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Major Storm to Clock St. Louis Region with Heavy, Wet Snow

| WashU Weather Reporter

Background

The string of busted snow chances this winter will likely finally come to an end Tuesday night into Wednesday. This article will help explain the setup and “ingredients” for the storm, known aspects and unusually large uncertainties remaining in this forecast, timing, snowfall amounts, potential impacts, and what lays ahead after this storm.

The Setup

The first half of January brought unusually mild and somewhat dry weather, a combination responsible for very few snow opportunities between December and now. However, the atmospheric pattern driving that weather has slowly been jolted out of place this past week. Despite not being impactful on the surface, the recent storm systems that passed our region ushered in a trough in the jet stream that has dug south of our region.

As many storms do, our developing storm will ride along the jet stream. The jet will guide the storm on a path roughly from southeast Texas up towards Memphis, Tennessee and then onwards towards Detroit, Michigan. Because it will pass close to the Gulf of Mexico, the storm will be able to draw ample Gulf moisture from the south, and allow cold air to rush in relatively unimpeded to the north.

With plenty of cold air and moisture — the two main ingredients for St. Louis snowstorms — to work with, this moderately-fast moving storm will have the capability to dump significant snowfall relatively quickly, especially near its track on the northwest side.

What We Know

There is nearly unanimous consensus that a storm will form in the south-central US and take a northeastward trek through the midweek. There is enough cold air for places north and west of the storm to see snow. The storm will not move quickly, so precipitation will fall for an extended time.

There is not a ton of cold air in place, so precipitation will likely start as rain across our region and many others. However, as the storm strengthens and is able to draw more cold air from both the northwest and its circulation center, precipitation is expected to change over to snow across our entire region. Although temperatures may not actually fall below freezing, there will be sufficient cold air in the upper levels of the atmosphere for precipitation to eventually fall as snow (Yes, it can snow at above 32 degrees!).

Furthermore, a narrow band of very heavy snow will set up near the storm’s center on the northwest side. The locations impacted by this band will pick up significant snowfall very quickly. Further to the north and west, accumulating snow is still expected, but the snow will not be nearly as heavy as areas closer to the storm. Conversely, enough warm air will linger to the storm’s south and east to keep precipitation as rain throughout the entire storm across much of the southeastern US.

Uncertainties

There are two major uncertainties that will greatly influence whether we see a minor snow event or major storm. The first is strength, and the second is track.

Strength: As previously discussed, the storm will be working with only marginally cold air, and will have to “pull in” and/or generate much of the cold air by its own strengthening. A stronger storm would be able to pull the cold air from the northwest over our region, and even generate its own cold air. This would allow snow to fall for the vast majority of the precipitation period in our region, leading to higher total accumulations. Conversely, a weaker storm will not be able to incorporate or generate colder air, and would therefore bring a longer period of rain before atmospheric temperatures are sufficiently cold to support snowfall. This would keep snow totals suppressed as our window for accumulation would be shortened significantly.

Track: Equally important as the strength of the storm is where the storm actually goes. As previously discussed, the location of heaviest snowfall will depend on where the center of the storm tracks. For the most snowfall in this region, we would want the storm to make its closest pass through southeast Missouri. If the storm tracks southeast of that benchmark, the heaviest snow will also fall to our southeast, and the lack of intensity of the snowfall will limit accumulation. If the storm tracks northwest of that benchmark, we will have to contend with more warm air, which may lead to a slower changeover from rain to snow, which would also lead to diminished accumulations.

Of course, the strength and track are not independent of each other. For reasons difficult to explain, a stronger storm will correlate with a further north track, and a weaker storm with a more southerly track.

Timing

We are confident that precipitation from this storm will fall from Tuesday night night into Wednesday morning with relative continuity. Precipitation should start as rain Tuesday evening, but once temperatures fall to around 35, likely by around midnight, precipitation will change to snow for the remainder of the storm. The heaviest snow will likely fall on Tuesday overnight.

Snow Accumulations

Currently, the heaviest snow is expected to fall to our southeast. However, roughly 90% of storms end up shifting north and west in the final hours before their arrival. Therefore, I do believe there is a significant chance St. Louis will fall in the narrow region of heaviest snowfall.

Expect accumulations to be between 4-6 inches. I am 60% confident in this prediction. However, as noted, there is a high upside for total accumulations if we get into the heaviest band, a fact that almost no weather outlets are reporting. Therefore, I believe there is a roughly 25% chance we receive between 6-10 inches of snowfall. There is a very small chance (perhaps about 1%) that we exceed 10 inches of snow. Conversely, I feel there is also some chance we get under 4 inches, perhaps around 15%.

Impacts

There will be 2 main impacts from this storm: travel disruptions and minor power outages.

St. Louis is relatively slow at clearing snow, and therefore a plowable accumulation (3+ inches) will likely snarl travel in the region. Expect significant delays on the roads and in the airports. However, since temperatures will remain near or above freezing, the snow should melt relatively quickly.

The heavy, wet nature of the snow may allow it to pile up on power poles, which can lead to sporadic power outages. These outages will largely be concentrated to where the heaviest band of snow sets up. If that happens to fall on St. Louis, we can expect some lights to go out.

Looking Ahead

The storm will likely be the most impactful, but not the last, storm we see in our currently wintry pattern. There will be multiple chances for cold air from our northwest to meet up with Gulf moisture to produce snowstorms as we head into early February. By mid-Feburary, the pattern looks to shift towards more tranquil, mild weather, which means that our best chances for more snowstorms will come over the next few weeks.

Follow @washuweather on Instagram for live, detailed, and accurate updates on this upcoming storm and St. Louis weather more generally.

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