diff --git a/.env b/.env
index 08bef7d..f144aa6 100644
--- a/.env
+++ b/.env
@@ -6,6 +6,9 @@ DATA_BACKUP_DIR="J:/Coastal/Temp/CKL/nsw_2016_storm_impact/data"
 # Location where the matlab interpreter is located. Required for a couple of data processing scripts.
 MATLAB_PATH="C:/Program Files/MATLAB/R2016b/bin/win64/MATLAB.exe"
 
+# Number of threads to use for multi-core processing. Used when calculating time-varying beach slope when estimating
+# total water level.
+MULTIPROCESS_THREADS=2
 
 # The settings below should be left as is unless you know what you're doing.
 
diff --git a/src/analysis/forecast_twl.py b/src/analysis/forecast_twl.py
index 70f357e..2fe1b0a 100644
--- a/src/analysis/forecast_twl.py
+++ b/src/analysis/forecast_twl.py
@@ -12,9 +12,17 @@ from src.analysis.runup_models import sto06_individual, sto06
 logging.config.fileConfig("./src/logging.conf", disable_existing_loggers=False)
 logger = logging.getLogger(__name__)
 
+MULTIPROCESS_THREADS = int(os.environ.get("MULTIPROCESS_THREADS", 4))
+
 
 def forecast_twl(
-    df_tides, df_profiles, df_waves, df_profile_features, runup_function, n_processes=4, slope="foreshore"
+    df_tides,
+    df_profiles,
+    df_waves,
+    df_profile_features,
+    runup_function,
+    n_processes=MULTIPROCESS_THREADS,
+    slope="foreshore",
 ):
     # Use df_waves as a base
     df_twl = df_waves.copy()