GFS and EC Monthly show a low coming North of Hokkaido, bringing heavy snowfall to both Hokkaido and Honshu on the 24th, continuing through the 25th, easing to moderate snowfall on the 26th.
EC Monthly continues the heavy snowfall with another bout of heavy snowfall on the 27th for Honshu and NW Hokkaido, easing by the morning of the 28th.
Apart from a few hours of moderate snowfall on the 29th, the next big snowfall is on the 1st of Jan with a weak Honshu Low on EC Monthly. The precipitation doesn’t penetrate the coastal regions, because of unfavourable winds, until the 3rd, but the 4th and 5th bring moderate-heavy snowfall, before it eases.
EC Monthly shows moderate-heavy snowfall for Northern Honshu and Hokkaido from later on the 6th, and the 7th, until it slowly eases later on the 7th.
In the stratosphere, the SSW has reached it’s peak, and it is starting to cool as a result of a warmer tropical stratosphere as a result of a weakening Brewer-Dobson Circulation, which means less ozone to warm the stratosphere. The colder stratosphere will push negative NAM/Arctic Oscillation index anomalies down into the troposphere to impact the Northern Hemisphere, with more troughs on it’s way to the North of Hokkaido to help bring snowfall there, and align NW winds into Honshu also. The timeframe for these stratosphere impacts look like the last week of Jan and the first two weeks of Feb, as the downwelling is still progressing.
The East Asian Mountain Torque will be negative over the next 3-5 days, but it will become more positive over the following 10 days, so the next 8-20 days will feature a strong jetstream off East Asia, that leads to East Asian highs, which assist with Honshu NW winds. This is part of a wider +AAM period in January and February, featuring several peaks and drops, but generally promoting stronger wind flow, which is good for Japanese snowfall.
The MJO is at Phase 8 at the moment, but the MJO is coming around through the COD, back to Phase 6 & 7 on both GEFS and EPS models. Phase 7 MJO is favourable for Japanese snowfall by enhancing the +AAM processes in what is known as the East Asian Winter Monsoon.
So in the last week of Jan & first week of Feb, the MJO and stratosphere look good. A +EAMT looks to improve the second half of January for snowfall for both Hokkaido and Honshu, the former via a stronger Aleutian Low. So the next month looks good in terms of snowfall chances for both Honshu and Hokkaido IMO.
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Disclaimer: There is low skill asssociated with using long range model forecasts to find snow systems.
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