by Mike Godsey, mike AT iwindsurf.com

Note: This blog uses Southern California as an example of picking the best model using ship reports but the basic idea of the blog is relevant to all regions in the USA.

On winter days as I walk through the La Ventana campground I am always impressed by the thoughtful kiters and windsurfers who print out and post the day’s forecast on clipboards right where folks stand to judge the winds. But I also feel a bit downcast when I see the models they pick to print. Most of the time they pick websites that use the old GFS model. The same is sometimes even true for the web sites for local resorts as you can see in this, all too true, warning about the model output from the Captian Kirks resort.

Don’t get me wrong. The GFS model is great for following large scale wind patterns and is one of your best choices if you are forecasting storms or if you are a mariner. The problem is that the GFS resolution is too low to properly factor in the topography that so strongly impacts the wind at most launch sites.

The trick in forecasting anywhere is to know which model, on any given day, is doing best at capturing the reality of the wind later that day. And that takes years of experience as well as knowing how to interpret satellite imagery, weather balloon reports and, and yes, even ship reports.

Let’s take a look at this Southern California day as depicted by the windy.com web site and how well the ECMWF 9 km model did forecasting an eddy compared to the NAM 3 km model.

The first part of the animation shows the ECMWF 9 km forecast for an eddy over the Southern California Bight. Notice how the forecast eddy spans almost the entire middle portion of the Bight. This large eddy is supported by the NNW winds you see in the upper left corner of the image. It is also supported by a thermal trough extending from a low pressure just north of the Sea of Cortez. Note how the modeled isobars of the thermal trough cover the entire Southern California Bight.

Such a large eddy could easily keep the afternoon beach winds very weak. So in forecasting, we have to determine the accuracy of this model output. Sometimes we can do this by looking at the swirl pattern of the marine layer clouds in the satellite imagery. But often that imagery is too fuzzy at 7AM. Another way is to look at wind reports from ships traversing the area.

As you examine the ship wind reports below notice that the furthermost south ship report does not jibe with the ECMWF modeled eddy.

Now, look at the NAM 3 km animation. Notice how the NAM has the isobars of the thermal trough more focused in the northern Southern California Bight. Now looking at the ship reports notice how their wind reports more closely follow the modeled eddy winds.

So, in this case, I would tend to forecast the eddy dying midday and wind peaking in the Long Beach area. Why, because I picked the right tools for the job which in this case was using the NAM output and ship reports.

Upper convergence and divergence the strength of the North Pacific High: 

by Mike Godsey, mikeATiwindsurf.com

ROUGH DRAFT

There is a tendency of many kiters and especially sailors to look at the wind from a 2-dimensional perspective. This means we only care about the direction and strength of the wind. Although kiters are sometimes aware that their kite is feeling winds that they are not feeling at the surface especially in gusty conditions.

Likewise, we are sometimes shocked when our airline flight time is much shorter or longer than the miles alone would predict. And if you live in the mountains you sometimes see evidence in clouds like the lenticular clouds in my banner and the image just above that there are powerful winds aloft even when it is nearly calm at the surface.

In our forecasts, we sometimes mention in passing upper-level winds or upper troughs and ridges but it is hard for us to connect these events to your surface winds in a single paragraph. So let’s take a more detailed geeky look at how those upper-level winds impact the high pressure and low pressures that often deliver our wind.

Simplified Upper-level Convergence and Divergence aloft in a simple 3D image:

First, let’s show  you a very simple 3D graphic of what convergence and divergence look like relative to surface high pressure and low pressure.

At the top of the image we are looking at the winds at about ≈ 18,000 ft. On the left side we are seeing Convergence of those winds. This makes the air more dense in this area so it sinks towards the surface augmenting the surface high pressure (Anticyclone).

On the top right of the image notice how the wind aloft is fanning out creating Divergence aloft. This makes the air less dense creating an area of lower pressure. And this encourages the surface air to flow upward which makes the surface low pressure stronger.

Model output and upper-level Convergence and Divergence in 3D image:

At the bottom of this 3D image for next Tuesday, April 9 and find the North Pacific High with its isobars compacted against the California coast. When the isobars are tightly spaced like this there is a strong pressure gradient on the coast so the NW accelerates. Note that there is also low pressure in the Great Basin and beyond which also contributes a pressure gradient that helps pull the wind down the coast and inland. Clearly, this is a very windy pattern for Northern California and perhaps Southern California. This is the sort of setup we will probably describe in our forecast next Tuesday.

Now let’s look at the back story why the North Pacific High is so strong next Tuesday and the Great Basin has such low pressure. Looking at the top part of the 3D image above notice the upper-level winds at ≈ 18,000 ft. The northward wind pattern is called an upper ridge while the southward pattern is called an upper trough. These are very strong winds that sometimes are over 100 mph.

Notice how these bands of upper-level winds are sort of like a river. In some location the wind band is wide and other locations it is narrow. For this to happen there has to be locations where the winds are converging like a river narrowing and other location where the winds are diverging like a river widening.

Remember that all the colored wind patterns are up at ≈ 18,000 ft. Now find the upper-level ridge and note how the upper winds approaching California are converging above the North Pacific High at the surface.

Now visualize all those rushing air molecules cramming into this zone. This will make the air over this region denser and since it is heavier it will sink in the atmosphere. This creates stronger high pressure along our coast. And as this air sinks it carries some of its motion towards the surface enhancing the North Pacific High’s surface NW winds.

Now check out what is happening over the Great Basin. The upper-level winds in that zone are fanning out or diverging. This means the rushing air molecules are further apart creating lower pressure aloft. And this, in turn, enhances the surface low pressure in the Great Basin.

This lower pressure creates a pressure gradient from the North Pacific High to the Great Basin strengthen the surface NW wind and encouraging it to move inland.

Since these upper ridges and upper troughs normally move from west to east the zones of convergence and divergence will shift eastward and the North Pacific High’s surface NW winds should weaken of following days.

by Mike Godsey, Mike AT iwindsurf.com

This symbol, omega, is the last letter of the Greek alphabet and a famous watch brand. But in meteorology, it is the shape of the omega symbol, written in our upper-level winds, that has locked most of the west coast into a weather pattern with little day to day change. Specifically, it has given the Gorge mostly blue skies while depriving the San Francisco Bay Area of NW clearing winds. While Southern California has sometimes benefited from mild NW winds curving into the beaches.

Take a look at the brightest colors in the image below and look for an “omega”  pattern in the winds at ≈ 18,000 ft. When this pattern of upper level ridges and troughs develop it largely blocks the normal movement of weather from West to East.

Let’s take a look at this diagram step by step.

1. First find Southern California, Bay Area and the Gorge.
2. Notice the isobar white lines. They represent the surface pressure at different locations.
3. Now notice the large storm off the west coast. It has been locked near that location for many days and has barely impacted our weather.
4. Then note how far the North Pacific High is from the Baja and California coast. Not much chance of NW wind on our coast!
5. The thick red line accentuates the Omega shape of the upper-level winds.
6. Notice how the upper troughs make up the”arms” of the Omega Block.
7. And between the 2 upper troughs, there is an upper ridge. Voilà, an Omega Block!

Once this type of block sets up it is slow to change and is keeping the North Pacific High from moving up and towards the west coast. It also makes for boring forecasting since things change little from day to day. Notice how this block does not have a perfect Omega shape and it slowly breaks up the next 4-5 days. As this happens my guesscast is that Southern California will see building NW winds Thursday while the Bay Area has to wait until Sunday or Monday.

Strong North Pacific High + low pressure in Great Basin + strong winds aloft = Epic winds!

by Mike Godsey, mike AT iwindsurf.com

Strong wind ruled the west coast on March 12, 2019. Here is a visual summary of the causes and the results:

1. Note the North Pacific High off the west coast.
2. Look at how the NPH’s isobars stack up along the west coast creating strong NW wind.
3. Check out the low-pressure zone in the Great Basin creating a strong pressure gradient.
4. Note how the NW wind curves inland towards the low pressure in the Basin.
5. Check out how many of the sensor in the San Francisco Bay Area have winds in the low 20’s to upper 30’s
6. 3rd. Ave had peak average winds to 37 mph.
7. And Southern California had sites with winds in the upper-teens to even mid 20’s.
8. C St. in Ventura had average winds to 26 mph.
9. A bit factor in the strong surface wind was the strong gusts to 40 mph just aloft over the Bay.
10. And Southern California had similar winds aloft.
11. On most days kites way outnumber windsurfers at 3rd. Ave. but not late Tuesday afternoon.
12. An for a change the Gorge was at the bottom of wind totem pole excepts for Dougs that hit a respectable 29 mph.