Email Alerts

Anyone can sign up to get an email when an emergency alert covers an address they care about. Each email says what is happening, what to do, and links back to your site for the latest.

Signing up

• The sign-up block: email, a US street address, and which alerts. Sentinel looks the address up with the US Census Bureau, then sends a confirmation email. Nothing is sent until the person clicks the link. Unconfirmed sign-ups are deleted after a week.
• On a user account: logged-in users can tick Email me when an emergency alert covers this location under Location for alerts on their account, using a saved address or the location they saved from the location block. No confirmation is needed: it's their own account's email address.

People can sign up several addresses, for example their home and a parent's house. Signing up the same address twice doesn't create a duplicate; Sentinel sends a link to manage the existing sign-up instead, so the form never reveals who is signed up. The sign-up form has a hidden honeypot field and allows 5 sign-ups per visitor per hour.

Which alerts

The emails

• New alert: the official headline, area, end time and instruction, the safety steps for that hazard, and leave-early ideas. When a flood warning covers a high-risk flood zone, the subject starts with FLOOD DANGER.
• Update: when the alert changes, for example a watch becomes a warning or a warning is extended.
• All clear: when the alert ends, is cancelled, or no longer covers the address.

Every email links to a live page for the subscriber's address, which refreshes every 2 minutes, and has a link to change or stop the emails. Mail clients that support one-click unsubscribe (RFC 8058) show an Unsubscribe button too. No email is sent twice for the same alert.

Emails are queued and sent on cron, so a storm covering thousands of subscribers doesn't slow down the fetch or the site.

Emergency mode

When a flood warning, or any Severe or Extreme alert, covers at least one subscriber, Sentinel switches to emergency mode:

• It fetches new alerts every minute instead of every 5 (both adjustable in settings). Cron must run every minute for this to work.
• It matches subscribers on their precise address, so only people inside the alert area are emailed. Other alerts use a square of about 100 meters around the address.

It switches back when no emergency covers any subscriber. The Subscribers tab shows whether emergency mode is on and how many subscribers are inside an emergency area.

Privacy

• Subscribers' addresses are stored precisely, to tell whether they are inside an alert area. The sign-up form says so, and people must tick a box agreeing to it.
• Links in emails carry a signed token, not a password. Anyone with an email's link can see that address's alerts and change or stop the emails, so the emails shouldn't be forwarded.
• Unsubscribing deletes the sign-up and its sent log. Deleting a user deletes their account sign-up.
• The Subscribers tab, which lists email addresses, needs the Administer Sentinel permission.