Hello to all those following the Japanese weather forecast this season!
As usual, I am writing a seasonal outlook, followed by several long term outlook updates over the winter. Today we will cover the models and the climate drivers that assist with the seasonal outlook.
There is an overwhelming consensus of the UKMO, ECMWF and the multi-model ensemble of Copernicus, and that is for ridging over Japan for the winter. This setup isn’t great for Japanese snowfall.
Sea Surface Temperatures
There are two points to make here:
- The warm SSTAs surrounding Japan demonstrate the potential for ridging around Japan for winter.
- The warm SSTAs in the Sea of Japan specifically allow a larger divide between warm sea surface temperatures and cold air aloft coming from Siberia. This would increase lake/sea effect snowfall in terms of strength and duration of events, rather than the exact timing of such events.
“The Siberian High is an important, yet relatively unknown driver for snowfall in the Japanese Alps. A strong, stable Siberian High provides very cold air down towards Japan, except if the High is too far west. This cold air from the Siberian High creates the lake effect, that provides the massive snowfalls in the Japanese Alps.Snowy Hibbo two years ago
The Siberian High is affected by a number of things, including snow cover in the region and the AO. A negative AO causes a stronger Siberian High. The Siberian High is also affected by the snow cover in Siberia during October and November. An above average Siberian snow cover creates an early, potentially more stable Siberian High. It also correlates with a negative AO.Snowy Hibbo two years ago
So this year is going to feature both lower than average Arctic sea ice cover and a higher than average Siberian snow cover, as you can see in the tweet below.
So this will have a positive effect for Japanese snowfall throughout the season. It could also indicate an earlier start to the snowfall season, with the Siberian High developing and strengthening earlier than normal.
I expect a weak Nino Modoki event to prevail into the winter. What this means for Japan is based on composites, a somewhat warmer Central Honshu and a colder Hokkaido. But it also predicts a somewhat strengthened ability for lake-effect snowfall to fall, with a noticeable increase in And it also shows a stronger Aleutian Low in accords with normal Nino expectations.
A negative East Asian Mountain Torque is not good for Japan, and it doesn’t help that we are in a -AAM phase, which is expected to continue into winter. But I expect frequent responding +EAMTs throughout the season to help counter this, so I don’t think this will be a negative factor IMO. A positive East Asian Mountain Torque increases the strength of the East Asian jetstream, which helps Japanese snowfall.
- Siberian Snow Cover is expected to be higher than average, so a -AO is more probable, which benefits Japanese snowfall.
- Arctic sea ice is lower than average, which also helps to force a -AO, in particular low sea ice in the Barents-Kara Sea.
- The QBO is expected to have an effect earlier in the season of subsiding stratospheric weakening, which encourages a +AO. But later in the season, the factors support a potential SSW, which helps a -AO scenario (or maybe sooner than we think, CFS forecasts a SSW in late November).
- The stratospheric polar vortex is expected to be stronger than normal going into November and possibly December.
- The Brewer-Dobson Circulation of ozone to the NH stratosphere is expected to be strong through both the NH and SH subtropics, and slightly above average over the tropics themselves.
- A SSW is probably most likely in January and February. But CFS is currently progging one in late November.
Because of the above factors, I expect a mild +AO to dominate early winter, unless the CFS forecast for a November SSW comes through. But I can see a -AO response in the latter two months of winter.
Expect an early start to the season. I’d also expect an above average season for Honshu. Hokkaido is probably more likely to be slightly above average. All the factors roughly line up for a snowy winter for Japan, which means there’s plenty of snow for skiing and boarding.
Thanks so much for reading.
Seasonal outlooks tend to have bias and errors, due to the fact that these forecasts are so far out. So don’t use these outlooks to make important decisions. These outlooks is meant to be interesting information, that can help to see what the season might be like.
This took a lot of work over the last fortnight, so I appreciate your support. Starting in late November, my long range outlooks for Japan will start! They will be produced every 2-3 weeks, looking out into the range of 10-30 days out. Stay tuned.
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