Three to six inches of rain fell in an area just two miles by two miles in Duluth late Monday evening. If you know your Duluth topography, it’s quite hilly with creeks running down them. That kind of water in just an hour or so really created rush of water.
So what the heck happened? It appears we had the perfect confluence of events. Think of lake effect snow meets thunderstorm… sort of.
First let’s take a look at radar leading up to the event. We had a cluster of showers and thunderstorms moving east toward the Duluth area:
Note, at the same time, there’s a nearly stationary couple of showers sitting near Duluth in the western tip of the St. Louis Bay. They then merge to produce the torrential downpour on top of Duluth’s hillside area.
What’s cool about the Duluth/North Shore areas is you often have collisions of what we call synoptic meteorological events (larger scale patterns) combined with micro climates or mesoscale features (on the order of Lake Superior size).
We’re heading into the time of year where Lake Superior can have an impact on local weather near it. The obvious thing we all think of is lake effect snow, but in Duluth they often get what’s called ‘lake enhancement.’ That means there’s already a weather feature moving through producing snow but it gets a ‘boost’ from the Lake due to a northeast wind moving over the relatively warm waters. True lake effect snow, like in the Upper Peninsula of Michigan, occurs when there’s now solely as a result of the lake, in other words a northwest wind blowing arctic air over 35 degree waters, producing snow inland. This can happen in Duluth, but it’s less frequent because you’d have to have cold air blowing in from the northeast, which doesn’t happen on a clear day very often.
Let’s take a look at he surface water temperatures of Lake Superior. They’re pretty warm when you compare it to the air mass moving in.
The latest satellite estimates are between 60 and 65 degrees from late Monday. At the same time, we had air that was in the 40s moving over the lake Monday evening.
It’s even more dramatic if we look at conditions just about 1,500 feet above the surface, which can give us a better idea of the instability being created.
Keep in mind the above temperatures are in degrees Celsius. We can see that we have 39 degree air blowing (hard) across Lake Superior from northeast to southwest toward Duluth, but at the same time the waters of the Big Lake are in the 60s. This generates convection, just like how thunderstorms form, but on a smaller scale.
It’s this set up, helping to produce showers already going on near the shore. We can see this on satellite imagery during lake effect or lake enhanced event as convective streamers blowing across the lake, aligned with the fetch of the wind. One of these streamers was evident still early this morning over the lake blowing into the shore:
It’s worth noting too that for the second half of the summer up to now, Lake Superior water temperatures are running warmer than normal. So, any seasonably cool air mass moving over the lake can carry a little extra energy with it.
So, while an event like this is pretty rare: a thunderstorm colliding with lake effect showers to create a boosted cluster of storms, this overall idea is why snowfall is much higher near Lake Superior, especially along high ridges where it gets an orographic (topography) boost too.
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