Climatology And Persistence Forecasting

Leaning on guidance provided by MOS and looking at temperature observations "upstream" from your hometown are not the only steps that you, as an apprentice forecaster, should take to create a forecast for high and low temperatures. Preparing a forecast for the high temperature at a given city or town on a given day should include a quick check of the date's average high temperature so that you always have "ballpark" numbers in mind. Prudent forecasters also are aware of record high and low temperatures for the date. Records are indeed made to be broken, but they don't usually fall from the history books very easily. Thus, forecasters who unknowingly predict a record high or record low in a mundane weather pattern do not show very good form. In short, weather forecasters worth their salt always keep climatology in mind.

The standard climatology report for an observing site includes the 30-year average and record highs and lows for the date.

I don't want to imply that you merely use the date's average high and low for your forecast. These daily "normals" are just a simple 30-year average for the date (to calculate the "normal" high at Chicago on July 4, for example, add up all the highs for July 4 from 1971-2000 and divide by 30). A prudent forecaster knows that temperatures can be highly variable at many places, particularly during winter, when temperature advection is the major controller. Thus, she will always put her temperature forecasts in the context of climatology.

Honolulu, Hawaii (located on the island of Oahu) is surrounded by the Pacific Ocean, which, like all large bodies of water, grudgingly resists large, fast changes in surface water temperature.

Obviously, there is more reliability in a forecast based solely on the date's average high temperature at cities where there's not much variability in day-to-day weather. For example, Honolulu, HI, surrounded by the Pacific Ocean (whose surface waters heat up and cool off very slowly), is a city where departures from daily highs (and lows) tend to be monotonously small. Even monthly variations in high and low temperatures are typically slight. Indeed, temperature forecasts based on climatology work very well for Honolulu.

For landlocked cities such as Minneapolis, Minnesota, however, high variability in day-to-day and season-to-season temperature patterns is more the rule rather than the exception. When forecasting for such cities, meteorologists must be a bit cautious about incorporating climatology into their temperature predictions.

Weather forecasters rely more heavily on climatology during the lazy, hazy days of summer, when changes in the prevailing air mass (associated with the arrival of mid-latitude low pressure systems and their fronts) slow to a snail's pace. With the now relatively weak steering winds of the summertime jet stream having retreated northward over Canada, a mT air mass can linger for days over regions of the contiguous states, setting the stage for persistently warm and humid weather.

The catalyst for protracted warm and humid weather over the Eastern U.S. is the Bermuda high, which marks the core of an expansive mT air mass that recurrently affects much of the region during summer. The Bermuda high, which has a nearly continuous presence on surface weather maps during summer, earns its nickname because its average position lies near the island of Bermuda off the Atlantic Coast.

The "Bermuda high" marks the core of an expansive mT air mass (left panel) that set up on June 12, 2000. Meteorologists often refer to the "Bermuda high" as a "heat pump" because the clockwise circulation of winds carry tropical air northward from the Gulf of Mexico. The right panel is a roll-over that shows surface temperatures and then dew points across the East on June 12, 2000.

When a maritime-tropical air mass grips the eastern U.S., high and low temperatures at many cities and towns can vary little over the course of a few to several days. Here I assume that sky conditions are generally clear to partly cloudy and that cooling afternoon or evening showers and thunderstorms (a staple of mT air masses) are only widely separated. In such cases, weather forecasters use a method called persistence, a method which simply uses one day's observed high and low temperatures for the next day's predicted high and low.

Forecasters sometimes apply a "modified" version of persistence forecasting, particularly when a relatively slow-moving cP air mass moves across a region during the cold season. In the sequence of surface weather maps below, the center of a continental-polar air mass, marked by a center of high pressure, built eastward toward the Middle Atlantic States in mid-November, 2001. With cool and dry northerly winds on its eastern flank and warmer, slightly more moist southerly winds on its western flank, such a high pressure system is like a chameleon, in effect changing color from blue (representing cool air on its eastern flank) to red (representing warmer air on its western flank.

Modified persistence can be used when a large, continental-polar air mass, marked by a center of high pressure, moves slowly across a region. With cool, dry northerly winds (blue shading) on its eastern flank and warmer, slightly more moist southerly winds (red shading) on its western flank, forecasters can simply adjust temperatures slightly upward as the high migrates eastward.

As the high approached State College, Pennsylvania, on November 11th, the high temperature observed at the Penn State Weather Station was 46 degrees. As the center of the high approached and passed east of State College, the high temperatures on the 12th and 13th were 49 and 54 degrees, reflecting a gradual and slight warming trend associated with the approach, arrival and passage of the core of the air mass. By the way, the low temperatures on the 11th, 12th and 13th were 32, 28 and 28 degrees respectively (there was some wind during the night of the 11th that limited the nocturnal drop in temperature).

I'll generalize the above set of observations so that you can use it as a forecasting tool. When continental-polar air masses gradually move across a region, professional weather forecasters will use the method of modified persistence, taking the high temperature observed on the first full day under the influence of the approaching high pressure center and then adding a few degrees to this reading to predict the high temperature on subsequent days. This method of modified persistence reflects the often gradual warming trend associated with the approach and arrival of the warmer western flank of a chameleon high pressure system.

Successfully implementing MOS, climatology and the methods of persistence and modified persistence to forecast high and low temperatures require practice, of course. With time, your temperature forecasts will improve, especially if you log your temperature forecast in a weather diary and then write a critique of your forecast once you verify the official high and low temperatures.

Now it's time to earn your G-11 rating as an apprentice forecaster. Good luck!