Here is an earlier than expected Japanese long term forecast.
EC Monthly is keen on a system from the 3rd (EPS Control is showing something from 1-3 Feb) for a 24 hour period, then slowly down on the 4th, and coming back on the 5th with a NW wind boosted by a low off the NE of Hokkaido, bringing moderate-heavy snowfall until about the 7th.
EC Monthly shows a low coming up the NE of Honshu, bringing a NW wind and heavy snowfall, after perhaps a bit of rainfall on the 9th, with major snowfall confined mostly to Honshu, and weakening over the 10th.
Later at night on the 11th, a Low and following NW winds bring moderate-heavy snowfall to Honshu and much of Hokkaido on the 12th, easing later in the evening
Apart from some light snowfall, this is the only system on the radar, that continues to draw off lows from East Asia, this case further south, meaning it brings rainfall to Central Honshu on the 17th unfortunately.
GEFS (and EPS) both show the NAM or AO being forecast to turn negative in the next week or so, sending us into a deep negative phase of the AO and NAO, created by the slow downwelling progress of the SSW, that is soon to come to fruition. This will assist with the strengthening of the Siberian High, also allowing more lows to gravitate down near Hokkaido and Kamchatka, bringing snowfall to Hokkaido, and helping to setup NW winds into Honshu, to bring snowfall there.
A -AO is associated with high latitude blocking in the Arctic.
My thoughts towards the MJO are heading towards a weakening in Phase 6, Phase 7 is the best Phase for Japan, to allow a strengthening of the East Asian Winter Monsoon. There is potential for some weak orbit of Phase 7 in the first days of February, so my thinking is that the tropical setup will be reasonably favourable, with some signals carrying out to Phase 7.
I expect the next two-three weeks to see an increase of westerly momentum, and a transition through GWO Phases 5, 6, 7, that all bring a stronger Aleutian Low that sets up troughing NE of Hokkaido to see gyres off the main low, bring snowfall to Hokkaido, and overall bring NW winds into Honshu, with a potential low-high gradient if we see a strong East Asian High. The way the NWP and climatology places these ideas, we should expect the Japanese Alps to get more favourablity in terms of extra westerly momentum. We should see a +EAMT continue over the next 5-8 days cementing a good period of snowfall over the next 10-12 days. There will be continued westerly momentum around, cementing a strong jetstream off East Asia, which should help the case for a strong setup in terms of highs coming more from the Siberian domain in Early February.
IMO the next 15 days will be very good in terms of snowfall for Honshu particularly, but also Hokkaido, and Mid February should still see slightly above average snowfall in Hokkaido and Honshu, probably favouring Hokkaido more.
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Disclaimer: There is low skill asssociated with using long range model forecasts to find snow systems.
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