JMA is the Japan Meteorological Agency. It is the official provider of forecasts and warnings for all types of natural hazards and severe events in Japan.
The long range forecasting system at JMA is designated as JMA/MRI-CPS2. It is the second generation modelling system, upgraded in June 2015.
All these forecasts are an average picture over 3 months (Dec-Jan-Feb), and show the general prevailing weather pattern. Even if the models would be completely accurate, it does not mean that such weather conditions would last for 3 months straight. There can still be cold fronts and snowfall in between such milder patterns in the mid-latitudes. The difference is that instead of the usual 10-20 snow days for example, you only get 3-6 snow days (as an example). So the models don’t suggest what the weather will be like for 3 months straight, but just how it might look 40-60% of the time.
Upper level forecasts.
We always want to know how the pressure forecasts look like. That tells us a lot about the overall global circulation and weather patterns. You can learn more about the importance of long range pressure forecasts in our previous post, that contains basic information that you need to know about long range forecasts, and graphics from the September model runs to compare:
Winter 2019/2020 early look (September) and LONG RANGE THEORY.
Just like ECMWF forecast, the 500mb geopotential height forecast from JMA is not really optimistic if you are a fan of winter from Europe or central/eastern parts of USA. We see a deep broad low in the North Atlantic, surrounded by the “highs” from east USA across the Atlantic into Europe. This is a classical positive NAO pattern that you can see on the long range forecasts. This is a typical pattern for windy and wet weather across the UK and Scandinavia, and mild conditions across central/southern Europe. In the USA, we have warmer and drier conditions in the far east, and mild and more wet in central/southern parts. We do see a trough extension into Western USA, which increases the warmth in the east, but like other models have suggested, can drive colder air down from Canada into western USA.
Staying at altitude, we have the 850mb temperature forecast, which is on average around 1.500 m altitude in the mid-latitudes. Here, we can see what we expected from the 500mb height forecast. Warm conditions across Europe and east USA, and a batch of colder air over west Canada, extending into parts of northwest USA, just like suggested by the 500mb pattern above.
Going down to the surface, we start with pressure again. This shows the pressure anomalies at the surface, which are more influential for surface wind conditions and temperature transports. Here we see the entire North Pole is under low pressure (positive AO – Arctic Oscillation), which means the coldest air is locked inside the Arctic circle, and is harder for it to move towards south into Europe for example. That is a typical sign of a strong polar vortex. Over the North Atlantic, we see a broad low pressure area, which causes the west air flow across the UK and Scandinavia, just as expected from the general circulation forecast above. This means stormy weather for UK and Scandinavia, with less chance for easterly conditions and so less chances for colder weather in UK. At the same time we have higher pressure in southern parts of Europe, which are drier and warmer.
In the USA, we can see a low pressure signal from Canada into Midwest, which drives the warmer westerly/southerly flow on its south and east side. But at the same time it can promote more northerly (and potentially colder) flow across the western parts.
One of the most used long range parameters is of course the 2-meter temperature anomaly forecast, also simply referred to as the surface temperature forecast. It is the main forecast to tell us what to expect in terms of temperatures (warmer/colder). The graphic below is made out of two parts, with the top one (TS) referring to the surface temperature. Warmer than average across Europe and most of the USA. We can see the hints of colder air coming down from west Canada into west/northwest parts of the USA. The graphic on the bottom, shows 850mb pressure anomaly, and the average flow, where we can see the strong westerly/southerly winds over Europe and east USA, and northerly flow across west/northwest USA.
To summarize, the JMA forecast from October, shows a typical positive NAO/AO pattern, with milder winter in Europe and USA, hinting at more colder periods possibly across northwest USA.
We still have the stratosphere as a major factor. Long range forecasts are generally not as good at forecasting stratospheric dynamics in detail, which means they tend to underestimate any potential sudden stratospheric warming events (SSW’s), since the final forecast is made out of many individual calculations, which have different ideas about the stratospheric development. An SSW event can have a major impact on the circulation and can cause major pattern changes in the Northern Hemisphere. So a potential SSW event is an important factor that can change the course of winter in either way across the North Hemisphere.
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