u/Neko_Dash

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.

Hey everyone and good evening.

As y'all know, we got a bunch of rain coming in. Well, it has already come in, as a good portion of the country is seeing showers at the moment. The JMA has issues some advisories, and I thought it would be a good to disseminate the information.

We have an area of heavy rain advisories (大雨警報, Ooame Keihou) in effect from midnight tonight through 6pm tomorrow for the following prefectures: Ishikawa, Toyama, Gifu, Nagano, Shizuoka and Kanagawa. Heavy rain can be expected from the incoming system during this time.

A strong wind advisory (暮風警報, Bofu Keihou) has been issued for the following prefectures:, in effect from midnight tonight through 6pm tomorrow Toyama, northern Niigata, western Yamagata, Akita, Aomori, Iwate, Miyagi, eastern Fukushima, and Hokkaido's western Hiyama region and Soya region. Chances are greater (in excess of 70% chance) for eastern Aomori and eastern Miyagi prefectures along the Pacific coast. Those ares are shown in a darker yellow.

Strong winds are already blowing in parts of western Japan, especially in Kansai's Osaka - Kyoto corridor. The winds will follow the storm as it tracks northeast overnight tonight.

Take care if you are in the areas affected.

https://preview.redd.it/8kn5s9x41xyg1.png?width=2584&format=png&auto=webp&s=02cea03c9f9c6070018eeaf07729f0a3444a2143

https://preview.redd.it/e7g4max41xyg1.png?width=2502&format=png&auto=webp&s=76e45af69efa7febe53a6dc7ae7612b3a3664ee2

The JMA advisory maps are at: https://www.jma.go.jp/bosai/map.html#7/43.06/140.436/&elem=wind&contents=probability

I put a spoiler in the title. Now, who's going to read the post? Y'all can do the Neko a solid and pretend to read the post, OK? Thanks in advance.

Hey - did you guys know it was hot today from Sendai on down? Yeah, if you stepped outside, you probably learned that one real quick and you don't need me to tell you about it. Hokkaido got away with some cooler air today, as they were getting some north winds thanks to the strong low heading off the coast today. For much of the country, from Hokkaido on south, daytime highs were generally anywhere from above normal to way above normal.

​​​​​​​Today's hotspot prize went to the town of Saga in Kochi prefecture, which rang in with 30.3c this afternoon. On top of that, a mostly gorgeous day, with some clouds moving in from west Japan, presaging another low which is getting ready to bring more precipitation to the country tomorrow evening and into Monday.

Highs Sunday and Monday will be a little cooler than today in much of the country, taking the edge off the early taste of summer we all got today. Cloudy from the midsection of Honshu on down to Kyushu, with rain coming ashore Japan's southernmost island from around noon. By 9pm, rain will cover most of the western half of the country.

The fast moving system will cruise out into the Pacific by mid-afternoon Monday, taking most of the rain with it, leaving some scattered areas of precipitation along Hokuriku and Tohoku.

** Our Water Situation**
The rainfall we got over Thursday and Friday really helped move the needle on our rain exposure this year. We actually came out with a water surplus in the area south and west of a line from Sendai to Niigata. North of that line came out about normal, with a plus/minus of about 5% of normal, according to ECMWF ensemble data.

That said, we are still in an overall deficit, albeit a smaller deficit, for 2026 rainfall. Early projections for this coming rainy season (梅雨, tsuyu) point to normal across the nation, starting in mid-May in Okinawa and moving up to Aomori in mid-June. Forecasts for Hokkaido have not been released yet, but a rainy season more in line with norms would help give us a better position for the upcoming summer as far as water stores are concerned. Heaven knows last year's rainy season was just about a non-starter, with a series of quick fading pop-up storms taking the place of the normal longer lasting frontal rain showers we normally have.

The low moving through Sunday and Monday will probably be the last system-derived change for rain we will see for at least 10 days. Scattered showers and a few pop-ups will be along the Hokuriku and Tohoku Sea of Japan coast and some inland areas, but that will be it for large-scale rain fronts for a little while.

Have a great day tomorrow.

Good evening, everyone. For those of you who have started Golden Week, I hope you have a fantastic, relaxing holiday. If you still have a few days to go, such as myself, then hang on just a little longer.

Hey, I want to say thanks to ManjiroPrime for filling in for me while I was in Reddit Jail. Long story, and I try to avoid politics on this sub, but I have never shied away from expressing my views. Thing is, there is a time and a place to express such, and I apparently have not learned the nuances.

Current rainfall (as of 21:20 this evening), from our good friends over at Weather News.

A pressure map showing that strong low moving over Japan on Friday.

Projected total rainfall for 1 May in Japan. More intense colors describe stronger rainfall.

https://preview.redd.it/05bo23ucr4yg1.png?width=2142&format=png&auto=webp&s=e1908545712e7da29bbd9b5949d2db22a5c1c551

Projected rainfall from afternoon of 3 May to afternoon of 4 May, using the same scale as the 1 May graphic. Heavier rain is expected along the Pacific Coast.

Anyway, we do have GW, as it is often abbreviated, come up soon. Unsettled weather, too, is going to be part of the equation through the vacation. Unless you have access to multi-million dollar government-level resources, the instability in the atmosphere makes things very difficult to call accurately any more than about four days out. Beyond that we can make conjectures (which is just a Y50,000 word for "guess"), but accuracy is suspect. So, let's try to break this down day by day.

=======

30 Apr: Overnight tonight and into tomorrow a low pressure system makes its run up from the southern East China Sea, between Taiwan and Okinawa, and approaches Kyushu. As of now - around 21:30, 29 Apr, national radar indicates this system is beginning to pitch rain up into Kyushu. By midday Thursday, rain will cover much of Kyushu, the entirety of Shikoku, and central and western Honshu from around Nagoya on down to Yamaguchi.

1 May: Overnight Thursday into Friday, rain will move into Kanto and Hokuriku as Shikoku's showers stop. Due to the rotation of the low, some rain bands will wrap around give northern Kyushu and western Honshu another round of rain. Basically, rain from Fukushima on down to Fukuoka. Holy Fuku...oka. By the evening of 1 May (say, around 18:00), the low will have tracked north, and western Honshu's rain will be ending. The party, however, will just be getting started up in Tohoku and Hokkaido.

2 May: Overnight, the low moves along the Pacific Coast of northern Honshu, giving Tohoku and Hokkaido some substantial evening showers. The low will finally track up northeast and move away from Hokkaido around noon on Saturday, and all residual rain should be moving offshore by around 20:00.

3 May: A few hours of stability come over Japan's skies before the next big system begins its run in almost the same exact track - hitting Kyushu first in the afternoon, then moving on to cover western Honshu and Shikoku.

4 May: If you live anywhere south of a line from Sendai to Niigata, just stay home. Unless you really, really love the rain. The low system which brought rain to southern and western Japan on 3 May will soak much of the nation on Monday.

And that's really as far as I'm willing to go for now. With so much instability in the atmosphere, data runs may (and probably will) vary. FYI, from my analysis, the second low (moving in 3-4 May) will be the stronger of the two, although the first low will also offer up chances of very heavy local rainfall in some areas, especially along the Pacific Coast.

Daytime high temperatures will start out more or less normal from tomorrow, but highs will spike on Saturday for everywhere except Hokkaido as the first low pulls up warm tropical air over Japan. The Pacific Coast side (Sendai, Tokyo, Nagoya, Osaka, Shikoku, southern Kyushu) will have to deal with a higher degree of elevated temperatures than the Sea of Japan side. Saturday's highs, for instance, are expected to be 22 in Sendai, 27 in Tokyo and 26 in Nagoya, while Kanazawa just barely hits 21.

Winds along the coast and inland plain areas will pick up in the path and immediate wake of the low, with sustained breezes in those areas of 22-30kph in advance and on the trailing edge of the system. Mountain side areas won't see as much wind as the low moves past, but some gusts of 10-15kph can be expected.

So, insofar as you can, have yourselves A Most Excellent Golden Week. (<-- Bonus points if you get the rather strained reference). For my part, I'll keep reading the skies and will try to tell you the story as it changes. And, I'll keep my big mouth out of the wolves' den. Have a great evening, everyone.