FORECASTING ISOBARIC CONFIGURATION

LEARNING OBJECTIVES: Evaluate isobaric configuration in preparation of surface charts. Isobars may be constructed on the surface prognosis either by computing the central pressures for the high and low centers and numerous other points on the surface prognosis chart and drawing the isobars, or by moving the isobars in accordance with the surface pressure 3-hour tendencies and indications.

A thickness prognosis is used in constructing the isobaric configuration on your forecast. The steps for constructing forecasted isobaric configuration by using the thickness chart are as follows:

1. At a selected point, determine the difference between the present 500-hPa height and the forecasted 500-hPa height. A forecasted rise at the 500-hPa level is positive; a forecasted fall is negative.

2. At the same selected point, determine the difference between the current and the forecasted thickness value.

3. Algebraically subtract the difference of the forecasted thickness value from the forecast difference of the 500-hPa level.

4. Convert this difference in meters to hectopascals by using the expression 60 meters equal 7 1/2 hPa. Be sure to assign the proper sign.

5. Add (or subtract if the value is negative) this value to the current sea level pressure. This is the progged sea-level pressure.

6. Repeat steps 1 through 5 for all the points selected, and sketch the isobars.

EXTENDED WEATHER FORECASTS

LEARNING OBJECTIVES: Recognize the need for extended weather forecasts. While most routine forecasting does cover a short period of time, generally 12 to 36 hours, the forecaster will, upon occasion, be required to provide outlooks for extended periods of up to 7 days or longer. Fleet Numerical Meteorology and Oceanography Center (FNMOC) and the National Weather Service prepare facsimile products to aid the forecaster in providing extended forecasts. Of major importance to the forecaster in preparing an extended outlook without the benefit of facsimile prognostic charts is the determination of the type of flow (zonal or meridional) that can be expected to persist during the outlook period. With a zonal flow, a steady progression of pressure systems moving regularly can be anticipated. With the change to a meridional flow, the introduction of cold polar or arctic air into the lower latitudes and mixing with the warmer air will create a number of problems to be considered. Among these will be the development of new pressure systems, deepening or filling of present systems, and movement of the systems.

The forecaster should also become familiar with the particular areas that tend to trigger cyclogenesis and/or frontogenesis, and use this information when preparing his/her outlook. Many of the climatological publications that are readily available make note of these areas.

Many local area forecasters handbooks contain detailed information on how various weather situations approaching the station affect the station weather well in advance. These should be used when available.

In this chapter we first discussed the formation of new pressure systems, and available tools used for their interpretation. The next topic discussed was that of the forecasting of the movement of surface pressure systems; lows followed by high-pressure centers. We also discussed forecasting the intensity of surface pressure systems. Topics discussed were extrapolation, isallobaric indications, relation to frontal movements, and lastly, aloft indications of deepening and filling. We then discussed forecasting the movement and intensity of fronts by using extrapolation and the geostrophic wind methods. We also discussed the importance of and the procedures used in the construction of the isobaric configuration on forecasts. Lastly, we discussed extended weather forecasts.

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