I think it is supposed to 'expire' when it's 'endTime' is over per the JSON "periods": [
{
"number": 1,
"name": "This Afternoon",
"startTime": "2017-08-02T14:00:00-07:00",
"endTime": "2017-08-02T18:00:00-07:00",
"isDaytime": true,
"temperature": 93,
"temperatureUnit": "F",
"temperatureTrend": "falling",
"windSpeed": "9 mph",
"windDirection": "N",
"icon": "https://api.weather.gov/icons/land/day/tsra_hi?size=medium",
"shortForecast": "Slight Chance T-storms",
"detailedForecast": "A slight chance of thunderstorms after 5pm. Mostly sunny. High near 93, with temperatures falling to around 90 in the afternoon. North wind around 9 mph."
},
I think the api.weather.gov server should remove that JSON period after the endTime and scoot up the other entries, but I've not studied it in detail and there's no docs available except for how to request the JSON itself.