u/Neko_Dash

The rain much of the country got today was a welcome relief, especially after a few days of over 300 reporting stations checking in with plus 30 temps. Highs today were anywhere from 4 to 12 degrees off the peaks we had on Monday. Looking at national radar overlaid on a Himawari satellite movie over the past 24 hours, you can see the main thrust of the rain today moving through the central part of the nation. The low, however, tracked a little more northerly than anticipated, which kind of gypped Kyushu and parts of western Honshu out of their predicted rainfall and, instead dropped buckets on Hokuriku and Tohoku.

Rainfall across the country in the 24 hours from 18:00 20 May to the same time 21 May.

Expected high tempeartues for the next four days. Green is cooler (up to 20c), shades of yellow denote 20-24c, and orange to red are upper 20s.

The low did track more northerly than anticipated, but still brought a good soaking to the midsection of the nation.

Some rain will continue on through tomorrow as the low moves offshore, but unstable air will hang around the home islands for the next few days. We can expect partly-cloudy to cloudy skies over the country, with cloud breaks letting in little splotches of sun here and there. Despite the cloudy weekend ahead, there will not be a strong risk of rain and we expect no system-driven storms at least through the middle of next week, but, that said, we could see a localized pop-up shower take advantage of the turbulent atmosphere.

Daytime highs are going to remain pleasantly cool nationwide, but starting from Monday, we will see warmer temps as high pressure once against moves into the picture. I am seeing nothing like the heat wave of this past week, but it will be definitely warmer than today and tomorrow.

Last note - No tropical storm activity yet, but I have seen two models suggest that a typhoon may start to bubble up in the Philippine Sea next week. I will certainly be keeping my eye out for any potential storm formation.

And with that, everyone, have a smooth and calm night.

Well, you would think. Scorching hot temps across much of the country today. Hokkaido got away with upper teens for the most part, but if you are in Hakodate on south, you were on fire. And you will be tomorrow, too.

​​​​​​The towns of Hita, Oita and Toyooka, Hyogo shared the "Good Heavens, WTF Is This? It's Only May!" prize today, both coming in at 35.3c, making it the earliest day in over 13 years that an "extremely hot summer day" (猛暑日, Mosho-bi) has been recorded. 35.3c on May 18th. Just roll that around a bit. Another 313 weather stations reported "mid-summer day" (真夏日, Manatsu-bi) temperatures of 30c or greater. We can look forward to another run at Mosho-bi-level temps tomorrow. So what's going on?

https://preview.redd.it/936wmt7ky12h1.png?width=1384&format=png&auto=webp&s=8769758bc0e500f798e8f9c9c1ef3cf96af53e1a

https://preview.redd.it/df58lr7ky12h1.png?width=496&format=png&auto=webp&s=06ed99a61694276d162fcb928f0116ec946caebb

https://preview.redd.it/2lm1ms7ky12h1.png?width=1030&format=png&auto=webp&s=9c7d9be266230686e936f801127075515e54a808

https://preview.redd.it/24knyr7ky12h1.png?width=1770&format=png&auto=webp&s=b15a30b55de72bf51cbd7eb3ce7e4a7338411217

https://preview.redd.it/spii0u7ky12h1.png?width=2782&format=png&auto=webp&s=6d826524c47bd11dde45205bd08d83d5a34ab314

https://preview.redd.it/0ttavs7ky12h1.png?width=2186&format=png&auto=webp&s=61085cece26f5f3461a7ea9a82ce0db15d5be074

Is it climate change? Yes. The atmosphere as a whole is warmer than it should be. But, at a more local level, the high pressure which came down late last week is still parked off the Pacific coast of the country, bringing in a southerly flow of [very] warm tropical air over the country. The effect is clearly visible on the 1500m temperature chart, where aloft temperatures of up to 18c (the areas in yellow) have moved over the northern part of the nation.

Honestly, the only saving grace here is that daytime humidities are still in the 40-60% range, giving us a nice margin of evaporative cooling. Good old fashioned solar radiation plays a factor in our first starter heat wave of the year, too, as we are nearing June, when the sun will be at its peak height in the northern hemisphere sky, which means the sun's rays are nearing their most intense.

The pattern will continue into tomorrow but will start to weaken on Wednesday as two lows, one from the Philippine Sea, and the other coming in from northeast China, begin to merge, and Thursday will see more rational high temperatures about 6-8c lower than today.

The move of the low pressure systems over Japan on late Wednesday into Thursday will also bring strong chances of rain over much of the country, with potentially severe storms in some areas. As things get closer, we will get a better idea of how turbulent the atmosphere will be and we can better pinpoint the rain profile.

For the next 24-36 hours, enjoy, to the best of your ability, this little preview of summer.

After a series of extensive late night consultations with Prime Minister Takaichi, the Bank of Japan Board of Governors, the World Meteorological Organization and the International Criminal Court of Justice, r/japanweather has settled on a new mod.

The lucky winner is u/IntoTheWilder25. I will let the new mod say a few words at their leisure (completely voluntary), but as the sub closes in on 10,000 members, it will be nice to have an extra pair of eyes making sure the nuts, bolts, chewing gum and bailing wire which keeps r/japanweather together behind the scenes stays together. Welcome aboard!

弾く続き、よろしくお願いします。

https://reddit.com/link/1tfhbtv/video/63mtag3x3n1h1/player

Just a little PSA here, but dogs don't usually wear shoes. The asphalt near my place today hit 55c. Would you walk barefoot on that? If you take them for a walk in the middle of the day, that's basically what you're doing.

Please walk your pets closer to evening or nighttime, when the streets are a little cooler.

And, yes, before anyone asks, I have seen people walk cats. It's not common, but it it is a thing.

Maybe more like 15 inches. We'll get to seven inches later this summer.

  Now that high pressure has settled down over the country, pushing out the low that sat over the nation previously and moving unstable air out over the ocean, we are in for about five days of really glorious, fantastic weather.  However, while the high dominates our weather picture, it will also drive up daytime highs to near early-summer levels.  While today (Saturday) was warm, but mostly tolerable, much higher temperatures will start to be seen around Japan, mostly in the interior of Honshu, from Sunday onward.

  Sunday, interior Kanto, from Utsunomiya on over to Iida in southern Nagano, and from Gunma over to Kansai, will see highs of approaching - or exceeding - 30c.  These temps will continue through Wednesday, 20 May.  Coastal areas won’t see much more relief as we expect Sendai to hit 28c, with Osaka and Tokyo getting up to 29c and Nagoya topping out at 32c Sunday.  Over on the Sea of Japan side, Fukuoka and Matsue will get up to 27, highs do taper off a bit from Kanazawa (23c) on up to Hokkaido, which will see highs in the mid to high teens while the rest of us get put in an air fryer. 

  Taking a quick check of the 1500m temp chart - my favorite chart since it isn’t subject to the day/night changes that you see on the surface - a small blob of 15-18c air will form over Japan around Monday and Tuesday as the high pressure allows air to sit and stagnate over the country.  This is unusually warm for that altitude and this time of year.  I would be expecting waves of 10-12c, but 15c+?  Dang, that’s warm.  That the warmth up at that altitude is going to translate into warmer than normal days for us down at the surface.

Projected highs this coming Monday. High 20s and on up into lower 30s for most of the country.

Unusually warm temperatures aloft, too. These numbers (15-18c) at 1500m are unusual for this time of year, but they will support this small heat wave that we will go through.

But, hey - good news. The weather will break by Wednesday night and we can expect a bit of a cool down. We will also see some storms from this change, too, but I'll post on that later.

The stagnant air that goes with this high, however, is going to increase the risk of poor air quality. Again, we should get a reset on this by late Wednesday to Thursday, but Tuesday and Wednesday will probably see a lot of suspended gunk in our skies.

  Winds are not expected to do much through Tuesday.  A little breezy on the coast, but once you get even a little inland, there won’t be much air movement at all.

  So, I’ve made two mentions of stagnant or still air.  That brings me to my last point.  While we do have this beautiful weather, industrial pollution and particulates will start to accumulate around the country and, especially in the industrial belt from Tokyo to Osaka, we will begin to see worsening air quality.  Projections from the Copernicus Data Space Ecosystem, which monitors environmental data via satellite, projects that some areas of Japan may approach “unhealthy” categories by mid-week.

  The good news here is that we expect this high to begin to break down and move on from around late-Wednesday, and we will see a shift in weather patterns at around that time.  Just get through the next few days, get a few ice cold glasses of your preferred beverage, maybe hold your breath from Tuesday to late-Wednesday, and we’ll be fine.

Central Honshu was the scene of our first real burst of springtime weather activity today as unstable air, catalyzed by a low off the coast of the country and a high up north near Sakhalin, spawned a line of thunderstorms much heavier than originally forecast.

Today's radar profile movie. Look at how the storm developed in the mountains and headed down the Pacific coast.

Pressure map for Friday morning, as high pressure settles in from the north.

The pressure profile early this afternoon.

​​About 3 days ago, I wrote in this very forum something along the lines of "A weak low will transit the country over Wednesday into Thursday, firing off a few showers mostly on the Sea of Japan coast from Hokuriku up to Tohoku, but little else." Yeah, didn't happen quite like that. The low did transit and it was supposed to be weak, but over the day today, it turned out to be a stronger system than forecast. That, and the introduction of dry air flow from the north gave us a bunch of storms this afternoon that rocked the main Japanese island from Kobe to Kanto.

There is a chance that tomorrow will bring another round of strong pop-ups as a high begins to slide down from the north. The clash, again, of cooler, drier air and warm moist air trailing off the low could setup another round of reactive storms.

We do look forward to more stable weather from Thursday evening onward as that high pressure dome takes up shop over Japan for the next few days.

For those of you who took one for the home team and put laundry out/washed your cars/walked your dogs when the downpours started, we salute you.

Have a great evening, everyone. 8:30pm and time for a beer. Cheers!

...but it's not gonna last long. Developmental prognosis is not favorable...which is a high-dollar way of saying, "It's not going to develop into anything beyond small storm."

It has way too much vertical shear, too much surrounding dry air, and the highs surrounding Japan for the next few days are going to keep it contained down south.

Not tracking this one.

https://preview.redd.it/dfjjem8wcb0h1.png?width=1198&format=png&auto=webp&s=2b78269f51bd260fc637de3f7529f4257d8103bc

As noted last week, Okinawa has, according to the head weather guys at the JMA, entered this year's rainy season. Projected dates have not been set for the rest of the country, but we can draw some comparisons to last year, look at the weather setups we see and make a bit of a guess as to what this year has in store.

Looking back on last year, those of you who were with us then saw a much drier than normal rainy season. We had a series of strong high pressure systems set up in eastern China from mid-May onward which kept much of the precipitation away from Japan. We did have a couple of traditional stationary front systems catalyze large-scale rain showers here and there, but for the most part, last year's rainy season was punctuated by pop-up thunderstorms which meant that rain coverage was much lower than usual. The east China highs were so prevalent, the JMA called an end to the tsuyu for much of the nation between June 27 to June 29, much earlier than the normal mid-July rainy season ends. We all know what happened after that - about ten weeks of brutal upper 30s/low 40s highs and insufferable humidity to match.

2026 Rainy Season Diary. So far, we only have Okinawa in the books.

This high will be the main feature of Japan's weather picture from the 14-18 May.

This year looks to be different. At this point, I'm not seeing the formation of a persistent high in eastern China. I am seeing movement in all the models - highs and lows passing through. That is good and it does encourage the setup of a more traditional rainy season pattern.

It should be noted, while we are on the topic of high pressure systems, that we do have a high parked out over the Pacific just east of the nation which will influence the skies over Japan through Tuesday. A weak low will transit the country over Wednesday into Thursday, firing off a few showers mostly on the Sea of Japan coast from Hokuriku up to Tohoku, but little else. By Thursday evening, 14 May, a new high will establish itself over the country and drift v-e-r-y s-l-o-w-l-y eastward, dominating our weather until around next Tuesday, the 19th. Okinawa, meanwhile, will get some rain storms here and there, as will parts of Kyushu. But for most of us, we have a splendid 8-10 days coming up.

From Kanto on south, highs in the mid-to-upper 20s, lows in the mid-teens. Tohoku on north, highs about 2-3 degrees either side of 20, lows in the low teens.

At least until the 18th, while we are under the high pressure dome, not much in the way of wind, either. Light gusts near coastal areas, but not much air consistent air movement at all until the low starts to move in from China next week and shake things up.

Have a great week, everyone!

See if you get this reference: [Looks around...] "You'll make a fine little helper. What's your name?"

"Charles DeMar."

Answer is in the comments. But, still looking for a mod to help run the well-organized machine that is r/japanweather.

The position of the low pressure system as of 21:00, 8 May.

Wind gusts as of 21:00, 8 May. Yellow is a range of 50-70 kph. The yellow-greenish shades are 30-50 kph gusts. Green areas are 10-30 kph.

That said, we got some gust action in much of the country this evening as a low pressure skates up the northern Sea of Japan and across Hokkaido. Sustained winds aren't that heavy, nationwide, but many areas are seeing quick gusts of between 30-70kph. The low is firing off some weather in the form of scattered showers up in Aomori and parts of Hokkaido, but most of the action is staying offshore. Clocking in at 998 hPa, the system isn't really rocking the pillars of heaven, but the pressure gradient is steep, and that's what's causing the windy conditions out there.

As this low moves on, a high pressure dome is going to settle in over the country for the next few days, giving most of us clear skies through at least Tuesday (12 May). Winds, too, will calm down. Some pop-ups may generate here and there over the weekend, but there will be no large, organized, system-driven rain storms.

High temps will ramp up nationwide as that high settles in, with some locations getting a nice jump in temps between Saturday to Sunday. Sapporo goes from 12 to 21, Hakodate jumps from 14 to 20. Aomori will drift up from 15 to 20, Akita lifts from 14 to 19 and Sendai shoots up to 24 on Sunday, after a Saturday peak of 19. There. I have just about exhausted all my snappy synonyms for "rise". Other places south of Sendai will see highs in the mid 20s Sunday, but the jumps will not be as drastic as those in the north.

Hey - stay tuned to r/japanweather. I'm putting together some data for this year's rainy season. I'm hoping we get some decent rainfall out of this year's monsoon season and there is some data that shows we will see a normal year this time. I'll try to publish the rainy season edition by Sunday. Got to take the cat into the vet over the weekend for an annual checkup, but I should be able to get the package assembled and published.

Have a great weekend. For those of you who are coming off an extended Golden Week holiday, enjoy the last few days of rest. For those of you who did work Thursday and Friday, then お疲れ様 for a week's worth of hard work.

Hi everyone. Got a little note from Reddit saying that r/japanweather is getting too large, in their opinion, to be a single mod operation.

I mean, yay, right? It’s success at some level. We are pushing 10,000 members.

Now, it is just a suggestion from the Reddit Mod Team - you know, the group that moderates moderators - and not a firm “must do”, but I’m inclined to follow the advice.

Thus, I would like to out the call out for mod or two to help out with the sub.

Interested parties, please DM me.

Must haves:
A) A passion about the weather up to and including large scale climate change.
A1) A recognition that continued fossil fuel burning is an 18th century technology which we need to outgrow ASAP.
B) A sense of humor. You take the data seriously, but you can frame it in a lighthearted way.
C) You want to build community.
D) Getting rich through Reddit is not on your immediate to-do list. Being a mod is a non-paid position. I do have other weather and science ideas in the fire which may make some cash, though.
E) You have a sense of empathy. When someone comments that they’re afraid of typhoons or earthquakes here in Japan, the goal is to comfort and educate them and get them used to it. We don’t call them names and laugh.
F) Be resident in Japan
G) Solid command of English + Japanese.

And I have several other editorial guidelines I’ve built up over the past two years, and we can chat about that later. Submitting posts as a mod is encouraged but not absolutely necessary.

DM me if you want to help mod the smartest, hottest Reddit community around.

[*EDIT: Edited “F”. The “playing with trolls” bit was not my best form. I was crafting this around midnight, and it seemed like a good idea in my drowsy state. Waking up and rereading it, yeah, not so much.
Apologies to all. ]

...eventful. From a weather perspective, it has been eventful. We got it all - rain, heat, wind. We had a lot to keep track of today. Let's unpack it all and see what happened today, and take a quick peek into what will happen for the next couple of days.

First, the temperatures. Seems like a decent place to kick things off. A real study in contrasts across the country today as Nerima, Tokyo took the hotspot honors topping out at 30.5c, while Hachioji, just down the road a bit, reported a high of 30.1, making the first "Mid Summer Day" (真夏日, manatsu bi) for the year. Meanwhile, snow was reported in north central Hokkaido, with up to 5-8cm falling in some areas. As of 21:45 tonight, Monbetsu, in Hokkaido was reporting 0.8c and snow.

Then there was the wind. Goodness gracious, the wind. Wind is caused by pressure differences between low and high systems moving across the surface of the earth. This difference is called a "pressure gradient". The low we had pass through yesterday, had a very steep pressure gradient. A steep gradient means higher winds, and that's what we saw across much of the nation.

The top (sustained) winds in Japan today were recorded in Murotomisaki in Kochi prefecture. The 23 m/s blows there equate out to 83kph. Just for comparison's sake, the JMA defines a typhoon as a storm with winds of 17 m/s sustained over 10 minutes. 8 of the top 10 windy places in Japan today recorded top speeds above that mark.

Then there was the rain. The storm didn't dump as much rain across the country as originally predicted, keeping most of the showers in Chubu, up the Hokuriku coast, up Tohoku and into Hokkaido.

So, what comes next? Tuesday (May 5) and Wednesday (May 6), things will settle down a bit. Kanto and the Pacific Coast temperatures will come back a bit more in line with normal, falling from the 27-30c range back down to the 22-25c area. The strong southerly pull of that low system will warm up Hokkaido over the next two days, with Sapporo seeing highs of 19 and 24, respectively. Winds up in Tohoku and Hokkaido will follow through, with that region getting their turn at the heavy blows and gusts most of us down south saw today. Clear skies expected nationwide tomorrow (Tuesday), and fair to partly-cloudy conditions on Wednesday. No new systems or storms are expected anywhere in Japan for the rest of Golden Week. Some pop-up showers here and there may show up, but that's about it.

https://preview.redd.it/vgss96fjj4zg1.png?width=1726&format=png&auto=webp&s=f1fd6d18a26d124b403d666da4dd475abe23d72e

The national high was 30.5 in Nerima today - a mid-July temperature.

North wind profile in Hokkaido from this afternoon

Pressure gradient map. The compressed low helped power our strong winds today. Tohoku and Hokkaido will feel those winds tonight into tomorrow while the rest of the nation settles down.

Top 10 winds today.

Wind profile from earlier this afternoon.

Rainfall in the 24 hours from Sunday evening to Monday evening.

Lastly, the JMA announced that Okinawa entered into their Rainy Season today, 6 days earlier than normal. The tropical islands there can expect rain tomorrow with highs in the low 20s. Rain becomes scattered on Wednesday, with highs still holding in the same range.

Have a great Golden Week. Looks like we're going to have some great weather as we close out the holidays.