Here is the first long term outlook for Japan, focussing on the climate drivers and the model forecasts.
GFS and EPS shows a common trend for a system right here. They both proceed with some sort of low from the south of Japan bringing rain in the 5th and 6th of Dec. Then the fetch from the NW comes down on the 7th, creating the potential for the first major Central Honshu snowfall event of the season, continuing into the 8th and 9th. This is a key period and setup to watch.
EC Monthly Control shows not a lot after the 7th-9th December, and shows a low coming from the south bringing rainfall on the 15th, with a chance for some moderate snowfall for a Central Honshu on the 16th.
After a rain-bearing low from the south on the 18th, EC Monthly shows an event of moderate-heavy snowfall on the 19th and 20th for Honshu and more moderate for Hokkaido.
This particular low brings heavy snowfall for Hokkaido in the 21st and 22nd of Dec, with potential for moderate snowfall on the 23rd, as per EC Monthly.
So in terms of the progression of the GSDM in the extratropics:
- We should expect positive torques and an increase in momentum in the next 10-15 days. This is observed by +EAMTs being prevalent, which increases ridging over China. This improves chances of snowfall for Japan, as it creates the NW fetch needed for the lake-effect.
- So the end of November and the first 10-12 days of December should be rather decent for Japanese snowfall, and could provide a solid early start.
- Mid-late December is likely to provide more chances for troughing to the South of Japan, bringing rain, and an otherwise relatively dry synoptic outlook. This doesn’t mean there is no chance of snowfall, just lower chances. Tropical and extratropical support for systems is unlikely.
- This support will enhance in the last days of December, and it is likely that January would have an improving atmosphere more conducive to snowfall in Japan.
The last days of November will be spent with a neutral MJO. Early December is likely to start with a return to (sort of) Phase 7-8 conditions for a short while, until we return to Phase 1 in the second week of December. Or it could be mostly Phase 1 forcing on the other hand. These will improve tropical forcing for Japan a little, and are reflective of the general state of momentum as discussed above. This is good for Japanese snowfall in the first week of December.
Apart from a short period of +AO in the troposphere, the Arctic Oscillation outlook is mostly neutral. So the Arctic domain effects upon Japan will be minimal at this point. There is SSW potential later in December or beyond, that may help Japan in January.
The Siberian High is currently quite strong and is expected to weaken to more normal strength.
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Disclaimer: There is low skill asssociated with using long range model forecasts to find snow systems.
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