diff --git a/examples/precipitation_accumulation_nowcast.py b/examples/precipitation_accumulation_nowcast.py
index e69de29..fd78178 100644
--- a/examples/precipitation_accumulation_nowcast.py
+++ b/examples/precipitation_accumulation_nowcast.py
@@ -0,0 +1,202 @@
+#!/usr/bin/env python3
+# -*- coding: utf-8 -*-
+import os, sys
+import pyximport
+import copy
+
+#from pysteps_custom_utils.utils import convert_dbz_to_mm
+
+pyximport.install()
+
+from tendo import singleton
+
+me = singleton.SingleInstance() # will sys.exit(-1) if other instance is running
+
+
+"""Stochastic ensemble precipitation nowcasting
+
+The script shows how to run a stochastic ensemble of precipitation nowcasts with
+pysteps.
+
+More info: https://pysteps.github.io/
+"""
+import datetime
+import numpy as np
+sys.path.append(os.path.join(os.path.dirname(__file__), '../'))
+
+import pysteps as stp
+import config as cfg
+
+# List of case studies that can be used in this tutorial
+
+#+-------+--------------+-------------+----------------------------------------+
+#| event |  start_time  | data_source | description                            |
+#+=======+==============+=============+========================================+
+#|  01   | 201701311030 |     mch     | orographic precipitation               |
+#+-------+--------------+-------------+----------------------------------------+
+#|  02   | 201505151630 |     mch     | non-stationary field, apparent rotation|
+#+-------+--------------+------------------------------------------------------+
+#|  03   | 201609281530 |     fmi     | stratiform rain band                   |
+#+-------+--------------+-------------+----------------------------------------+
+#|  04   | 201705091130 |     fmi     | widespread convective activity         |
+#+-------+--------------+-------------+----------------------------------------+
+#|  05   | 201806161100 |     bom     | bom example data                       |
+#+-------+--------------+-------------+----------------------------------------+
+
+# Set parameters for this tutorial
+data_source="gimet"
+
+## import data specifications
+ds = cfg.get_specifications(data_source)
+
+
+## input data (copy/paste values from table above)
+archive_dir=ds.root_path+"/"+max(os.listdir(ds.root_path))
+last_fname=max(os.listdir(archive_dir))
+print(last_fname)
+startdate_str=last_fname[-18:-10]+last_fname[-9:-5]
+
+print(startdate_str)
+
+## methods
+oflow_method        = "lucaskanade"     # lucaskanade, darts, None
+
+## forecast parameters
+n_prvs_times        = 6                # use at least 9 with DARTS
+n_leadtimes         = 6                # use 6 for one hour extrapolation and 12 for 2 hours extrapolation
+r_threshold         = 0.1               # rain/no-rain threshold [mm/h]
+unit                = "mm/h"            # mm/h or dBZ
+transformation      = "dB"              # None or dB
+adjust_domain       = None              # None or square
+output_time_step = 1
+a = 316.0
+b = 1.5
+
+# Read-in the data
+print('Read the data...', startdate_str)
+startdate  = datetime.datetime.strptime(startdate_str, "%Y%m%d%H%M")
+# startdate = datetime.datetime(2019, 10, 7, 18, 40)
+## import data specifications
+ds = cfg.get_specifications(data_source)
+
+## find radar field filenames
+input_files = stp.io.find_by_date(startdate, ds.root_path, ds.path_fmt, ds.fn_pattern,
+                                  ds.fn_ext, ds.timestep, n_prvs_times, 0)
+
+## read radar field files
+importer = stp.io.get_method(ds.importer, "importer")
+R, _, metadata = stp.io.read_timeseries(input_files, importer, **ds.importer_kwargs)
+
+# Preprocess the data
+# R = convert_dbz_to_mm(R)
+
+# Prepare input files
+print("Prepare the data...")
+
+## if requested, make sure we work with a square domain
+reshaper = stp.utils.get_method(adjust_domain)
+R, metadata = reshaper(R, metadata, method="pad")
+
+## if necessary, convert to rain rates [mm/h]
+converter = stp.utils.get_method("mm/h")
+R, metadata = converter(R, metadata, a=a, b=b)
+
+## threshold the data
+R[R<r_threshold] = 0.0
+metadata["threshold"] = r_threshold
+
+## convert the data
+# converter = stp.utils.get_method(unit)
+# R, metadata = converter(R, metadata)
+
+## transform the data
+# transformer = stp.utils.get_method(transformation)
+# R, metadata = transformer(R, metadata)
+
+nan_mask = np.ma.masked_invalid(R).mask
+R[nan_mask] = metadata["zerovalue"] # to compute optical flow
+
+# Compute motion field
+oflow_method = stp.motion.get_method(oflow_method)
+UV = oflow_method(R)
+
+# Perform the nowcast
+extrap_kwargs={}
+extrap_kwargs['allow_nonfinite_values'] = True
+
+# apply nan mask back
+R[nan_mask] = np.nan
+
+# extrapolate at ten minute time step
+extrapolate = stp.nowcasts.get_method("extrapolation")
+R_calc = extrapolate(R[-1], UV, n_leadtimes, allow_nans=True)
+
+R_all = np.append(R, R_calc, axis=0)
+
+R_all_ref = copy.deepcopy(R_all)
+
+# zeros_mask = np.ma.masked_less_equal(R_all, 0).mask
+# R_all = np.ma.masked_less_equal(R_all, 0)
+
+# per minute precipitation
+# R_all[R_all<0] = 0
+# R_all = R_all/60
+slow_UV = UV - (UV/10)*(10-output_time_step)    # one minute motion vectors
+
+R_computed = np.zeros((R_all.shape[0], R.shape[1], R.shape[2]))
+
+ten_min_sum = np.zeros((R_all.shape[1], R_all.shape[2]))
+
+# compute sum at 10 min timestep
+for i in range(R_all.shape[0]):
+    if i == 0:
+        R_computed[i] = R_all[i]/60
+        ten_min_sum = R_computed[i]
+    else:
+        prev_min_nowcast = R_all[i]
+        for j in range(10):
+            one_min_nowcast = extrapolate(prev_min_nowcast, slow_UV, 1, allow_nans=True)
+            ten_min_sum += one_min_nowcast[0]/60
+            prev_min_nowcast = one_min_nowcast[0]
+        R_computed[i] = ten_min_sum
+        ten_min_sum = R_all[i]/60
+
+print(R_computed.shape)
+
+# compute sum at one hour
+R_final = np.zeros((R_all.shape[0]-6, R.shape[1], R.shape[2]))
+for i in range(R_final.shape[0]):
+    if i == 0:
+        R_final[i] = np.sum(R_computed[:6], axis=0)
+    else:
+        index = i+6
+        R_final[i] = np.sum(R_computed[index-6: index], axis=0)
+
+## export precip accumulation to file
+filename = "%s/%s_%s.ncf" % (cfg.path_outputs, "precip_accum_nwc", startdate_str)
+timestep  = ds.timestep
+shape = (R_final.shape[1], R_final.shape[2])
+metadata['unit'] = 'mm'
+n_leadtimes = R_final.shape[0]
+
+# # set -1 for nan
+R_final[np.isnan(R_final)] = -1
+export_initializer = stp.io.get_method('netcdf', 'exporter')
+exporter = export_initializer(filename, startdate, 10, n_leadtimes, shape, 1, metadata,
+                              incremental=None)
+stp.io.export_forecast_dataset(R_final, exporter)
+stp.io.close_forecast_file(exporter)
+
+# export 10 min precipitation nowcasts to file
+filename = "%s/%s_%s.ncf" % (cfg.path_outputs, "precip_nwc", startdate_str)
+shape = (R_calc.shape[1], R_calc.shape[2])
+metadata['unit'] = 'mm/h'
+n_leadtimes = R_calc.shape[0]
+
+# # set -1 for nan
+R_calc[np.isnan(R_calc)] = -1
+export_initializer = stp.io.get_method('netcdf', 'exporter')
+exporter = export_initializer(filename, startdate, 10, n_leadtimes, shape, 1, metadata,
+                              incremental=None)
+stp.io.export_forecast_dataset(R_calc, exporter)
+stp.io.close_forecast_file(exporter)
diff --git a/examples/precipitation_nowcast.py b/examples/precipitation_nowcast.py
index 3f003dd..6ca5427 100644
--- a/examples/precipitation_nowcast.py
+++ b/examples/precipitation_nowcast.py
@@ -2,6 +2,9 @@
 # -*- coding: utf-8 -*-
 import os, sys
 import pyximport
+
+from pysteps_custom_utils.utils import convert_dbz_to_mm
+
 pyximport.install()
 
 from tendo import singleton
@@ -68,7 +71,7 @@
 
 ## forecast parameters
 n_prvs_times        = 3                # use at least 9 with DARTS
-n_leadtimes         = 5
+n_leadtimes         = 40
 n_ens_members       = 3
 n_cascade_levels    = 8
 ar_order            = 2
@@ -82,6 +85,7 @@
 transformation      = "dB"              # None or dB
 adjust_domain       = None              # None or square
 seed                = 42                # for reproducibility
+output_time_step = 1
 
 vel_pert_kwargs = dict()
 vel_pert_kwargs["p_par"] = [ 0.38550792, 0.62097167, -0.23937287]
@@ -91,7 +95,7 @@
 # Read-in the data
 print('Read the data...', startdate_str)
 startdate  = datetime.datetime.strptime(startdate_str, "%Y%m%d%H%M")
-
+# startdate = datetime.datetime(2019, 10, 7, 18, 40)
 ## import data specifications
 ds = cfg.get_specifications(data_source)
 
@@ -103,6 +107,9 @@
 importer = stp.io.get_method(ds.importer, "importer")
 R, _, metadata = stp.io.read_timeseries(input_files, importer, **ds.importer_kwargs)
 
+# Preprocess the data
+# R = convert_dbz_to_mm(R)
+
 # Prepare input files
 print("Prepare the data...")
 
@@ -133,6 +140,10 @@
 oflow_method = stp.motion.get_method(oflow_method)
 UV = oflow_method(R)
 
+# Change motion field according to timeStep
+if output_time_step < 10:
+    UV = UV - (UV/10)*(10-output_time_step)
+
 # Perform the nowcast
 extrap_kwargs={}
 extrap_kwargs['allow_nonfinite_values'] = True
@@ -140,22 +151,26 @@
 # apply nan mask back
 R[nan_mask] = np.nan
 
-nwc_method = stp.nowcasts.get_method(nwc_method)
-R_fct = nwc_method(R[-3:, :, :], UV,
-    n_leadtimes,
-    n_ens_members,
-    n_cascade_levels=8,
-    R_thr=-10.0,
-    kmperpixel=metadata["xpixelsize"]/1000,
-    timestep=5,
-    decomp_method="fft",
-    bandpass_filter_method="gaussian",
-    noise_method="nonparametric",
-    vel_pert_method="bps",
-    mask_method="incremental",
-    seed=seed,
-    conserve_radar_mask=True,
-    vel_pert_kwargs=vel_pert_kwargs)[-1, :, :]
+extrapolate = stp.nowcasts.get_method("extrapolation")
+R_fct = extrapolate(R[-1], UV, n_leadtimes, allow_nans=True)
+
+
+# nwc_method = stp.nowcasts.get_method(nwc_method)
+# R_fct = nwc_method(R[-3:, :, :], UV,
+#     n_leadtimes,
+#     n_ens_members,
+#     n_cascade_levels=8,
+#     R_thr=-10.0,
+#     kmperpixel=metadata["xpixelsize"]/1000,
+#     timestep=output_time_step,
+#     decomp_method="fft",
+#     bandpass_filter_method="gaussian",
+#     noise_method="nonparametric",
+#     vel_pert_method="bps",
+#     mask_method="incremental",
+#     seed=seed,
+#     conserve_radar_mask=True,
+#     vel_pert_kwargs=vel_pert_kwargs)[-1, :, :]
 
 ## convert all data to mm/h
 converter   = stp.utils.get_method("mm/h")
@@ -177,7 +192,7 @@
 # # set -1 for nan
 R_fct[np.isnan(R_fct)] = -1
 export_initializer = stp.io.get_method('netcdf', 'exporter')
-exporter = export_initializer(filename, startdate, timestep, n_leadtimes, shape, 1, metadata,
+exporter = export_initializer(filename, startdate, output_time_step, n_leadtimes, shape, 1, metadata,
                               incremental=None)
 stp.io.export_forecast_dataset(R_fct, exporter)
 stp.io.close_forecast_file(exporter)
diff --git a/examples/probabilistic_nowcast_with_export.py b/examples/probabilistic_nowcast_with_export.py
index 47b52a4..f306bad 100755
--- a/examples/probabilistic_nowcast_with_export.py
+++ b/examples/probabilistic_nowcast_with_export.py
@@ -177,7 +177,7 @@
 # set -1 for nan
 prob_array[np.isnan(prob_array)] = -1
 export_initializer = stp.io.get_method('netcdf', 'exporter')
-exporter = export_initializer(filename, startdate, timestep, n_leadtimes , shape, n_ens_members, metadata,
+exporter = export_initializer(filename, startdate, timestep, n_leadtimes, shape, n_ens_members, metadata,
                               product='precip_probability', incremental=None)
 stp.io.export_forecast_dataset(prob_array, exporter)
 stp.io.close_forecast_file(exporter)
diff --git a/pysteps/io/exporters.py b/pysteps/io/exporters.py
index f70bf15..699effd 100644
--- a/pysteps/io/exporters.py
+++ b/pysteps/io/exporters.py
@@ -413,20 +413,16 @@ def initialize_forecast_exporter_netcdf(filename, startdate, timestep,
 
     var_time = ncf.createVariable("time", np.int, dimensions=("time",))
     if incremental != "timestep":
-        if product == 'precip_probability':
-            var_time[:] = [i*timestep for i in range(1, n_timesteps+1)]
-        else:
-            var_time[:] = [i*timestep*60 for i in range(1, n_timesteps+1)]
+        var_time[:] = [i*timestep for i in range(1, n_timesteps+1)]
 
     var_time.long_name = "forecast time"
     startdate_str = datetime.strftime(startdate, "%Y-%m-%d %H:%M:%S")
-    var_time.units = "minutes since %s" % startdate_str if product == 'precip_probability' \
-        else "seconds since %s" % startdate_str
-
-    dimensions = ("time", "y", "x")
+    # var_time.units = "minutes since %s" % startdate_str if product == 'precip_probability' \
+    #     else "seconds since %s" % startdate_str
+    var_time.units = "minutes since %s" % startdate_str
 
     var_F = ncf.createVariable(var_name, np.float32,
-                               dimensions=dimensions,
+                               dimensions=("time", "y", "x"),
                                zlib=True, complevel=9)
 
     if var_standard_name is not None:
diff --git a/pysteps/io/importers.py b/pysteps/io/importers.py
index 410ff6f..a894922 100644
--- a/pysteps/io/importers.py
+++ b/pysteps/io/importers.py
@@ -115,7 +115,7 @@
 except ImportError:
     netcdf4_imported = False
 try:
-    import PIL
+    import PIL.Image
 
     pil_imported = True
 except ImportError:
diff --git a/pysteps/nowcasts/extrapolation.py b/pysteps/nowcasts/extrapolation.py
index f0dff8a..bc6971a 100644
--- a/pysteps/nowcasts/extrapolation.py
+++ b/pysteps/nowcasts/extrapolation.py
@@ -17,7 +17,7 @@
 
 def forecast(precip, velocity, num_timesteps,
              extrap_method="semilagrangian", extrap_kwargs=None,
-             measure_time=False):
+             measure_time=False, allow_nans=False):
     """Generate a nowcast by applying a simple advection-based extrapolation to
     the given precipitation field.
 
@@ -73,6 +73,7 @@ def forecast(precip, velocity, num_timesteps,
     extrapolation_method = extrapolation.get_method(extrap_method)
 
     precip_forecast = extrapolation_method(precip, velocity, num_timesteps,
+                                           allow_nonfinite_values=allow_nans,
                                            **extrap_kwargs)
 
     if measure_time: