diff --git a/CHANGELOG.md b/CHANGELOG.md
new file mode 100644
index 0000000..e36aad9
--- /dev/null
+++ b/CHANGELOG.md
@@ -0,0 +1,13 @@
+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [Unreleased]
+
+### Fixed
+
+- Switch to using np.arctan2 rather than np.arctan for Probabilistic Advection [#2](https://github.com/dmidk/SolarSTEPS/pull/2), @KristianHMoller
+
diff --git a/Models/ProbabilisticAdvection.py b/Models/ProbabilisticAdvection.py
index ecf667a..87d3875 100644
--- a/Models/ProbabilisticAdvection.py
+++ b/Models/ProbabilisticAdvection.py
@@ -62,8 +62,10 @@ def ens_forecast(self,
         else:
             raise Exception('angle_pert_dist must be either normal or vonmises')
 
-        pert_motion_field[0, :, :] = pert_abs * np.cos(np.arctan(y / x) + beta_noise)
-        pert_motion_field[1, :, :] = pert_abs * np.sin(np.arctan(y / x) + beta_noise)
+        # Use arctan2 for angle calculation to handle all quadrants correctly
+        base_angle = np.arctan2(y, x)
+        pert_motion_field[0, :, :] = pert_abs * np.cos(base_angle + beta_noise)
+        pert_motion_field[1, :, :] = pert_abs * np.sin(base_angle + beta_noise)
 
         yhat_maps = self.extrapolate(self.last_map,
                                      pert_motion_field,