Avery

WEATHER DISCUSSION: SSW, THE POLAR VORTEX & EL NIÑO — PLUS CALIFORNIA'S HALLOWEEN WEEKEND STORM!!! Followers - There has been A LOT of discussion lately about a potential Sudden Stratospheric Warming (SSW) event, the Polar Vortex (PV), and whether this could interfere with the developing El Niño signal heading into winter. I've been posting about this relationship for a while now, ever since I met with @judah47 in August. The big question isn't simply whether #ElNiño will dominate or the #PolarVortex will take over. It's how these competing influences interact, how the atmospheric response evolves, and whether changes high above the Arctic eventually work their way down to the weather we experience at the surface. If you don't want to read about it, scroll down to the bottom, where I discuss the weekend storm in #California. Let's put some context around this, because the terminology can get confusing. WHAT EXACTLY IS AN SSW, AND HOW DOES IT RELATE TO THE POLAR VORTEX? First, the Polar Vortex isn't a giant ball of cold air waiting to drop into the United States. It's a large-scale circulation of strong westerly winds that develops around the polar region during the colder months. There is a stratospheric polar vortex high above the surface, and a tropospheric polar vortex lower down. They are related, but they aren't the same thing. A Sudden Stratospheric Warming is a major disruption of the stratospheric circulation. Temperatures high above the Arctic can rise dramatically over a short period as atmospheric wave activity transfers energy into the stratosphere. That warming can weaken, stretch, displace or, during a major event, split the stratospheric Polar Vortex. In a major SSW, the normally westerly winds in the stratosphere reverse direction. Here's the important part: the warming happens high in the atmosphere, not at the surface. If the resulting circulation changes propagate downward, they can influence the jet stream, atmospheric blocking and the persistence of weather patterns over the following weeks. Sometimes that favors colder outbreaks in parts of North America or Europe. Sometimes the impacts are limited, delayed or concentrated in different regions. An SSW is not an automatic snowstorm forecast. It's a potential driver of a larger atmospheric pattern change. WHY I'VE BEEN WATCHING THIS ALONGSIDE EL NIÑO El Niño influences the atmosphere through changes in tropical Pacific convection, the Walker circulation and the large-scale planetary waves that help shape the jet stream. In a typical El Niño response, the Pacific jet can become more active or extend farther east, increasing the potential for storm systems to reach parts of the southern United States and California. But the strength, location and timing of that response vary. Now introduce a potential early-season stratospheric disruption. If the Polar Vortex weakens significantly and that disruption couples with the troposphere, it could help reshape the background circulation. That may change where ridging and troughing develop, where blocking becomes established, and how persistent certain weather patterns become. This is where the discussion gets interesting. The SSW/PV signal could modify the way the El Niño pattern expresses itself. That doesn't mean it switches El Niño off, or cancels its influence or guarantees a cold winter. Think of El Niño as one major influence on the atmospheric playing field. The Polar Vortex, the stratospheric winds, the QBO, the MJO and other teleconnections can all influence how the game unfolds. I discussed this in my highlight posts over the summer. And yes, these signals can sometimes work against one another. The developing stratospheric disturbance is particularly interesting because forecast guidance has been pointing toward an unusually early weakening of the vortex in early November, with some solutions suggesting a much more substantial disruption. But there's a difference between a forecast showing a weakening or stretching vortex and a confirmed major SSW. The details matter, including whether the stratospheric winds actually reverse and whether the disturbance propagates downward. That's what I'm watching—not just the headline or one dramatic model image. Again, the OG of the PV is @judah47. He discussed on his blog that this #elnino year, might resemble the winter of 2009-2010. Again, it is anyone's guess, but this could happen. Stay tuned! NOW, LET'S BRING IT BACK TO THE WEATHER OUT WEST While we're discussing the bigger winter pattern, California has a much more immediate weather story developing. An upper-level low is forecast to pinch off from a larger trough over the eastern Pacific, then move toward northern Baja. The exact speed and track remain important, and the latest guidance still shows some differences. As this feature approaches, the combination of cooling aloft, changing wind fields and lift associated with the upper-level circulation will support precipitation across California, with the current signal favoring Southern California. Basically south of Pt. Conception. In plain English: the Pacific is sending us a storm system, and the setup is becoming increasingly favorable for a wet Halloween weekend. This isn't something we need to attribute directly to the Polar Vortex or declare proof that El Niño has arrived. The immediate forecast is driven by the evolving Pacific trough and upper-level low, sterring in a slug of moisture, with not great dynamics, over #socal. The larger pattern tells us what to watch over the coming weeks. The short-range dynamics tell us what's likely to happen this weekend. Two different time scales. Both worth following. CALIFORNIA HALLOWEEN WEEKEND: THE LATEST SETUP 📍 Southern California: Rain chances increase Friday, with the more meaningful precipitation currently expected Saturday into Sunday morning. The latest forecast favors the period from Saturday evening through early Sunday for the main event. 📍 Los Angeles County: Current estimates generally favor around ½ inch to 1 inch, with a broader range of roughly ¼ inch to 2 inches possible across the region. Foothills and mountain slopes could receive higher totals. These are estimates, not guarantees. 📍 San Diego and areas farther south: The southern track of the system makes this part of the state worth watching closely, although local totals will depend on the storm's evolution. 📍 Bay Area and Northern California: The latest guidance favors a much weaker precipitation signal farther north. Some showers remain possible, but don't assume this will be a statewide soaking. 📍 Sierra Nevada: Some snow is possible at higher elevations, but this is not currently a signal for a major Sierra snowstorm. Snow levels and the exact track will determine the outcome. One important caveat: a small change in the upper low's speed or track can shift the timing and distribution of rainfall. The forecast is becoming clearer, but the details are not locked in. If you have Halloween plans, have a rain plan. The heaviest rain may favor Saturday night into Sunday morning, but don't build your entire forecast around a single model's timing. I have posted some Euro AI images below. Take a look and I will refine it tomorrow. Some areas might get light showers in #socal tomorrow, so pay attention to the NWS, at weather.gov, should the situation change. One thing I am not expecting is major hydro-logical issues in #socal. This is not an AR or a #elnino fused storm......it is just a slug of moisture, being steered into #socal by the upper level low. WHAT I'M WATCHING NEXT 👀 Three things stand out: - The stratosphere: Does the forecast weakening develop into a major SSW, including a reversal of the stratospheric westerlies? And if it does, does the signal propagate downward? - The Pacific pattern: Does troughing continue along the West Coast after this weekend, or does ridging rebuild? One storm doesn't establish a persistent wet pattern. - The El Niño response: Does the expected tropical forcing translate into a more recognizable Pacific jet response as we move through November and into winter? I'll be watching the actual atmospheric response, not just ocean temperatures. This is why I have been discussing the SSW/PV and El Niño relationship for a while. The interaction could be important, but it is still evolving. We need to see how the atmosphere responds before drawing big conclusions about the winter. BOTTOM LINE: California has a legitimate rain setup for Halloween weekend, with Southern California currently favored for the more meaningful precipitation. Meanwhile, the potential early-season Polar Vortex disruption is a fascinating wild card for November and winter. It could modify the larger circulation and influence how the El Niño signal plays out, but it is not a guaranteed cold shot or a winter forecast by itself. Below I have posted the 850MB temps for the long range and you can see the SSW in the Article circle. Also, sent some model runs of the NBM and the Euro AI of the total precipitation totals for this weekend. Again, it is a complex forecast. More later........... #Weather #PolarVortex #SSW #ElNino #CaliforniaWeather #SoCal #cawx #wxtwitter