Impressive Storm System to Impact Upper Midwest, Snow over 1ft Possible across Wisconsin
By Hunter Anderson
January 20, 2018 - 10:23 PM CST | 161 0
Good Saturday evening everyone. I hope you all have bought your bread and milk because this upcoming storm system is going to be a doozy. Alright, I don't mean to scaremonger anyone, however snowfall amounts will likely exceed 8" across a region from northeast Nebraska through southern Minnesota and up into Northern Wisconsin and adjacent U.P. of Michigan. The heaviest snow will fall across Wisconsin Monday morning (01/22) into Tuesday morning (01/23.) Below I'll go into more detail on how dynamic this system is and what it'll most likely be doing as it traverses the central plains and Great Lakes regions.
Some weak cold air advection (CAA) is currently sitting over Minnesota and Wisconsin at the 850mb level (~5,000ft) as of 6AM this morning. Please disregard the misprint on the top right of the image and the following 3 SPC upper air analyses with regards to time. It should say "6AM", not "12PM." To be fair, on the synoptic scale things won't change too much within the 6-hour time frame, but it's still a dumb error on my part. Some decent warm air advection (WAA) is located over the central plains but a stable layer of air has inhibited any cloud formation across that region today.
The closed low on the 850mb map over CO-KS-NE will hang around until the 700mb trough (black dashed line) reaches the high plains. This trough is associated with a larger scale trough in the upper levels that will eventually traverse the United States these next few days. The trough as of 6AM this morning is tilted positively which typically isn't a sign of a strengthening system. However as we move forward with time (as you'll see later) the trough axis will shift to a more negative tilt.
The 500mb map shows 2 main areas of jet streaks within one main belt. The stronger jet streak over the northeast U.S./southern Quebec is associated with the upper level trough over eastern Canada and the other jet streak over the southwest/Rockies is associated with the trough that's on the west coast. As we move forward with time these two jets will split further apart and will create separate regions of enhanced upper level divergence and opposing convergence.
The 300mb map has plotted calculated divergence (yellow contours) and a lot of yellow can be noted with the approaching western trough (circled.) This tells us that once this feature reaches the Rockies, the potential will exist for lee cyclogenesis. This developing cyclone will be the weather maker that impacts the upper Midwest and Great Lakes with snow, freezing rain, sleet, and above freezing convection further south.
Surface temperatures this evening across Wisconsin range from the mid 30s near the U.P. border to low 40s near the Iowa and Illinois borders. These above average temps will last for one more day downstream of the future surface cyclone. Some of this (warmer) air will become modified as the low pressure system approaches and traverses the region which will make the rain/snow line location tough to predict. This will also have an effect on the snowfall amounts since some melting aloft will definitely occur.
Lets look at some computer model outputs now... This is the projected wind speeds and geopotential heights at 250mb on Sunday afternoon. Using the thermal wind law, I found jet level to be around 270mb, so we'll be using 250mb plots instead of 300mb. Jet streak dynamics place the area of upper level divergence in the left-exit and right-entrance region of a cyclonic/anticyclonic, or straight jet streak. This is the area where mass is being distributed outward in the upper levels and filling itself in at the surface (convergence.) This region of convergence at the surface is where we expect the low pressure center to be.
The tricky part about this system is that two streams are present at jet level--there's a northern stream over which extends from Minnesota out to eastern Canada, and a southern stream consisting of the confluence between the tropical jet and main polar jet upstream of the trough axis. This coupling of jets over the southern plains is what will ultimately get this party started. I expect the lee cyclone to develop around the Oklahoma panhandle-southwest Kansas area. However, a lot of questions remain as to how this surface cyclone will interact with each stream.
The phasing of these streams is complicated and tough to forecast, however current indications suggest that the upper level low (larger "L") and surface low will be at it's deepest (strongest) point over Iowa before the system occludes entirely. The latest GFS
shows it's nestled in the left-exit region of the cyclonically-curved southern jet. But after looking at calculated upper-level divergence (thank you Wyoming Weather Web), the low center appears to be lagging behind the strongest diverging motions aloft by noon on Monday.
The following images show plotted divergence (greens) and convergence (blues/purples) at jet level over the next 36 hours.
The quasi-coupling of both streams is creating a region of increased divergence and associated strong convergence.
I placed a red line over the region between the strong divergence (rising) and convergence (sinking.) This is approximately where I'd expect the precipitation shield to end. In other words, I don't expect much precipitation north of the red line. This also correlates with the approximate location of the axis of dilatation. This is where the tightest snowfall gradient will be located--so a low pressure track shift of 50 miles south or north could be the difference between 0" or 10" for Superior, WI (for example.) So now that I've touched on some of the upper-level dynamics, I'll now discuss the impressive thermal advection that'll be associated with this storm.
By noon on Monday frontogenesis (strengthening of a front) will be taking place over northeast Iowa, central Wisconsin, and northern lower-Michigan. Some weaker warm air advection is expected to occur across northeast Nebraska as warm air gets wrapped around the system. The contributing frontogenesis will produce heavy precipitation but the issue is that this will be over an area that may not be above 0F by 12PM on Monday still. On the northern fringe of the 850mb warm front I expect some intense snow banding to occur and possibly some embedded thundersnow. Higher omega will reside right along this frontal axis and will enable the collision of suspended frozen hydrometeors. If vertical mixing can occur then a few episodes of thundersnow is possible.
The most favorable thermal advection at 700mb will reside over the U.P. of Michigan and southern Ontario which is offset a little bit from the lower level advection but overall the region of strongest temperature advection will lie across northeast Nebraska, central Wisconsin and eastern U.P. of Michigan (as previously mentioned.)
To add a little more icing on the cake of synoptic scale goodness, lets take a peek at the 500mb vorticity on Monday (right.)
In response to the waves of cyclonic vorticity advection, pressures will be falling at the surface and heights will be falling aloft--another sign that our system would still be deepening. The ample amount of vorticity associated with this cut-off low also tells us that this system won't stay around for long. If the amount of vorticity present was less than what's being depicted, then the mid-latitude cyclone would propagate slower to the northeast. This quicker movement of the trough will keep snow totals less overall. Another tool I'm just beginning to divulge into is potential vorticity (PV.) PV is the product of absolute vorticity and stability, so areas of high stability or high vorticity will have high values of PV. If higher values of PV are present, then (strong) cyclonic flow is assumed to be associated with that feature. The next 4 images are PV model outputs that go out to Wednesday morning (01/24.) Watch how the black streamer pivots it tilt around the northern periphery of the mid-latitude cyclone (red "L") and essentially kicks it out to eastern Canada/northern Maine rather quickly. The red streamer tries to phase with the eastern system overtime and is associated with a weak shortwave over the Midwest downstream of the next ridge which is situated in the west.
Alright, enough about upper-level processes, lets discuss some trends in the placement and timing of the surface low... Wednesday's (01/17) runs had the low pressure center over Eau Claire by noon on Monday. Over the past few days, the track has shifted by approximately 220 miles (depicted by the red line) south-southwest. The low is now expected to be around the Iowa City, IA area by noon on Monday. This will put a hold on some of the precipitation by about 6 hours or more in some locations. This is why we never like to create/publish snowfall amount maps 3 days out from the event since a track change is inevitable. This is a great example of a system seemingly slowing down with future model runs (typically see that with the GFS anyways so not surprised by this.) The ECMWF, another global computer model has also been consistently south with this storms track, however it still remains somewhat of an outlier so decided not to favor its low position much.
The amount of moisture this trough will be pulling up from the south will be borderline record-breaking as well. Precipitable water (PWAT) values will be around .7" mark. If this were to be realized at GRB (Green Bay's National Weather Service sounding site) then that would break the all-time PWAT value for January 22! The Quantitative Precipitation Forecast (QPF) is also showing this heightened amount of moisture available across the upper Midwest/Great Lakes region.
Finally, I wanted to take a look at areas around central Wisconsin and diagnose the atmosphere when precipitation is expected to begin. The issue with central Wisconsin is that it's going to fall right along the rain-snow-mix line initially before transitioning to all snow by Monday evening. Temperatures aloft will be warm enough initially for hydrometeors to melt some as they descend towards the surface, however temperatures at the surface could be below freezing. This would cause freezing rain/drizzle to occur depending on how saturated the profile gets and if collision coalescence is the preferred process or not. The high moisture content and warmer temps will make snow compact easier and ratios around 10:1=>12:1 seem probable.
Here's a forecast sounding for Wausau, WI valid at midnight tonight. This is when I would expect precipitation to begin to fall, give-or-take a hour or two. The dendritic growth zone is saturated (although it's not a deep layer) so ice crystal growth is the preferred method. Some cirrus is noted around 300mb so some seeder-feeder processes could be occurring as well. The freezing line is highlighted in blue, and the temperature profile never fully crosses it but gets really close at 900mb so some minor melting could definitely occur. By 6AM Monday morning Wausau will be positioned well into the colder air so snow is the certain precipitation type.
We expect the heaviest snow to occur across western and northern Wisconsin where totals could approach 1ft or more. On either side of the highest values a tight snowfall gradient will occur so (as mentioned earlier in this post) a location could receive 0" but is located only 20-30miles away from the heaviest swath. Stay updated with our current thoughts on this system and upcoming winter storms on our Facebook page ( https://www.facebook.com/WIWXGROUP/ ) and website here. Justin P will be posting an updated snowfall map sometime in the next day.
Lastly, I'd like to recognize the dedicated workers at all National Weather Services across the nation. Weather never takes a break, even when the idiots at Washington do, as crazy as that sounds. #sarc
I hope you all found this post to be meaningful in some way or another. Thanks for reading through if you made it this far! I'll see you at the next storm. -HA