by Mike Godsey, mike AT iwindsurf.com
Take a look at the powerful NW winds in the mid 20’s to low 30’s at the ocean buoys predawn today when I was doing your forecast. Then look at the model output for 6AM. Both see really promising for a wicked strong NW winds for the coast and NW favored sites within the Bay. At a glance this looks like the sort of day that Marin windsurfers would think about heading to the waves at Tubamancha as seen in my banner above.
But my 7:30 AM forecast is for mostly weak or unreliable winds:
7:30 AM: A beefy summer type 1500 mile wide North Pacific High pushes an extension or ridge into Northern California and all the way to North Dakota. This causes its isobars to stack up over California from the Oregon border to near the Bay Area. So North wind is blowing the Central Valley and just aloft over the Bay while our ocean winds are NNW in the mid to upper 20’s just offshore. Meanwhile i can see the reflection of the Golden Gate towers in the mill pond like bay waters. So what are the odds of getting that powerful ocean wind to the coast and Bay? We have 2 unfavorable factors: 1. The N to NNE winds aloft continue all day and will fight against the NW wind trying to curve into the Bay. 2. Relatively air from the Central Valley will encourage a bulge in the valley low pressure to stay over much of the Bay Area so we have a weak pressure gradient. So the only weak winds at most Bay sites. Waddell and Coyote have the most promise.
So, why the pessimism?
This animation shows the strong NW winds as modeled by a medium res. model. Note at 6AM the strong NW winds are almost at the Golden Gate. But then mid morning strong N to NNE winds roar in from the Central Valley. This is due to a ridge from the North Pacific High that moves in over Northern California from near the Oregon border to near Sacramento.
While this imagery shows the surface N-NNE winds there is similar wind just aloft. Note how the NW ocean wind becomes weaker and more patchy as the day progresses. This happens as the 2 winds battle just offshore. At the same time the northerly flow encourages the Central Valley thermal low to bulge over the Bay Area keeping the pressure gradient weak.
But also notice how the N-NNE wind weakens some around 2-4 PM. And how the NW ocean wind builds and moves closer the coast.
This makes for very tricky forecasting since if the N-NNE winds were to fade early Waddell and Coyote and 3rd. Ave. could see strong winds.
by Mike Godsey, mike ATiwindsurf.com
Check out the reflections at Pt. Diablo in my banner above and the reflection of the south tower of the Golden Gate this morning. It is rare to see such calm conditions even in fall.
It is even weirder since there are very strong NNW to NW ocean winds just west of the ocean buoys today. But only a hint of those winds will curve into the Bay this afternoon. So let’s take a look at the story behind these reflections.
In the first animation follow the time from 4AM to 5PM. Note how the North Pacific High’s wind are NNW in the AM. Winds from this direction have a much harder time curving into the Bay since they are parallel to the coast.
Notice in the PM that the ocean winds turn a bit more NW and actually become WNW over the Peninsula. These wind direction hit the coast at an angle and readily move into the Bay if there is a pressure gradient.
But also notice the AM wind coming from the Central Valley is N to NE to NNE. This wind direction tends to push the ocean wind away from shore and makes the PM winds later and weaker.
Equally important is that when we have NE flow the low pressure in the Central Valley often bulges out over the coast.
Watch the time in the upper left and note the location of the isobars of the Central Valley thermal low.
The lowest pressure is in the Central Valley but from 2AM to mid day you can see that the isobars show a lobe of low pressure that extends over the coast almost to the Farallon Islands.
This means very weak AM wind since there is no pressure gradient over the Bay Area at the surface.
The image at the bottom of this page shows the overall pressure situation in the AM.
However since this is relatively warm air when it hits the coast range north of Sherman Island it tends to rise and most of the wind goes over the Bay Area. This NE flow helps to keep the low pressure over the ocean and also blocks the NW ocean wind.
In the PM the NE wind typically fades and as this happens the isobars of the Central Valley thermal low pull back to about mid bay. And this allows mild ocean wind to curve into sites like Waddell, Crissy, Coyote & 3rd that are on or near the coast.
By Mike Godsey
Don’t you hate it when the wind forecast is full of wishy washy “ifs” and “maybes”! Today forecast was a classic weasel word forecast:
“Now at 11:30 AM Southern California looks on track as does San Diego. The Pismo forecast hinges on the NE winds just aloft and at the surface. Lopez and Nipomo Dunes are seeing weak unfavorable N to NNE winds while Branch Mountain just to the west is seeing NE winds averaging 19 knots. All of this will make it hard for the ocean winds to curve into the Pismo beach reliably. So…. I am sticking to the current forecast for UP AND DOWN low to possibly weak mid teens BUT if you see the sensors just inland staying NE all bets are off.”
So what does all that jargon mean? First look at the wind graph inset into the graphic above. Notice that from 1 PM to about 2:30PM the winds at the Pismo sensor (green circle) did reach the forecast values and were pretty up and down as forecast. But why did they fade so fast by 3PM and why up and down? And why were the WNW winds so powerful at the nearby Red Diamonds?
The local cause is the weak NE winds you see in the purple circle in the top imagery.
But the large scale answer starts far away from Pismo. Today there is high pressure in the Great Basin. That is creating NE weak offshore winds for much of California today. Those winds were funneled over the coast range and reached very close to Pismo. You can see this is the animation above. Look from the upper right corner to Pismo and you can see the NE wind flow at 2PM. Meanwhile as the pressure gradient to towards Cuyama Valley & Los Olives picked up in the afternoon the North Pacific High’s surface NW winds started curving in towards Pismo which you can see in the right side of the animation.
Now look that the area where the text says “Wind battle zone” In this area the WNW ocean winds and the NE winds from the interior collided. This NE flow tends to delay the arrival of the ocean wind and when there is a NE gust the winds ocean winds are blown away from shore so you experience a lull. But when there is a lull in the NE winds the WNW ocean winds slam back to shore and you experience a strong period.
Trying to put all this into a forecast is impossible hence all the weasel words in my brief forecast this morning.
Banner photo by S. Anderson
by Mike Godsey, Mike AT windsurf.com
We have already seen one strong east wind event this fall and that event had devastating consequences for the Eagle Creek fire causing it to sweep from Eagle Creek to almost the outskirts of Corbett.
As you can see in Ben’s forecast for today we are in for big east winds again today. And as Ben says we can only hope this event does not stir up the fire.
So where do these east winds come from and why are they so tightly focused from Home Valley to Rooster Rock while rest of the Gorge barely has any wind?
First we have to look at the big picture. The first animation shows the west coast and much of the eastern Pacific.
Find the center of the North Pacific High WSW of the San Francisco Bay Area. The North Pacific High is further south than it was in the summer since it has begun its annual migration to its winter time home closer to the equator.
Typically in the summer the North Pacific High is more oval shaped but sometimes when a low pressure system passes to the north the NPH extends a ridge towards the Pacific Northwest.
But in this animation you can see that the North Pacific High has a narrow extension or ridge that extends into eastern Oregon and Washington. And you can see the winds in the Gorge are EASTERLY. Also note the low pressure trough along the west coast. So we now have a E. to W. pressure gradient.
This happens as fall progresses since low pressure systems take a more southerly trajectory and the ridge from the North Pacific High moves way inland and sometimes high pressure ends up in the Columbia River Basin out past the Tri Cities and even into the Great Basin.
Hence we have high pressure EAST of the Gorge rather than to our west as we see in the summer.
Now let’s zoom in and look at the isobars of the region.
Picture the North Pacific High as a dome or mountain of high pressure air. You are probably familiar with topographic maps land where lines of equal elevation show you where the terrain is flat or steeper.
So think of isobars as lines showing you areas of equal pressure. Where the isobars are packed closely together there is a abrupt change in the barometric pressure. And this means there is a stronger pressure gradient and hence stronger wind.
The next image shows the isobars line of the North Pacific High’s surface ridge and those of the low pressure thermal trough along the coast.
The orange area and the isobars around it show the high pressure from the North Pacific High’s ridge.
The yellow area shows the low pressure thermal trough and its isobars.
In the last animation you can see the isobars from the high pressure to the east and the low pressure trough along the coast are packed tight near the crest of the Cascades in the Rooster Rock to Home Valley region. While the corridor from Viento to out East has very few isobars.
This happens because the crest of the Cascades acts as a barrier for the two pressure zones so the isobars stack up here.
Also note how the isobars are only tight in morning then become further apart in the PM. This means the strongest winds are typically in the morning and early afternoon.
by Mike Godsey, mike AT iwindsurf.com
As you work to understand the central coast winds, I thought You might be able to explain the winds the last couple of days at Arroyo Laguna. If you look at the 11am to 3pm time period on Wednesday and Thursday for Arroyo Laguna, you see wind speeds on the 15-20mph range and a fairly normal direction. However the reality was that we were sailing 5.2-4.2 and max powered on Wednesday and 3.8-4.7 max powered on Thursday. The winds were also a bit more side-off than usual with the strongest wind inside blowing the tops off the surf.
Was the true wind direction different than normal and Piedras Blancas was not picking up on the strong winds at Arroyo? There was definitely a disconnect between the two as Piedras Blancas normally would be a bit windier than the reality of Arroyo. Let us know what you think. Thx Hitech.
But in this case I can make a fairly useful analysis. Many sites along the coast such as Waddell, Piedras, Cabrillo. Leo etc have their winds slightly accelerated from a one sided venturi effect. This happens when the ocean winds compresses against the coastal hills or bluff and locally speeds up.
In your case the venturi is most commonly focused near Piedras when the ocean wind is more WNW-NW which, as you can see in our archive data, image #1 is the most common Sept. winds at our Piedres sensor.
But on Thursday image #2 from our Piedras sensor and the San Martin buoy #3 and had winds more NW-NNW. (this occurs when the NPH extends a ridge into the Pacific Northwest.)
When this wind shift occurs the max venturi will move further south which probably caused the acceleration you noticed at Arroyo. And with the venturi weaker at Piedras the sensor would read low compared to the wind at Arroyo. If you look carefully at the time slot the previous evening (red diamond) you can see when this wind shift occurred.
The last two images also show this effect. Looking carefully at #4 you can see how the NW wind would focus a venturi at Piedras while more NNW winds #5 would focus it more to the south.
There are hundreds of such effects we have to learn before we can do really useful forecasts for a region. After 20+ years I am still learning lots of new things about such effects in the Bay Area.
It is not so much that I am a slow learner but rather that our wind pattens are changing from year to year compared to the norm 20-30 years ago.
by Mike Godsey, mike AT iwindsurf.com
Below is a map I prepared using our weatherflow.com sensor data and a fire map from the Incident Management Team 2. Note that the most catastrophic fire expansion happened on Sept 5. when the fire swept over 11 miles of the Gorge in 12 hours driven by winds over 30 knots between Stevenson and Rooster Rock. Click on the map for a larger version. Click on the map for a larger version.
Yesterday I had to drive to PDX so I missed the very strong Hatch winds. But I dropped by the Hatch mid day to check out the wind just as the 10+ day fire went crazy just west of Stevenson and also made a lunge towards Hood River. From the Hatch I could see helicopters dumping water on the far western outskirts of Hood River. So as I drove to PDX I took a series of photos of the fire spreading in our beloved Gorge.
Last night I could see hundred foot bursts of fires just past Starvation Creek as conifer trees exploded.
Driving down the Gorge looking towards Oregon it seemed that the damage to the forests was patchy along Hwy. 84 and up the cliffs. Some areas were devastated while others were intact. It looked like from 84 the Gorge will seem not horribly disfigured. The real damage is up in the canyons where the spectacular hiking used to be found.
by Mike Godsey
Sept. 11. Updated Sept. 21, 2017
We had a “mystery” wind event early Monday morning Sept 11, 2017. The previous Sunday evening while preparing the Bay Area 7 PM wind forecast I noticed that our 1 km WF-WRF model had a baffling 1-2 hour burst of very strong wind well before dawn.
To the right you can see this modeled burst of strong wind with average velocities in the animation that Jeff Copeland of our Denver model crew retrieved from our 1km WF-WRF archives.
In the next animation, from our 3km WF-WRF archives, you can see this event as it originates on the Central California coast south of Monterey.
This seemed very weird since other than winter storms and occasionally Sherman Island night and predawn winds the Bay Area is usually near windless until after sunrise. At a glance there I saw nothing in the overall pattern to cause this brief intense wind but I did note some rare monsoonal convective clouds to the south were dying. And since few of our customers need wind information pre dawn I only touched on these predawn winds in my forecast just as a curiosity.
The next day the SF Gate had the following report:
“Meteorologists are baffled by strong wind gusts that ripped through concentrated pockets of the Bay Area early Monday morning, while other areas remained completely still. The National Weather Service reports that a gust of 54 mph roared through Pebble Beach at 2:30 a.m., while only 10 minutes away in Monterey conditions remained completely calm.”We had a very unusual event happen overnight,” says Charles Bell, a meteorologist with the National Weather Service Bay Area. “We had isolated strong periods of winds. It’s really remarkable. Usually wind storms are more widespread, while this wind occurred in concentrated pockets.”
The first gusts roared through Pebble Beach and the southwesterly winds traveled north reaching up to 40 mph in Santa Cruz, up to 37 mph in Los Gatos and at the San Jose Airport.
The weather service recorded the last gust from the event at 3:30 a.m.; it was a 35 mph wind at the Golden Gate Bridge. Bell says there are anecdotal reports of high winds in parts of Marin. “We left our window open because of heat and winds were so strong our interior doors were slamming shut and flying open!” Thais Derich of Mill Valley wrote on Facebook. The winds knocked down trees and power lines. PG&E reported 11,000 customers without power at one point. The event lasted about two hours.”
Later Monday 3PM the SF Gate had the following story update:
None of our Bay Area sensors fully caught these winds but many of them show a very unusual predawn spike. Note both the wind spike and direction.
by Mike Godsey, mike AT iwindsurf.com
Update: This first video is the eye of Irma with average winds of 185 mph Tuesday Morning.
What is clear is this Tuesday afternoon is that this hurricane has peak winds around 180 mph as measured by Hurricane Hunter aircraft and a dropsonde into its eye found a pressure of 927 mb. and dropping. This places it well above the threshold for a Category 5 storm.
Weak wind shear aloft, very warm 85F water temperatures and increasing humidity are factors favoring intensification.
This animation show how one model, the ECMWF, projects the trajectory of Irma from today, Tuesday Sept 5, through next Tuesday. This ECMWF model animation is not optimized for hurricane forecasting but if you look at the optimized imagery in the second image you can see that there is a lot of agreement regarding Irma’s trajectory.
Notice that some of the forecasts have Irma passing right over the mountainous backbone of Cuba. While this would be devastating for Cuba the mountains would greatly weaken and disorganize the storm which would benefit the USA.