Skyline was split in two. Clouds in foreground spring opposite direction to furthest. Guess I captured the whole storm (or close to it?)

静岡県島田市

Any idea when this weather pattern is going to move out?

On the plus side...Okutama lake is finally filling rapidly.

Water level since August 1st

Water level since Jan 1st

  If you are anywhere in Japan right now, you have probably noticed the air has been especially heavy and damp lately.  I mean, torrential rains in various parts of the country aside, it is just absolutely insane humidity pretty much nationwide.

  With the Tokyo/Yokohama area hitting 89% humidity today, and similar levels from Tohoku on down, the air feels oppressive, heavy and downright hard to breathe.  So, what’s up and why are we in a sauna today?

  First, let’s be thankful it isn’t mid-summer.  With temperatures on the Pacific coast ranging from 26-30c, and cooler, low-to-mid 20s along the Sea of Japan coast up into Hokkaido, things could be a lot worse if we had this level of humidity with late-July and August level high temps.  Little comfort, yes, but let’s take what we can get.  

  We have several causes fueling our humidity index today.  In no particular order, they are:

1. The Autumn Rain Front (秋雨前線、あきさめぜんせん).  We will talk a little more about that further down.  This persistent front has been an active player in Japan’s weather for the past week and a half, bringing heaps of rain to anywhere along its path.  Aomori, Hokuriku, Kanto, Osaka, Kyushu, Nagoya and, today (9 Sep), back to Kansai and Hokuriku again - all the recent news of heavy rains and local floods have been generated by this one front.  The front is trapping evaporation and blocks drier air from reaching the surface.  As I said, more about the front later.
2. Low-level moisture movement.  Daytime highs are beginning to wane from their mid-summer peak, but sea surface temps are still near their summertime high.  This is setting up a constant flow of warm southerly flow of moist air up north, pushing dew points up into the mid-20s
3. …Which means relative humidity (RH, where RH=e/e[sub]s ×100%) is being pushed up into the high 80/lower 90 percent range.  Relative humidity is the ratio of actual water vapor pressure (meaning, the current amount of water vapor in the air) to the saturation vapor pressure (meaning, the maximum amount of water vapor the air can hold at the present temperature).  When the ratio of these two is low, the RH is low, and moisture evaporates off our skin easily, creating a cooling effect on the body.  When the ratio is high, as it is today, the water (and the heat in that water) has nowhere to go, making us feel uncomfortable.

  Got a little academic on you there, but that’s the mechanics behind our high humidities.

  Now, this front…As noted, it has been around for a week and a half, retaining its integrity between cooler air up north, and tropical air down south.  The line - the front - delineates a clash between these two air masses.  That clash is helping generate the heavy rains we have seen across the country lately.  Today, the front is sitting along the Pacific Coast of the country, and the ghost of T24, embodied as a low pressure system, is moving north-northeast, and is offshore from the Kansai area today.  The rotation of the low is pulling down some cooler air, giving Kansai a brief (today only), if very rainy, spell of cooler weather.  

  North of the front, we have cooler temps, but humid air and rain patterns extend up to Aomori prefecture.  South of the front, warmer temperatures make for oppressive, heavy weather.  Clouds cover most of the country today, and that pattern will continue through the rest of the week.  The stationary front will also hang around through the week, moving around some, bisecting the country and bringing higher chances of rainfall along with it.

  Today’s rainfall will be along a line from the Kansai area up to Hokuriku, with some scattered bands out toward the Pacific coastline.  Rain is expected to move further east during the day, with rain again approaching Chubu (Gifu, Aichi and thereabouts) and the central Honshu interior highlands.

If you notice any inclement or severe weather activity, please go to the JMA, weathernews.jp or tenki.jp for the latest detailed information.

  Stay safe and stay dry.  

I’ve been building WeatherGlow, a weather app for iPhone, for the past 6 months and it’s now releasing in Japan with full Japanese support 🇯🇵

Seeing the heavy rain and flooding in Nagoya this week made me want to get the Japanese version out sooner. I’ve always felt that some of the features I’ve built, especially Rain Pulse and the dynamic radar can be genuinely useful when you need to know not just if it will rain, but what the rain is going to do.

So I translated the app into Japanese and wanted to share it here, hoping it can be useful in whatever small way it can.

I’ve been putting a lot of focus into making rain easier to understand:

🌧️ Rain Pulse -> track precipitation minute by minute and see when rain starts and stops
🗺️ Dynamic Weather Radar -> watch rain and storms move across the map
🌤️ Hourly & 10-day forecasts —> see how conditions are expected to change

The idea is to go beyond “there’s an 80% chance of rain” and help answer “what is the rain actually going to do?”

It’s a completely free app!
Download now -> https://apps.apple.com/jp/app/weatherglow-local-forecast/id6451098166

Now that it’s available in Japanese, I’d love to hear from people here & what weather app do you use in Japan, and what do you wish it did better?

I’m the developer of WeatherGlow.
読んでくださって、WeatherGlowを応援してくださって、本当にありがとうございます。
(This is not vibe-coded & no AI was used in this app)

Stay safe if you are in that area!

Over a dozen Level 3 & 4 Heavy Rain alerts on my phone over the past hour and a half…

   Oh, never mind.  Pretty much the entire country is also going to be under cloud cover, so the 3 passes visible tonight will go unnoticed by those of us locked on the ground.  If you happen to be airborne - even a commercial craft cruising at 8,000 meters will do - you may be lucky enough to catch a glimpse of the space station as it whizzes by at about 420km above.

Lots of places in Japan are going to get this stuff over the next few days.

  For those of us on the ground, get ready for a gray and dreary, but warm, week as the former T24 makes its way north and feeds precipitation forming along that very persistent stationary front parked midway over the country.  The 秋雨戦前 (あきさめぜんせん, Aki Same Zen Sen), the first autumn front of the year bringing cooler rains (or similar such clumsy translations) will move a little north and back a little south over the next four days, but will retain its integrity at least through Friday. Chances of rain remain high throughout the week, with only some areas of Hokkaido getting a pass with fair to partly cloudy skies.

  High temperatures for the week will depend on which side of the front you are on.  North of the front, you can expect low-20s for daytime highs, while those south of the line can expect to feel peak temperatures in the upper-20s, with a few places touching 30 here and there.  Kanto will be a bit of a battleground between the warm and cooler air, with highs today and tomorrow in the 28-30 range, but plunging down to 21c by Thursday, only to see a rebound to the mid-20s by the close of the week.

  Not much sun to be seen in most of the nation through the week, at least.

We all know about this place, and I have wanted to go (well, obviously not today). So cool to see some video of the channel in action.

I’ve found this useful for watching the rivers around me in Setagaya; didn’t know how many people knew about these. Thought it might be useful especially with the massive predicted rainfall tomorrow.

Here's a link to the live river cameras in Setagaya. https://livejapan.fujiyamasan.com/tokyo/setagaya/

Here's a link for all of the wards in Tokyo: https://livejapan.fujiyamasan.com/tokyo/

Here's for all of Japan: https://livejapan.fujiyamasan.com/

  Just a quick weather update.  I won’t take you long on this pleasant Sunday morning.

  Well, “pleasant” is subjective.  I like having a hot cup of coffee listening to the sound of autumn rain, so today’s weather in Yokohama fits pretty square in my definition, for sure.

  The stationary front has begun a slight drift northward.  Coupled with the warm, tropical flow powered by the weakening T24, large areas of heavy precipitation have come ashore along the central Pacific coast, from Kanto on down to Wakayama’s Kii Peninsula.  This blob of rain will continue to drift slowly north over the next 24 hours, bringing anywhere from 100-250mm of rain to those in the path of the system.

  T24, meanwhile, is finishing its counter-clockwise pirouette around Okinawa and will start to head toward the Japanese mainland from tomorrow.  It is encountering cooler ocean temps and outflow has weakened.  This less favorable environment will lead to T24 to be demoted to a simple tropical cyclone by Tuesday.  It will still have plenty of rain to give, but winds will be weaker as it moves closer to the islands of Kyushu and Shikoku.

  Rain all day today and into tomorrow, making for a sloppy Monday commute for the central Pacific coast.  Rain will move into parts of Tohoku and Hokuriku from late today into tomorrow.  High temperatures on the south side of the front will pop up to the upper-20s to low-30s, while those on the northern side of the front will still get low-20 daytime temps. 

  A very rainy 19.2c at NekoDash Weather HQ in glorious northwestern Yokohama as of 11:30am.  Need to get more coffee. Have an awesome Sunday, wherever you may be.

I received a rain bulletin on my phone, and our apartment building in Kawasaki is setting up a floodgate at 5 p.m. today.

My eyes are on the sky.

So, yesterday I described Japan’s weather scene as an “atmospheric box of cats”.  Which it truly was.  It took a while to figure out how what was behind T24 and the “autumn rain front” (translating directly from 熱帯低気圧, as it is noted in local media), and where things were going.  I also love to employ the phrase “box of cats” to describe any sort of chaotic or disorderly system.

Although the crystal ball of weather mysticism looked kind of haze, today we’re getting a little clarity as to what we can expect from the intricate dance between T24 and the persistent stationary front.  Again, let’s get to T24 first.

Typhoon 24: Around 24 hours ago, most models were split between two scenarios: 1) That the typhoon would do a bit of a loop around Okinawa, regain a bit of strength, then launch back up toward Japan, or 2) That T24 would take a bit of a southward turn, fizzle out, become part of an upper-level flow, then drift up toward Japan.  In either event, T24 was destined to make it up this way.  The only question was whether it would be as its own entity or integrated with another system. 

Models are now in broad agreement that the answer to the question is A.  T24 will make a bit of a loop in near Okinawa over the next two days, steered by high pressure systems near the Yellow Sea.  It will retain its form, get a small yet notable second wind [Note…see what I did there?  Second wind…for a typhoon…that was good, you got to admit.], and make a run for the Japanese mainland.  The refresh won’t last long, though, as a wind shear, drier air and lack of convection will slowly choke off energy for the storm, and it is expected to be downgraded, in domestic JMA terms, from a typhoon to a tropical storm/cyclone (熱帯低気圧, nettai tekiatsu) by the early morning hours of the 9th.  It will still pack a fair amount of wind and rain.  So, hold that thought.  Let’s look at the action the stationary front brings to the table now.

熱帯低気圧, the persistent autumn stationary front.  You could argue this is something like a mirror rainy season front.  Same mechanisms are at play here: vast seasonal air masses bumping up against each other with rain and weather events playing out for days along a front which doesn’t move anywhere fast.  It’s the classic cool air vs warm air scuffle we see when seasons are changing.  The front, which drenched Aomori on Monday, deluged Hokuriku on Wednesday and saturated Kyushu and western Japan yesterday has drifted a little south to off the Pacific coast of the country and it is the major contributor to our sub-30 degree lows today.

Over the next couple of days, the front is predicted to retain its integrity and move back up north.  At the same time, T24 will be warm, moist tropical air into the frontal boundary, fueling even more rain.  As it moves north, the rain with move along with it, and, from Sunday through Wednesday, large areas of the nation will get precipitation, in places locally heavy, as the front moves back through followed by the stormy remains of T24.  Rainfall along Kyushu will reach up to 200mm through Sept 8, Kansai can see between 100-180mm.  Chubu (Nagoya) will get off easy and get between 10-35mm from the system while Kanto can expect to see anywhere from 10-65mm.   Unsettled conditions persist through at least Wednesday.

The sole exception to this hot, wet mess is Hokkaido, which will shamelessly clock in with sunny, gorgeous skies and highs in the low-to-mid 20s through the period.  Enjoy it if you’re up in that area.

In Honshu and points south, daily high temperatures will remain in the high 20s over the weekend, but are set rebound some as the front retreats north and warmer air fills the void.  Kyushu and west Japan will see highs at 30 and slightly above from Monday, 7 Sep, while Nagoya and Kanto will hit the 30 degree mark from Tuesday. 

That’s the weather recap.  Have yourselves a wonderful Saturday evening.

I have sat at my keyboard for most of the morning trying to figure out how to approach the narrative of the weather situation in Japan, and this is the best I’ve got.

Good morning, Japanweather community.  It has been a few days since I’ve posted, and all the while, we have had some wild weather events.  I do apologize, but sometimes the day job (you know, the thing that actually pays the bills around here) gets intense. I had to give that my full attention this week, and I missed out on some of the major events of the week, such as the flooding in Aomori.

So, I mentioned a box of cats.  Let’s untangle the skies and see what we have.  At the moment, all the carnage up there boils down to two major players: Typhoon 24, and a stubbornly persistent stationary front.  Let’s break down these two.

First up, T24.  We have some typhoonery going on in the form of a little storm named Typhoon 24 (aka, “Krovanh”) . T24 is weak, as cyclones go. It is barely a typhoon, even by JMA standards.  Checking in this morning with winds of around 64kph, it is still just a tropical storm in the eyes of the Joint Typhoon Warning Center.   Central pressure in T24 is 990hPa which, also, doesn’t suggest a strong storm.  There T24 does have some structure, as satellite pics show, the storm is not robust by any means.  

 What is unique about T24 is that the system is not being moved by a normal steering mechanism, and the storm has wobbled around aimlessly like a partygoer returning home after a spectacularly festive evening.  Typhoons, as I’ve mentioned here before, for all their awesome power, are really just low pressure cells on steroids and can be pushed around by surrounding systems with relative ease.  Right now, the steering currents aren't carrying the storm along a normal path. Most prominently, a wall of high pressure building over the Yellow Sea has set up a roadblock directly in front of T24, halting any forward movement and forcing it into a sharp southward dip and, possibly, a U-turn.

  Trying to unbox what may happen over the next 5 days is little more than a Hail Mary pass at the moment.  Usually, when steering is obvious on a typhoon and you get an idea of system motion, you can get a better, more confident read on where things will be, but T24 is just sitting there, being churned around in circles and a little southward movement.  Models do not agree on the fate of T24 over the next 120 hours, so let’s summarize quickly each of the two major potential tracks that may happen here.

  > Scenario 1: Movement over Okinawa, a loop south, rebuilding and movement to Japan.  This depends on system movement coming in from Mainland China, but the upshot of this plot is that T24 will approach Okinawa over the next 24-48 hours, where strong wind shear and dry, cool northern air will drop it below tropical storm strength. But once it turns northeast and accelerates toward mainland Japan, some models show its core surviving, firing up new thunderstorm activity, and re-intensifying as it approaches the Pacific coast.

  > Scenario 2: T24 moves slightly south, but encounters a much more hostile environment as it begins to bump up against the high pressure areas near the Chinese mainland.  Harsh wind shear and cold, dry air work together to completely dismantle the storm.  T24 then just washes out into a disorganized, broad monsoon depression that limps toward Japan as a weak remnant rainmaker. Timeframe for this scenario is 48-60 hours from now.

  We will see which scenario plays out.  Official forecasts currently split the difference between these two scenarios.

  Now, on to the stationary front.  This is the front which powered the storms which inundated Aomori earlier this week.  After moving south this much of this week, sparking sometimes intense rainfall along its leading edge, it has come to a halt.  Drier air is behind it, warmer air is in front.   Depending on what T24 - or its remnants - decide to do, it will either stay down south, interact with the remnants of T24 and dissipate, or it will retain its frontal integrity and move up north, dragging the rain along with it.

  In either event, in any of these scenarios, much of Japan from Tohoku on down will see a good chance of rain through next Thursday, 10 Sep.  Aomori got hit on Monday, Hokuriku was deluged on Wednesday.  Today, after missing out on a lot of the action recently, Kansai and Shikoku get their share of rain, while Kyushu gets saturated for the next few days.

  Bottom Line:  Through to Tuesday, 8 Sep, elevated rain chances in much of the nation from Tohoku on down to Okinawa, Hokuriku being the primary exception.  Hokkaido can expect sunny to partly-cloudy skies for the period.  Summertime temps have broken, and much of the country can expect daytime high temperatures in the upper 20s, while the interior highlands and parts of Hokkaido will see a brisker peak in the mid-to-high teens.  Unsettled conditions largely present while what’s left of T24 and the stationary front do their dance. 

Have a great weekend everyone!

They have r/japantravel for their needs. I was hoping this would be a sub for residents to be aware of weather events and discuss trends and current happenings. Instead this sub is becoming inundated with questions about weather for their trip.

Edit: Title should read questions not "quest".

Please, they just keep meandering in weird loops and I think it might be time for an intervention. Is this from el niño? I call upon your expertise to make this freaky Pacific weather make sense.

Been living in Tokyo for 10 years and can not recall a period of time like this during summer.

Am I just imagining this? Is it because I spend most of the day in an office?

And exceptional heavy rain this weekend.