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my question concerns the definition of the grib keys 'validityDate' and 'step' for monthly statistics of seasonal forecast data. In particular, the values for the grib keys 'validityDate' and 'verifyingMonth' in the monthly statistics to me seem to be inconsistent with each other. Take the following example request to get monthly forecasts of 2-m temperature from the UKMO model initialized on 1 July 2022 (nominal start date):

c.retrieve(
    'seasonal-monthly-single-levels',
    {
        'format': 'grib',
        'originating_centre': 'ukmo',
        'variable': '2m_temperature',
        'product_type': 'monthly_mean',
        'year': '2022',
        'month': '07',
        'leadtime_month': [
            '1', '2', '3',
            '4', '5', '6',
        ],
        'grid': [1.0,1.0],
        'area': [89.5,0.5,-89.5,359.5]
    },
    't2m_2022_07_ukmo_.grib'
)

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Inspecting the grib keys of 't2m_2022_07_ukmo_.grib' for the different ensemble members (as suggested here)

grib_ls -p origin,type,shortName,number,indexingDate,indexingTime,dataDate,dataTime,validityDate,fcmonth,verifyingMonth t2m_2022_07_ukmo_.grib

shows that 'validityDate' and 'verifyingMonth' are different. While 'validityDate' (to my mind) suggests that there is no data for July and the first 60 members are valid for August, 'verifyingMonth' indicates that the first 60 ensemble members are valid for July (which makes more sense to me).

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