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Forecasting Technique: Know your station, and know your trajectories.
First, you must decide whether it's going to be a clear, calm night. If so, you need to know how well the particular station "radiates", or forms a temperature inversion with very cold air at the surface. The best way to tell is through experience: what happened to the temperature at night under similar circumstances in the past?
During the day, or if the night will be cloudy or windy, the air arriving at a station will have passed over several tens (or hundreds) of miles of territory. Was it land or over water? Was it over the city or across the countryside? Subtle differences in the wind direction at a station may lead to a change of several degrees in the daytime or nighttime temperature, even if the large-scale weather conditions haven't changed much.The objective techniques derived from numerical models are not very good at taking these effects into account. They tend to have a very "blurry" idea of what's going on at the local scale. It therefore makes sense to adjust your forecast by a few degrees if there's a significant change in land characteristics coming into play.Here's a question that brings together everything else in the module: In the hypothetical city shown here, which wind direction would correspond to the largest diurnal temperature variation? That is, when would you have the biggest difference between the minimum temperature and the maximum temperature?
How much to adjust temperatures? Again, experience is the best guide. If it's a coastal station and you're expecting an overwater trajectory, you should always check the water temperature. While we receive some reports of water temperatures by computer from buoys on the oceans and Great Lakes, the most efficient way of checking water temperatures is by looking at a map of sea surface temperatures on the Internet. These maps also incorporate satellite data into their analyses (recall from the satellite tutorial that IR imagery can detect sea-surface temperatures). One site for sea surface temperature maps is http://www.ospo.noaa.gov/Products/ocean/sst/contour/. Often, you will find a sea-surface temperature map that has the info you need. Otherwise, check the North American surface weather maps. The offshore stations report the temperature (in Fahrenheit) to the lower left of the station location.
If you're worried about a change in land type, or the like, again experience is your best bet. If you're lucky, you can find nearby stations that experienced similar air mass trajectories the previous night or a few nights ago. Otherwise, you're on your own. Play it conservative, and learn from the weather when it happens.
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