Looking for an Edge: The Cyclone Model Comes in Handy
Once forecastersgain a degree of confidence in one or more of the medium-range models, they assess the positions and strenths of troughs and ridges on 500-mb charts. In the context of numerical weather prediction, we'll focus on 500-mb troughs, ostensibly because they promote the development of surface low-pressure systems.
Indeed, you learned that short-wave troughs play a pivotal role in cyclogenesis, while long-wave troughs just provide the cyclonic background on which short-wave troughs can do their thing. You can think of short waves as speedy tailbacks in football, while slower long waves resemble bulky offensive linemen; the long-wave troughs, as offensive linemen, open the way for the tailback short waves "score" by initiating cyclogenesis. Like tailbacks who carry the football, short-wave troughs possess vorticity maxima. By way of review, closed contours of absolute vorticity typically mark short-wave troughs (there's usually a well-defined vort max that's associated with a short-wave trough). In contrast, the pattern of absolute vorticity in a long-wave trough tends to be strung out and spread over a broad area. At the very most, any vorticity maxima are weak and diffuse, unless, of course, a short-wave trough travels through the long-wave trough. To confirm what we're saying, check out the 156-hour 500-mb forecast by the GFS initialized at 12Z on August 2, 2009 (valid at 00Z on August 9). Note the general lack of vort maxes in the long-wave trough over the West Coast. The vort max predicted to be off the coast of northern California, for example, was associated with a short-wave trough moving throug h the long-wave pattern.
Once forecasters locate short waves on a medium-range 500-mb prog (see the prog above), they look for the corresponding surface lows to the east of the upper-air vort maxes. Sometimes, the sea-level pressure pattern will have closed isobars indicating a well-developed low, as you can see on the prog below over the British Columbia Coast and in eastern Colorado. Other times, only a trough in the sea-level isobars will locate a weak or embryonic low. Note the area of "bagginess" in the sea-level isobars predicted to form over Minnesota and south-central Canada in response to a relatively weak 500-mb short wave over North Dakota (revisit the 500-mb prog above). Lesson learned: There wasn't any closed surfac e low to the east of this 500-mb trough, just a "baggy" trough in the sea-level isobars.
Once forecasters locate surface lows (and highs) located on medium-range progs, it's time to draw fronts. Forecasters look for the telltale signs, such as tight packing of thickness lines near surface troughs in the vicinity of lows, all the while keeping the cyclone model in mind. Upon completion, they are ready to make a forecast based on how the progs predict low- and high-pressure systems to approach or depart the local forecast area.
Even though we discourage using single, specific numbers for daytime highs and nighttime low temperatures in extended forecasts, meteorologists can still get a reasonable range for temperatures. The place to start is climatology. In other words, forecasters look at the city's average daily highs and lows. Then they adjust these averages up or down on any given forecast day, depending on the season and where the city lies in relation to traveling high and low-pressure systems and fronts. For example, if it's summer and the local forecast area lies to the north of an approaching warm front on day six of the medium-range forecast, then meteorologists will lean toward a cooler-than-average daytime high because they expect clouds and precipitation.
There are other forecasting features to glean from the medium-range models. For example, if the 500-mb contours are packed close together and strung in a general west-to-east direction, the flow will be speedy and zonal (like this fast 500-mb flow into the Pacific Northwest; note the several weak vorticity maxima embedded in the tight height gradient to the west of Washington and Oregon). Such a pattern favors numerous, weak, fast-moving, west-to-east traveling disturbances, leading to changeable weather. Because of the quick flow, confidence on timing of systems is low ("Fast flow, weatherman's woe").
If, however, the flow pattern at 500 mb is more meridional with areas of pronounced north-south flow of air (like this high-amplitude 500-mb pattern over the United States; note the compact but potent closed low over the Southeast), then forecast confidence in timing is higher, given that there usually are fewer, slower-moving weather systems. In this case, storms also tend to be stronger and tend to produce heavier precipitation.
Another powerful tool that helps meteorologists to gauge the consistency and degree of confidence in computer models is ensemble forecasting, a relatively new approach to short-range and medium-range forecasting.