A Shot of Fall (9/25/17)

Is it fall yet?

Meteorologically, yes. Temperature-wise? Absolutely not. It’s 9:30pm in southern Wisconsin and 74 degrees. It’s been blazing hot the last few days (relatively speaking of course). If you’ve been hunting in this weather kudos to you. I personally went out fishing this weekend with my Fiance and hit some golf balls. We had a great time:

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I have some great news, though, that I’m sure you’ve caught some wind of by now: one of those gifts from the north is on its way to most of the central US. The Midwest and Great Plains will all see at least a 15 degree temperature drop by Wednesday and some places quite a bit more.

Let’s look at some models.

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Here’s the 7am temperatures on Wednesday according to the 1pm run of the NAM model (7pm run wasn’t finished by the time of this post). Now that looks more like fall! I’m pretty excited for this. I went out last week to do some scouting and took the stand/bow with me just in case I decided to hunt. The wind died down as soon as I got out and I got annihilated by mosquitos (big surprise in WI right?). This may be the cold front that finally tames them down for the year. I want to enjoy the woods in piece without swatting for hours. I hear I need to get a Thermacell but I haven’t decided if they’re the real deal yet.

This cool weather should hang around for a few days. I’m going to say at least until Saturday or so we should see high temperatures no more than the mid 70s, with some areas only in the 60s for most of the week. Winds will be out of the north to northeast at 5-15 mph (not accounting for gusts) along and directly behind the front. They will become more nebulous the farther you go away from the front.

Unfortunately I think summer will make one last hurrah before fall is here to stay. The jet stream is forecast to mostly have what we call a “zonal” pattern (flowing mostly from west to east) or possibly some ridging (high pressure), which makes for generally quiet weather as discussed in previous posts. I’m going to discuss briefly what different jet stream patterns look like so you can have an idea for your future reference.

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This is what’s known as a 500 millibar chart. It shows the air flow at a specific pressure of 500 mb (synonymous with altitude in our case) in the atmosphere. 500 mb is generally considered half-way up in the atmosphere and is one of the best pressure levels for diagnosing the overall flow of the jet stream. The colors represent the high wind speeds in knots and the black bars represent the height in meters at which you will find 500mb. By mapping things this way we can get an idea of where the high and low pressure are. In the case of the map above there are no major highs or lows. There is a weak low pressure system of the southwest coast of California (note the wind spinning counter-clockwise) and a weak high pressure over the southeastern US (winds spinning CW). Remember that the wind barbs point like an arrow flies, so the wind over Dallas, TX on this map is out of the southwest. This map is an example of mostly “zonal” flow, because most of the air is moving from west to east.

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This map is an example of what is known as a ridge (synonymous with high pressure). Notice how the jet stream looks like a hill and most of the flow has a clockwise look to it. This is why we call it a ridge. This flow pattern generally makes for high pressure underneath the ridge. This type of pattern will arise next week and bring another shot of summer-like conditions back for a brief period of time. Here’s what the GFS model thinks that could look like:

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This is 200 hours into the future, so again not great for exact details but it gives a solid ballpark idea. This is especially true for things like temperature, which are less affected by meteorological subtleties. As you can see, it’s not the 90s like we’re experiencing right now but it does warm back up a bit from the low 60s that this week’s front will bring.

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This type of flow pattern is what’s known as a trough. These are associated with low pressure sytems; notice the CCW flow. These are the most exciting features of a jet stream for meteorologists and for deer hunters alike. They bring change to the weather by creating low pressure centers at the surface, which bring warm fronts, cold fronts, and drylines. All of these can lead to interesting weather depending on where you are in relation to them and the time of year. Interestingly, often it is a strong trough and a strong ridge working in tandem that bring the really big cold fronts down from Canada. If you look at the case above you’ll notice the steep ridge directly west of the trough. In between the two you can see how the wind is flowing fairly strong directly out of the north. It is this situation (only over the central US) that usually brings the really strong fronts down. This is why we have yet to see a major front move in. The troughs that we have seen so far have all been weak and haven’t had the strength to pull that cold arctic air down south. It won’t be long though. As a matter of fact, just a few more weeks and it will be getting into the rut up here in Wisconsin.

If you don’t hear from me again this week check back on Monday (10/2) for another forecast discussion.

It’s finally time! Good luck.

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