West Coast Wind Blog: Sudden arrival of SW winds aloft blows forecast.

WC39SWstromMLfastUnexpected early arrival of SW winds aloft sends streamer of marine layer clouds into Bay Area and brings ZZ74A3ECF6 sudden spikes in wind.

As I mentioned in a blog a few days ago the wind patterns over the pacific are a mess with will fluctuations in the upper level winds. In that state forecasting becomes difficult and sometimes, like today, $#!+ happens. Today the storm to NW unexpectedly sent a pulse of southerly wind over the Bay Area.

Notice in the top video there was not even a hint of marine layer clouds even in the mid afternoon But as the winds aloft arrived they quickly sent a streamer of clouds rushing through the gate and over the Marin Headlands through the slot all the way towards Pt. Isabel.

Looking at the next set of images you can see these winds and the consequences for the wind at different sites..

 

Posted in San Francisco

West Coast Wind Blog: Late summer storm winds hit Gorge

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ZZ47B5C290How often do you see forecasts like this in the Gorge? I mean we do sometimes do see summertime low to mid 30’s but what is with all the VERY GUSTY AND UNSETTLED….UNRELIABLE stuff.

Well the winds Benjamin is forecasting for this Saturday are very different from typical summer winds. As I pointed out in a recent blog the Pacific weather patterns are a mess and Saturday you will meet a small part of that mess up close and personal.

Let’s look at the first animation taken from the GFS model. In the first frame the winds in the Gorge and out in the near shore waters of the pacific look mild. But in subsequent frames you an see a potent extremely early season low pressure storm and its SURFACE winds. Notice the counter-clockwise spinning surface winds of the storm and how organized they are as they approaches the Oregon coast.GorgeStorm

The air in these winds is “unstable air” which is geek talk meaning that the air at the surface is about the same density as the air aloft. Out at the ocean that does not impact the surface wind speed much. But notice how the wind pattern breaks up as the storm comes over the coast and hits first the coast range and then the Cascades.

As this wind hits topography it rises and since its density is about the same as the air aloft there is little to coax it back to the surface. So towards the coast people will notice blasts on the ridges but valleys will be more wind protected. So the overall flow of the storm wind becomes disorganized compared to the wind flow over the Pacific.

In the corridor this effect will be very noticeable and the winds will be unreliable with blasts and lulls. Once the storm wind passes the Cascades and encounters the less topographically complex land near the eastern Gorge it will settle out a bit and become stronger.GorgeStorm2000ft

But there is a second factor behind the VERY GUSTY AND UNSETTLED forecast.

Take a look at the second animation. This image shows the winds at 2000 feet above sea level. Note how these winds are shown in a red to blue color indicating that they are stronger than the winds in the first animation.

As these winds, and similar winds at higher and lower levels hit the mountains they become turbulent and create up and down down stream oscillations much like
ocean waves but in this case the waves are composed of fast moving air.

Sometimes if the dew point is perfect you can actually see these waves as the water vapor in the rising part of a wave condenses forming a cloud while the clouds evaporates in the descending part of the wave. But tomorrow these waves will sometimes descend to near the surface. When that happens the powerful winds aloft transfer GorgeStorm2momentum to the surface wind and you experience a crazy gust.

Then a few seconds later the wave of storm wind lifts and you experience a major lull. If you are lucky and hit the water at the right place at the right time you might have an epic session. But at most sites the winds are going to be what Ben euphemistically calls UNSETTLED.

To increase your chances of finding wind that does not suck think of launch sites with barely any topography nearby. See you there!

Looking at the 2nd image can you figure out why there are such strong winds in the Willamette Valley as the storm hits the coast?

Update at 1:20PM Saturday. The new satellite video below shows 3 mile canyonthe storm
hitting the Gorge. As expected the strong winds are focused out east with 3 Mile Canyon averaging a steady 35 mph. Notice the wave clouds I mentioned in the blog.

 

Posted in Columbia River Gorge

West Coast Wind Blog: Old North Pacific High fades while new NPH develops

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The pacific weather pattern is a mess!

Any veteran windsurfer or kiter, whether from the Gorge, Bay Area, California coast or Southern California or even Hawaii, will attest that this has been a weird summer season. The proximal cause of the atypical winds is the unusual shape and locations of the North Pacific High. In the Bay Area this has resulted in unreliable winds for some sites for some of the normally most reliable sites and an epic season for sites that are typically less reliable. While in the Gorge the winds have been less frequent, weaker, gustier, hotter and briefer than normal. And both Southern California and Hawaii have also seen atypical winds. The ultimate cause of this wind weirdness is a complex interaction between the strange Pacific Heat Blob, the Pacific Decadal Oscillation (PDO) a maturing El Niño and the North Pacific High and its prodigy the Golden Gate eddy.  That complex interaction will be the focus of a future blog. Right now I just want to show you today’s weirdness out in the Pacific.

MultipleUpperRidgeTrough

Looking at this first model animation of the winds at the 500mb level at ≈ 18,000 ft. from http://earth.nullschool.net find the Bay Area and The Gorge. The immediate weather maker is the large Cut-Off Low you see just west of the USA. Right now the Cut-Off Low is still connected to its parent upper trough but it looks like it will become complete Cut-Off by tomorrow. Once that happens it will  be free to wobble around in the Pacific near the west coast. Look at both the Gorge and the Bay Area extended forecasts and you will  see that the forecasters are being unusually wishy washy about the winds the next few days since the fate of any Cut-Off Low is tricky.

The second thing to notice in the animation is the extreme northward and southward extension of the upper troughs and upper ridges. This is something that is becoming much more common in the last decade or so. Why is it important? The air mass within an upper trough is cool air that is extending southward from polar regions so it tends to bring cooler weather. For example the Gorge has been cooler than normal in recent days due to the upper trough you see in the animation. Likewise the air mass in the upper ridges you see in the animation is warm air extending from the tropics. For example Southern California is going through a real warm up due to the upper ridge you see over the USA.

More importantly these greater than normal extensions of the upper troughs and upper ridges also have climatic consequences. In normal times upper troughs and ridges move from west to east over a given point in a few days. So our weather is constantly changing. But when the upper troughs and upper ridges become elongated like you see in the animation they move very slowly over the continents and may even stall for long periods. We saw this happen this last winter when the entire west coast had a very mild winter as an upper ridge lingered over western North America much of the winter. Likewise when an upper trough stalls over a region the winter will be unusually cold. These exaggerated N. and S. extensions of the upper level winds is the main reason you are hearing about more extreme weather patterns in the future.OldNewNPH

Oops! we are getting on a side track since our focus is suppose to be wind. So what does all this have to do with wind? Check out the second animation. Notice the “old” North Pacific High. This is the remains of the NPH that has spanned the waters from the coast of the Pacific Northwest to Hawaii the last few weeks. But in recent days it has been battered by storm systems and been greatly distorted and reduced in size. Today it is being crushed against the west coast by a surface storm system. Now glance back at the first animation. Notice that this surface storm is below the Cut-Off Low. This alignment greatly enhances the strength of the surface storm and as it moves towards the coast it will destroy the last remains of the “old” NPH. So Southern California and the Bay Area will lose the NW wind component of their wind machine while the Gorge loses the high pressure critical for making strong wind.

Now look war to the west and you will see a new North Pacific High forming. Once this surface storm crosses the Pacific Northwest this new North Pacific High will move towards the west coast. Normally this would mean NW clearing winds on the California coast. But more storm systems will batter this new NPH and I don’t expect much of it’s NW wind to reach the California coast or to impact the Gorge winds until next Monday or so.

Posted in Columbia River Gorge, Los Angeles, San Diego, San Francisco

West Coast Wind Blog: Monsoonal moisture and clouds coming over Southern California.

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ZZ395EE792Upper level high pressure sweeps moisture from the Gulf of Mexico over Old and New Mexico to
Southern California.

Looking aloft today you will see scattered high clouds but over the Southern California deserts the clouds are much more dense. Most of the isolated showers from this monsoonal moisture will only create isolated showers in the interior but all the clouds over the deserts, along with the  large Catalina Eddy will probably limit today’s Southern California coast winds.

This is a very minor monsoonal event but this blog will show the origin of these clouds.

In the first image from 11AM today you can see that there are scattered clouds over much of the coast with especially dense clouds over the San Diego region.

But looking over the interior you can see the much denser clouds which will limit the pressure gradient.

The first animation shows the monsoonal moisture heading towards Southern CaliforniaMonsoonalSouthern California. This imagery is from a satellite with a sensor that detects water vapor at levels above about 10,000 feet. The green areas show lower levels of moisture while the orange areas show dense concentrations of water vapor.

The second  animation shows the upper level winds at ≈ 18,000 ft. First find the upper level high pressure centered over the 4 corners area. Notice the clockwise winds circulating around this high pressure. This high pressure tends to heat the land below it hence the toast temps today in the interior of Southern California.

Now notice how the winds of the upper high pressure sweep across the Gulf of Mexico and end up bringing moist air to Southern California. This is this is the source of the high clouds you are seeing today.

upperlevelMonsMoistuerThe next animation shows the winds at ≈ 18,000 ft. The mass of clockwise moving air over

Posted in Los Angeles, San Diego

Atlantic Basin Tropical Tools – What Do Meteorologists Use?

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By WeatherFlow meteorologist Shea Gibson.

Ever wonder how a meteorologist might study the Atlantic Basin for tropical activity? Well in this article we hope to show you some great tools to use so that you may find your own bearings on how to do perform your own “tropics watch”. There are many, many products, apps and tools out there to use, so we are not giving all of them here…but just a few things we may use to make educated evaluations on what is going on in the tropics and what may happen given the data and empirical evidence we gather.

When we study the tropics, one of the first things to think about is getting a general perspective of the entire basin, which includes the North Atlantic Ocean(north of the equator), the Caribbean Sea and the entire Gulf of Mexico.  It’s good to see everything in motion to view any potential areas of rotation. As you should remember, Low pressure in the Northern Hemisphere spins counter-clockwise. Conversely, High pressure spins clockwise.

So here is one way we can see everything at once with the Atlantic wide view found here the NOAA “GOES” (Geostationary Operational Environmental Satellite) site to find some useful information:

http://www.ssd.noaa.gov/PS/TROP/trop-atl.html

You’ll need to choose which media outlet you prefer (flash player, java, HTML5, gif’s) to use or what works for you, but there are some additional features within the flashplayer and HTML5 on each product in the list. Remember the visible satellites become infrared when darkness sets in. You can see the sunset edging on the eastern side of this map over the Cape Verde Islands off the African Coast. You can also find other smaller regions to choose from for up close looks at those areas.

VIZATL

Here is the “Water Vapor” imagery with the Tropical Forecast Points (track of storm) for named storms and HDW -High (upper level winds) activated. Just a way to see the path and the possible shearing factor for areas of disturbance or tropical cyclones.

WVATL

The next tool that is naturally useful for us at WeatherFlow…is the map in Wind Alert to see if we can get any readings from sensors at or near areas of interest (includes ocean buoys, near-shore buoys, coastal stations and all sensors in the public domain). The other products/applications such as SailFlow, FishWeather, iKitesurf, iWindsurf and the professional product Datascope also have the 50,000+ stations available to check worldwide.

www.windalert.com

wa

From here, you can go to the National Hurricane Center website to check for any areas of disturbance they have. This can be done first in the order, but if you really want to test yourself… you can study those Tropical Atlantic maps and the other tools below to see if there are any areas you can point out first… and then do this part lastly.

http://www.nhc.noaa.gov/gtwo.php?basin=atlc&fdays=2

They can provide you with areas of interest – sometimes knows as “Invests”. Some of these areas are given a percentage for chance of development in the next 48hrs…and the chance of development over the next 5 days. Here we have Invest 97 (#1), Tropical Storm Danny, Invest 98 (#2), and Area of Disturbance #3 coming off the African coast.

Here is an active Atlantic Basin shot on Sunday, August 23, 2015.

nhc1nhc2

Ok so now we have some information to go on as you identify “areas of interest” on your personal radar. Now we will take a peek at the SST’s (Sea Surface Temperatures) to see if water temps provide the fuel tropical systems need.  There is quite a bit of variation in what constitutes ideal temperatures for tropical systems. The general idea here is that the warmer the waters are, the more favorable it is at the surface for development. There are many products out there pulling from NOAA/NASA satellites, NOAA buoys and other coastal instrumentation to find out what the SST’s are…and there are many ways to find it. Here is one I like to use – and it is fairly new from NASA…called Sport SST – and it is very user friendly. :)

http://weather.msfc.nasa.gov/cgi-bin/sportPublishData.pl?dataset=sst&product=sport_nhemis_sstcomp

You can choose the region to focus on and animate from past to present in 20 day increments. Makes for a pretty neat visual!

sstAlright so now you are on your way to being an amateur.  Now we can move into the intermediate levels. Saharan Air Layer. This is where Saharan desert dust blows out over the Atlantic Ocean at the mid levels of the atmosphere and travels Westerly all the way to South America and parts of the Caribbean (even into the Bahamas and southern Florida).  This is important to know if you are watching waves come off the African Coast as this dust can fluctuate light to heavy…with heavier dust events able to dry out the air so much that tropical systems cannot develop.  If you are in the mood for reading more about it, I did a 3 part study about this last year 2014. Part I: http://blog.weatherflow.com/sal-ty-atlantic-the-saharan-air-layer/ , Part II: http://blog.weatherflow.com/sal-ty-atlantic-the-saharan-air-layer-part-ii/ and Part III: http://blog.weatherflow.com/sal-ty-atlantic-part-iii/

Here is a good product to use that is animated. You can see the dust levels and where it is in the atmosphere. The heavier the dust levels, the less likely a storm can develop near it. Below is Tropical Storm Danny 2015 to the far west (Invest 97 well to the north), Invest 98 behind him to the East, and Area of Disturbance #3 presently coming off the African coast.

Cooperative Institute for Meteorological Satellite Studies
Space Science and Engineering Center  /  University of Wisconsin-Madison

http://tropic.ssec.wisc.edu/real-time/sal/splitE/movies/splitE5.html

sal

Next we like to see what the “Precipitable Water” looks like and where the drier air is located outside of what the Saharan dust levels are showing. Along the ITCZ (Intertropical Convergence Zone), you typically see quite a bit of moisture pushing along the east to west belt of troughing, where areas of disturbance are fed into from the southwest, south and southeast. You can also see where areas of interest are wrapping moisture into the centers of these Low pressures as the animation provides a great visual.  This is further evidence of possible enhancement to the system(s).

http://tropic.ssec.wisc.edu/real-time/mimic-tpw/natl/main.html

pwats

Ok so now you are becoming intermediate in what you are doing. Want to follow the rabbit hole even further? Come on let’s take a look at some advanced stuff…

This tool is the Advanced Scatterometer – or ASCAT METOP A and METOP B satellite information. This allow for vertical readings from space as Scatterometers operate by transmitting a pulse of microwave energy towards the Earth’s surface and measuring the reflected energy. The main purpose of this technology is to find out the near surface winds over the ocean.  It allows for meteorologists to see what the winds are doing in these areas of disturbance…and can be attributed to naming of systems before the NOAA Hurricane Hunters fly in or buoy readings capture speeds at the surface.  The satellites orbit and do what are called ascending and descending passes over the Earth. As they do, they “swath” the planet with these microwaves…which looks like ribbon strips. Within these strips are enormous amount of datasets that are very useful to meteorologists around the world.

Here is a great source from STAR (Center for Satellite Application and Research). You click on the ribbons to see the data. Wind barbs are important to know, so here is a quick wind barb chart:

windbarbspeeds

http://manati.star.nesdis.noaa.gov/datasets/ASCATData.php

ascat

 

Another great ASCAT source is from the Ocean and Sea Ice Satellite Application Facility. The below screenshot shows where Invest 98 is trying to develop closed Low pressure from the counter-clockwise wind barb directions.

http://www.knmi.nl/scatterometer/ascat_osi_25_prod/ascat_app.cgi

ascat2

Next up we have a fairly new tool developed by a brilliant individual named Cameron Beccario. He has created the beautiful wind mapping product http://earth.nullschool.net/ . Using this product, you look over areas for what winds are doing at most levels of the atmosphere. As things change higher up, we always have to look for upper level shear to weaken storms as cloud tops get blown off by stronger winds aloft. This limits vertical stacking and prevents tropical cyclones from developing further…and many times strips them of surface storm energy from remaining wrapped around the core.

Here is a screenshot of small Tropical Storm Danny just east of the Leeward Islands:

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And here at roughly 34,000 feet (250hPa)..we see the upper level shear from the S and SW that is currently weakening him.

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Ok so now you have the ASCAT information, visual satellite imagery tools (cloud types and patterns), infrared imagery, rainbow/color infrared imagery for coldness of cloud tops and shear patterns. Using this data, we may be able to deduce from what is called the Objective Dvorak Technique if you have a tropical system or not…and then put your findings to the test by going to the NHC site listed near the top of the page and seeing if they have that area circles or highlighted for observation.

You are now equipped with some good tools to use in your adventures on learning how to see what is going on in the tropics. As far as forecasting, well that may be a story for another day.

BUT, if you really want to get to an expert level, it wouldn’t hurt to look at Global patterns to see how the Hurricane Season may play out from year to year. Here are two sources to check out for a taste of what the global scientists look at:

El Niño/Southern Oscillation (ENSO) Diagnostic Discussions at:

http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/index.shtml

And the Madden Julian Oscillations (MJO):

http://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/mjoupdate.pdf

 

Cheers from us at WeatherFlow,

Shea Gibson

Forecast Team, SE Region/East Coast

 

 

 

 

 

 

 

 

Posted in Coastal Georgia, Coastal South Carolina, SouthEastern North Carolina Tagged with: , , , , , , , , , , , , , ,

West Coast Wind Blog: Video of elongated eddy spoiling coast & Peninsula winds in Bay Area. Plus the NPH just will not settle down.

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ElongatedEddy10DaysLIfeNPH

Posted in San Francisco

West Coast Wind Blog: Dissection of how the Gorge wind goes from MILD to WILD.

WCWB4Inbound shortwave deepens marine layer and jacks up
pressure gradient.

Compare Crysta’s iwindsurf.com forecast for Wednesday and Thursday. How can conditions change from limp winds to upper 20’s   overnight?

ZZ288A79C2First let’s review what is causing the current limp winds by looking at the imagery in my last blog. The heat producing ridge you see in that imagery has been heating up the Gorge, weakening the pressure gradient and crushing the marine layer clouds hence the weak winds.

But over the pacific at ≈ 18,000 ft. feet an upper trough is moving towards the Pacific Northwest and should come over us Wednesday night and Thursday. Think of an upper troughs as southward extending loop in the upper level winds. The air within this loop of is cooler and at lower pressure than in an upper ridge. In this case the loop is an especially tight loop spanning only a few hundred miles.

Thursday as inbound shortwave at ≈ 18,000 ft.comes over us several things ShortwaveGorgewindshappen:

1. The wind flow at the leading edge of the shortwave spreads out or diverges creating lower pressure at ≈ 18,000 ft. that intensifies the surface low pressure in the Columbia Basin. So the pressure gradient suddenly goes up  which enhances the wind.

2. As the shortwave bumps the heat producing upper ridge eastward we begin to receive cooler air. And as the coast cools the air becomes denser creating high pressure. This further enhances the pressure gradient between the coast and the Basin.

3. In recent days the upper ridge has been compressing the marine layer cloud and as the upper shortwave comes over the coast the inversion lifts so the marine layer deepen and move from the coast over Portland and into the western Gorge. This mass of cooler denser marine air further enhances the pressure gradient and since it is heavier air IsobarsGorgeit tends to flow like molasses and sticks better to the water so the winds will be somewhat steadier than if there was no marine layer presence.

4. As the strong winds of the shortwave come over us they hit the mountains of the Cascades. This creates waves in the upper level wind that sometimes transfer momentum to the surface pressure gradient winds. This adds a big GUST factor which is somewhat moderated by the marine layer clouds. If conditions are just right you will sometimes see these waves as wave like patterns in the cloud out east.

The video above will allow you to visualize some of these processes.shortwaveArrivesGorge

Posted in Columbia River Gorge

Upper level high pressure creates surface low pressure… what is happening here?

WCWB17How expanding low pressure impacts Southern California, UpperRidgeUpperTroughHeatWaveBay Area and Gorge winds.

“There’s something happening here
What it is ain’t exactly clear”
Buffalo Springfield

Sometimes meteorology seems to jargon ridden to wind seekers that it is hard to figure out what is actually happening. It is hard enough to make sense of all this high pressure and low pressure and pressure gradient gibberish only to find out there are upper high pressures and upper low pressures that also impact the surface winds.

Since the wind will be impacted for the next several days  at most west coast sites by upper level high pressure and surface low pressure it is good time to discuss how those factors impact the distribution of the wind.

1. The Gorge is moving into a pattern with very weak winds but Jones Beach closer to the coast BLOWS!

2. The Bay Area transitions into a pattern with mild wind at most sites but Crissy, Candlestick, Coyote and 3rd. Ave. are likely to BLOW!

3. The Southern California beaches BLOW but Belmont and Seal Beach are threatened by the dreaded “Heat Bubble”

Could there possibly be a connection between the wind patterns at sites separated by up to a 1000 miles. You bet! All three venues are going into a heat wave. Looking at this first animation of tomorrow weather pattern you can see the causes of the heating. First remember that the winds you are seeing in the animation are up at ≈ 18,000 ft. or more technically the 500mb level. UpperHighSurfaceLowBayGorgeSC What is important is what these wind patterns do to the surface temps. First note the Upper Level Trough that is exiting to the east taking the recent cool air with it.

Then note the Upper Level Ridge that is moving eastward coming over the western USA. As it comes over us this Upper Level Ridge brings warm higher pressure air over the west coast. Also note the clockwise spinning Upper Level High over Southern California which will bring hearing to the inland valleys and Southern California deserts.

At this point you may be thinking “I don’t care about heat I am looking for wind” Let me connect the dots: all the heat from these upper level high pressure systems tends to focus in the interior areas far from the cooling effect of the Pacific. This is especially true for valleys since there is less wind circulation in a valley. So the California Central Valley, the Columbia Basin, the Great Basin  and the Southern California deserts begin to cook with an upper ridge aloft.

As the valley land cooks it heats up the surface air. This causes the surface air to expand and rise creating low pressure. So far this sounds promising for wind since we all know that low pressure is critical for creating strong wind. The problem is that with an Upper Level Ridge overhead there is so much surface heating that the expanding air balloons beyond the basins or valleys towards the coast and once it comes over the sailing and kiting sites the pressure is low both in the valley and ALSO at the sailing/kiting sites. So the pressure gradient fades away as does the windLet’s see how this ballooning impacts the premiere west coast wind venues:

Bay Area: Looking at this second animation let’s look at the Bay Area first. Notice how the Central Valley thermal low pressure trough in BLUE has expanded over the coast in the AM. This means there is almost no pressure gradient for Bay  sites in the AM to mid afternoon. Then notice how the low pressure retracts near the coast in the PM. This means the isobars of the pressure gradient is tightly focused over sites in the Crissy to 3rd Ave. zone. Since the resulting local sea breezes have to move over land they are in wind shadow near shore and focus mostly outside. Meanwhile sites more inland like Pt. Isabel and Sherman Island see only faint wind.

The Gorge: Looking at the animation notice the low pressure, in GREEN,  is first confined to the Columbia Basin. But as the Upper Level Ridge comes over the Pacific Northwest notice how the low pressure trough expands over the Gorge and its edge ends up on the Jones’s Beach zone. So each day as the pressure gradient builds in this area the winds ramp up Jones blows while Gorge sites from Stevenson to Three Mile Canyon only see weak winds.

Southern California: With an upper level high pressure overhead the temps in the Southern California deserts and the inland valleys moves towards the century mark. As the land heats the interior low pressure trough deepens and balloons towards the coast. Now things get really tricky. If the low pressure’s isobars stay just at the coast many sites see solid upper teens to even low 20’s wind. But if the low pressure thermal trough (aka “heat bubble”) balloons over the coast the pressure gradient fades and the winds become weak and/or unreliable. Cabrillo is usually protected from the “heat bubble” since it is out on the Palos Verdes Peninsula surrounded by relatively cool water that blocks the “heat bubble”).

So when does normal wind return to the Gorge, the Bay Area and Southern California? Well since the upper level ridge you see in the first image is drifting eastward it will soon depart from the west coast. And when this happens all that heat in the Columbia Basin and California Central Valley will still be there ready to create a powerful pressure gradient as the North Pacific High moves back towards the coast. So I am expecting a SW marine surge towards this coming Friday for the Bay Area. Something similar should happen for the Gorge. While Southern California sees weaker winds as the low pressure moves further away.

 

 

Posted in Columbia River Gorge, Los Angeles, San Diego, San Francisco

Is Your Name Eddy?

by Meteorologist Kerry Anderson

This morning one of our spotters on Catalina Island took the following series of photographs of the marine layer and reported that from his position at Two Harbors that it appeared that the marine layer was moving southward and he wanted to know if there was an eddy.

Catalina 4 pictures AUG 14, 2015

Series of views from Two Harbors, Catalina looking west on August 4, 2015 starting after sunrise. The marine layer was observed to appear to be moving southward.

Actually it is more likely that the movement he saw was from the strong NWeseterlies that have built on the outside of the Bight but since he brought up the question of eddies I am more than happy to oblige with a conversation since it has been topic among our West Coast forecasters.

You may have noticed that we more frequently describe eddies and discuss how long they will last.  Have you wondered if eddies are forming more frequently?  No that’s not it. Actually the change comes because we now have the ability to spot and track small eddies due to recent advances in the forecast models provided through WeatherFlow.  These small eddies can change the wind pattern for beach locations and can many times keep the beach winds from building until they dissipate.  We also report these small scale winds in our Pro forecasts because these are not just used by Wind and Kite Surfers, who are hoping from strong winds in the afternoon but these forecasts are also disseminated to fishermen, sailors and many different outdoor wind enthusiasts.

Actually this morning the inside waters of the Bight did experience a few small eddies for awhile.  We could not see them on the satellite because the skies were clear and they were too small to detect just using our network of observations.but the HRRR computer model detected them.  

Below is the HRRR from this morning at 7pm.  You can see the strong NW winds blowing on the outside of the California Bight while inside the winds have been cut off from this flow and just like in a stream we see small pockets of circulation.

Eddy Aug 14a

HRRR forecast for the Southern California region on August 14, 2015. Notice the circular motion of the winds “eddies” near the coastline.

Up until the advent of this model, last year, we would not have seen all these small scale features. The HRRR delivers a new forecast every hour using a small scale grid.  It does this by ingesting new observations such as  radar, airplane reports, surface observations every hour and then running these numbers and delivering a forecast for each hour for the next 18 hours.  This combination of small grid, new observations and a forecast for each successive hour allows us to see smaller eddies that only last a short time and forecast how long they will last

So our big question is what to call these circulations. In Southern California we are used to talking about eddies when we see a Catalina Eddy and people know to expect low clouds and cool weather well inland.  So should we call these small wind patterns eddies too? Dictionary.com describes an eddy as either 

  1. a current at variance with the main current in a stream of liquid or gas,especially one having a rotary or whirling motion
  2. a small whirlpool.
  3. any similar current, as of air, dust, or fog. 

Based on these definitions obviously what we see are clearly eddies but they are not the big eddies people are used to hearing about.  So as science advances we are caught in the dilemma of how to describe these patterns without overusing a term that has become synonymous with larger features.  That debate may continue for awhile but for now we will be careful to describe the features we see, let you know how long they will last and how they will affect your outdoor activities.

Posted in Columbia River Gorge, Los Angeles, San Diego, San Francisco, Weather Blog

Cut-Off Low ejects over next 24 hours and Gorge and Bay winds improve.

WCWB10

Watch as Cut-Off Low opens up into an upper trough then exits the West Coast.

You are used to hearing about the passing upper trough that pass over the west coast and occasional upper ridges that bob around from the Great Basin. These upper features are upper winds wind around the earth. WhenCut-Off LowGorge& BAy these winds loop southward they allow warmer air and low pressure from the south come over the Bay Area and the Gorge. And they may also allow surface low pressures to develop. Either way they tend to push the North Pacific High away from the Northern California coast so the NW winds fade.

Rarely the loops of upper troughs become very tight loops which may pinch off from rest of the upper level winds leaving a counter-clockwise spinning mass of air at ≈ 18,000 ft. that is known as a Cut-Off Low. Since a Cut-Off Low is disconnected from the overall flow of upper wind from west to east the Cut-Off Low meanders about to our west. And this disconnect makes the trajectory of the Cut-Off Low very difficult to forecast.

The current Cut-Off Low has meant that  the Bay Area has had less than optimal winds south of the Bay Bridge for several days while most sites in the Gorge have had weak or unsettled up and down winds. In the Bay Area this has been caused by several surface low pressure spawned by the Cut-Off Low while in the Gorge the issue has been unstable air and moisture aloft.

The Cut-Off Low at ≈ 18,000 ft. which has been responsible for this situation can be seen ejecting eastward in this video. Once this Cut-Off Low departs we should see more season typical winds in both the Gorge and the Bay Area.

Posted in Columbia River Gorge, San Francisco