problem with a conditional statement

I am struggling to get a CASE WHEN statement to work within another
CASE WHEN. Here is my original code:

SELECT CASE WHEN w.station_id = site_near.station_id THEN w.obs_id ELSE
s.obs_id END AS obs_id,  site_near.station_id, site_near.longname,
w.year, w.doy, w.precip, w.tmin, w.tmax,

--replace missing solar values (-999) with the average of all solar
values from that month (s.month)

--CASE  s.par WHEN -999 THEN AVG( s.par) ELSE s.par END
--FROM solar s
--GROUP BY s.month;

FROM site_near INNER JOIN solar s ON
site_near.ref_solar_station_id = s.station_id  AND site_near.obs_year
= s.year
INNER JOIN weather w ON site_near.ref_weather_station_id =
w.station_id AND site_near.obs_year = w.year AND s.date = w.date
WHERE w.station_id = 211630;

I have commented out the troublesome bits in the middle of the code.
All I am trying to do here is to replace missing values with averages
from the same day of the year for all years. Does anyone see what I
am buggering up here?

Thanks in advance