diff --git a/dartwrf/evaluate_obs_space.py b/dartwrf/evaluate_obs_space.py
index 7f393853cbafa2cfbe618556b55e4d91dc6e8507..7c6a1eef951ea1d72d087e3ed14f64c6e399dae0 100755
--- a/dartwrf/evaluate_obs_space.py
+++ b/dartwrf/evaluate_obs_space.py
@@ -23,15 +23,23 @@ def evaluate_one_time(assim_time, valid_time, use_other_obsseq=False):
 
      # does an observation exist at this time?
      f_oso = valid_time.strftime(aso.pattern_obs_seq_out)
-     f_oso = cluster.archivedir+valid_time.strftime("/obs_seq_out/%Y-%m-%d_%H:%M:%S_obs_seq.out")
+     f_oso = cluster.archivedir+valid_time.strftime("/%Y-%m-%d_%H:%M_obs_seq.out")
 
      if os.path.exists(f_oso):
           # use the existing file
           shutil.copy(f_oso, cluster.dart_rundir+'/obs_seq.out')
      else:
-          if False:
+          try:
+               # generate the observations for the specified valid_time
+               aso.prepare_nature_dart(valid_time)
+               osq_out.generate_obsseq_out(valid_time)
+          except:
+               print("failed to generate observations from a nature file")
+               print("-> trying to evaluate posterior with dummy observations")
                # use an old obsseq file and overwrite obs/truth values with "missing value"
-               f_oso = cluster.archivedir+valid_time.strftime("/obs_seq_out/%Y-%m-%d_%H:%M_obs_seq.out")
+               f_oso = cluster.archivedir+valid_time.strftime("/diagnostics/%Y-%m-%d_%H:%M_obs_seq.out")
+               if not os.path.isfile(f_oso):
+                    raise RuntimeError(f_oso+' not found. Cannot create dummy observation.')
           
                from dartwrf.obs import obsseq
                oso = obsseq.ObsSeq(f_oso)
@@ -40,10 +48,6 @@ def evaluate_one_time(assim_time, valid_time, use_other_obsseq=False):
                oso.df['observations'] = -888888.0
                oso.df['truth'] = -888888.0
                oso.to_dart(cluster.dart_rundir+'/obs_seq.out')
-          else:
-               # generate the observations for the specified valid_time
-               aso.prepare_nature_dart(valid_time)
-               osq_out.generate_obsseq_out(valid_time)
      
      aso.evaluate(valid_time, f_out_pattern=cluster.archivedir + "/diagnostics/%Y-%m-%d_%H:%M:%S_obs_seq.final-evaluate")