def freshet_start(tas, thresh='0 degC', window=5, freq='YS'):
    r"""First day consistently exceeding threshold temperature.

    Returns first day of period where a temperature threshold is exceeded
    over a given number of days.

    Parameters
    ----------
    tas : xarray.DataArray
      Mean daily temperature [℃] or [K]
    thresh : str
      Threshold temperature on which to base evaluation [℃] or [K]. Default '0 degC'
    window : int
      Minimum number of days with temperature above threshold needed for evaluation
    freq : str, optional
      Resampling frequency

    Returns
    -------
    float
      Day of the year when temperature exceeds threshold over a given number of days for the first time. If there are
      no such day, return np.nan.

    Notes
    -----
    Let :math:`x_i` be the daily mean temperature at day of the year :math:`i` for values of :math:`i` going from 1
    to 365 or 366. The start date of the freshet is given by the smallest index :math:`i` for which

    .. math::

       \prod_{j=i}^{i+w} [x_j > thresh]

    is true, where :math:`w` is the number of days the temperature threshold should be exceeded,  and :math:`[P]` is
    1 if :math:`P` is true, and 0 if false.
    """
    thresh = utils.convert_units_to(thresh, tas)
    over = (tas > thresh)
    group = over.resample(time=freq)
    return group.apply(rl.first_run_ufunc, window=window, index='dayofyear')