Six week forecasts were able to track the cool summer


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The verification of the summer’s six week forecasts has started. So far it seems like the six week forecasts were able to predict the cool weather conditions in Finland last summer.

At the moment the verification has been targeted especially at the skill of the six week temperature forecast and the growing degree day sum forecast. Also, preliminary evaluation has been made for the thunderstorm outlook. The skill of the six week forecasts can be evaluated by studying whether the forecasts predicted the upcoming summer weather conditions better than the climate statistics would have.

“Both the climatology and the six week forecasts are compared to the observations, and then we calculate which one performed better”, explains Natalia Korhonen who has been doing the verification of the CLIPS six week forecasts at the Finnish Meteorological Institute.

Better than the climatology?

Summer 2017 was unusually cool in most parts of Finland. “So far it seems like the six week forecasts were able to foresee the summer’s unusually cool weekly mean temperatures more skillfully than a forecast based only on the climatology”, tells Korhonen. Therefore when it comes to this summer, the six week temperature forecasts scored better than the statistics of the average Finnish climate.

In the summer season the temperature parameter was used in the sport and beach weather outlooks and the forecasts estimating the probability of a sultry heat spell and the development of the growing season. In the autumn the temperature is part of, for example, the biking weather outlook and forecasts of the amount of days with slippery conditions or temperature sinking below zero degrees (frost days).

Growing season was predictably weak

The growing degree day sum was even exceptionally low in some parts of the Southern Finland. Six week forecasts were able to catch this trend in the Southern Finland, as the forecasted growing degree day sum was more skillful than the climatology throughout the whole six week periods.

Also in the Northern Finland the growing degree day sum was a bit lower than usually. In the Northern Finland the six week forecasts of the growing degree day sum scored better than the climatology for the first three weeks of the forecasts.

Thunderstorm season was tame both in forecasts and observations

In the summer the somewhat empty thunderstorm outlook forecast maps puzzled the viewers. Six week thunderstorm outlook hinted in the right direction in predicting less thunderstorms and lightnings than usually. The thunderstorm season of 2017 was very tame and there were exceptionally low amount of lightnings observed during the summer season.

“In the thunderstorm season of 2017  the amount of lightnings was very close to the statistical minimum. Only in 2015 there were fewer lightnings observed than this year”, tells researcher Terhi Laurila.

Verification continues in the autumn

Based on one summer it is not possible to verify, whether the temperature, growing degree day sum and thunderstorm forecasts performed more skillfully than the climatology because of the cool summer weather or because the forecast model simply has a good skill. Therefore it is important to study also the skill outcome of the past years.

Verification is continued by observing the skill of six week forecasts also in the past years and growing seasons. So called reanalysis data is used in this task. Reanalysis combines outcomes of the atmospheric modelling and available observations and provides a data set which can be used in evaluating the past weather conditions.