Eddy creates AM southerly flow then fades in the afternoon.
by Mike Godsey, email@example.com
The tiny eddy that is often created west of the Golden Gate is actually more west of Pt. Reyes tomorrow and, unlike today, it is more likely to die before the afternoon winds ramp up. The animation below shows the relationship between the NPH and the tiny eddy tomorrow July 2:
A visual guide: What’s with those dirty thin clouds above the marine layer clouds.
by Mike Godsey, mikeATiwindsurf.com
Yesterday Ben did a great job showing the impact of the current monsoonal pattern on Southern California so today I will cover the Bay Area.
As the marine layer clouds burn back today you will notice 2 things.
First looking upward you will notice thin hazy monsoonal clouds far aloft.
Then looking at your device you will notice my wishy-washy wind forecast with vague mumblings about monsoonal clouds making for an iffy forecast.
So lets look at these monsoonal clouds and figure out how they come to be over California today.
The first satellite animation from predawn to about 9AM today shows these mid to upper level clouds as they stream from the SE over the Bay Area. The same cloud flow is happening over Southern California today. Looking carefully can you see why the Gorge is finally free from monsoonal clouds today.
The second animation shows the flow of water vapor above 10,000 feet. Watch carefully the rotation of the water vapor since we will come back to that.
As these clouds come over us their winds do not directly impact our surface winds since the SE winds are 10K to 18K above the surface. But they can impact the surface winds indirectly in 3 major ways:
1. Shadows from these clouds can partially shade the surface which decreases surface heating and the pressure gradient. Unfortunately the where and now much of this effect is impossible to forecast in advance.
2. As the most air in these clouds comes over mountains they can create brief showers and even
thunderstorms that very briefly produce strong local winds. For example parts of San Diego saw very brief winds to over 40 knots yesterday. The exact location of these convective events is also difficult to predict in advance. And check out this wind graph from 3rd. Ave
3. And lastly the monsoonal clouds can cause the marine layer clouds to mix out which can influence the wind distribution.
So what generates these monsoonal clouds and wind?
Looking at this 3rd animation first find the upper high over the Great Basin and note the clockwise rotation of winds around this high. Now find the upper low pressure west of Baja and note the counter clockwise winds spiraling from the low. Next notice how these two flows converge over California.
This convergence means that we have SE upper flow over most of California at the 10,000 to 18,000 foot level. Now look carefully and you will notice that these winds are pulling in moist warm air from the Gulf of Mexico and from the waters near Baja.
As this moisture partially condenses in the cooler upper levels it forms thin clouds that you see overhead. And when conditions are right this moisture can form thicker masses of clouds near the surface, showers and even thunder storms.
Lastly look to north of the Great Basin and note the northward jog in the upper level winds. That puppy is called an upper ridge and it is responsible for the warmer than typical conditions today. So when will this monsoonal pattern subside? As soon as the Great Basin upper high moves east of the Sierra.
So, we’ve all heard about the monsoons that bring flooding rains to the South Pacific and southeast Asia. But that’s it, right? Monsoons don’t impact the U.S., do they? Well, that depends upon how you define monsoons. Traditionally, we like to define monsoon as “a seasonal reversing wind accompanied by corresponding changes in precipitation.” So, in short, the Southwest and even Pacific Northwest are, at times, impacted by monsoonal flow in summer. Well, if you live in southern California or Arizona you’ve probably heard about this. But just how does monsoonal flow get set up?
It all starts when there is a big high pressure ridge over the Great Basin. This upper high pressure ridge continues to lift and expand northward. Now, this helps pack the heat across much of the West. Meanwhile, as it does this, the upper flow over northern Baja California, southern Arizona, and southern California turns E to SE. This flow, known as monsoonal flow, begins to tap into mid and upper level moisture from the Gulf of California or even from the Gulf of Mexico. (In one noteworthy case a couple of years ago subtropical moisture from the remnants of a tropical storm that hit far south Texas and Tamaulipas, Mexico migrated westward enough to get drawn into the established monsoonal flow. The result was pretty heavy rains for New Mexico, Arizona, and southern California.)
Now, as shown in the image, the current monsoonal flow setup is being reinforced by a vague upper low well off the southern California coast. This helps pump in subtropical moisture as well. The result is that convective clouds build over the deserts and mountains and thunderstorms can form on a daily basis.
Typically, the coast is pretty unaffected by the monsoonal flow. The easterly winds are more aloft and may impact the deserts and mountains. The monsoonal rains and thunderstorms often don’t push west of the coastal mountain ranges. In fact, the strong upper ridge often helps to shrink and strengthen the
marine layer. This results in a shallow, but well formed marine layer that burns off slowly. For beaches near San Diego, sea breeze winds tend to struggle in these monsoonal patterns. This is because the NW flow across the southern California bight is absent and the beaches must rely solely on the mild sea breezes to drive afternoon winds. Since the immediate coast remains cool under the shallow inversion, these winds are generally unimpressive. (NOTE: The sea breeze winds for sites around L.A., Ventura, and Santa Barbara can often be healthier during this pattern.)
But, what happens when the monsoonal thunderstorms push over the coastal mountains and march right to the coast? Well, check out these satellite images from Tuesday, June 30th.
By midday the developing storm system pushes over the mountains, moving westward as monsoonal storms do, and begins pushing toward the coast. By 2:30PM a strong line of thunderstorms has developed and begins to push past the coast. These storms wipe out the marine layer inversion and bring heavy showers and thunderstorms to San Diego County.
As mentioned above, monsoonal flow often has little direct impact on the coast winds. But in this case, just how did the monsoonal storms impact winds around San Diego? Check this out! All WeatherFlow sites around San Diego saw a spike in the winds as the storm front pushed through. The strongest recorded wind spike was a Mission Bay North Jetty, reaching a sustained NNW wind of 40 mph and a max gust to 45 mph!
In this blog series, we explore the vast realm of Sea Breezes. What are Sea Breezes? How do they work? What would they look like if I could see them? Is there only one kind? How do you forecast for them? These are the questions we hear frequently and these are the question we constantly have to ask ourselves when we look over the region on a daily basis.
The first question is “What are Sea Breezes?” By definition: The breeze that blows from the sea toward the land during the day, as air rising over the warmer land is replaced by cooler air from above the sea. Now other than being winds that blow in off the ocean, they are very complex in nature and require specific sets of criteria for the various speeds they are able to generate. The land and sea interface is very unique and very sensitive to the temperatures of both land and water. You’ve probably been out at the beach and noticed the winds almost dead calm and then all of a sudden they crank up out of nowhere without storming present…or a slower more gradual build until the sand is seemingly starting to cut you off at or below the knees. Or at other times you see clouds building and storms issuing out their rumbles…but never make it to the beaches. This leads us to the next question…
“How do they work?” First let’s take a look at what a typical setup is for the SE coastline: The Sea Breeze is in all actuality, is really a unique type of cold front. In most cases, it sits offshore at night while land breezes (offshore Westerly winds down-sloping the elevations of NC, SC and GA) are generated from land cooling faster than the water. As the sun rises and the land heats (especially during our hotter months), the Sea Breeze “front” advances onto land and penetrates inland. The distance varies with the overall atmospheric profile of the area at the time, which is rarely the exact same from day to day. As the front advances, the warm air parcels known as “thermals” rise into the atmosphere from radiational heating (land heating) to help clouds build inland where low level moist air circulations mix with the hotter air along the head of the front. Additionally, urban heating over industrial zones, cities, asphalt parking lots, rooftops, etc… very often accelerate this process just inland or along the coast where air temps become significantly higher than what the normal temperature is. Now when we have heat rising and moist ocean air or inland residual moist soils from prior rains and/or moist vegetative areas inland, we will see low level circulations start to mix the two types of air to create large areas of instability – which creates convection. Opposing directions of mid level and upper level winds frequently drive these inland areas of convective clouding/storms towards the ocean, where they are met by the Sea Breeze front. This is known as the convergence zone. This is an area of enhanced convective activity and/or thunderstorms… and at the higher levels, the overflow of unstable air aloft pushes out as stabilized return flow back out over the ocean that cools, condenses and falls back to the surface…. which gets pulled right back into the Sea Breeze front again. This is what knows as a classic “Sea Breeze circulation”. Many times with a stronger Sea Breeze front, we see the inland storms get held back and explains why those storms sometimes never make it to the coast.
Here is a great pic from the NOAA/COMET Program that shows what I like to call the “coastal wind wheel”.
Winds from S/SSW/SW Sea Breezing along the coast today, June 30, 2015 up until inland storms breached the coastline in the evening.
Red = gusts , Blue = average and Green = lulls
Isle of Palms
Charleston Harbor- just broke 30mph in the gusts. Notice the computer forecast models were quite a bit lower on their predictions.
Here are examples of the urban heat island effects – which is why sometimes the Sea Breeze convergence zone can show heavy rains in one specific area and half a mile or less away, we see little to no rain.
Now we have a very broad coastal plain at sea level for GA/SC and Southeast NC. As we head inland, the elevations increases very gradually instead of abruptly, which allows the Sea Breeze to penetrate further inland. This explains why our thunderstorm lines develop so far inland at times. This is called the “I-95 Corridor” from south GA into Hilton Head, SC. For Charleston, SC, I can say after living here for so many years and making observations, Peninsular downtown Charleston is a hot bed of activity, as well as Rivers Avenue/North Charleston as the true “Thunderstorm Alley”. To the north of Charleston, it’s pretty much up Highway 17 where storms are fed by urban commercial/residential heating downwind to Awendaw, McClellanville – and ultimately follow Highway 17 North up into NC, where inland Sea Breeze frontal surges follow the inside curves of the outer capes.
Here is the Sea Breeze front has penetrated inland along Southeast NC…where the dryer more stable air is behind it along the coast and the convergence zone is relatively quiet (at that time).
And a great .gif animation of the Sea Breezes stretching from NC down through SC. On some days, we can see this line stretch from northern FL all the way up into OBX.
Here is a snapshot I took in North Charleston, SC right along the vicinity of the convergence zone. 1st pic is facing South towards the ocean – notice the clearer and pleasant weather (pretty much clear past the bridge). In the 2nd pic, immediately to the west and north at the exact same time, you can see the convection building. Those clouds are heading towards the ocean and stalling directly overhead.
Another look inland of Folly Beach/Morris Island, SC at the build-up over West Ashley over to North Charleston. Pic by C-17 Air Force Pilot Sam Johnson (thanks again Sam!).
Ok so next we talk about the counter-part to the Sea Breeze – which is called the “Land Breeze”. At night, land cools faster than the water (radiational cooling) and the winds switch to offshore Land Breezes – or in most cases for GA/SC/NC we see some sort of Westerly wind overnight. The warmer air above the water continues to rise, and cooler air from over the land replaces it, creating a night time breeze. Depending on the amount of cooling, winds could be light to moderate. These are typically stronger during seasonal shifts in the fall and spring for the Southeast. Weaker in the summer months where low temps peak in the 80’s.
Video below shows the general ebb and flow of Sea Breezes to Land Breezes…and back.
Ok so that does it for what the Sea Breezes are and how they basically work. Next part in the series, I will dive into the other questions of “What would they look like if we could see them?”, “Is there only one kind? and “How do we forecast for them?” I will show the anatomy of the Sea Breeze front and break down the types of Sea Breezes (yes we have 4 types believe it or not) as we progress through this series.
The Bay Area has seen several day of SW flow and the waters from Treasure Island northward to Sherman Island have rocked with southerly winds in the low to even upper 20’s. A major factor in these atypical marine surge winds was a large eddy that first formed south of the Golden Gate and gradually moved up the coast. Today, Sunday June 28 will be the last day of this SW flow.
Why? Well beyond creating southerly flow along the coast and inside the bay the eddy has also been pushing the North Pacific High’s NW winds far away from our coast. As the you can see in the is video as the eddy dies the NPH’s NW winds are able to slide eastward towards us. This means that site like Waddell, Coyote and 3rd. Ave. which have been largely wind deprived will see the NW wind curving in as NW to WNW wind.
SSW wind through Muir Bay & Gate focuses San Pablo Bay robbing Sherman Island of peak winds.
by Mike Godsey, mikeATiwindsurf.com
Forecast note at 7AM today: “As the marine surge ramps up the flow will first be diverted into Napa in the so watch the San Pablo Bay sensor for S to SSW wind. If this happens Sherman will not pick up until the San Pablo Bay sensor turns SW.”
Well as I write this at 4PM the winds are still SSW at our San Pablo Bay sensor and Sherman Island is only seeing upper teens winds not the mid 20’s potential forecast.
The imagery at the bottom of the page shows how S to SSW winds at the San Pablo Bay sensor shunts the wind that should go to Sherman Island into Napa. If this happens Sherman will not pick up until the San Pablo Bay sensor turns SW.
Rest of the sites are in the forecast range or stronger with Crissy and Treasure Island more upper 20’s to 30 rather than the mid 20’s forecast. I suspect few are complaining except for those who went to Sherman Island for epic winds and did not read the Forecast note at 7AM.
The 2 key players in the Bay Area summer winds are the North Pacific High’s NW winds and the Central
Valley thermal. Today and especially tomorrow the NW wind are only bit players. So the Central Valley commands the weather stage.
Paradoxically this thermal low can either kill the wind or make it howl depending upon how it’s isobars dance across the map.
First where does the Central Valley thermal low come from? Looking at this heat animation watch the time line to find the moment of dawn. Notice how the Bay and most of the Central Valley are relatively cool. Now watch how fast the Central Valley heats up. As the land heats it transfers heat to the surrounding air. As this air heats it expands creating lower pressures.
(For weather geeks: One of the questions I get asked most often is why Sherman Island blows in the morning unlike rest of the Bay Area. Or why it sometimes blows all night. If you look at the heat animation very very carefully looking at the temps at night in the Bay Area vs. Redding and Bakersfield you will have the answer. Still at a loss? I will try to explain what is going on in a future blog.)
Since the Central Valley is an landlocked valley it contains this low pressure. The only sea level connection between the cool coastal air and the Central Valley thermal low is the gaps in the coast range around the Bay Area.
When temperatures are in the normal range the isobars of this low pressure are mostly inside the valley and extend a bit towards the Bay. Hence our typical thermal winds as the oceans cool high pressure air flows towards the low pressure in the valley.
Now look at the next image showing the isobars over the Bay Area at 11AM today. In a heat wave the Central Valley thermal low expands and balloons over the Bay and even out into the sea. You can see this in image as a bulge of low pressure going about 3o miles west of the Bay Area.
This bulge causes the pressure gradient to drop to near zero and the Bay winds fade away in the AM. Now look at the second isobar image. By 4PM as the suns angle lowers the low pressure retracts a bit eastward. Today those isobars concentrate in the Crissy to Treasure Island zone hence my forecast for upper teens to 20 knots winds outside for those venues.
Ok, let’s go to the 3rd isobar image for 11AM tomorrow. Notice how different the isobars look. With a marine surge and cool southerly flow coming in from the ocean the Bay cools down. And the isobars are beginning to concentrate over Sherman Island. It this was a normal marine surge the winds will be ripping over Sherman Island at dawn. But I suspect that the inversion will be too strong and the marine layer clouds to shallow for the isobars to really stack over Sherman Island in the AM.
But looking at the 4th image you can see how the isobars tighten over Sherman Island Friday afternoon. Hence my forecast for strong Sherman Island winds Friday afternoon and Saturday.
Now go back to the top animation. Watch carefully how the how air hangs over most of the Bay Area today. Then look how it retreats Friday. That is the cause of the very different wind forecasts for today vs. tomorrow.
In the summertime you’ll almost always see the forecasters at WeatherFlow mention in their forecasts the possibility for afternoon sea breeze development. At first glance, the sea breeze appears to be a simple phenomenon, but forecasting the strength and overall evolution of a sea breeze is quite complex. There are actually several specific geophysical parameters that directly influence sea breeze evolution. The two most influential parameters are the differential heating between the land and water and the synoptic scale wind regime (the overall wind field and whether it is offshore, onshore, or parallel shore).
Figure was created by Nick Luchetti
For those who need a quick refresher, the sea breeze is a small scale wind circulation brought about by differential heating between the land and the water. On a hot summer day with little synoptic wind flow, the land heats up faster than the water. This creates a localized pressure gradient where low pressure develops inland and high pressure develops out over the water. Due to the pressure gradient force, wind always blows from high to low pressure. This creates a breeze that flows from the sea , hence “sea breeze”. Ahead of the sea breeze a front can develop and if moisture and instability are present then storms can fire off this front. The average circulation depth is roughly 1 km, which gives you an idea of how localized and mesoscale this phenomenon is.
Where things get interesting is how the overall synoptic wind field regime influence this tiny, localized wind circulation.
Figure created by Nick Luchetti
During offshore flow, the synoptic level winds (usually look at the 925-850mb levels, just above the surface) oppose the sea breeze. This inhibits the sea breeze front’s ability to penetrate inland, which in turn allows the circulation to maintain a strong thermal gradient between the land and water. Because of this, offshore flow induces a strong sea breeze circulation. However, there is a threshold for optimal circulation strength. Offshore winds greater than 14 knots have been shown to significantly weaken sea breeze circulation.
During onshore flow, the synoptic level winds flow with the sea breeze. This allows for greater inland penetration. However, the cool ocean air filters in over the land, which in turn weakens the thermal gradient. This aids in a weakening of the sea breeze front. It takes very light onshore winds to maintain sea breeze circulation. It has been observed that winds as light as 2-8 knots can fully suppress a sea breeze.
During parallel flow, the synoptic level winds flow alongshore. This aids in a generally weak sea breeze front that tends to stall along the coast. Additionally, we tend to see a later arrival of the sea breeze during parallel flow.
From a forecasting perspective, sea breeze evolution is quite difficult to discern. Models do not do a phenomenal job forecasting this phenomenon. However, understanding how the synoptic level wind flow regime influences the strength and evolution of sea breezes is something forecasters can keep in mind to provide better accuracy in their forecasts.
Visible sea breeze front on radar reflectivity. Created using the NOAA Weather and Climate Toolkit.
From an observational standpoint, the sea breeze front can be visibly seen on radar reflectivity. You can see on the radar image above a very fine “thin line” that stretches from Wilmington, NC northeast towards Greenville, NC. This visible “thin line” is essentially a collection of insects and dust particles that have been ingested into the updraft along the sea breeze front. During a particular strong sea breeze event, observations have shown inland penetration as far as Raleigh, NC.
Forecasting sea breeze development, evolution, and strength continues to be an enigma for coastal forecasters. However, due to the re-curring nature of sea breezes, observational analysis is highly feasible. As observational and model development increases there is hope that more accurate sea breeze forecasts will be attainable in the future.
(Arrit, 1993; Grisogono et al., 1993; Gilliam et al.,2004, Porson et al., 2007; Crosmen and Horel, 2010)
North Pacific High ridge, eddy & upper low may trigger a marine surge.
by Mike Godsey, mikeATiwindsurf.com
Last Sunday in the extended forecast I issued a Marine Surge Watch for this Friday. This is my jargon to give you a heads up that there is a chance of a surge of southerly wind and fog pouring through the Golden Gate on June 26. The top image shows how one model has the surface wind from Tuesday though Friday changing from NW to South. Check the time line at the bottom and you can see the southerly marine surge wind developing. I should be able to fine tune the ETA of the marine surge about 12 hours in advance. Right now it looks like Friday PM.
Typically marine surges happen at the end of a heat wave when Central Valley thermal low begins to pull back from the Bay towards its home in the Central Valley. As this happens a low pressure area often remains in the North Bay counties creating a South to North pressure gradient triggering southerly winds along the coast.
But the marine surge I expect sometime this Friday will be more complex than. This blog takes a brief look at some of variables behind this possible marine surge and why its arrival time will be hard to pin down.
Looking the first 3 images covering the surface winds today though Friday first find the North Pacific High. Notice how today today and Thursday the high is focusing NW winds along the Bay Area coast. But also notice that extension or ridge the NPH is pushing into far Northern California.
By Thursday the ridge has created North winds in Oregon to California’s Cape Mendocino. By Friday the NPH ridge is into the Pacific Northwest contributing to a massive heat wave there.
More importantly notice that with the ridge in that position the ocean winds near the Bay Area turn from NW to NNW or even N. Winds from that vector help create a huge counterclockwise eddy from Bodega almost to Southern California. So the Bay Area coast sees southerly winds and deeper fog.
However the typical low pressure in the North Bay is absent so the marine surge would be quite weak and would be unlikely to reach Sherman Island. I would hardly issue a Marine Surge Watch if that was all there was to this story.
While all of this is happening at the surface at about 12,000 feet a sort of Cut-Off Low is forming towards Southern California. As you can see in the animation below this spiraling mass of air slides up the coast towards the Bay Area. By Friday I expect this mid level low pressure to create southerly flow on the ridge tops and jazz up the surface southerly winds from the ridge produced eddy.
The tricky part of the forecast is since this is a sort of Cut-Off Low and its trajectory could vary from the model output. And if it ends up in the wrong location the marine surge would fizzle. Hopefully the picture will clarify in the next 24 hours. Plus we have to worry about monsoonal moisture and clouds the Cut-Off Low may bring over the Bay Area which could weaken the pressure gradient.