diff --git a/scripts/run_unit_tests.py b/scripts/run_unit_tests.py
index 4254f2f8e6b9..869a520fb081 100644
--- a/scripts/run_unit_tests.py
+++ b/scripts/run_unit_tests.py
@@ -30,6 +30,7 @@
 from script_utils import get_changed_packages
 from script_utils import in_travis
 from script_utils import in_travis_pr
+from script_utils import local_diff_branch
 from script_utils import travis_branch
 
 
@@ -119,6 +120,8 @@ def get_test_packages():
     Filters the package list in the following order:
 
     * Check command line for packages passed in as positional arguments
+    * Check if the the local remote and local branch environment variables
+      have been set to specify a remote branch to diff against.
     * Check if in Travis, then limit the subset based on changes
       in a Pull Request ("push" builds to branches may not have
       any filtering)
@@ -129,12 +132,15 @@ def get_test_packages():
               need be run.
     """
     all_packages = get_package_directories()
+    local_diff = local_diff_branch()
 
     parser = get_parser()
     args = parser.parse_args()
     if args.packages is not UNSET_SENTINEL:
         verify_packages(args.packages, all_packages)
         return sorted(args.packages)
+    elif local_diff is not None:
+        return get_changed_packages('HEAD', local_diff, all_packages)
     elif in_travis():
         return get_travis_directories(all_packages)
     else:
diff --git a/scripts/script_utils.py b/scripts/script_utils.py
index 370408e6faa3..b64f897d4a93 100644
--- a/scripts/script_utils.py
+++ b/scripts/script_utils.py
@@ -163,6 +163,23 @@ def get_changed_packages(blob_name1, blob_name2, package_list):
     return sorted(result)
 
 
+def local_diff_branch():
+    """Get a remote branch to diff against in a local checkout.
+
+    Checks if the the local remote and local branch environment
+    variables specify a remote branch.
+
+    :rtype: str
+    :returns: The diffbase `{remote}/{branch}` if the environment
+              variables are defined. If not, returns ``None``.
+    """
+    # Only allow specified remote and branch in local dev.
+    remote = os.getenv(LOCAL_REMOTE_ENV)
+    branch = os.getenv(LOCAL_BRANCH_ENV)
+    if remote is not None and branch is not None:
+        return '%s/%s' % (remote, branch)
+
+
 def get_affected_files(allow_limited=True):
     """Gets a list of files in the repository.
 
@@ -195,11 +212,7 @@ def get_affected_files(allow_limited=True):
         if in_travis_pr():
             diff_base = travis_branch()
     else:
-        # Only allow specified remote and branch in local dev.
-        remote = os.getenv(LOCAL_REMOTE_ENV)
-        branch = os.getenv(LOCAL_BRANCH_ENV)
-        if remote is not None and branch is not None:
-            diff_base = '%s/%s' % (remote, branch)
+        diff_base = local_diff_branch()
 
     if diff_base is not None and allow_limited:
         result = subprocess.check_output(['git', 'diff', '--name-only',
diff --git a/tox.ini b/tox.ini
index c0783749c4c4..69118912797b 100644
--- a/tox.ini
+++ b/tox.ini
@@ -121,7 +121,7 @@ skip_install =
     py34: True
     py35: True
     isolated-cover: True
-passenv = TRAVIS*
+passenv = TRAVIS* GOOGLE_CLOUD_*
 
 [testenv:isolated-cover]
 commands =