Here Comes The Wind
October 16, 2012 7:40 PM
After a tranquil fall so far, its about to become a lot more active. Our next storm system will be pulling through the area Wednesday and Thursday which will not only cool off the temperatures and bring us some chances for showers, but its really going to increase the winds. You can see what our futurecast is showing for sustained winds on the left. High wind events are pretty common across our area through the fall months as the northern hemisphere transitions from summer to winter. On average, South Dakota is involved with 3 during the fall, one in each month from October to December. However, in active years, these large wind events can top 10 through the course of the fall.
Thankfully, this will be our first major wind event of the season, but it could be a doozy. Right now it looks like the area of low pressure responsible for this wind will drastically strengthen as it heads our direction out of southern Canada. This will create very gusty winds on the backside of this low-pressure center that will catapult the cold front forward overnight Tuesday and send a river of cooler air into the state. So why is this storm creating more wind then the ones we have already seen? Well this low is far stronger then many we experience in the upper Midwest, which creates a larger gap between low and high pressure across the country and therefore the winds have to travel faster to try and fill the void in order to keep the planet stable. Remember that the earth is constantly striving to have equal pressure, but the sun disrupts that equality giving us the weather that allows us to survive. The earth is constantly trying to combat what the sun disrupts, and that is where weather comes into play. Here’s an example… picture a landscape where you have a valley in between 2 hills. Your hills are high pressure and your valley is low pressure. You are standing at the top of one of those 2 hills with a tennis ball. You release the tennis ball and it begins its slow roll down the hill. This would be equal to a weak area of low pressure. Now lets take that exact same scenario and make the hills 10 times higher. Now you have 2 very large hills that drop quickly into a valley. Now when you release the ball it starts at the same speed but ends drastically faster thanks to the sharper gradient and the steeper decline. The same can be said for our upcoming low pressure. It is much stronger then most, creating more of a gradient (AKA a large hill) and therefore you have stronger winds. Below is an image from one of our computer models showing forecast sustained wind speeds for Wednesday and Thursday.
25-35mph sustained winds can be expected both days according to this model. However, this model typically underestimates wind speeds, and with many of the other computer models showing sustained winds even stronger then this, I’d say its ok to conclude that sustained wind speeds will be at least this high. But it’s not so much the sustained wind speeds that are going to be the issue. Yes they are strong and will make doing anything outdoors more challenging, but these are more of a nuisance then actually dangerous. The winds you really need to watch out for will be the gusts.
The forecasting computer models only show us sustained wind speeds for the surface, so we have to go to levels above the surface to get an idea of what our wind gusts will be. This is because the wind gusts we experience originate from above the surface. These winds are forced down to the surface by the turbulence in the atmosphere that the sun creates. This very same turbulence is what you experience when you are flying in a plane. It is the vertical motion of the air. Remember that when the sun rises and heats the ground, the air near the ground heats up as well. Warmer air rises so this creates rising motion in the lowest levels of the atmosphere. Considering that the atmosphere acts like a fluid, air cannot be removed from one location without immediately replacing it with air from a different location. In this case, from higher up in the atmosphere. Well if winds are typically stronger the higher you go, the heating and cooling allows these winds to get forced down to the surface as air is replaced at ground level.
Here is a look at the forecasted sustained wind speeds for about 5000 feet into the air during Wednesday afternoon from 2 of our computer forecasting models.
These are the wind speeds that could potentially be thrown down to ground level. Both of these images show wind speeds topping 50mph with the strongest winds located in the central Dakotas. This is how we know what the potential wind speed could be at the surface. Now if you think those winds are strong, check out the next 2 images. These show the same thing except for Thursday afternoon.
Notice the core of winds are quite a bit stronger, but they also don’t line up with each other. The first image is very similar in placement to the ones above. But the second image shows the strongest winds have moved further east into the I-29 corridor. This leaves us with some uncertainty on just where the strongest winds will end up on Thursday. Because of this, the east should be prepared for 60+mph winds during the day even though they may not materialize.
Anyway you slice it, its going to be windy. So hold on because we are in for a wild ride!