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Features January 2012 Issue

Forecast-Wind Follies

Case Study 2: A Cold Front Can Blow the Temps

A strong cold front at the right location can wreak havoc with the temperature forecast in the official FBWinds. For example, a 300-mile IFR flight during the morning from Rochester, Minn., (KRST) to Topeka, Kan., (KTOP) passes right over the Mason City, Iowa (MCW) winds aloft station shortly after departure. Based on the 0900 UTC surface analysis chart, a cold front is approaching from the northwest and is positioned just to the west of the proposed route.

Shown here is a partial list of the latest FBWinds bulletin, including MCW located in the last row. Given cloud cover over northern Iowa, structural icing is a concern. Based on this forecast, the freezing level over Mason City is expected to be at 12,000 feet. Therefore, a proposed altitude of 10,000 feet should be sufficient to stay below any icing potential. That’s great, but the departure isn’t until 1430 UTC.

Here’s the problem: The FBWinds bulletin is valid at 1200 UTC, but it is considered “for use” between 0800 and 1500 UTC. With a strong cold front such as this, two or more hours can make a significant difference in the freezing level given a departure that is over two hours beyond the valid time of the forecast.

Instead, examine the temperature profile on a Skew-T log (p) diagram. Shown here is the RUC model one-hour forecast sounding for Mason City valid at 1200 UTC. The zero-degree isotherm is highlighted here in red for clarity.

Notice the forecast environmental temperature shown in red crosses the zero-degree isotherm around 12,000 feet MSL. This matches the temperature in the official FBWinds forecast which is also valid at 1200 UTC. So far, so good.
However, as the cold front passes through the Mason City station shortly after 1200 UTC, the temperature aloft will begin to plummet. The RUC model has a much better temporal resolution than the FBWinds forecast, given that it is refreshed hourly and provides hourly forecasts out to 18 hours.

In fact, the forecast sounding for Mason City valid at 1500 UTC shows the environmental temperature crossing the zero-degree isotherm around 9000 feet MSL. So where the closest time window on the FBWinds predict +7 degrees C, the RUC shows it’s more likely to be zero degrees. That could make a significant difference in your flight experience. —S.D.

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