Is El Niño finished?


by WeatherFlow Meteorologist Shea Gibson

As many now, we had quite a strong El Niño develop in the central and eastern Pacific Ocean as Sea Surface temperatures soared to 3.1°C above normal (tying the previous record from 1997).  Normal is considered as 28°C or ~82.4°F. The focus now is shifting towards a La Nina this year, in which the equatorial central and eastern Pacific is now cooling rather quickly.

First thing is to identify with the Niño regions and where they are.  The ONI (Oceanic Niño Index) is derived from the largest and most influential region, Niño Region 3.4, over an overlapping 5 season period for consistency. This is done in 3 month increments.

Let’s take a quick look at the latest Sea Surface Temperatures, which show an obvious cooling streak along the equatorial eastern and central Pacific. This is due to cool water up-welling from deeper below the surface along what is called the “thermocline” (underwater body of cooler water).


Here is a good example of what the thermocline is about from

And here is the latest from the Climate Prediction Center showing the cool water upwelling towards the surface and sending pushing warmer waters back to the west:



Here is the anomaly of temperature departures which shows the significant drop in temps recently along that belt.


Equatorial upper ocean heat that shows cooling starting in March and going well into May (from the latest CPC ENSO report).

And the graph of all 4 regions (from the same report) showing the steady decline:

Here are the latest CDAS SST’s by region:
Region 4:


Region 3.4 (Oceanic Nino Index) – almost at normal:
Region 3:

Region 1+2:

So what are the chances of a La Nina year? Pretty good case for it to develop by summer 2016 and a 75% chance by the fall/winter.

Notice the NCEP CFSv2 aggressively bringing it down May/June/July:

And the Euro (ECMWF) being less aggressive and keeping a more ENSO Neutral Phase by late summer into fall:

Bottom line is that the Sea Surface Temps are falling and a La Nina is favored. It certainly appears that we have enbtered a ENSO Neutral phase, but we will have to wait and see what the ONI values say once overlapping and other details give researchers the information they need to determine the index. After such a strong El Nino, it should be interesting to see how the Atlantic responds. Waiting on the 3 month ENSO Diagnostic to come out in June from the CPC and watching as temps continue to fall. We’ll be keeping an eye on this and the tropics- the Atlantic Basic Hurricane Season starts up on June 1st and we are already seeing an area of potential near the Bahamas. Let’s see what happens this year!


Shea Gibson
WeatherFlow Meteorologist /Wind Forecaster
New Station Projects & Outreach
SE Region / East Coast
Twitter: @WeatherFlowCHAS

Tropical Tidbits:







Posted in Baja Guide, Cape Cod, Chesapeake Bay, Coastal Georgia, Coastal South Carolina, Columbia River Gorge, Connecticut, Delmarva, Long Island/SE New York, Long Island/SE NY, Los Angeles, Massachusetts North Coast, New Jersey, Ontario- Simcoe, Ontario: Lake Erie, Ontario: Lake Ontario, Outer Banks of North Carolina, Rhode Island, San Diego, San Francisco, SouthEastern North Carolina, Tidewater Virginia

Tropics Waking Up Early for Hurricane Season 2016?


by WeatherFlow meteorologist Shea Gibson


We are already watching for potential tropical development along the SE Region just a week before the start of the Hurricane Season 2016 that begins June 1st and runs through November 30th.  With Atlantic basin waters warming up and overall air temps on the rise, we sometimes begin to see areas of activity develop earlier in middle to late spring. Some of these systems originate over the Caribbean/Bahamas and head north around the first series of our warmer month Bermuda High pressures.   Other times, they may originate from stationary boundaries or tail ends of cold fronts that have pushed off of the SE region.

Bermuda High pressures position out in the Atlantic near Bermuda or expand further west to become larger Bermuda-Azores High’s. This is known as a “blocking pattern” and generally steers surface winds and general surface features in a clockwise rotation.  For us along the SE coast, the surface steering sometimes takes any closed Low pressures or swaths of tropical waving very close if not directly into the coast. Of course this often depends on many other factors, but we always have to count that one as a priority.

(image from


This is no new news for the SE region to see this activity early. For example, we saw early developments (pre-season) in 2012 with Tropical Storm Alberto (60mph) and Tropical Storm Beryl (70mph) that affected the Carolinas on the weekend before Memorial Day and the weekend of Memorial Day.

Recently, the GFS (American), GEFS (ensemble), ECMWF (Euro) and CMC (Canadian) medium range models have been picking up on a disturbance lifting north up near or into the Carolina coast. These models are showing guidance, but nothing is for certain yet. One thing to remember is that until we get an actual area of closed Low pressure, we won’t be able to see trajectories or true potential for strengthening.  Currently, the GFS keeps the Low weaker and takes it into the coastline, the Euro develops it slightly stronger and brings it north up into southeast NC and the CMC develops this into a tropical system and takes a good pull through southeast NC as well.

Here is the GFS from 6am (06z) this morning that goes from this Thursday through next Tuesday – keeps it as a weak Low:
GFS 06z

GEFS Ensemble, which stalls it along the GA/SC coast:
gefs 06z

Here is the ECMWF (Euro) model from midnight last night (00z)- notice I ran this model further out to show potential for another very similar system for next week following right in its footstep.
EURO 00z

Here is the CMC (Canadian) model run from Thursday, May 26th through Thursday, June 2.
CMC 00z

Other factors to watch for:

Sea Surface Temps are warming up in the Gulf Stream. If this system stays over it, tropical characteristics could be in play. Surface temps there are between 80°-85°, with the hotter temps right down the middle.


The shelf waters of the coastal Carolinas are significantly cooler, which would bring cooler surface air into the system for possible subtropical characteristics. Surface temps are between 74°-75° along SC/GA…and cooler at 72.5° up at Wilmington, NC.


Slightly cooler air might also be spilling down along a ridge that develops in the midwest – and the GFS also sees the Bermuda High erode as a northern Atlantic High builds up along New Brunswick/Nova Scotia. This makes for a stalling mechanism over the SC area and a possible weakening process with it being nearly stationary over cooler waters dry air from the NW (and possibly a little bit of shearing along the western side).

Either way, with this much agreement from major model sources, we’ll be watching for development off the SE Coast this week and next week. Until then, take care and stay tuned in to the tropics with us.

Oh and don’t forget – the name “Alex” was already used Jan 13-14 as it became a hurricane near the Azores. So the next storm name will be “Bonnie”.


Shea Gibson
WeatherFlow Meteorologist/ Wind Forecaster
New Stations Projects & Outreach
SE Region/ East Coast
Twitter: @WeatherFlowCHAS


Outside Sources:, NASA Sport SST and Levi Cowan’s Tropical Tidbits


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

Yet another Cut-Off Low out in the pacific but…


This Cut-Off Low spawns a surface storm that shoves the North Pacific High’s winds to the Northern California coast.

by Mike Godsey

Recent days have seen light winds at most sites in the Bay Area as well as clouds and unsettled weather. This has been the result of the Cut-Off Low at ≈ 18,000 ft. that came over the western USA last week and continues to linger as a upper trough. These events have kept the North Pacific High’s surface NW winds several hundred miles away from the Bay Area with its winds focused towards Southern California.

Yet another Cut-Off Low on the far horizon!

Looking at my banner imagery above you can see a surface storm and its counter-clockwise spinning winds about 1200 miles  west of Northern California. Directly NPHstormCompactsCoastabove this surface low at ≈ 18,000 ft. is yet another Cut-Off Low. But this Cut-Off Low does us a favor. As it drifts to the NE towards the Gulf of Alaska it steers the surface storm you see in the banner towards Northern California. This in turn press the North Pacific High towards Northern California. As this happens the isobars of the NPH compact along the coast and the NW winds slowly ramp up each passing day. You can see this happening in the animation to the right. Note how the winds really ramp up this Friday.

Always remember that this scenario is from a low res. model that covers the entire globe so I can not yet guarantee strong winds Friday. For those strong NW winds to reach into the Bay Area we need 2 additional ingredients: 1. For the Cut-Off Low to behave according to the model and 2. For a pressure gradient to develop to the Central Valley and/or the Great Basin to cause the NW wind to curve into the Bay.

Meanwhile the Cut-Off Low itself is modeled to head into Canada so it does bring clouds and rain to Northern California. So the North Pacific High’s surface NW winds linger for a while along the Northern California coast.

Posted in Los Angeles, San Diego, San Francisco

Why does today’s forecast call for: “STRONG, SHIFTY, GUSTY sometimes UNRELIABLE wind”


NPHupperTrough2ZZ1DE9472CThe answer is way aloft!

by Mike Godsey

In yesterday’s blog I talked about why I expected strong NW wind on the Northern California coast today as the North Pacific High was nudged closer to the California coast. At the time I suggested that the winds might be unreliable at some sites and today I added This phrasing “A combo of strong NW surface winds StrongNW windand a mixture of NW and SW wind aloft will make for SHIFTY, GUSTY and for some sites UNRELIABLE wind.”  Plus warnings like this “Due to Gusts, Lulls and Shifts near shore use caution when launching!”

So why the purple prose? First let’s look at this animation.

In the first part of the animation notice the strong surface winds from the North Pacific High which is centered way out in the Pacific.

Still looking at the first part of the animation note the strong winds heading across California towards the Great Basin.

Also note who these winds
curve as they go over the Bay Area. The exact location of that curve in the wind is not modeled very well even in the highest resolution models. And if that curve occurs near Coyote and 3rd. Ave. the winds could go from WNW to an unfavorable W.

Now watch the 2nd. part of the animation which shows the winds at ≈ 18,000 ft. These winds comprise an upper trough that is in the process of turning into a Cut-Off Low.

Often we associate Cut-Off Lows as creating southerly winds aloft for the Bay Area. But this one is mostly north of us so we see strong WNW to SW winds aloft. The peak winds are in red and are in the 70 knot range.

This entire Cut-Off Low is trending towards moving to the east but being a Cut-Off Low it is unpredictable.

Right now it looks like the portion of the Cut-Off Low with SW winds is directly over the Bay Area.

Look at the map of the Bay Area above and you can see strong NW wind on Mt. Tam in Marin but gusts to 47 knots on East Bay hilltops.

The Bodega profiler image shows the very strong NW winds aloft. So that curve will not be an issue for the coast.

The last image shows the amazing strong pressure gradient to various points in the Great Basin.

This pressure gradient guarantees strong surface winds but the winds aloft make it gusty, shifty and possibly unreliable at sites like Berkeley, Treasure Island and Coyote and 3rd. Ave.

Posted in Los Angeles, San Diego, San Francisco

Mysterious E/ENE undercut in spite of solid W/SW’rlys around Duxbury 5/9

Mysterious E/ENE undercut in spite of solid W/SW’rlys around Duxbury 5/9

Tim Kent


A day like many other spring days with Low pressure spinning well to the North of the region scheduled rotate a short wave through the region late morning or early afternoon.  High pressure set up in the Great Lakes, producing solid W’rly gradient over the New England region.

See forecast below:

Picture 4

As you can see I was anticipating the W’rly flow to back SW’rly and couple with Sea Breeze influences as the shortwave approached, a classic Canal Trough feature (a la Matt Corey) was likely to develop as full sun encouraged strong surface heating.  Therefore, I bumped up tables beyond guidance in the early afternoon to reflect what I suspected would be a pretty solid afternoon of SW’rly flow.  See Tables Below:

Picture 5

See AM guidance vs Observations for Duxbury:

Picture 2

As you can see AM guidance does not suggest a NE/ENE’rly turn with the exception of the hourly updating HRRR which doesnt count since it incorporates hourly observations into each new run and it still doesnt suggest that the turn happens until hours later anyway.

So what happened???  lets take a look at some urface analysis maps just to confirm the scenario.

Picture 6

The 12z Analysis from WPC shows exactly what we have described prior.

Picture 3

SPC’s surface objective analysis at 11AM does reveal more pronounced trough like feature at the surface right along the Mass. coast.  Not unusual in and of itself especially considering the Upper Level shortwave that was in the area and the general pattern in place.

But why were pressures so low over just W of Duxbury as to induce a reversal within a solid W/SW’rly gradient????

See Wind Map:

Picture 1

So lets look into this further…..

Surface METARS for Mass.



STN     TIME PMSL   ALTM   TMP DEW RH  DIR SPD GUS PEAK  VIS  CLOUDS                  Weather      MIN MAX P01  PCP 
     DD/HHMM hPa    inHg   F   F   %   deg kt  kt  ddfff mile                                      F   F   in   in  
==== ======= ====== ====== === === === === === === ===== ==== ======= ======= ======= ============ === === ==== ====
TAN  09/1552 1013.5  29.93  66  36  33 260   9  17       10.0  SCT065
TAN  09/1452 1013.8  29.94  64  36  35 260  14  20       10.0  FEW060  FEW090
TAN  09/1352 1014.0  29.94  61  37  41 250   9  19       10.0  CLR
TAN  09/1252 1014.4  29.96  57  37  47 240   7           10.0  SCT100
TAN  09/1152 1014.3  29.95  51  35  54 230   4           10.0  SCT070  OVC090                       46  51
TAN  09/1052 1013.8  29.94  51  34  52 240   3           10.0  BKN065  OVC090


Marshfield (Duxbury)


STN     TIME ALTM   TMP DEW RH  DIR SPD GUS VIS  CLOUDS                  Weather      P01 
     DD/HHMM inHg   F   F   %   deg kt  kt  mile                                      in  
==== ======= ====== === === === === === === ==== ======= ======= ======= ============ ====
GHG  09/1555  29.93  52  41  66  60   9     10.0  FEW055  BKN070  BKN075
GHG  09/1535  29.94  52  41  66  60   9     10.0  FEW055  BKN065  BKN075
GHG  09/1515  29.94  52  39  62  60  11     10.0  FEW055  BKN065  OVC110
GHG  09/1455  29.94  54  39  58  50   7  13 10.0  SCT055  BKN065  BKN090
GHG  09/1435  29.92  59  36  41 260   9  19 10.0  FEW050  SCT070  SCT095
GHG  09/1415  29.93  59  37  44 270  10  18 10.0  CLR
GHG  09/1355  29.94  59  37  44 260   8  16 10.0  CLR
GHG  09/1335  29.94  57  37  47 250  10  17 10.0  CLR
GHG  09/1315  29.94  54  36  50 260  10  15 10.0  FEW055  SCT070  BKN080
GHG  09/1255  29.94  54  36  50 240   9  16 10.0  FEW060  BKN080

New Bedford

1453Z 8 May 2016 to 1553Z 9 May 2016

     DD/HHMM hPa    inHg   F   F   %   deg kt  kt  ddfff mile                              F   F   in   in  
==== ======= ====== ====== === === === === === === ===== ==== ======= ======= ============ === === ==== ====
EWB  09/1553 1013.5  29.93  64  38  38 250  11  21       10.0  CLR
EWB  09/1453 1013.8  29.94  62  38  41 280  15  23       10.0  FEW095
EWB  09/1353 1014.0  29.95  59  38  45 240   9           10.0  BKN090
EWB  09/1253         29.95  55  39  54 240   9           10.0  BKN100
EWB  09/1153 1014.3  29.95  52  37  57 240   5           10.0  OVC070                       44  52
EWB  09/1053 1013.7  29.94  50  35  56 230   5           10.0  OVC095


Based on these observations it is not clear that surface pressures were lower in the interior portion of Mass., than they were on the E or S facing shores.  We do however, note a simultaneous appearance of overcast conditions or scattered clouds between early morning and 15z,  suggesting, that the Shortwave was approaching. There is however no notable signature in the pressure charts:

Picture 8

Minor Sea Breeze front velocity signature on local radar

So what do we think??  It appears that surface pressures were more or less the same at the time of reversal and the only difference is surface temperature, which was nearly 10F between the beaches and the interior.  Is this enough to overcome ~20mph of W/SW’rly gradient??? Ideas, thoughts, concerns, talk amongst yourselves….

Posted in Cape Cod

A tiny shift in the North Pacific High makes a big change in the winds.

WC30Passing upper low bumps NPH towards the west coast so NW winds rip!NPHmay18,19anim

by Mike Godsey

The California coast has seen several days of relatively mild winds as the North Pacific High has held a ridge towards the Pacific Northwest. NPHarrivalOFnwWindANIMThis tends to angle the NPH away from the coast, make the ocean winds NNW and favors eddy formation in Southern California.

Tomorrow, Thursday May 19 an inbound upper trough bumps the NPH a bit closer to the California coast and as you can see in the following animations this makes a big difference in the strength of the coast wind.

Unfortunately I am seeing signs that the wind quality may be less than stellar in terms of gusts and direction especially for Coyote and 3rd. Ave.TEMP2

Update at 4:44PM Wednesday:

Looking at the imagery below you can see that today at 4:30PM the Bodega ocean buoy, about 12 miles west of Bodega just picked up the leading edge of the inbound NW wind you see in the animation above.

Also note that the the Bodega 449Mhz profiler picked up the leading edge of the NW winds aloft at 2000 feet several hours earlier.

These winds should reach near the Bay Area coast by tomorrow morning.

Posted in Los Angeles, San Diego, San Francisco

May Grey brings Winds our Way.

by Meteorologist, Kerry Challoner Anderson

Temps and clouds

May 10 Visible Satellite Picture for Los Angeles Basin with local temperatures.

The Marine Layer has been the starring attraction of our forecasts the last few days. I know I have devoted a lot time determining the depth of the layer, how far it will move inland and when it will burn off and then a of print space communicating that information in the forecast.  If you think we may have given it too much play these animations will likely change your mind (if all them cycling at once doesn’t give you an apoplectic fit first).

During May and June the Los Angeles Basin sees the most number of hours of marine layer clouds, so many that we locals talk about May Grey and June Gloom.  It is important in wind forecasting to be able to predict just how long these clouds will cover the area because they have such a dramatic effect on the local temperatures and hence the sea breeze development.  The animation above was taken on Tuesday, May 12 showing the visible satellite picture and the temperatures.  As the clouds clear from the Basin temperatures jump quickly rising almost 10 degrees in 2 hours while the coast sees little change.

Clouds and temps close

Clouds and winds close

Zooming in a little closer the animations above show the same time period looking down on the Palos Verdes area.  Note how quickly the sea breezes develop as the local thermal gradients increase. The clouds hung around the coastline for much of the afternoon keeping temperatures cool at the beaches while the local valleys warmed further and Cabrillo experienced an extended period of upper teen winds.  You can see from the graph below that the winds built very quickly starting around 1pm as those marine clouds started to peel back from the LA Basin.

Cabrillo Tuesday

Cabrillo wind graph , Tuesday May 10, 2016

Today a High Pressure ridge has built in over the area.  The marine clouds are not as deep and have cleared the Basin much faster.  With the descending and warming air with the ridge and more hours of sunshine the Basin is heating much faster as the animation below shows. Chino was already near 90 degrees by noon. As expected the sea breezes got an early jump start.

Clouds and temps close May 12


Clouds and winds close May 12


Cabrillo Thursday

Cabrillo wind graph Thursday May 12.

Cabrillo’s winds had climbed to 10 mph by 11am and to 20mph by 1pm.  With these strong thermal gradients in place the forecast calls for these winds to continue through the afternoon.   Enjoy!

Posted in Los Angeles, San Diego, Weather Blog

A “Guesscast” about the return of solid NW ocean winds to the Bay Area

bay3 West Coast Wind Blog

NPHdeathBirthLast week in my blog I made a tentative guesscast that strong NW wind would return this coming Sunday. I am sticking to my guesscast with winds beginning to build late this coming Saturday and peaking Sunday & Monday.




In the top image you see the pacific as depicted by the GFS model yesterday. Note the NPH in a crazy position for May in the Gulf of Alaska. Also note how tiny it is compared to its normal size spanning the pacific from California to Hawaii.




Looking carefully you can see a vague clockwise spinning mass west of Southern California there a new NPH is building.



In the second image the old NPH is clearly dying. Notice the counter-clockwise spinning wind of a storm tracking towards the Pacific Northwest.


The big news for California is the increased bulk of the new NPH west of Southern California.



Also note how the storm is becoming disorganized.







In this third image note how the new NPH has bulked up and is now sending strong NW wind near the Southern California coast.












In the next 2 images you can see the North Pacific High make a move on the California coast.








So I expect NW wind to ramp up fast Saturday night and continue to build Sunday and Monday.




Posted in Los Angeles, San Diego, San Francisco

That Mischievous Subtropical Jet!


By WeatherFlow meteorologist Shea Gibson

Yesterday, May 10, 2016, seemed like a fairly quiet day in the Charleston weather world with seemingly overcast/cloudy skies keeping the hotter air temps down while weakening the Sea Breeze along the coast.  However, things are not always as simple as they may seem. What type of clouding was it and what was going on aloft in the atmosphere?  How did it affect winds at the surface? We find that these are a couple of things that WeatherFlow coastal wind forecasters have to watch for in our daily analysis and predictions. Some of these “quieter” days can be very interesting if you pay close attention.

The original forecast presented by WeatherFlow meteorologist Tom Allen noted a modest Sea Breeze with mention of “short waves” to host some weakening effects on the Sea Breeze gradients, which is exactly what happened.

A line of showers was noted on radar by midday into the afternoon slowly approaching the coast. This seemed out of place since there was no surface cold front registering,  nor was there one expected in the SE Region (for GA/SC respectfully). Areas of GA/FL showed thicker convection. A more saturated lower atmosphere saw this rain hit the ground, and others areas did not – especially up into SC.

Further analysis showed the Subtropical Jet Stream lifting up into the SE Region into a freezing layer aloft.  The height of the Subtropical Jet Stream is roughly 10-16kms or 33,000-52,000ft up.  This jet is known to carry abundances of moisture along it, which are sometimes known as “atmospheric rivers”.  This portion of the subtropical jet was carrying just enough to assist with convective feeds to those areas with more abundances of sunshine and strong thermal lifting to mix just underneath the stratus cloud decking.

The subzero temps aloft kept the liquid in a frozen state in this case. It certainly looked like the radar signature was showing this moisture reaching the ground near Charleston; however, we see in the Charleston weather balloon reading that there was a subzero degree saturated column aloft likely made up of ice crystals and a very dry slot below that.


It was so dry that the moisture (ice crystals) falling would sublimate, or turn from solid to gas. (These areas of convection were causing short waves to develop overhead).  This resultant sublimation of ice to gas is is called “Virga”.  From the ground, it can appear as very subtle, and other times can appear more dramatic. This was from North Charleston around 3pm.
Notice the mammatus clouds associated with these short waves. They are the bulging clouds in the center of the picture. These don’t always have to be associated with thunderstorms as there are exceptions to their development, such as this example.

And this was later over Breach Inlet between Isle of Palms and Sullivan’s Island around 7:00pm for another shot of virga.

You can see how over time, this precipitation process slowly saturated the atmosphere below it, but not enough to develop liquid to reach the ground.


So what does this have to do with surface winds for the Charleston area? Let’s take a look at the entire day wind profile from the Fort Sumter Front Range Light in the outer Charleston Harbor and walk through it:

  1. The nocturnal jet burned off early as usual with the sun coming up.
  2. There was an initial build of the Sea Breeze as a break in the clouds late morning allowed for thermals (rising parcels of warm air) to show some increases in speeds.
  3. Once the clouds filled back in, the winds came down again.
  4. Then we see the convections give increases to winds in small spikes due to warmer air surface convergence lifting up towards the cooler air aloft (warm air advection).
  5. Clouds break and we see a slight nocturnal jet rebuild with weaker speeds later.


There are many who watch the wind forecasts frequently for their respective purposes, so we try to learn something new every day about the daily pattern and what to expect. As a colleague of mine meteorologist Joey Sovine from Live5 news in Charleston, SC, once said (and as we understand in the field), “There are no two days alike”.  Never a dull moment in meteorology!


Shea Gibson
WeatherFlow Meteorologist/Wind Forecaster
Outreach & New Station Projects
SE Region/ East Coast
Twitter: @WeatherFlowCHAS

Posted in Coastal South Carolina

Inverted! Is it an old school windsurfing or kiteboarding trick?

a Inversion marine layer

Early arrival of summer type dense marine layer.inversionPhotos

by Mike Godsey

Dawn today in coastal Southern California or S. F. Bay Area found dense fog almost to the surface. These type of marine layer clouds are usually typical of June Gloom and their early arrival is triggered in part by the unusually warm pacific waters this year.

Looking at the first 4 photos it looks like a miserable day for a hike up the hills. Dreary fog at the Golden Gate, Ocean Beach and over the bay waters. But hikers in the know have learned that when the marine layer is dense like this the hilltops are basking is warm glorious sunshine as you can see in the next 2 photos from early this morning.

All of  this may seem like hiking trivia but it actually reveals a lot about Bay Area and Southern California winds. Looking at those same 2 photos notice how the top of the marine layer clouds is relatively flattened. That zone is known as the inversion point. Sometimes the inversion point is so defined that you as walk downslope it almost looks hard enough that you could walk across the top of the marine layer. What makes the inversion and why is it important for wind? Hang on…

Now let’s take at the satellite imagery from the same time this morning. Looking at both Southern California and the Bay Area notice how the marine layer clouds have oozed well inland overnight. But also notice how you can see how the fog has flowed inland following the contours of the topography. At leading edge of the marine layer you can see every valley and gap in the coast range. It almost looks like a giant hand squashed the marine layer clouds down into the valleys.

Next look at the imagery from the 449 MHz profiler at Bodega Bay in the North Bay. This vertical radio signal detects both wind direction aloft and the air temperature at different levels. Notice the very cool air that extends from the surface to about 300 meters or about 1000′. Then note the abrupt transition to much warmer air. The point where these two very different air masses meet is the inversion. The cool air near the surface is relatively heavy dense air and if wind blows in this layer the wind tends to be very steady since as it encounters hills or trees etc. it lifts and then quickly reattaches to the surface. So if you go hiking on a day like  this you will find warm still air above the inversion but cool windy air below.

So why are the marine layer clouds so compacted towards the surface today? The answer starts far to the east. Looking at the last image note the high pressure in the Great Basin  area. This high pressure area is moving closer to California. Within this high pressure zone the air is descending so it compresses as it encounters the surface. If you have ever filled a SCUBA tank, or vaguely remember the gas laws, you know that when you compress air it heats. So today as the warm air descends over California it is compressing the marine layer clouds and creating a moderate inversion above the marine layer.

So all we need is a strong pressure gradient towards the valleys and we will see steady moderate wind in both the Bay Area and Southern California. And looking at Kerry’s 7 AM forecast today it looks like both Southern California and Bay Area sites favored by SW flow will reach the upper teens!

Posted in Los Angeles, San Diego, San Francisco