What will tomorrow's temperature be at 12Z?

12Z is noon (standard time) in Greenwich, England. (The "Z" stands for "zero", the longitude of Greenwich expressed in degrees.) The Pacific time zone is 8 hours behind UTC time (when we are on Pacific Standard Time) or 7 hours behind (when we go on daylight savings time, or Pacific Daylight Time). Hence, 12Z is 4 am PST or 5 am PDT.

If you examine enough meteograms (such as the latest meteogram for the forecast location or the next-latest one), you can probably convince yourself that on many days, air temperature near the earth's surface tends to follow a semi-regular cycle over 24 hours. It usually reaches a peak somewhere between early and late afternoon, then declines gradually through the rest of the afternoon, into the evening, and through the night until the sun comes up again, after which the temperature rises to a new peak the next afternoon. (There are exceptions to this pattern, of course, depending on which physical processes are affecting the temperature.)

The time at which the sun rises depends on the time of year—it comes up after about 6 am (standard time) for half the year (fall and winter) and before about 6 am (standard time) the other half of the year (spring and summer). It rises close to 6 am (standard time) at the spring and fall equinoxes (March 21 or 22 and September 21 or 22, respectively, in the Northern Hemisphere, and the opposite in the Southern Hemisphere).

Regardless, at many midlatitude and low latitude locations in the U.S., 12Z comes within a few hours before sunrise. Hence we might ask, how much does the temperature typically change from 12Z to the time of the minimum temperature (near sunrise)? The 12Z temperature should typically be at least as high as the lowest temperature but not too much higher, since the temperature is usually cooling relatively slowly in the last few hours before sunrise. Examine several meteograms (such as the latest and next-latest meteograms for the forecast location) and compare the minimum temperatures and the 12Z temperatures on the same day to get a feel for the typical difference between the two.

Now, if we have a forecast of the next day's minimum temperature (the "low")—which we can get from the National Weather Service, for example—we might make a pretty good forecast of the 12Z temperature simply by estimating the difference between the forecast low and the 12Z temperature based on what has happened in the recent past.

If your 12Z temperature forecast is within plus or minus 5°F of the actual 12Z observation made the next day, you'll get credit for a "correct" forecast (3 points). Otherwise you'll get 1 point for trying.

In summary, one tactic is to:

  1. Examine recent meteograms for the forecast location to see how the 12Z temperature has related to the minimum temperature recently.
  2. Look up the NWS low temperature forecast for the forecast location for tomorrow.
  3. Add a correction to the low temperature forecast to estimate the 12Z temperature.

Resources: From the class iLearn Web site, look under Forecasting Assignment and go to Weather Maps and Images > Surface Analyses. There you'll find links to: