San Luis Wind Sensor is wacky on directions

[ctuna]

San Luis Wind Sensor is wacky on directions
I think they need to replace or move the Sensor to
get it to read more like the Pacheco pass sensor.
I have seen this thing reporting up to 180 degrees out
from the Sensors in the surrounding area. Seems to be worse
on South directions. Sometines it changes 180 degrees in an instant.
went back and forth between North and South a coupe of times
in the last half hour. starting at 10am .
I have observed this in the Last Year or so I wonder if the sensor
has been moved or if something has been put up in front of it.
verified with other users also I have the iwindsurf windmeter bluetooth
version it was saying wsw to west all afternoon today .
Mederios (south launch) Other people at the site verified West
at the direction all day today.(6/4/2017)

[windfind]

Hi ctuna,

You are not the only one to notice the "wacky" San Luis readings. No we have not moved the sensor and there are no new structures near the sensor. But something has changed and that is the Bay Area wind patterns.

I know the wind patterns around San Luis very well since I was employed by GoogleX to do the forecasts for their Project Loon launches in that area:

https://x.company/loon/

So let me develop a hypothesis for you:

More and more frequently we are getting spring and summer northerly winds coming down the Central Valley. This change is related to increased # of NPH ridges we keep mentioning in the forecasts as well as the Golden Gate Eddy.

So what does this have to do with wacky San Luis winds? Take a look at the animation below and watch the battle between northerly and more southerly wind over San Luis.

The second illustration makes it clearer. The details vary from day to day and with the relative strength of the N. and WSW wind but basically:

1. In the morning the marine layer is often deep in Pacheco Pass and WSW wind blows at the sensor. See GREEN arrows and GREEN diamond on wind graph below for Pacheco Pass.

2. If there is northerly wind in the Central Valley (see RED arrows) it hits the hills just north of the sensor creating northerly turbulence. This northerly turbulence spreads randomly over the Forebay area. So at some locations all you sense is the WSW wind from the pass while other areas get mostly northerly wind while others get a mixture. The higher your location and the more easterly your site the more northerly wind you will feel. You can see this by comparing all the archives from the surrounding sensors. But at our sensor so there is a battle between the N. & WSW winds. Hence the "wacky" readings (see ORANGE boxes)

3. If the northerly wind is fairly strong the sensor shows N. wind for a while.(ORANGE BOX)

4. Then in the early afternoon as the Central Valley pressure gradient goes up the WSW wind blows stronger at the sensor and the Northerly wind fades away. As this shift happens there may be a second period of "wacky" wind and the winds battle. Then the more southerly winds prevail. (2nd. green diamond.)

If you look at the San Luis archives for the month of May you will see the pattern of SW wind then N wind then SW wind punctuated with "wacky" readings very frequently. And on each of those days there was northerly wind in the Central Valley.

The reason you see different directions at different nearby sensors is the different frequency they update. Our sensor updates more frequently than most of the other sensors so it shows more of the "wacky" changes.

Of course my hypothesis could be nonsense and actually it is a wacky osprey that lands on the wind vane each day to avoid the northerly winds and then takes off when the marine winds start.



Hopefully no one will ask why we are seeing more ridges, eddies and northerly wind!

Mike Godsey
iwindsurf.com/ikitesurf.com
Weatheflow.com

[ctuna]

I have the iwindsurf wind meter bluetooth version.
I am not seeing the same direction of fluctuation in direction
on the lake from either the North Shore or South Shore
the the iwindsurf station is measuring . It always seem more like
the 152 sensor. Yes I know those don't update as often.
Made three measurements yesterday they appeared on the site info
under local info measurements.

[windfind]

[ctuna]

Where is the Sensor exactly?
It looks like its in the campgrounds below the dam on the
dam side(as oposed to the Forebay side) of the Highway.
Thanks for taking the time to respond so thoroughly.

[windfind]

Hi ctuna,

Below is the wind graph for the Rt. 152 sensor which is closest sensor our IW sensor. You can see in the image below that the Rt. 152 sensor shows almost the exactly the same "wacky" periods at the start and the end of the N. winds.

I am guessing that the dam is funneling the northerly wind while at locations further away from the dam you are mostly getting WSW-W wind that lifts over the dam. That would explain your different readings at different locations

Map below shows the IW sensor location.
Mike

[ctuna]

Since the above dam water level is nearly full,
does this mean that more wind comes over the top of
the dam spilling into the sensor area and mixing with the northerly
components of wind?

[jcscott]

I keep seeing a similar thing. The San Luis sensor is frequently about 90 degrees off the two closest sensors, near the north and south ends of the dam. See the attached map from a few minutes ago. I realize that the readings are about 1/2 h apart, but this is not the time of day when northerlies blow.

Assuming the sensor is accurate, this implies that there is another phenomenon at work - perhaps the dam itself creates a low-level flow along its front at the ranger station. My experience is that the other two sensors give much better accuracy in direction. So one question is how far from the dam does the effect go.

Even more important is what this does to the windspeed forecast. I am accustomed to the fact that the long range (7 day) forecast usually underestimates the short range (3 km) forecast on high wind days. But the past two weekends (just 5 days, so not a great sample) the short range forecast was 20 mph and above, while the long range and the actual windspeed was more like 12-14 mph. The San Luis sensor is within 3 km of the sailing area, and so is probably heavily weighted in the short-range forcast.[/img]

[windfind]

Hi jcscott & ctuna,

Damn good idea about the dam being full vs empty. I will run a wind tunnel app this week looking at the effect on the wind flow of the dam being empty vs being full. This could be the issue of the weird readings with northerly wind since the WSW flow over the dam would be very different.

The northerly winds over the length of the Central Valley are much more common this year and last hours longer. My first graphic is at 3PM with northerly winds.

The problem with long-range forecasts vs. short range forecasts is that the short range forecasts are from high resolution models that can better resolve the topography that accelerates the wind. Limits of computer power make it presently impossible for these models to cover large areas of space or go many days into the future.

The long range models are much lower resolution and can not resolve gaps in the coast range line Pacheco Pass so they underestimate the wind.

Long range models are for seeing to movement of large weather patterns like the location of the North Pacific High and overall wind over a week or more. They are not for forecasting wind at specific sites.

While the high res short term models can give you an idea about the wind at specific sites but do not give the overall picture so they can be wrong if there is something developing beyond their boundaries.

Mike
iwindsurf.com/ikitesurf.com
Weatheflow.com

[jcscott]

Hi Windfind:

Thanks for looking into this.
I have been sailing at O'Neill for 30 years, and have been constantly frustrated by the difficulty of making accurate forecasts. Now this is about the only place I sail anymore, and like to plan a trip with a one or two day outlook.

So I really like the idea of a wind tunnel simulation. I wonder if your model includes the two "chutes" from the N and S ends of the dam, converging close to the Ranger/Weather Station. See the Google map image attached. Rte 152 follows a gully on the north side, and the access to the Basalt Campground is along another gully to the South. It is possible that these features guide competing westerlies toward the weather station.

Then my earlier question maybe even more relevant - how far over the Forebay could such an effect extend.

Thanks again, Campbell[/img]