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I want to take a few minutes to explain this windstorm compared to last November’s. Last year, November 17th we had a historic windstorm in the Inland Northwest and now we are expecting two very vigorous storms starting Friday morning and a second on Saturday. Both are expected to produce strong winds around the Inland Northwest, but nothing compared to last year.
Let’s talk about last year to start. We forecasted the windstorm several days out and the day leading up to it, we called it potentially historic, and it was. Winds topped at Spokane International Airport at 71mph that is the strongest wind gust ever recorded there. To ease minds right away, we expect peak winds to be about 45-50mph, not the strength of last year.
So what is the difference? Well, I have borrowed a few pictures from the National Weather Service and from UW Professor Cliff Mass to show in detail the difference between the track of the center of low pressure between these two storms.
Lets start with why we get strong winds so we all have an understanding as to why this happens. First off, we have what is known as pressure differences. You often hear “high and low pressure.” Stormy weather is associated with low pressure and nice weather associated with high pressure. Reason being is that with low-pressure air will rise, and with that, condensation, clouds and rain can form. High-pressure, however, pushes down not allowing that to happen, which creates clear skies. Now the lower the pressure, the stronger the winds. So for the November windstorm we had a strong area of low pressure moving over us, and a strong ridge of high pressure to our south and this, in turn, created pressure differences. Those differences in pressure are what cause winds. We measure these differences in millibars or inches of mercury. For this, I will stick with millibars. (General sea level pressure is 1013mb).
So then in theory, these differences in pressure create winds, the stronger the difference, the stronger the winds. So back to the November windstorm! This was really the perfect storm, the ingredients were ripe and at play for this and it caused extremely dangerous and strong winds. We look at the pressure gradient first off to create these winds. We see we have a 989mb storm to the north of us in southern British Columbia and right over the border between WA & OR a millibar of 1016 giving us a difference 27mb, and that equals STRONG winds. Look at the picture below. A good reference, the lines, the closer they are together the stronger the winds will be.
You may also notice the arrows moving from west to east. That is the track of the center of low pressure. The reason this is significant is because when storms track this direction, it sets up for the strongest winds right over top of Eastern Washington. The track for our storms this weekend (yes two) are a different path and will be much less damaging. Still bad… but not quite to the severity of the 2015 storm.
Let’s get to this weekend’s storms now. First off winds. Yes, it will be quite windy. A WIND ADVISORY is already in place for gusts possible to 50mph. Last year, it was a high wind warning. Already an improvement. But look at the picture below, there are two pictures, the one on the left is Friday morning, the one on the right Friday afternoon. Just with the eye test you can see it is much different. First, it is out over the Pacific Ocean meaning we will not see the brunt of the winds. Secondly, it is a stronger storm pressure wise, but it is so far west that the strength here in the Inland Northwest will not be as strong. And then you notice the second picture, notice how far apart the lines are? The difference in pressure is showing the storm weak. This is tonight and early Friday morning’s storm.
You may also notice it is moving from the southwest to the northeast… Also, a sign that our weather will not be as bad as the 2015 windstorm. Yes, as I mentioned, quite windy but nothing compared to the Nov. windstorm. Now, to recap, this is the first in a series of storms. With this first one, expect winds of upwards of 45-50mph likely. So while I say it is not as bad as the Nov. windstorm, we can still expect power outages and few downed trees but again, nothing like last year.
The second system is set to arrive on Saturday this one skirts a little closer and I actually think might be slightly stronger with possible winds of 45-55mph as the pressure gradient is almost the same even possibly slightly stronger. Again, power outages likely and a few downed trees but nothing like last year’ windstorm. The track of the Saturday storm is expected to be from the southwest to the northeast once again which is good news. The ones we need to watch very carefully are the ones that track from west to east across southern British Columbia.
Either way, I still want to stress caution. If you live near trees just keep an eye on them, tune into social media and the news. If you lose power NOAA provides weather radio updates that you can listen to as well.
Finally, I just ask that people stay safe, stay informed and stay tuned. As we get closer to these systems moving in we will, of course, keep everyone informed!