OBJECTIVES OF CHART ANALYSIS
As an analyst youíll be expected to take a com-plex maze of observed synoptic data and perform the following tasks:
1. Delineate the current state of the atmos-phere using lines (isopleths) and/or symbols. This delineation must be made according to accepted analysis standards.
2. Determine what particular atmospheric processes are involved in the production of the various kinds of reported weather.
3. Complete the analysis in the least amount of time to facilitate its use for briefing and forecasting purposes.
Objectives one and three can readily be accom-plished with practice and experience. However, objective two cannot be properly accomplished without a three-dimensional model of the atmos-phere. This model is developed using upper air analysis in conjunction with the surface chart. Upper air charts and their analysis are covered in unit 8.
Accurate prognoses and forecasts donít just happen. They are primarily the result of successful analysis. The successful analyst doesnít just hap-pen either. The better analysts are usually the most experienced, but we all begin as amateurs. To help you along as you gain experience, Iíve listed the following guidelines:
Review past history
Question and evaluate doubtful data
Look at more than the level in question
Use satellite information
Be able to support your final product REMEMBER: All prognoses and forecasts stem from analyses. Whether those analyses are good or bad determines the quality of weather predictions.
Reviewing Past History
Before ever sitting down and putting your "analystís pencil" to the chart, you should ex-amine the most recent analysis along with several previous analyses to establish continuity. Know-ing previous positions of fronts and pressure systems is your first clue toward locating these and other features on your chart. In most weather offices, past history is traced onto the current chart before the chart is ever plotted. Review and use it. It will give you a basis from which to begin analyzing the latest information.
Evaluation of Data
There is always the possibility of an error in plotted reports because of faulty instrumentation, observation, transmission, or plotting. Also, even though correct, surface reports are sometimes not representative of the surrounding area or of con-ditions aloft. You should understand diurnal ef-fects and be constantly on the alert for errors and nonrepresentative data; the quality and usefulness of the analysis depends on your judgment and keenness of perception when interpreting reports. If your plotter is well trained, experienced, and conscientious, many errors will be spotted and corrected before your analysis begins.
NOTE: The plotter must always check with you before making any changes to reports. The types of errors frequently caught before the actual analysis are as follows: communication garbles or errors, continuing pressure errors in successive ship reports, pressure tendencies that do not agree with reported pressures, and ship winds that do not correspond to the magnitude or direction of the sea waves.
DIURNAL VARIATIONS.óOur weather is constantly changing, and even when it appears to repeat itself day in and day out, changes are occurring. The changes that occur in the daily 24-hour cycle are known as diurnal variations. These variations must be known to more effec-tively evaluate the weather data on the surface chart .
. Temperatures reach their maximum in midafternoon and their minimum near sunrise. The swing in temperatures is much greater over land than at sea, and in general, as the distance from water increases, the average diurnal temperature range also increases.
. Pressure variations in low- and mid-latitudes are characterized by two maxima oc-curring near 1000 and 2200 and two minima occurring near 0400 and 1600. They are greatest and more uniform near the equator. In fact, the 3-hour pressure tendencies in the tropics are of little value because the diurnal variation is so great; diurnal changes completely mask the much smaller synoptic pressure changes.
. Wind velocities reach their maximum and minimum during early afternoon and early morning respectively over land. Over the oceans, the diurnal effect is considered minimal because of the small diurnal change of seawater temperatures. Along coasts and lakeshores, pressure differences created by heating and cool-ing set up what we know as sea and land breezes. The sea breeze reaches its maximum in early or middle afternoon and is normally stronger than its counterpart, the land breeze, which occurs before sunrise.
. Precipitation at most inland stations, par-ticularly those in the tropics, shows a maximum in the afternoon. This coincides with the time of maximum convective activity. Over tropical oceans, coasts and islands maximum precipitation occurs at night.