What you need to know for this week’s winter storm

| Staff Writer
A yellow front-loader construction vehicle plows snow from a path in front of a glass-windowed building. The sky is blue behind it and the bare branches of a tree overhang the photo with a lamppost to the right of the yellow vehicle.

A vehicle clears snow from the Washington University campus last winter. (Photo by Clara Richards / Student Life)

Over the next three days, from midday Tuesday through Thursday, the greater St. Louis metro area will be under siege by a vicious storm system that will dump significant snow, freezing rain and frigid air over a long duration of time. This article will focus on walking you through the setup and “ingredients” of the storm, what we know and what is still uncertain, two possible storm scenarios and the precipitation amounts and timing (including the snowfall forecast).

Data has been adapted from The Weather Channel, Accuweather and the National Weather Service. All forecast models reference can be found on Tropical Tidbits and Weather US.

The setup

The northeast snowstorm this past weekend shifted the jet stream trough into the eastern U.S., allowing for a ridge to form in our region. The ridge is pumping warm, moist air from the Gulf of Mexico upwards into the midwest. This will produce above average temperatures Monday and Tuesday. However, to our north, a potent cold front will begin charging southward.

The contrast of warm and cold air will produce precipitation. While the cold front is still north of us, temperatures will be sufficiently warm for rain for the majority of Tuesday. The high will be near 50. The rain will be moderate and possibly heavy at times. Do not let the occurrence of these warm temperatures and rain dissuade you about the upcoming snowstorm. As the front dives south and increasing cold air is thrown into the storm, precipitation will change to freezing rain, then to sleet which will likely linger through much of Wednesday, and then finally to snow. The exact timing is uncertain, but two scenarios are sketched later in this article. By Wednesday, temperatures should be around 30 and not rise further. The front will stall just to our south, and therefore, the storm associated with this frontal system will remain in place through much of Wednesday.

Simultaneously, a secondary storm system will form off of the Pacific Ocean to our southwest. The stalled frontal system will act as “train tracks,” on which the secondary storm will ride. That leg of the storm would arrive Wednesday overnight into Thursday, and has the potential to bring our heaviest snow. There is the largest uncertainty with this part of the storm.

The secondary storm will usher in the coldest air of the season. Essentially, the storm opens the path for arctic air to pour in from our north and west. As the snow comes to an end on Thursday likely around midday, cold dry air and a brisk wind will fill its place. Thursday and Friday’s highs will both be in the low 20s, and the lows in the single digits.

What we know

Let’s first go through what there is little doubt you can expect. Firstly, precipitation will start as rain Tuesday. At some point, precipitation will change to freezing rain and sleet and then snow as the cold front dives in. We are also highly confident that both parts of this storm will form. Lastly, there is little doubt very cold air will plunge on Wednesday and remain through Friday.

Cars in the snowy right lane of a road wait for a red light as the oncoming lights of another car are seen across the intersection. Green streetlights can be seen in the distance.

A blizzard envelops the intersection of Forest Park Parkway and Big Bend Boulevard in this file photo. (Photo by Matt Mitgang / Student Life)

The uncertainty

There are plenty of uncertainties involved with that storm that make forecasting the amount, type, and timing of precipitation exceedingly difficult (although the snowfall amounts are decently predictable, for reasons explained later in this article).

Firstly, there is little consensus on how long into the storm each precipitation change will occur. While cold air will dive southward, it is not yet clear how quickly this will happen. Lingering warm, moist air from the Gulf of Mexico will be stubborn to leave. If the cold air runs atop the warmer air but does not push it out, there will be a longer period of freezing rain and sleet, and therefore less snow. There is a small and decreasing possibility the warmth will linger too long for any snow to fall in this part of the storm. If the cold front overtakes the warm air quickly, precipitation will change to snow earlier, which will lead to greater snow accumulation.

Secondly, there is no unanimity regarding the exact final placement of the frontal system, and therefore the track of the secondary storm. Since the storm will ride along the cold front, its track will basically be where the cold front stalls. A more powerful front will lead to a further southward push, which in turn would make the storm track further south. This could leave a majority of the precipitation, including the snow, to our south. However, a weaker front will allow the secondary low pressure system to take a more northward track, bringing a swath of potentially heavy snow on Thursday.This uncertainty basically leaves us with two “sample” scenarios. I will give a walkthrough of what the storm could look like if each scenario was to occur.

Scenarios

The first scenario would be that the cold front is strong, and pushes significantly south of our region before stalling. In this situation, rain Tuesday would change to sleet and freezing rain overnight and to all snow by Wednesday afternoon. There would be a long period of light to moderate snow that then pulls out overnight Wednesday and leaves us with a cold, dry, blustery Thursday. While flurries are possible, the majority of the second storm would stay to our south. This scenario would leave us with slightly colder air behind the storm than the next scenario. Note this scenario is slightly less likely to occur than scenario 2, below.

The second and more likely scenario would be that the cold front is not as strong and the Gulf moisture dominates. The front in this case would stall roughly over our region. After rain Tuesday and Tuesday night, Wednesday would bring more sleet than snow, with the chance the changeover to snow does not occur until Wednesday evening or even overnight. However, the secondary storm would track us closely and tap into the now entrenched cold air to produce moderate to heavy snow on Thursday. The storm would then depart and leave behind a trail of cold air, but not as potent as the first scenario. Note this scenario is slightly more likely to occur than scenario 1.

Precipitation amounts and timing

Each scenario and nearly everywhere in between has us getting a mixed bag of rain, freezing rain, sleet and snow. Since both scenarios and the possibilities in between will allow for a period of snow, making the accumulation predictions less difficult than the uncertainty may suggest.

Here is a general outlook of how much precipitation to expect:

  • Rain: 0.25-0.5 inches
  • Freezing rain: 0.05-0.1 inches (note that this can be enough to slick surfaces significantly) 
  • Sleet: 0.25-0.5 inches
  • Snow: 6-10 inches (equivalent to 0.6-1 inch of precipitation)

Here are possible schedules for the precipitation in accordance with each scenario. Note that darker shades of a given color indicate heavier precipitation. Rain is denoted in green, freezing rain in pink, sleet in purple, and snow in blue. Times are given in 4-hour increments from 4 p.m. Tuesday to 8 p.m. Thursday.

A three-columned chart shows four-hour time chunks from Tuesday through Thursday, with the second column showing 'Scenario 1' and the third column showing 'Scenario 2' with various types of precipitation listed in rows below, with background colors of boxes corresponding to certain types of precipitation (green as rain, pink as ice, blue as snow).

Josh Warner’s two potential scenarios for different types of precipitation during four-hour chunks of the storm.

As you can see, both scenarios entail a significant period of moderate to possibly heavy snow. While the first scenario would give us a longer duration of snow, the second scenario would bring snowfall with higher intensity. These balance each other out enough that my snowfall forecast will be the same for each; 6-10 inches of snow. My confidence in the snow total falling in this range is about 80%; meaning there is about a 10% chance we get less than 6 inches of snow, and about a 10% chance we get more than 10.

If we get over 8 inches of snow, this will be the largest snowstorm in St. Louis in 8 years (St. Louis Yearly Snow Record). Snow will be hand-measured by me and the total will be reported in my Studlife weather blurb on Friday regarding the weekend’s weather.

Stay tuned to @washuweather on instagram for key real time updates and possible livesteams. 

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