As well stated by my friend Rajesh Sankaran “While I have you here, what is up with the weather? Why is it behaving like an angry confused teenager going through puberty here in Chicago/Midwest? This hot and cold is not fun.”. First, it’s not due to COVID19. Yes, as I said on WTTW Chicago Tonight, there are clear impacts on the atmosphere and our ability to predict it however, the variability that saw us go from shorts weather to snow is a normal seasonal phenomena. In previous posts I have compared the

upper level winds to “railway tracks” these winds are referred to at the Jet streams or jet stream pattern. At the moment these patterns are very buckled sending storm systems bearing tornadoes and hail into Alabama and southern states while pulling arctic air down over Chicago. If this were mid winter it would be a Chibera, however because things have been warming we are seeing snow and just above freezing at the surface… What is crazy about these snow systems is we are seeing snow at up to +5 degrees celsius! This is because the temperature is cooling so rapidly with height that snowflakes are not melting on their trip from the freezing level to the surface… Silver lining? well hopefully the snow is good for stay at home orders and social distancing.. meanwhile I am getting back into baking to make the house warm…

This winter has been a rollercoaster! I was riding outside yesterday with temperatures in double digit celsius (55F). Well this was the peak of the ride and now we are about to come crashing down. Whenever you have unseasonal warmth there has to be a gradient in temperature somewhere and nature likes to destroy those gradients with nice juicy weather systems. The story for this system is all in the middle of the atmosphere.

The above figure shows the vorticity of the winds. Or how much they are spinning. There is a whole heap of math I could lay on you but simply put vorticity helps air go up. And rising air (also called unstable air) allows clouds to form as it cools as it rises becoming saturated. So the other thing we need is moisture.

The figure above shows temperature (red) and dewpoint (green, a measure of moisture content) throughout the atmosphere and is referred to as a Skew T Log P diagram. To the right of the temperature trace you can see the wind barbs. The direction is from the “flags” to the tip. In this case we can see winds from the North East up to about 3km. These are coming right off the lake! The lake is running very ice free at the moment so those cold winds will be fully saturated as they move over the water. And with the aforementioned vorticity forcing the air to rise we should see very effective “wringing out” of the air like a sponge! Now, there is one more ingredient. To get real good snow you need there to be plentiful saturated air in the dendritic growth zone (see previous post).

A second forecast sounding (from the “North American Model, NAM“) , shown above, is truly remarkable. For one the vorticity is forcing ascent so effectively you have a rapid cooling of temperatures with height. This is very unstable air. Second, you have a deep slab (~3km) of saturated air between -10c and -15c. If this verifies this will be a snow making machine!

The only real part spoiler will be if the models (simulations) are being initialized with bad data. This is entirely possible (and I think the NWS in Chicago are thinking this given their forecast) and the surface layer of air is way warmer that the model analyses (simulation time step zero). Prediction: If it pans out the way the NAM/GFS are predicting we will see rain turn to light snow Tuesday morning around 4am. Snow will remain light as the vorticity maximum remains to our west until around 3pm when snow will begin to intensify (if it is not rain). Overnight the party really starts with the most intense snowfall rates right in time for Wednesday morning’s commute. Again, big uncertainty is rain versus snow. My bet is for that cold air to replace the warmer airmass we have over us pretty quickly. Despite the warm weather our closest soil temperature measurement in St Charles shows 4in temps of 33F.  So we will not see much heat coming from the soil into the air. The Chicago region could see from 8 to 14 inches for this event. However this is a complex system and small errors in the location of that high vorticity air could have a big impact. FUN TIMES!

This is basically a dump from our Departmental Slack channel… Nice compact system shaping up for the next 24-48 hours. Normally this would not get my attention but we have been so starved for snow and real winter weather this year I’ll take anything. Very interesting MSLP as shown in Figure 1 from the NAM (12Hr FCST). Juicy intense cold front over the Appalachians with copious moisture being fed in (SPC has a SLGHT chance for Severe.. Worried about flooding) Note the solid block high over central Canada and then a second trough squeezed on in over our region. Second figure shows the 500hPa winds. VERY VERY broad jet dipping down. Chicago is sitting under, kind of,  the left entrance to the NE propagating jet streak. Take a read of this primer on the impact of jets on instability. The left entrance is a region of unstable air and will be enhancing snowfall over our region… Kind of…

Next figure is a forecast sounding taken at the time of maximum snowfall rate, as forecast by NAM, at +12hours, or at around 6pm Chicago time. Green line is the dewpoint or “water content” while red is temperature. Where the red and green lines are close the atmosphere is saturation and clouds can form. Note the temperature axis is skewed (This is called a SkewT Log P chart) so from 1500m to ~4000m the saturated atmosphere is around 10 degrees celsius… This is borderline temperature for nice snow formation (aka the Dendritic growth zone, DGZ). the DGZ is a range of temperatures at which the saturation vapor pressure for air over ice is much less than that for air over water. This allows a process called Wegener–Bergeron–Findeisen where, basically, ice can very effectively suck all the moisture not only from the atmosphere but surrounding drops.. Suffice to say, when you look at a plot like the above and you are looking to forecast snow, look for deep saturated layers at ~-10 to -15c. Also note the steep lapse rate (decrease in temperature with height) from the surface to ~1km… This layer is what we call “Conditionally unstable” which could allow convective clouds to form… This time is right at the change over from mixed precipitation to snow.. So our total accumulation in Chicago will depend on the exact timing.. Expect a NASTY WET SNOW at the peak commute home 🙂

Finally, let’s look at the actual NAM (the 4km “Nest”) forecast above… Hopefully you have gathered from my discussion that model estimates for snow  are very uncertain. But if the model (simulation initialized with atmospheric observations) is to be believed we could be shoveling 4 wet inches of snow.. But, wait… There is a kicker here.

The above animation is from NOAA’s High Resolution Rapid Refresh simulation which is a type of simulation (Model) that we call “Convection permitting”. This means it is high enough resolution to resolve smaller storm systems. But, as it is costly to run it only simulates 18 hours in the future. Note the interesting small features at the end of the predicted radar image. This is a small mesoscale (~100km in size) snow squall.. Behind the squall the winds are turning north east, or off the lake and on to the western shore of lake Michigan. This means we may see some lake enhancement at the end of the event. My take is that we could see 4-7 inches from this event depending on 1) How long we get rain before the snow and 2) If we get some lake enhancement at the end…

The final image is from Michigan State University and NOAA and shows lake temperatures near Chicago. Just above freezing… So there may be a sweet spot for snow accumulations  ~5km inland from the lake edge… it will be touch and go in the city to begin with.

Weather plots courtesy of the amazing COD NEXLAB site!

What a difference a few months makes. Back in January I was visiting Utqiagvik as a guest of Argonne’s Education and Outreach team. When we arrived it was -20F and lets just assume the dewpoint was -20F. Today I went for a run in Nanjing, China with a (5am) temperature of 70F and a dewpoint of 65F. In China each kg (roughly a cubic meter has 13.37 grams of water in the form of vapor. In Utqiagvik it was a mere 0.35g. From the dry to the dripping wet!