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Down Slope Wind Event Strongest In 9 Years

Sunday February 3, brought with it a rare wind event to the Coachella Valley, creating a ton of damage including a roof being blown off a Travel Lodge hotel in Palm Springs. What the west end of the Coachella Valley, Twentynine Palms, andhigher elevations experienced wasis commonly referred toby meteorologists as a “Mountain Wave”.For the publica”Down Slope Wind Event”. Most pilots are aware of these because they are a very real risk to aircraft and create conditions that cause aircrafts to lose lift and crash.

The First Alert Weather team was the onlyweather team in the Valleyto warn the area Saturday evening February 2, that conditions were prime for a major Down Slope Wind Event to occur on Sunday.

The high resolution forecast model “Pinpoint Microcast Winds” on Friday eluded to the fact that such an event was possible Sunday. The reason “Pinpoint Microcast” was able to forecast this event so well is because the Pinpoint Microcastforecast modelis exclusive to KESQ andhoused right in the television station in Palm Desert, so we are able to personally adjust variables tocontinually increase itsaccuracy. No one else in the Valley, let alone Southern California has a forecast model with the capability to warn of Down Slope Wind Events, only KESQ with “Pinpoint Microcast Winds”.

Meteorologist Peter Cannella continued to review atmospheric data Saturday afternoon and confirmed conditions were quite favorable for a Down Slope Wind Event.He wenton-air Saturday evening showing the “Pinpoint Microcast Wind” forecast and warning everyone that winds and damage could exceed that of theJanuary 28 wind event with wind gusts of 50 mph possible locally in Palm Springs andgusts to 65 mph in higher elevations on Sunday.

Down Slope Wind Events are not like the normal windy conditions. They cangenerate a lot stronger wind gusts than a typical wind event and result ina lot more damage. Thereal difference though is in how the stongwind gustsare actually created, resulting in the Down Slope Wind actually being able to produce stronger gusts than the typical wind.

Your typical strong wind is created by a pressure difference, a high pressure system and low pressure system being to close to one another. In a minute we will explain how the Down Slope Wind Event forms.

Down Slope Wind Events occur a couple times a year here in Southern California. They are more common on the leeward (east side in the Northern Hemisphere)side of the Rockies where winds have been recorded to gust to near 120 mph before.What makes the February 3 event stand outis itsstrength. The difference in the atmosphere Sunday was that there were a lot stronger winds in the middle part of the atmosphere at about 8,000 feet and they approached the mountains at the most favorable angle for the strongest wind gusts to occur, the perfect combination. Figure 2.

Wind gusts in Palm Springs gusted up to 55 mph at approximately 4 p.m..The last timewe recorded wind guststhat strong was back in June of 1999 with a gust to 58 mph. In Burns Canyon winds gusted to 96 mph which is a rare event there, but not as rare as the 55 mph wind gusts in Palm Springs. Sustained winds of over 45 mph were common in Twentynine Palms. In the west end of the Valley winds remained steady between 30-35 mph from 3 and 5 p.m., toppling trees, power poles, ripping shingles off roofs, and destroying fences.

There is one distinct aspect of the atmosphere Sunday that had to be present for this event to evolve. That was the presence of a “stable” layer of air at approximately 15,000-20,000 feet above the ground. We will use several analogies to simplify this, the “stable” layer acts as a brick wall. It is simply a layer of air that is not buoyant (doesn’t rise).

As strong winds of 40 mph at 8,000 ft.(blue arrows) come in off the Pacific Ocean perpendicular (90 degree angle) to the San Jacinto Mountainsthey hit the mountains as Figure 2 shows.This air acts like a rubber ball and bounces off the west face of the mountain and up into the higher parts of the atmosphere.

On its journey up it ran into the “stable” air (brick wall) at approximately 15,000 to 20,000 feet and is bounced back to the Earth below.At 15,000 to 20,000 feetwinds were blowing at approximately 90 mph, that bouncing ball of air will get sped up if you will while at that altitude and then bringits new acquired speeddown to the Earth below. Now it won’t totally inherit all of the 90 mph wind speed.A rough estimate would be to average the starting speed of the air, 40 mph, with the 90 mphwind speedin the “stable” layer,and you come up with a gust possible of 65 mph. Which is approximately what was recorded in Pinyon. Burns Canyon’s speeds are amplified by the constricting space of the Canyon so we don’t want to use that as a example of max wind gusts.

Thenas the air or “rubber ball” as we call it,crashes into the ground it creates strong wind gusts at random times and locations west of the base of the mountains asFigure3shows in the areas in red. This isn’t the end of the story for this rubber ball though. It’s acting like a rubber ball after all!!! We all know that rubber balls can’t bounce just once. After hitting the ground the air bounces off the ground and heads back towards our “stable” air or brick wall.

It again is on a crash course with the “stable” layer of air. It hits it again!!Figure3shows this, as the air (blue arrows) rises, like all pockets of rising air that have enough moisture in them, the rising air creates a cloud. Like before though it bounces back towards the Earth and this air is sinking, so cloud do not form.

What is taking place is that our air is maintaining its momentum and has become trapped between the ground and the “stable” layer (brick wall). As a bouncing ball does, this air bounces up and down between the ground and the “stable” layer. It’s what we call a trapped wave.Figure 3shows this pocket of air or “bouncing ball” bouncing eastward from the base of the San Jacinto Mountains. The strongest wind gusts will always occur with in the first or second crash with the ground. In Figure 4 you can see the cloud that formed at the “highest” point of the trapped air’s “bounce”. This image is looking northwest towards the Banning Pass. The cloud extended south down in to Mexico as the satellite image in Figure 5 shows.

These pockets of air can be trapped and travel for several thousand miles from the bases of mountains before they run out of energy like the rubber ball and basically disappear. Creating nice uniform rows of clouds (rising air-“ball bouncing up”)and empty gaps (descending air-“falling ball”)traveling east, like waveson the ocean. How far these rows of clouds can travel away from the base of the mountainis partly dependant on the speed of the winds hitting the west side (Northern Hemisphere) or windward side of the mountain.

Sunday’s event was a text book example with the wave or “row of clouds” traveling several hundred miles away into central Arizona. As you can see inFigure5 a visible satellite image from the early afternoon on Sunday February 3, rows of clouds spread east from the base of the San Jacinto Mountains across Riverside and Imperial counties and into Arizona. Yellow arrows have been placed to show the equal distances between the rows of clouds. The cloud rows wiggle from north to south and are not in a straight line because they form with the contours of the mountains.

The equal space between the rows of clouds are not by accident. The atmosphere can be explained by complexed calculus and physics and there is actually a formula to predict the distances between the clouds-the crest (top) of a wave. It is a trigonometric function like Sine or Cosine and thedistances between thecrests of the wave (where the clouds form) are dependant on the initial speed of the winds coming off the ocean.

Earlier we said that the wind hit the San Jacinto Mountainsat a 90 degree angle along with our “stable” layer being in place. Those are the two major ingredients that do not come together to often around here.

Most of the time the winds don’t hit the mountains at the correct angle-90 degrees, or a “stable” layer isn’t in place, or the winds at 15,000-20,000 feet are not as strong, or the winds at 8,000 feet are not as strong. On February 3, all of those ingredients were in place to create the strongest wind gusts we have experienced in nine years herein the West Valley. This is a once in10-20 year event, making it extremely rare!!!

Other mountain waves or down slope wind events that occur in the Coachella Valley several times a year usually do not have as strong of winds at 15,000-20,000 feet and they don’t hit the San Jacinto Mountainsdirectly perpendicular to the mountain face. Both of those factors greatly reduce the resulting wind gusts.

The wind hitting the mountain square at a 90 degree angle is like a car (wind) running directly into a wall (mountain), it comes to a stop and thenbounces backwards, but the car wanted to continue traveling forward but the wall got in the way. Figure 6 shows the airdoes almost the same, but it can’t not go backwards so its forced up and over the mountain with most of its momentum (wind speed)in tack (while gaining extra speed from the”stablelayer”winds)still traveling forward.

Where as ifthe wind hits at a smaller angle, say 45 degrees, or from the northwest, thewind speed(momentum)is distributed to the north and south along the west face of the mountain, so the winds that actually make it up and over the mountain moving southeast are not as strong. Figure 7 helps visualize this effect, with windson the west side of the mountains moving at slower speeds.Making the car analogy, thecar (wind)having a glancing blow to a wall (mountain), will continue inthe general direction it was traveling but at a slightly different angle and it keeps going…..in wind terms, thewind gets stuck on the west side of the San Jacinto Mountains.

Questions, email pcannella@kesq.com

Check out this site for more info on mountain waves: http://moe.met.fsu.edu/~rhart/mtnwave.html

KESQ News Team

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