It’s going to be hot this coming week, for SoCal particularly Monday through Thursday. Why is it happening and how bad will it be? Daniel Swain, climate scientist from UCLA, explains in his latest iteration of Weather West. https://weatherwest.com/archives/7289
I don’t think the coast at least in SoCal is quite as threatened by the heat wave as he makes it sound, but he was highlighting the better chances for coastal heat in NorCal. The frequency of occurrence map seems to bear out that thinking. I like his explanation of the stationary blocking pattern. Only one month until summer, but we’ll get pretty summery before May is done.
While the coronavirus itself does not influence the weather, the global pandemic caused by it can influence the weather forecasts. Just what we need: another setback caused by the pandemic. It might be an interesting and possibly frustrating hurricane season unless we can get the full complement of observations going into the weather and hurricane models.
Today we have a marine layer on steroids. That means some low pressure aloft is moving into the region, the onshore flow (pressure gradient from coast to desert) is increasing (gradient is steepening), and the marine layer is deepening. The upper level trough is featured on the GFS model image for this afternoon (below).
Next, check out a nice visible satellite loop of today to illustrate how deep it was (below). Look closely at the first few frames and you’ll see the clouds were banked up against the mountains up to 5 or 6 thousand feet deep. Inland areas cleared, but then a plume of moisture (the cloud extending from roughly Orange County to the southwest) has moved in. This has enhanced the marine layer to the point of producing showers (see radar loop further down). Several things to notice, the initial marine layer rather stratiform (flat) clearing inland, then more convective, bumpy unstable clouds filling the coastal basin again. These appear brighter because they are thicker. The Elsinore Convergence Zone is firing up, the bright band of clouds over the Inland Empire at the end of the loop. The Antelope Valley is showing some wave clouds, lots of standing lenticulars that are without doubt very beautiful. I hope we see some sunset photos of these. Finally, the convergence zone cloud band around Hesperia. That’s probably producing virga, rain not surviving the dry journey to the ground.
In my upcoming book, I explain what’s happening.
When the marine layer responds to approaching low pressure aloft and/or a coastal eddy, it can deepen sufficiently and stretch the marine layer and its clouds vertically. There may be just enough vertical growth of the clouds to produce drizzle or even raindrops from the customary lifting and cooling mechanism. As the cloud deck itself grows thicker than about 1,500 feet, those tiny water droplets can grow just large enough to fall as drizzle. The water droplets are still very small compared to raindrops. In fact, it can drizzle all day, even making roads wet and windshield wipers thwack, but the rain gauge measures only a hundredth or two to show for it.
If this lifting mechanism continues, and the cloud deck grows over 2,000 feet thick, these drizzle droplets can grow bigger into actual raindrops to produce legitimate rain showers. All this can happen below the glass ceiling within a deep marine layer. Typically, these events are the result of a marine layer growing deeper than 3,000 feet. Measurable rain is possible, but usually less than a few hundredths of an inch. Extreme marine layers that grow over 6,000 feet deep happen a few times a year during any part of the year, but most likely in the spring. These are usually the result of the combination of lowering pressure aloft and a coastal eddy. Flat stratus clouds turn into their lumpy alter-ego, stratocumulus. Showers are widespread, but still not heavy, unless you look to the coastal slopes of the mountains. These foothills can harvest a great deal of rainfall because of the persistent onshore flow that forces the orographic, upslope effect. Where the lowlands get a tenth inch or so, Mt. Wilson or Crestline can pick up over an inch! In rare cases, the marine inversion can rise up into the mountains above the freezing level and produce light snow instead of rain.
These wet marine layer events that come from a very deep marine layer brought on by a low-pressure trough aloft begin to blur the lines of definition. After such an event, one may well ask: Is this an extremely heavy marine layer rain event? Or a very light winter storm? There are elements from each kind of weather event, so a hybrid of the two might be the best answer.
With some careful observation and practice you can look to the sky and tell the difference between rain-making clouds and rain-preventing coastal stratus. If the clouds are not continuous, meaning lumpy, bumpy, and very dark on the bottom, they can produce showers. If the cloud deck is quite uniform in color and more like a consistent cloud sheet, it’s stable stratus. A good zoomed-in satellite image can also help you make that call.
Check out the radar loop below from this afternoon. Notice the band of very light showers moving through Orange County. Also notice the orographic enhancement, the mountains that force the winds upward, producing deeper, taller clouds to drop more rain on the coastal slopes of the mountains. Finally, there’s a line in the inland empire from southwest to northeast that forms at the end of the loop. That’s the Elsinore Convergence Zone. Winds through Corona crash into winds through Temecula force air upward in the convergence zone, forming clouds and an organized band of showers near Hemet and San Jacinto.
And this is all what can happen when the marine layer goes on steroids!
My friend and mentor Jan Null of Golden Gate Weather and formerly of the NWS has studied how cars left in sunshine hear up, how fast, and their awful implications for people and pets left in them. Dr. Marshall Shepherd is an renowned and esteemed meteorologist who might be most famous for hosting the Weather Channel’s Weather Geeks program. His scientific integrity is rarely questioned and he might be the nation’s best at distilling important meteorological science into useful information in lay terms.
Things are definitely cooling and calming down this spring here in SoCal. After unprecedented April storms and two heat waves in late April and early May, we’ve entered a more seasonal pattern.
Weather West blogger Daniel Swain (climate scientist at UCLA) has shared some good discussion about the current trends. Lest we’re too myopic to our transition to seasonal weather in SoCal, Norcal is gonna get a decent storm this weekend. That’s good; they need it.
Last week I was invited to join a Zoom meeting of Poway third graders and their teachers learning about weather and meteorologists. One of the questions they asked me is “what’s your favorite weather?” My answer: “whatever weather is happening that day.” Now I’m sure that was somewhat disappointing, as nine-year olds would probably want to talk about tornadoes and hurricanes. But I told them the truth. As I say in my book and as I say in everyday conversation, I don’t need the fancy, eye-catching, attention-grabbing, newscast-leading, severe, deadly weather to get my interest. In fact, my interest in weather is less about adrenaline and excitement and more about curiosity and the fun of figuring stuff out.
That’s a good intro to my entry today. Bear with me; you’ll probably need to pull out your weather geek equipment to stay with me on this. Overnight, while at my forecast desk at the National Weather Service, I was doing my job monitoring the weather and making forecasts. I was briefed by the Sunday evening shift that today would probably be a lot like Sunday, a slow gradual clear of clouds toward the coast, but probably not clearing at the coast much if at all. That sounded right to me as I glanced at the satellite image that showed nighttime low clouds most clearly. The clouds had pretty much filled the coastal basin (coasts and valleys up to the mountains) completely by 11 pm.
Then weird stuff happened. At about 2-3 am, big chunks of clearing began over the ocean and some beaches and inland San Diego County. Was the marine inversion weakening? (that would be one most likely reason for the interesting busting-up pattern). No, our balloon sounding from Miramar at 5 am still showed a decently strong inversion. What else is going on? Waves. If you look at a satellite loop going fast, you’ll see subtle waves passing over the marine layer’s cloud deck. Maybe some upper level feature was creating some atmospheric waves just above the inversion, like a flag flapping in a horizontal position. The downward vertical motion could bump into the cloud deck, introduce drier air mixing into the marine layer, and clearing the clouds, at least temporarily.
The satellite loop shows infrared imagery before sunrise this morning, and visible imagery after. Watch it a few times going fast and you’ll see some wave action traipsing across the cloud deck. Some of that wave action mixed drier air down into the cloud deck, evaporating the clouds in somewhat random fashion. After sunrise, some surface based convection (sun heating ground to create a shallow rising motion) actually regenerated some clouds where they had cleared before sunrise. As frustrating and difficult as it is to forecast these very subtle, nearly invisible waves that cause clearing, it’s also stimulating and engaging to me. The whole marine layer and low cloud regime of coastal California can drive one crazy, but I take it as Mother Nature’s way to reminding scientists like me just how much we don’t know. Think with my years of experience forecasting this stuff I’d have more of it figured out. It keeps me humble, and I prefer to be humble instead of frustrated.