Convert TafData into a string for text-to-speech

def taf(data: TafData, units: Units) -> str:
    """
    Convert TafData into a string for text-to-speech
    """
    try:
        month = data.start_time.dt.strftime(r'%B')
        day = ordinal(data.start_time.dt.day)
        ret = f"Starting on {month} {day} - "
    except AttributeError:
        ret = ''
    return ret + '. '.join([taf_line(line, units) for line in data.forecast])