HARD TO FORECAST:
LOCAL VARIATION
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METEOROLOGIST JEFF HABY
Often forecasts are made for a specific location but the residence of the city and surrounding area are scattered over a wide ranging spatial
scale. Thus, the conditions and future weather for the downtown or airport of a city can be very different compared to nearby cities. This
is important since residence in nearby cities often base what they think will happen at their house based on the forecast for the downtown
area or airport. Variations in weather over short distances occur since weather, especially wild weather, occurs over localized areas. Examples
of these localized events include hail storms, tornadoes, thunderstorms, heavy snow bands, micro-bursts, and fog events. Reasons for local
variation include elevation changes, vegetation changes, changes in the amount of urbanization, changes in latitude, changes in the nearness
to lakes/ocean, and the small area that many weather events take place over.
Local variations can have a significant impact on the high and low temperatures and also the precipitation probability and expected
accumulation. It is common for residence that live a little ways from the city to say for example “the temperature is often cooler
here than the city” and “the elevation is 500 feet higher in my city thus it is often cooler here and more snow falls”. The larger
a forecast area is, then the more likely a forecast is going to have problems if the same forecast is given for the entire area. Because
of local variations, the forecast area will often be broken up into regions where the forecast numbers are adjusted for each region
given the numerous local impacts that can influence the forecast. Two big influences are latitude and elevation. Temperatures tend
to be cooler with increasing elevation in many forecast situations (although the opposite can be the case when a shallow layer of
cold air pools into the lower elevations). Temperatures tend to be cooler in higher latitudes. Thus, for example, it is common for
the northern part of the viewing area in a state like Oklahoma to be cooler than the southern part of the viewing area. Elevation
can influence precipitation probabilities. For example, in some cases the precipitation chance will be higher closer to mountains
where topographic lifting takes place. It is important to keep local variations in mind when making a forecast for an area, especially
if the area is large in size. When making a forecast it can be important to emphasize the localized nature of certain weather events. This
will help residence to be aware of the potential of certain weather events such as a damaging storm without expecting damage everywhere
within the entire forecast area.
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