What weather observations are used for determining the Daily High and Low?
Sites used are from data networks or providers deemed reliable with statistically rigorous data quality assurance, including but not limited to sensor sensitivity, placement and timeliness. This may include other government-run agencies with weather observation networks within the Lower 48 states and may incorporate some local, state-run or those considered reliable through peer reviewed/refereed journal articles. This includes:
This may include non-routine reports that a NWS Forecast Office disseminates via their official social media accounts (Twitter or Facebook). How often is the Daily High and Low produced?
Every six hours and distributed at 0050, 0650, 1250 and 1850 UTC.
Note: At 01 and 13 UTC, the High and Low are also added to the Selected Cities Product. Was the Daily High and Low observed on that specific calendar day? Yes and No.
Yes: The Daily High is considered the highest reliable temperature reported in the Lower 48 states, between LOCAL midnight to midnight.
No: The Daily Low is actually considered from LOCAL noon the prior calendar day to noon of the official report date. The vast majority of Low temperatures DO occur early in the morning on the date itself. However, on rare occasions, the Low may have occurred prior to midnight the day before. For example, a Low may occur at 11pm on Tuesday, but the temperature rose afterward. If this is the coldest reliable location, the Low at 11pm would be considered the Low for WEDNESDAY, not Tuesday. When does the Daily High/Low temperature get reset for the next day?
Given the Low may fall on prior calendar day (see question above), the daily High and Low get reset for the calendar day at different issuance times during the day; 0050 UTC for the High and 1250 UTC for the Low. For example, the first issuance of Tuesday's High will be at 0050 UTC on Wednesday (which is on Tuesday in all US time zones). As such, the last issuance of Tuesday's High will be at 1850 UTC on Wednesday.
Similarly, the first issuance of Tuesday's Low will be at 1250 UTC on Tuesday, making the last issuance of Tuesday's Low at 0650 UTC on Wednesday. How is the data vetted? How could a weather observation potentially be missed?
The determination of the daily maximum and minimum temperatures at WPC is NOT fully automated. There are automated methods to compile and visualize the vast amount of Maximum and Minimum temperatures, but there is always a forecaster doing the first level of quality control. However, some data may arrive afterwards, even one or two days later (more likely for NWS CO-OP reports). As such, we may not have all the data and so the results are considered Preliminary. Official records will be kept by the National Center for Environmental Information (NCEI). I found a colder/hotter weather report for today.
This is quite possible. Here are a few possible reasons:

1. The observation is from a weather site that WPC believes may have some data quality concerns.
2. It is a carry-over observation. A "carry-over" is typically associated with a 24hr Low temperature that occurred PRIOR to Noon (i.e. the Minimum temperature for the prior calendar day). This may even occur with automated ASOS and AWOS stations. This is quite often the most difficult observation to fully deduce precisely when the Low occurred and may lead to errors.
3. It is a late-arriving CO-OP report. For example, a morning observation is received at 8am reporting the weather to NWS forecast office staff with the Max/Min temps, precipitation, etc...for the last 24hrs or even sometimes longer (if the observer missed one or two days).

Why isn't my home, school, or local TV station's data used?
Even though there are weather observations that may be very reliable, well-sited and show high statistical correlation to surrounding reliable networks, there is often too much data to sift to ascertain its reliability. If you wish your data to be considered for National Hi/Lo, consider becoming a NWS Cooperative Observer (CO-OP) Why is a location with very low or no population used? Why is a mountain peak used for the Low temperature? In the past, there were requirements for city population and elevation. Yet, this excluded places of interest, such as Death Valley, Mount Washington in New Hampshire, Mount Mitchell in North Carolina, etc... Also, populations can change both over years but also seasonally. Additionally, mountains in the East fell below the prior elevation criteria resulting in unsatisfying and inconsistent application of rules. For simplicity it was determined to remove the requirements that were reducing timeliness and first-order quality assessment of candidate observations.