April Snowstorm Round Two
It is no secret that the Sioux Empire has seen its fair share of snow this April. In fact, after April 18th’s snow, Sioux Falls is now at 5th all time for seasonal snowfall sitting at a mark of 70.4″. That is a lot of snow folks, with 31.4 of those coming in just this month of April. While we all felt the affects of “The Blizzard of 2018″, this storm didn’t have as wide of impacts, with most of the Tri-State seeing the heaviest of those snowfall totals.
Let’s start with the snowfall totals that we have so far. As more amounts are submitted to the National Weather Service. In no particular order:
Orange City 8.5″
Dimock 10.2″ (Highest Total To Come In)
Sioux Falls 6.2″
Spencer, Iowa 7″
Some of those totals are very impressive. As you may know, we, at KDLT Weather, did not forecast for amounts this high. This storm packed a bigger punch than we anticipated, and there are reasons behind this.
First, forecast models started to agree on this being an accumulating snowfall event on Sunday night, April 15th. The heaviest amounts shown by models varied between 7″ and 15” at the time. This can create some confusion with the forecast, but being a few days out from the system, this is normally the case. Credit for all model images goes to Tropical Tidbits.
Even though the locations of the heavy snow were different, this gave us a good idea that we needed to keep a close eye on this system. Getting closer to the event, these same models really scaled back on these totals: Note the change in location of the system as well.
Models many times miss dynamics. They can’t solve the atmosphere, and are therefore not the most accurate. They are a fantastic tool for us meteorologists to use, but we as humans need to know all dynamics and what is going on with current conditions. These model runs were from Tuesday night, April 17th, right before the event. Again, these models are just a tool, and cannot be fully trusted.
Convection within bands of snow is something we look for when trying to predict where the heaviest snow will fall. There are a lot of dynamics that can contribute to this. Let’s start with near the surface.
Notice that bright purple streak. This is what is called a lower level jet. It is essentially an area of concentrated high winds. Even though this isn’t directly over our area, this lower level jet was essential in eliminating dry air at the surface Wednesday morning. Models didn’t pick up on this.
Now lets look at frontogenesis. This term sounds fancy, but it is just the intensification of warm/cold fronts, not at the surface. In the lower, to mid levels of the atmosphere at 850mb and 700mb are two places we look for this. The red shade in both of these images is warm air advection, and the purple lines areas of frontogenesis.
Two things to point out with these images. At 850mb, the strong presence of frontogenesis (purple lines) across N Iowa, this helped intensify precipitation and create heavier bands as this low continued to develop pushing into the Sioux Empire. Then, at 700mb, there is less frontogenesis, but warm air advection (red shaded area) is present across eastern South Dakota. These two combined to create some lift, and higher moisture content, which in turn created higher snowfall rates throughout the morning hours of Wednesday.
The last images to look at to complete the dynamics of this system, are at 500mb. 500mb is a part of the upper levels of the atmosphere, and we will be looking at vorticity (energy) as well as winds.
Show below is that the Tri-State area is experiencing very strong Positive Vorticity Advection, meaning it is not directly over our area, but was being pushed in. You want this if you want stronger snow bands within a system, as the PVA only increases the energy our atmosphere uses, as well as helps create a little bit of lift, creating some more convective snow bands. Notice also the strong flow coming from the SW, that is what helped push that concentration of energy into the Tri-State area Wednesday morning.
Finally, shown below is the angle at which we have this trough. Think of a trough as convex, like below, and a ridge as concave. This trough that we experienced Wednesday morning was tilted negatively, shown by the two drawn lines. This is important because at the axis of that trough we find great energy, as well as a deeper pool of moisture, which is why low pressure systems tend to rapidly strengthen when developing within a negatively tilted trough.
Models don’t always pick up on these things, and sometimes these dynamics can be hard to spot as meteorologists. We certainly do the best that we can! All of these reasons along with a few others combined to create a system that packed some punch! Eliminating the dry air in place at the surface so soon also helped in getting the snow to develop into the Sioux Empire much sooner.
Alex, Blaise, and Joe take a lot of pride in providing you with the most accurate forecast. KDLT Weather can’t thank all of you enough for trusting us as your weather source! With the forecast we have, let’s hope this is our last snowfall of the season.