query help

I need some help with rewriting a query. I have a fairly complicated
query (for me anyway) that dumps daily climate data, filling in
missing data with monthly averages (one line per day).

I want to output monthly averages (one line per month). I am having a
hard time wrapping my head around this. Particularly how to deal with
the doy column (day of year). I have tried several approaches and my
forehead is starting to get my keyboard bloody.

Thanks in advance for any suggestions.

Here is the daily query:

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,
--replace missing values (-999) with the monthly average
CASE w.tmax
WHEN -999 THEN avgtmax.avg
ELSE w.tmax
END,
CASE w.tmin
WHEN -999 THEN avgtmin.avg
ELSE w.tmin
END,
CASE s.par
WHEN -999 THEN avgpar.avg
ELSE s.par
END,
CASE w.precip
WHEN -999 THEN avgprecip.avg
ELSE w.precip
END
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)
INNER JOIN (SELECT MONTH,
round(avg(tmax)::numeric, 2) AS avg
FROM weather
WHERE tmax != -999
GROUP BY MONTH) AS avgtmax
ON (w.month = avgtmax.month)
INNER JOIN (SELECT MONTH,
round(avg(tmin)::numeric, 2) AS avg
FROM weather
WHERE tmin != -999
GROUP BY MONTH) AS avgtmin
ON (w.month = avgtmin.month)
INNER JOIN (SELECT MONTH,
round(avg(par)::numeric, 2) AS avg
FROM solar
WHERE par != -999
GROUP BY MONTH) AS avgpar
ON (s.month = avgpar.month)
INNER JOIN (SELECT MONTH,
round(avg(precip)::numeric, 2) AS avg
FROM weather
WHERE precip != -999
GROUP BY MONTH) AS avgprecip
ON (w.month = avgprecip.month)
--select station to output climate data by id number
WHERE w.station_id = 219101