to be near or directly over my area and that should keep the worst of the snow focused on central Iowa, SE Minnesota, and western Wisconsin. That is where the trowel or moisture feed sets up around the stacked upper low perched over eastern Iowa. That's why the Winter Storm Watches are posted off to the NW.
In that area the snow will be accompanied by winds of 30-40 mph creating a potent storm with 5-10" totals and blowing and drifting. The Winter Storm Severity Index shows the worst conditions (moderate impacts) in orange on the map below.
These are the odds of at least an inch of snow as posted by the Weather Prediction Center. You can see how the position of the storm favors my western counties for higher snow potential than those east of the Mississippi. The best odds though are currently shown out of my area.
The graphic below from the NWS in Des Moines shows a dramatic increase in potential snow amounts NW of a line from Dubuque to Cedar Rapids and on to Ottumwa.
Once again, the issue with the storm is that instead of moving west to east across the region carrying the snow with it, the storm occludes and turns nearly stationary directly over eastern Iowa. That injects a dry slot into much of my area while just north and west vorticity and snow spins around the system as it slowly fills while moving little. In fact, there is a period where the system actually backs up and moves west for a time which is fairly unusual. The bottom line is my far northern and western counties stand the best chance of impactful snowfall (1-4") while the rest of my area, escapes with minimal snow accumulations of an inch or so assuming there is no major change in the track.
Regarding the track, the 0Z model runs have shown a tendency to dig the system a bit further southwest. The EURO is now quite bullish with significant snows Thursday night and Friday of 4-7" for much of my area. That is a new trend and I am quite suspicious of it but its more southerly track would make it possible. Were it not the EURO I would dismiss the idea but as the best model on the planet (24 hours before the event) it deserves some respect and I will be anxious to see if the trend holds in coming runs. A similar solution was shown on the 3k NAM. However, the GFS and several other models favor the dry slot and lower amounts (and so do I). I would have hoped at this late juncture to see better consistency amongst models but unfortunately that is not the case as of early Thursday. Confidence in snowfall totals Thursday night and Friday is currently low to moderate, especially over the NW half of my area.
Here are the latest available snow depictions as of late Wednesday night. I will say it again, these are not forecasts, just individual model guidance that forecasts are made from. What we want to see are consistent signals among models regarding amounts and placement to construct a reliable forecast. That said, here's the raw model output. I will start with the EURO and 3K NAM which are the most aggressive snow producers locally, but in my opinion overdone and unlikely at the time of this posting. This is a tough one.
The 3K NAM
The GFS, 12K NAM, and GEM are all in pretty good agreement and I favor their solutions. Here they are.
The Canadian GEM
The NAM 12K
Last but not least, my new book Derecho 911, Iowa's Inland Hurricane completely sold out. However, due to continued demand I have authorized a limited second printing of 250 books. They are now on sale but it will be at least a month before they are available. If you are interested in obtaining a copy of this historic storm you can get yours at derechobook.com. Roll weather...TS