Bay Area Wind Blog: NPH ridge kills winds: Gorge and Bay Area

Heat producing ridge kills most sites but Jones Beach and Bay sites near coast still see wind.tempsUpperRidge

by Mike Godsey, mike@iwindsurf.com

All signs point to a real warm up and light wind forecast for the Gorge and most the S. F. Bay Area for the next 3-5 days. To understand how this results in mostly weak winds let’s start far aloft.

This first graphic shows the upper level winds at about ≈ 18,000 ft. Notice the exiting upper trough that has kept most of the west coast relatively cool the last few days.

Next note the incoming upper ridge. Also note that today the hot surface areas, in red, are most confined to the California Central Valley and the Columbia Basin.

Now look at the second image. You can see that the cool upper trough has exited to the east and the heat producing upper ridge covers much of the western USA. Looking at the surface temps you can see that the hot are has expanded over the Gorge and even into Canada. And if you look carefully you can see that hot air has expanded very close to the Bay Area coast.

OK so we are seeing warmer weather but how does that result in weaker winds. In the area of an upper ridge you have descending air. And as this descending air reaches the surface it compresses and heats. Hence the higher temps is the coming days.

As the surface heats the air expands creating low pressure. Each day as the heating cranks up the low pressure in the Great Basin expands and first comes over the corridor of the Gorge and then over Portland. This means:

1. The marine layer clouds evaporate

2. With low pressure over the Gorge the pressure gradient and winds fade away.

Meanwhile down in California the Central Valley thermal low expands over the Bay Area coast each day.NPHridgeGorgeBay

This means:

1. The Bay Area sees a warm up especially inland of the coast.

2. The marine layer clouds evaporate.

3. No pressure gradient in the AM so Sherman Island is dead.

4. The North Pacific High’s surface NW winds are pushed far from the coast so the Bay Area loses that part of the wind machine.

Now let’s drop to the surface and take a look at the action.

In the animated 3rd image find the North Pacific High which is a surface high pressure area in RED. Picture the NPH as a mountain of high pressure air. Typically it is a round to oval mountain of air but sometimes it forms an extension or ridge like you see from a rocky mountain. In this animation you can see each passing day it is pushing a ridge into far Northern California and into the Pacific Northwest.

As it extends this ridge you can see several things happen:

1. Note how the North Pacific High’s surface NW winds move away from the California coast.

2. Notice how the Central Valley thermal low begins to balloon over the California coast leaving a very weak wind zone over the coast.

3. Then watch how the North Pacific High moves towards the Oregon coast and strong N. winds develop.

4. As the days pass watch the low pressure from the California Central Valley expands northward and merges with the Columbia Basin low pressure.RidgeIsobarsGorgeBay

5. Then this low pressure balloons over the Gorge killing the wind.

Why does Jones Beach and sites like Candlestick, Crissy, Coyote and 3rd. Ave. still see wind even during a heat wave?

To forecast winds we look at the concentration of isobars that all of this produces. The last animation shows how the isobars dance around over the next 4 days. This image is a bit complex since the low pressures expand and contract each day with daylight and nighttime.

First let’s look at Jones Beach just west of Portland. Looking carefully you can see the expansion of the Columbia Basin low pressure over the Gorge each day. But in the afternoon you can also see that those isobars pull back to near Portland about the same time as the NPH’s isobars push inland to near Portland. This is a perfect set up for strong winds at Jones.

And in this weird wind summer there is even a chance that this isobar dance could bring some mild wind to the corridor despite all the hot air.

Now look carefully at the isobars near the Bay Area. Each day you can see the Central Valley thermal low expand over the coast. But in the LATE afternoon those isobars  retract slightly to right along the Bay Area coast. This means there  is a pressure gradient over sites right at the coast. So there is a chance of mild wind at the Stick, Crissy, Coyote and 3rd.

Posted in Columbia River Gorge, San Francisco

Piedmont Trough sparks moderate Sea Breezes in SC.

By WeatherFlow meteorologist Shea Gibson

Watch as Piedmont troughing/storming helps increase the Sea Breeze by providing stronger return flow up high over the ocean as storms move from WNW to the ESE.

Posted in Coastal South Carolina

A View of a Tricky Forecast w/ Tropical Discussion

By WeatherFlow meteorologist Shea Gibson

 

Posted in Coastal South Carolina

Bay Area Wind Blog: Why is Año Nuevo clearing so important?

Año Nuevo clearingThe marine layer clouds clearing in this area sets the stage for strong winds from Waddell to Natural Bridges.

by Mike Godsey, mikeATiwindsurf.com

Those of you who make the drive to Waddell and points south are well acquainted with Año Nuevo clearing and the racing of your heart as you finally round a fog shrouded curve and see impossibly blue skies near Año Nuevo. But for Bay sailors the frequent comment about Año Nuevo clearing in the forecast may seem cryptic. So I made this animation so you can see what it looks like. The reason Año Nuevo clearing is so important to Santa Cruz area winds is complex and one day I will  do a blog on super critical flow and  the magic Waddell’s complex wind machine.

And even if you do not ply the coast waters early Año Nuevo clearing is a good sign for Crissy, Coyote & 3rd. since it typically is a harbinger of clearing at those sites.

Posted in San Francisco

Always watch those East-West fronts in the south.

7-23-15 DS 0645

By WeatherFlow meteorologist Shea Gibson

As we are seeing an undulating front ripple through the SE Region, we always have to watch for Low pressures tracking along them to become better organized. With High pressures to the north and to the south, the rotational profile increases to help them develop. Once they enter the ocean, they have a better chance of becoming further organized for possible tropical development.

High pressure to the north is only at 1012-1016 millibars, which is fairly weak overall for “top spin”. It does have room to build – and if it does build to 1020mb’s or higher, we could see stronger divergence feed a stronger overall rotation.

7-23-15WSI LOW

And here is a current area of Low pressure in its first stages of development. We can see based on surface wind readings where it is.

7-23-15 DS 0645

Sea Surface Temps are currently in the low to mid 80’s, making it a favorable environment over the warmer waters.

SST

 

al_rTCFP_048Just something to keep an eye on for now…

 

Cheers,

Shea Gibson

WeatherFlow Forecast Team, SE Region

Posted in Coastal South Carolina

Heat Wave Shows Rapid Severe Storming in SC.

DS IOP

By WeatherFlow meteorologist Shea Gibson

July 22, 2015:

We saw a very rapid intensification of a thunderstorm breakout as areas of convection met the urban heat of downtown Charleston and industrial zones just to the north. The general flow at all levels for the day was West to East except at the coast where we had a Sea Breeze front steadily holding mid teens SSW flow. Overall air temps hit mid/upper 90’s inland for the fueling…with humidity causing heat indices to reach 110-115 degrees. Over the cities, we could easily have found 100-105 degrees with higher indices near 120.

The day started out mild with heat building throughout the morning.  There lower mid level blanket of strata (slight inversion) about 10,000-12,000ft up pushing from the west over the ocean at about 20kts by midday. It met the Sea Breeze at the coastline, which slowly dissipated it. Heat trapped at the surface slowed the process a bit at first then came up quickly. Once the stable layer cleared, we saw a healthy build start for the afternoon as thermals were able to rise higher.

Here is the stable layer that showed through midday…this is facing South towards the ocean from about 12 miles inland up the Cooper River. 

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Immediate following it, skies cleared as thermals started to lift and begin the cumulus formations…with air becoming more unstable. This is facing to the West. 

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By 3:23- 3:27PM, we started to see some activity brewing to the south begin to spread northwards quickly.  A quick first very small pop-up cell swept Sullivan’s Island as a head’s up. Radar didn’t pick up on it until it was already overhead. Winds switched direction so fast a local kiteboarder’s kite flipped out into the ocean and headed offshore (amazingly, this individual did get it back via jet ski after the entirety of the storm had passed). 

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And six minutes later by 3:33PM we had a bow echo develop and this storm swept across the Chas Harbor and over Sullivan’s Island/Isle of Palms.  

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View from Sullivan’s Island at Station 28.5. This was the northern end of the storm. 

 

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Speeds really came up. Here is the graph from Isle of Palms Pier just 3 miles away to the north. The first spike is straight line winds that reached 42mph from the SSW inline with the Sea Breeze zonal flow. The second spike was a direct result of very strong downdrafting, which pulsed up to 70mph. There was small hail that fell just minute or so before this to the south along Fort Moultrie. Possible funnel cloud within the squall? 

DS IOP

Here’s a little vid I took from my car during the squall for some effect. 

https://www.youtube.com/watch?v=eFhnyZJACIE

A Hobie Cat was seen on the Isle of Palms Carolina Coast webcam upending down the beach and finally stopped at the pier. 

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Notice the drop in air temps as well.  

DS AIR IOP

Following the storm, air temps came back up as the sun returned, the Sea Breeze front returned and everything went back to as if nothing had happened.  This is a prime example of what happened during a heat wave with a West-to-East storm flow that picks up quick momentum as urban heating fuels areas of convection.  Here is a pictorial of how the urban heating works to supplement heating to storms.

heatisland-main_0

Just goes to show how fast these things can happen.  Keep an eye to the sky during heat waves!

Cheers,

Shea Gibson

WeatherFlow Forecast Team, SE Region/EC

 

Posted in Coastal South Carolina

Sea Breezes in the Southeast Region: Part II – Types of Sea Breezes

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By WeatherFlow meteorologist Shea Gibson.

In Part I, we discussed the general understanding of how Sea Breezes work. In part II, we will look at the different classifications of Sea Breezes and how each one works.

In order to fully understand the “Is there only one kind of Sea Breeze?” question – and since they are not all the same, we will need to look at the 4 different current classifications – with 3 of the 4 being the main ones…”Pure Sea Breeze”, “Corkscrew Sea Breeze” and the “Backdoor Sea Breeze”.  The  “Synoptic Sea Breeze” is a bit different and sometimes limited to weak overall system patterns, so we’ll take that one last. Let’s get started with the first.

Pure Sea Breeze:

This breeze is very common and occurs without a vast synoptic flow (large scale flow as in a strong Bermuda High for e.g.) …and is usually perpendicular to shore.  For GA, it would be a general ESE flow perpendicular to the ESE facing beaches. As we head north into SC, this takes on a more SE direction…. and SSE/SE up into Southeast NC.  This breeze relies heavily on air-to-water temperature differential.  The smaller the differential, the weaker and later the Sea Breeze may be. By default, even with rains and storms traversing the areas, we can still see a ridge of High pressure build back along the immediate coast to get the light to modest flow going again. At times with a stronger synoptic flow, a warmer air wedge can settle out over the water and act as an artificial shoreline.  This would make 2 Sea Breeze fronts where there is calm in between – this happens quite a bit in the spring where marine layering/sea fog hugs into along the beaches and is clear just inland where Sea Breeze front is elevating winds (due to warm air from land streaming downwind)…and also where the Gulf Stream is helping to fuel the offshore breeze from behind. We could even call the Gulf Stream a secondary artificial shoreline in some cases with its much warmer surface temps.  The Pure Sea Breeze, overall, can many times limit the builds to modest speeds as moisture streams in from the ocean to contaminate what would be a convergence zone inland. In this case, precipitations would be freely allowed to occur almost anywhere, which would keep any kind of buildup inland very minimal and at times, non-existent. Other times, it can show higher numbers inland just along the front and weaker values behind it as the front penetrates further inland. In most cases for speeds  (over years of observations) the limitation along the immediate coast tends to be 9-13kts; however, there are the very infrequent exceptions to the rule if the gradient is tightened over the area to increase speeds…or is influenced by tropical activities lifting from the south.  This is very seldom as the Pure Sea Breezes generally bring lighter overall flows.

We can clearly see this type of breeze when subtropical ridging takes effect where High pressure in the Atlantic flattens and places a ridge axis along the coast to provide a weaker overall pattern. See this article for details and meaning of subtropical ridging: http://blog.weatherflow.com/subtropical-ridging-effects-in-the-se-region/

Picture below represents flows of winds. Pure Sea Breezes are shown in figure (a). Green shading represents land where straight offshore Land Breezes (3 black arrows) are eventually undercut by straight onshore Sea Breezes ( 2 blue arrows).

qj2484-fig-0001

Here is an example of a weather pattern that presents the ridging effect which prompts Pure Sea Breezes – this one showing the bulk of effects to the FL/GA/SC coastline where winds are weakest near the axis as High pressure is squeezed and flattened in between two frontal boundaries:

Subtropical-Ridging-4-12-141

 

And an example of what occurs with this weakening pattern at Oak Island, NC where the South direction is directly perpendicular to the coastline in that section of Southeast NC. 

4-12-15OakI

Corkscrew Sea Breeze:

The Corkscrew Sea Breeze is the most common of the Sea Breezes along the SE Region – especially during the warmer months.  This breeze can start out with an offshore Land Breeze that eventually turns onshore as the Sea Breeze pushes ashore and heads inland, giving it a helix shape. Sometimes the land breeze can become so aggressive that the Sea Breeze struggles to undercut the land breeze, where this corkscrewing effect creates a very shifty and punchy quality in the wind along the beaches until one wins out the flow (Sea Breeze usually does unless surface High offshore is weaker). With land to the left of water in the Northern Hemisphere, surface divergence (surface Bermuda High pressure for example) encountering land friction allows the Sea Breeze to gain momentum and creates this helix, or corkscrewing effect.  Once the Sea Breeze has set in along the coast, it builds and in many cases begins to veer (or tilts) due to the Coriolis effect. For the SE Region, this is usually around 5 degrees of backing per hour. The bend goes until the wind is 45 degrees to shore (somewhat parallel) and remains fixed until fading in to the evening. So that would explain SC winds backing from S or SSW over to SW – especially just inside of coastal breaks, harbors and sounds where heated land masses fuel the hotter air. This type of breeze also is a powerful generator of convergence zones inland as the gradient tightens behind the Sea Breeze front and thermals increase vertical stacking of clouds. The result is stronger convection and/or thunderstorm lines along the head of the front.  Outflow becomes optimal as the offshore winds aloft help to send air back out over the ocean (called “return flow”) where it cools, condenses and circulates back into the Sea Breeze. This goes faster and faster depending on offshore upper air movement working in unison with the onshore air movement below.

Here is an example of a helix with a healthy Corkscrew Sea Breeze circulation. Immediate coast is typically dry and void of clouds overhead during this setup – at least until storms develop along the convergence zone. Then the SB Front can either hold the storming inland or sometimes it is just too powerful and overruns the coast and forces the Sea Breeze front (where clouds are building) to retreat quickly back offshore. 


220px-Helix.svgseabreezeNOAA

Picture below represents flows of winds. Corkscrew Sea Breezes are shown in figure (b). Green shading represents land where angled offshore Land Breezes (3 black arrows) are eventually undercut by straight onshore Sea Breezes (2 blue arrows).

qj2484-fig-0001

Here is a good example of the Westerly flow over the SE Region prompted by moderate northeast Gulf of Mexico High pressure. Sometime this NE GOM High can become stronger and make the land breezes more aggressive.  The Bermuda High is providing a Southerly flow to encounter it at the coast, where the “helix” or corkscrew occurs. With inland troughing/thunderstorm activity, this breeze builds to moderate and strong levels – typically stronger further up the coast. 

6-24-15

Here are the results from 6/24/15 at Isle of Palms, SC. You can see the morning WSW’erlies fade and the SSW’rlies build and hold throughout the day. 

6-24-15IOP

Backdoor Sea Breeze:

This breeze is one of the hardest to forecast speeds for – and one of the most fascinating (at least to me) to watch the real-time observations as it can show an explosion of strong winds.  It’s relatively easy to see coming, but the wind strength it generates and the timing of arrival catches us off-guard many times each year – and it certainly does catch beachgoers/boaters off-guard. Computer models do fairly well on directions, but very little for guidance on speeds as they do not always catch the microclimate thermodynamics of what occurs along our immediate coast. These are stronger when the waters are warming up in the spring and we still see cold weather trying to make its way down from the north as the Polar Jet recedes. Adversely, we see this in the fall and during winter when the Polar Jet is advancing south to wedge these colder air masses into the Southeast Region.

Backdoor Sea Breezes are generally associated with cold front/backdoor cold front passages and cooler NE winds pressing down from High pressures to the north wedging down behind them (Canadian/Great Lakes/NE High’s).  The stronger the High, the stronger the gradient and ultimately, the stronger the winds. For examples…1024 millibars is moderate…1032-1040mb’s  brings strong winds and when we start to see 1042-1051 High pressures, we can reach gale force speeds. Residual coastal troughing or offshore low pressure(s) from these dissipating fronts interacting with this strong surface sinking of air with a healthy clockwise flow tightens the gradient as winds speed up between the two systems. In other words, and by the Buys-Ballot law, the gradient tightens and a convergence zone is created at the coast (instead of inland). The result is cooler air “advecting” (in many cases this air is slanting down at an angle into shore, or wedging) down over the warmer waters, which creates an additional unique temperature gradient where air is forced into the hotter land. The warmer land acts as an accelerator and winds are specifically stronger just along the land-and-sea interface – many cases the stronger flow is restricted to beaches.  The barrier islands and lee-side marshlands usually feel the brunt of these accelerations as winds start to fade while heading inland as the air lifts.  You can sometimes catch a visual when associated clouding in the area has a single sided wedge shape -and you could almost think of this as winds swooping down to the surface and scooping back up (a dip in the winds). Storms can be triggered along the coast with heavier troughing and weaker High pressure, but this is not a typical Sea Breeze and does not require circulations. This is an “oscillation” where the winds are generated onshore only without a return flow overhead. In other words…this is more of a forced wind into the coast.

Picture below represents flows of winds. Backdoor Sea Breezes are shown in figure (c). Green shading represents land where angled onshore breezes (3 black arrows) are eventually accelerated by Sea Breezes (2 blue arrows) – or the cold air advection process.

qj2484-fig-0001

It is very common for winds to linger as offshore Northerly values before land heating starts to bend the wind field from Coriolis effects as the sun rises further in the sky. As winds “peek” around the corner and become side-shore, they accelerate quickly as the warmer water now becomes its fuel. As winds lean more onshore at a 45 – 75 degrees for SC/GA, the jumpiness usually settles into a more consistent flow. NC has leans according to their land masses, but many locales with southerly facing beaches remain offshore and weaker for these events. The outer capes and E/SE facing beaches feel the acceleration effects. It is not known if there is a 5 degree shift per hour as in the Corkscrew Sea Breezes (could be a good theory to investigate), but we do know that these winds generally will lean perpendicular to shore later in the day as air temps/land temps cool. For the Southeast, this would be an Easterly wind, which becomes more ocean saturated and weaker in nature – especially later in the day as heating dissipates. Specifically, Charleston SC is known for its extreme increases in speeds…and I firmly believe that this is largely due to the 3 river system allowing for earlier advection of the offshore Northerlies to gain momentum. There is also a large open body of water that curves well inland to the north of Charleston (Bulls Bay) to allow for the side-shore increased momentum and the urban heat island effects to help make that Sea Breeze shift to from NNE/NE–>ENE  all the more inviting.

Typical surface map of what this might look like as NE/ENE winds wedge down behind backdoor fronts. This one from September 6, 2013. 

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Here is an example of how fast the Backdoor Sea Breeze can come up in Charleston, SC along the immediate beaches. Some of this has to do with station placement reading weaker with land shadowing from offshore values earlier; however, if we look at the Charleston Harbor, we can see the elevations came up at the same time, which further proves this is a rapid acceleration process.  This particular event caused the cancellation of the Charleston Race Week finals as the officials were well tuned in to the human forecast for the area versus the model forecast.

Isle of Palms Pier – highest gust during peak heating – notice the jump just before 10AM -that’s  a 10-15kt jump in speeds all at once! 

4-21-13

Charleston Harbor (Front Range Light centered near the middle of the inner harbor mouth)

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Here’s  classic example of the NE–>ENE–>E flow fading as winds lean more and more East over time during the cooling process. 

5-14-15 IOP

And lastly here is a wild day on April 26, 2015 at Isle of Palms/Sullivan’s Island, SC where we saw an elevated Corkscrew Sea Breezes shift to a Backdoor Sea Breeze as a backdoor cold front crept down the coast from the north.  Visual observation was angular clouding. 

4-26-15CS into BD IOP

 

Synoptic Sea Breeze or “Default Sea Breeze”:

This one applies to when there is a weak overall pattern in place and we see a High pressure ridge develop along the coast by default. This usually starts from the south and spreads to the north up the immediate coastline. This type of Sea Breeze oscillates into the coast with a very mild or modest onshore temperature gradient only.  A classic example of this we can see in the winter time when Northerly and Westerly winds diminish to at or below somewhere under 5 – 6kts. Ever wonder how those S/SW’rlies appear out of nowhere? With cooler waters and somewhat warmer land, we systematically see winds switch up from the S/SSW/SW and remain light to modest at best. Since there is no inland convergence or thunderstorming available within the abundantly cooler overall air, this breeze simply flows on the one leg of a pseudo Corkscrew Sea Breeze –  or the temperature gradient to be more specific. You could say this type of breeze could be a hybrid type of Pure Sea Breeze with corkscrew-like characteristics. We also see this more pronounced during the spring when fronts decay overhead, warm fronts lift or when marine layering hampers development over the cooler, more stable shelf waters.  High pressure tends to develop along southern SC/mid to northern GA in the curve or “coastal pocket” where heat fluxing creates this surface High pressure ridge that expands up the coast to SC and SENC (very similar to the Northeast Gulf of Mexico High).  We could in theory call this the “Carolina Coastal Ridge Breeze” per our coastal sensor observations and mesoscale analysis showing winds barbs pointing up the coast. It also can receive a little help from warmer Gulf Stream waters to the east as it eddies in closer to the coast at times…or simply acts as an artificial land mass with its warmer surface temps.  This would provide an offshore gradient able to make it onshore at times as it meanders in and out of the coast.

This is a very tricky one to catch as it tends to be so subtle, the coastal High pressure ridge many times does not even appear on surface maps. So it’s kind of a “default ridge” that develops without notice. It’s almost as if the air refuses to remain still along the land and sea interface – and it is very rare to see completely calm winds (usually only in heavy fogging or short-lived periods of 100% stability).

Here’s one instance where we can see the coastal ridge on a larger scale (this one was actually picked up by surface maps) assisted by an eastward meandering northeast Gulf High.  A frontal boundary had recently fizzled along the coast associated with the Low pressure to the northeast. 

 

3-30-15

 

And here are the two days of March 15th and 16th, 2015, where the coastal ridge set up light to somewhat modest Southerly winds as Northerly winds weakened below 5-6kts (I was able to catch and sketch it in on the surface map for March 16).
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Results for the beaches – this one from Isle of Palms, SC. You can see the shift from Northerly to SW’rly each day.  

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The last type of Sea Breeze mentioned here, the Synoptic Sea Breeze or as we could name it…the “Default” or “Carolina Coastal Ridge Breeze” is definitely up for discussion as it seems to be very elusive in studies. This particular classification will require more observations from our WeatherFlow mesonet and further discussion amongst our meteorologist team and partners.  The 3 main types of Sea Breezes (Pure, Corkscrew and Backdoor) have been classified from intensive studies and profiling, which helps to better understand how Sea Breezes work along our coastline.

Stay tuned for the next part in this series where we will look at “What would the Sea Breeze look like?” I will present the anatomy of what the Sea Breeze front and its components would look like if viewing from the side –  and how each part of the front works.

 

Cheers!

Shea Gibson

WeatherFlow Forecast Team, SE Region

 

 

 

Posted in Coastal Georgia, Coastal South Carolina, SouthEastern North Carolina

Dolores has more coming

by Meteorologist Kerry Anderson

Here in Southern California we are once again in a very unusual pattern for summertime as we watch the flow from the south bring up subtropical moisture.  All week Dolores, now a Tropical Storm, has been followed closely as the models have shown her moisture to be heading our way.   She is well south of us off the tip of Baja.  The National Hurricane Center projects that Dolores will continue to weaken as she tracks northward and by Tuesday morning will be southwest of the California coastline as a Tropical Depression.

Dolores track

So why so much moisture already?  The following satellite loop shows the moisture in the mid levels of the atmosphere.   You can see Dolores off Baja.  If you look over Texas there is a dry area where we have a large High Pressure system.  The flow around this system is pulling moisture northward into Southern California and will continue to do so through the weekend.

Dolores 7.19.2015

 

The sounding from San Diego this morning shows the arrival of all that moisture.  It is not unusual for us to see this monsoonal moisture each summer for the desert areas on the east side of the San Bernadino mountains but it is very unusual for the flow to drive the moisture west of the mountains over San Diego, Los Angeles and the coastal waters.

San Diego Upper air 7.18

 

With this moist air in place the atmosphere is unstable and by early today we have seen towering cumulus and radar is picking up plenty of precipitation.  Much of the rainfall this morning was not hitting the ground because of the dry layer near the surface but that will change over the next few days as we get more subtropical monsoonal moisture and then more moisture from Dolores. So watch the skies, the radar and our forecasts for the next few days as you head onto the water.  Thunderstorms with strong winds and lightning are expected.

Convective clouds San Diego Radar 7.18.2015

 

Posted in Los Angeles, San Diego

Bay Area Wind Blog: Anatomy of a long range marine surge forecast.

NPHridgeMarine surgeLots of the signs of a marine surge towards the weekend. ZZ492C0DA4What could go wrong? In one word: Dolores.

by Mike Godsey, mikeATiwindsurf.com

It all looks so simple in the top animation. The winds are NW today and tomorrow and turn into a SW marine surge by Friday and Saturday. But the reality is far more complex and far more iffy. So let’s dig but a bit into the anatomy of this marine surge forecast.

Today, Monday July 13 we are in a NW wind regime as the North Pacific High returns to the Northern California coast. However looking towards the weekend I am seeing signs of a surge of marine air and SW flow for the Bay Area. At the same time there are signs that convection could disrupt this marine surge before it even matures.

Here is the story in pictures and I will fill in the text as the pattern resolves.

The next image shows an upper trough at the 500mb level which varies around 18,000 feet. This upper trough or southward extending loop in the upper level winds brings cooler air to Northern California and lifts the inversion allowing the marine layer clouds to deepen and move further into the Bay Area. It also brings SW flow far above the Bay Area. All of which helps a marine surge event.

ZZ1FAF198FHowever there are also a chance of wind spoiling convection this weekend. Why? Check out the possible tracks of tropical storm Dolores off Baja. The models are pretty unclear which way she goes but if Dolores tracks to the north the storm will pump moisture into those SW winds you see streaming over California from the upper trough in the images above.

That would mean increasing high clouds, a weaker pressure gradient in the Central Valley and a disruption of the marine layer clouds which would cause my proposed marine surge to fizzle.

The next image looks at  the GFS model output at the 850mb level which averages at about 5000 feet. This model suggests that an elongated eddy forms just off the entire Central California and part of the Northern California coast. If realized this flow would really enhance the southerly winds of a marine surge.

NPHridgeNW2NNEmarSurgThe next image is an animation of the winds of the North Pacific High from tomorrow to this coming Saturday. Notice how the position of the North Pacific High in the early part of the week favors NW winds on the California coast.

Then the high moves to the NE as it pushes an extension or ridge into the Pacific Northwest. This ridge helps pump up the wind in the Gorge.

But as the week progresses notice how it causes the NW winds along the Bay Area coast to turn from NW to NNW to N and pull away from our coast.  Given the eddy at 5000 ft. the Central California coast this is a perfect set up for a marine surge as southerly winds develop along the coast south of the Bay Area. The next animation shows a close up of this happening.

All of this is tentative at this point and the marine surge hinges in good part on the fate of track of tropical storm Dolores off Bajamarine surgeVSstorm

Update Tuesday July 14. This next animation shows the winds at 5000 feet. Notice the blow off from tropical storm Dolores heading towards California. This marine surge is supported by low and mid level “eddies” that I think could entrain some of the moisture from Dolores and bring it over the Bay Area and nearby Central Valley which would dampen the marine surge Saturday.

I actually a pretty certain the marine surge will happen as forecast but it is risky forecasting a marine surge 5 days in advance so Dolores allows me to have plausible deniability if there is a forecast fiasco.

Posted in Los Angeles, San Diego, San Francisco

Bay Area wind blog: The North Pacific High returns from vacation.

Stronger winds for the Peninsula and Waddell area.NewNPHmovesEast

by Mike Godsey, mikeATiwindsurf.com

The North Pacific High, long absent from our waters, has reformed NE of Hawaii and will impact our winds in the next day or so.

These will not be the powerful NW winds of spring but they will definitely improve the winds on the Peninsula and the Waddell zone.NPHsunTues

 

Posted in San Francisco