The answer depends on what the POA document says. Look for a paragraph titled "Effectiveness." It will give the procedure, which might be a requirement of TWO doctor signatures, or it could say that the POA was effective immediately when signed by your father.
Disclaimer: This answer does not create an attorney-client relationship. You must not rely on it for legal advice. The answer might be different if more facts and details were known. Procedures and standards vary for each individual case.
The power of attorney document should, as Mr. Burkhart pointed out, specify when it becomes effective. Powers of attorney fall into broad categories. Some are effective upon signature. Other, so-called "springing" powers, are triggered by a subsequent event. The "trigger" varies from one attorney to another. Mine generally are triggered by two licensed physicians familiar with the principal's condition signing certificates to the effect his medical condition meets the legal standard for incapaciity. Others require a court order.
Make sure in WA that it is actually "durable" and has the appropriate language. Make sure also that it contains any specific powers you will need.
If there is no specified "trigger," the clinic note, accompanied by a physician's affidavit, may suffice. But the practical issue in utilizing any durable power of attorney is - will the third party accept it? I doubt a bank, other financial institution or title company will accept a mere doctor's note unless the officer is very familiar with your father.
If it was an immediate DPOA, you may run into the "staleness" objection. Some financial institutions will require that you update it. They may have to be persuaded that your father's incapacity makes that impossible. They really shouldn't do this except in the case of limited powers.
If the DPOA is a "springing" type and was not drafted to comply with the 2005 changes to HIPAA, you may have a problem with doctors and hospitals providing medical records. This can result in a "Catch-22" where you need a doctor's certificate to "trigger" the DPOA and they can't talk to you about capacity unless and until it is "triggered."
If your father still has "good days" you may consider having him execute a new DPOA effective immediately. This enables him to make sure it contains the necessary specific powers and is updated for HIPAA. But it also avoids written evidence of incapacity. This may be useful if he wants to retain the authority to change his will, etc. The standard for capacity to decide which of your family members to grant a durable power of attorney to is relatively low. The decision to delegate authority has actually been viewed as evidence of some capacity being left. Or, have him make any changes before the DPOA is "triggered."
If there are troublesome family members, he may wish to consider prohibiting them from bringing a petition to require an accounting or replace the attorney in fact.
Finally, if your brother will be dealing with real estate, the title company will probably require that it be recorded in the county where the realty is located. I usually don't otherwise record.