Written by Sig Silber
We published the NOAA Long-Term Forecast Part I on August 20, 2020. Here we compare the NOAA forecast for Alaska and CONUS with the JAMSTEC forecast. It is easier to see the disagreements (which are primarily in the area of precipitation) by comparing the maps which we show side by side in a table with a brief summary of the comparison. Obviously, the farther out you look, the less confidence you have in the forecasts and thus the differences in the forecasts. Also provided are the JAMSTEC World Forecasts. In Part I, we showed the differences in the assumptions with respect to ENSO (JAMSTEC is more convinced than NOAA that we will have more than a borderline La Nina) which may explain some of the differences in the forecasts. JAMSTEC may see the La Nina being shorter in duration.
I thought of providing the updated assumptions but both forecasts are based on the assumptions at the time they made the forecast so I have decided it would confuse the issue if I provided an update on the assumptions. I may do that at the end of the Month when NOAA updates their August forecast or I may wait until NOAA updates the ENSO Status on September 10, 2020.
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C. Comparison of the NOAA and JAMSTEC Forecasts
Below is the comparison of the NOAA and JAMSTEC temperature and precipitation forecast maps for three time-periods and from left to right the NOAA forecast for Alaska and CONUS (the contiguous mid-latitude U.S) and then JAMSTEC for North America (which includes Canada and Mexico). The NOAA forecast maps can be clicked on to enlarge. The JAMSTEC maps in the table are not set up to be clicked on to enlarge (because we have no larger version of them). We have concluded that these smaller images work fine for comparison purposes. Later in the article, we show the World Forecasts.
JAMSTEC works with three-month seasons: Fall: SON, Winter: DJF and Spring: MAM. Out of each three months, there is one where the months in the two forecasts align perfectly for the first time period. This is one of those months so for the first period we are comparing SON for both JAMSTEC and NOAA.
In addition to the value of comparing the JAMSTEC and NOAA forecasts, the JAMSTEC forecast by showing North America provides more context for the Alaska and CONUS Forecasts as the temperature and precipitation patterns cover North America, not just Alaska and CONUS.
Map Comparisons and our Comments
Temperature
| NOAA Alaska Plus CONUS | JAMSTEC North America | |
| FALL SON 2020/2021 |  |  |
WINTER DJF 2020-2021 |  |  |
SPRING MAM 2021 |  | |
Precipitation
| NOAA Alaska Plus CONUS | JAMSTEC North America | |
FALL SON 2020 |  |  |
WINTER DJF 2020-2021 |  |  |
SPRING MAM 2021 |  |  |
JAMSTEC World Forecasts
This month our comments are taken directly from the JAMSTEC discussion (the translation may be a bit stilted). Now that JAMSTEC has a number of models we are not exactly sure of the best one to display and they vary somewhat. We assume that the JAMSTEC discussion considers their various models and provides their assessment having considered their suite of models.
Fall which is SON 2020
| Temperature |
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| Precipitation |
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Winter which is DJF 2000/2001
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| Precipitation |
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And Spring which is MAM 2021
| Temperature |
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| Precipitation |
 |
D. Conclusion
As usual, there is substantial disagreement between NOAA and JAMSTEC. It is difficult to relate the differences in the forecast to differences in assumptions on ENSO. But JAMSTEC tends to consider other factors that may not be considered by NOAA. On the other hand, the new versions of the JAMSTEC model are early in their usage and may not have been fully calibrated. I do not think I have gone into it in detail but when it comes to models we need to recognize that there are limitations and NOAA and JAMSTEC use very different approaches which I have summarized in the below table.
| Shorter Term | Intermediate-Term | |
| NOAA | Deterministic | Statistical |
| JAMSTEC | Deterministic | Deterministic (may also use statistical methods) |
| Assessment | Generally Considered to be reliable for 14 to 28 days | Errors build up in deterministic models and statistical models generally have insufficient historical data to be reliable |
So it is kind of a pick your poison choice. But both agencies have great skill at employing approaches that have inherent limitations.
JAMSTEC Discussion
We provided the full NOAA Discussion in Part I. The much shorter JAMSTEC Discussion was published on August 13, 2020 and we included it in Part I but we are repeating it here also.
Aug. 13, 2020 Prediction from 1st Aug., 2020
ENSO forecast:
Observation shows that a La Niña is developing at present. The SINTEX-F predicts that the La Niña-like condition will persist in the latter half of this year with weak anomaly in December-February.
Indian Ocean forecast:
Observation shows that the tropical Indian Ocean is slightly warmer-than-normal at present. The ensemble mean suggests that the warmer condition will persist in autumn, then will return to a neutral state from winter.
Regional forecast:
On a seasonal scale, the SINTEX-F predicts that most part of the globe will experience a warmer-than-normal condition in boreal autumn, except for southern Australia, India, and some parts of northern Africa. In boreal winter, the model also predicts that most part of the globe will experience a warmer-than-normal condition, except for western Brazil, India, and some parts of Southeast Asia.
As regards to the seasonally averaged rainfall in boreal autumn, a drier-than-normal condition is predicted for southern/central part of U.S.A., Mexico, some parts of the South American Continent, southern Africa, West Africa, southern China, some parts of the Russian Far East, and most part of Europe. In contrast, western Canada, India, some parts of East Africa, Southeast Asia, Philippines, Indonesia, northern Europe, and northern Russia will experience a wetter-than-normal condition. In boreal winter, a wetter-than-normal condition is predicted for Alaska, northwestern coastal area of Canada, northern Europe, northern part of the South American Continent, most part of Australia, Philippines, and some parts of Indonesia. In contrast, western/southern U.S.A., Mexico, Argentina, DRC, Mozambique, southern Europe, eastern China will experience a drier-than-normal condition.
The model predicts most part of Japan will experience warmer-than-normal condition in autumn and winter as a seasonal average. As regards to the seasonally averaged rainfall, most part of Japan, except for Hokkaido, will experience wetter-than-normal condition in autumn and drier-than-normal condition in winter.
E. JAMSTEC, NOAA and other Agency Indices
We showed these in Part I which can be accessed here.
But we are showing a few here that are especially important
Here is the JAMSTEC forecast
NOAA has their own proprietary model which they rarely use. But it is consistent with their other forecasts this month.
Right now it is clearly forecasting La Nina and looks a lot like the BOM forecast. But the JAMSTEC forecast has gone above Neutral by MAM 2021 which is the key point.
Here is the Australian Bureau of Meteorology (BOM) forecast.
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