Q: Language and region prefs are set to US but Today Notification reads in Celsius not Fahrenheit.

When you click on the Advanced tab in Language & Region, do you have the Measurement Units set to "US"?  If it's set to Metric, it will give you the temp in Celsius, not Fahrenheit.

Why don't we try this — Deleting the System Preference or other .plist file .  As it says, it will allow you to completely reset your system preferences, which should make your temperature settings match the rest of your Language & Region preferences.  Let me know if this helps. 

Hover the cursor over the temperature and you should see an I with a circle around it. Click that. That will give you an option to add a location and on the right side you should see C/F. Click F and then done.

Is this on the iPad? Today Summary temperature setting is stored in the Clock app.

See screen capture here:

Re: How do I change celsius to fahrenheit temperature in today summary in notification center?

In Language & Region preferences, click the Advanced... button and change the Measurement units to suit your preferences.