METEOROLOGIST JEFF HABY
It is important to have a forecast verification that is both fair to
you and your viewers / clients. Forecast
verification can be either an objective or subjective analysis of how well your forecast is compared to the
actual conditions that occur. If a forecast does not have a fair verification it may be either because it is
too general or too specific.
The "too general" forecast has a good chance of verifying because the
precipitation forecast has too large of a spread. The advantage of the too general forecast is the fact is
has a good chance of being correct. The disadvantage is that is does not properly prepare the viewers / clients
for the weather. An example includes: "The high will be in the 50's today with a 50% chance of precipitation".
This forecast gives a 10-degree spread on temperatures and gives no characteristics to the rainfall such as
intensity, amount, duration and timing.
In comparison, the "too specific" forecast does not have a good chance
of verifying because there is little room for even the slightest error. The advantage of the too specific forecast
is that you will look like a genius if it verifies. Forecast error is usually the result. An example includes:
"The high will be 53 degrees today and it will rain 0.67 inches between 3:00 and 5:00 pm". There is no room for
error on the temperature and the chance of having the precipitation forecast pan out is very unlikely. Other
examples of too specific forecasts include predicting the exact county a tornado will occur before the severe
thunderstorm forms and the exact time clouds will clear during the day.
A forecast that hits between the too general
and too specific forecasts is best. An example of this is: "The high will be in the mid-80's with a 70% chance for
afternoon convective thunderstorms. Rainfall amounts will range from 0.50 to 1.00 inches." Although exact
temperatures can be used to get the point across on what temperature to expect, a small range such as the
mid-80's has a much better chance of verifying. If the forecast says the high will be 75 degrees and the actual
high is 74 degrees, is the forecast a bust? I think not, Clearly this is better than saying the high will be in
the 70's especially when forecasting for a point location such as an airport. Mesoscale effects will often
temperature gradient over the forecast region. This makes it important to distinguish which regions
will be cooler or warmer. This becomes a problem especially for large forecast regions.
It is wise to keep a
forecasting journal to see how well your forecasts verify for temperatures, precipitation and other weather
events you deem important. A high or low within 2 degrees on the today, tonight and tomorrow forecast is
sufficient accuracy. Precipitation is more difficult to access forecast accuracy. Using both an objective and
subjective analysis are beneficial. The objective analysis would include predicting the precipitation amount.
The subjective / descriptive analysis would be describing the character of the precipitation (i.e. occurring around noon,
heavy at first then becoming lighter, how the rainfall will change temperatures, precipitation type, and the
intensity / duration).
Becoming a great forecaster series: http://www.theweatherprediction.com/greatforecaster/