The National Weather Service typically updates their forecasts four times per day, at approximately 4am, 11am, 4pm, and 11pm local time. If you want just the morning forecast, hover over your handle in the upper right corner, choose "My Email and Text Alerts" and select the "Settings" link to the right of the city you are interested in changing the settings for. Use the pull-down menus to choose a time between 3am and 8am to make sure you catch the morning forecast. If you notice you are still getting too many forecasts, you can always restrict the delivery hours to a narrower time range.
In order to stop the alerts completely, just hit the "OFF" button next to the city you no longer want to receive alerts for, or if you are done with it completely, you can hit the "Remove" link. These can be found on the main Email an Text Alerts page.