How to fix a bowled Threshold?

My first thought is to put a gasket (sweep) at the bottom of the door. I'm not sure that's a permanent fix, though. This link may help:

https://www.thisoldhouse.com/doors/21016488/how-to-make-your-doors-draft-free-with-weatherstripping

Here is a list of products that offer good options for your situation, take a look and read on what they can do for you:

https://www.aidsquilt.org/best-adhesive-to-bond-wood-to-concrete/

Low cost way is to install a door sweep to the bottom of the door. Make sure you get one that is high enough. Measure one side from the concrete and about one inch above the bottom of the door.

https://www.homedepot.com/b/Hardware-Weather-Stripping-Door-Sweeps/N-5yc1vZc7kg?NCNI-5&searchRedirect=door%2520sweep&semanticToken=310r20r00f22000000_202207160145464045124812049_us-east4-m9sq%20310r20r00f22000000%20%3E%20rid%3A%7B875ad15d2cce3c63ec6a8145dbd0cf40%7D%3Arid%20st%3A%7Bdoor%20sweep%7D%3Ast%20ml%3A%7B24%7D%3Aml%20ct%3A%7Bdoor%20sweep%7D%3Act%20nr%3A%7Bdoor%20sweep%7D%3Anr%20nf%3A%7Bn%2Fa%7D%3Anf%20qu%3A%7Bdoor%20sweep%7D%3Aqu%20qr%3A%7Bdoor%20sweep%7D%3Aqr

Another option would be to see if the threshhold can be pried out and replace it with a metal surface threshold. Use construction adhesive to hold it in place

https://www.homedepot.com/s/threshhold?NCNI-5

If the threshold can be removed and is flexible you can pry it out. Pour some cement in the groove, then replace the threshold using a level to make sure its straight and touching the rubber on the door bottom. Let the cement dry.

Looks like whatever support is under that threshold had deteriorated, Probably need to replace it.

You might be ahead to purchase a new metal threshold cut to the precise size of your doorway. Here's some info......

https://www.thisoldhouse.com/doors/22726997/how-to-build-exterior-door-threshold

Hi Lisa Jo, hope this video helps you out,

https://www.youtube.com/watch?v=D8KfOZSLdHE

Door sweeps have several benefits beside being a cheap and easy fix.

1. Protects from Pests

2. Mitigates Moisture

3. Negates Noise

4. Economizes Energy

5. Accentuates Air Quality

6. Confers Comfort

Both Lowes, Home Depot sell metal door sweeps. You can have them cut it to size for you. Get one that has a doorsweep. This will help with the cold air. https://www.google.com/search?q=how+to+install+a+door+threshold&oq=how+to+install+a+door+threshold&aqs=chrome..69i57.13431j0j7&sourceid=chrome&ie=UTF-8#kpvalbx=_yyLTYrbNAoOnqtsPt8uTiAg84

How about a Flex Seal product?

Put a small step there it will also stop the water go right over the saddle or replace the saddle.

As it looks as it is rather worn as well as damaged, I would replace it with something less likely to suffer the same fate. Brick, Metal, concrete etc.

Agree with John, the subfloor looks like its damaged from probably water exposure so will need to pop up that wood trim, check level and integrity of the wood, apply some waterproof sealant if not damaged, then a new piece of trim also waterproofed; or replace damaged wood, Then add the rubberized door sweep to prevent future damage. This will an area you will need to maintain annually with some waterproofing.

Well then you should only need to replace that bowed piece of wood. And waterproof it.

You might be able to take several clamps and old in place for an extended period of time. But that would mean leaving the door open. So you might consider removing before straightening.