There is no such time as 12:00 PM.
It's either 12:00 Noon or 12:00 Midnight. There’s no confusion about 12:00 Noon. That occurs at midday. To avoid confusion about which midnight is being referred to, it is usual to specify 11:59 PM on a specific day or 12:01 AM of the next day.
Too many people blithely assume that everyone will know what is meant by "12:00 PM" when in fact it's impossible to know what the speaker intends. (In this case, the NWS message clearly stated "12:00 Noon" but the people at alert.pa.gov substituted 12:00 PM.)
Why does it matter? Is this a grammar Nazi situation? Certainly not! After the obsolescence of sundials and the development of ever more precise methods of measuring time, it became desirable for people to have a standard by which to schedule activities.
It was the railroads growth which led to the standardization of time zones. Before the railroad, local time was used and if every locality set their clocks to “High Noon” when the sun reached its highest point in the sky, each town would have a different time because for towns further west, the sun would be at its highest after the towns to the east.
It was crucial for the railroads to have ALL their trains and personnel on EXACTLY the same time to keep trains from crashing in to each other. This led to the creation of the time zones we use today. It also led the railroads to adopt the practice of never having a timetable arrival or departure of 12:00 o'clock (as stated below in the Wikipedia reference)
— The time was always either 11:59 or 12:01. Further, the times in Bold Face type were always the PM times.
“The 30th edition of the U.S. Government Style Manual (2008) sections 9.54 and 12.9b
recommended the use of "12 a.m." for midnight and "12 p.m." for noon
totally contradicting their 29th edition (2000) which recommended use
of "12 p.m." for midnight and "12 a.m." (formerly "12 m.") for noon.”
“While computers and digital clocks display "12:00 a.m." and "12:00 p.m." these notations provide no clear and unambiguous way to distinguish between midnight and noon. It is technically improper to use "a.m." and "p.m." when referring to 12:00. The abbreviation a.m. stands for ante meridiem (or before the meridian) and p.m. stands for post meridiem (or after the meridian), with the meridian being 12:00 noon. For this reason, neither abbreviation is correct for noon or midnight. The length of the error is determined by the smallest unit of time: 12:00:01 p.m. would be correctly notated, as would even 12:00:00.00001 pm.”
The most common ways to represent these times are:
to use a 24-hour clock (00:00 and 12:00, 24:00; but never 24:01)
to use "12 noon" or "12 midnight", although unless the person is referring to a general time and not a specific day, "12 midnight" is still ambiguous
to specify midnight as between two successive days or dates (Midnight Saturday/Sunday or Midnight December 14/15)
to use "12:01 a.m." or "11:59 p.m." This final usage is common in the travel industry, especially train and plane schedules, to avoid confusion as to passengers' schedules [emphasis added]
Sometimes there really is a rhyme or reason for some things.