SURFACE FRONTAL ANALYSISTHUMB RULES
If one previous location of a front is known,its current position is downwind (referring to the wind in the cold air) at a distance, depending roughly on the speed of the wind in the cold air. Generally, cold fronts are found to the east and/or southeast of previous positions, warm fronts to the north and/or east of previous posi-tions, and quasi-stationary fronts in about the same position as before.
When the isobars on the previous map areparallel to the front, very little, if any, movement is expected.
When two successive previous positions areknown, an approximate past rate of frontal move-ment is established. In this case the quickest method of locating the probable current position is to move the front the same distance as indicated by the past movement.
Do not make major adjustments in locationfrom what would be expected by past history unless the wind and/or pressure gradient in the cold air has changed a proportional amount. Probably the most important criterion of a front in flat terrain (oceans included) is the cyclonic wind shift. Do not be misled by nonrepresentative winds such as those along coasts, large lakes, in valleys, and so forth, and directions of light winds (under 10 knots) on morning maps.
For a moving front, 3-hour pressure tendencydifferences over land are very helpful, especially in mountain areas where unrepresentative pres-sures exist. From a moving ship, they are of no value unless a dependable correction for the movement of the ship is made.
Pressure tendency characteristics may be oflittle help, particularly in the summer. Characteristics of the barograph trace (3 hour) may be helpful when the amount of the change is large by comparison with diurnal effects, dynamic effects of convective activity, and so forth. The test of usefulness is if the particular characteristic is organized along a line. Temperature and dew-point values are likely to be unrepresentative near a front due to horizontzl mixing, heating or cooling from below, and evaporation from below near the shallow edge of the cold air mass. Temperatures and dew points bordering on the cold air side of the front may closely approach those of the warm side. In these circumstances the stationís wind direction, if representative, is usually the determining factor. Surface temperatures are generally of little value in frontal analysis on morning maps and are most representative on afternoon or evening maps. Temperature or dew-point gradients are more reliable with weak gradients (homogeneity) in the warm air mass and strong gradients on the cold air side of the front.
Formation and Dissipation of Fronts
Scan the map for indications of frontogenesis and frontolysis. Keep in mind that certain geographical areas, during specified periods of the year, are favorable for the formation or dissipation of fronts. Also, certain portions of frontal systems are favorable for frontogenesis, such as the trailing edges of long cold fronts that are oriented east-west, and the isobars are either anticyclonically curved or parallel to the front.
Application of Computer Products
Just as it is important to refer to satellitepictures, the computer surface chart should also be used during the initial phase of the analysis and should be compared with the finished map before finalizing. Frequently, these charts are used in place of the hand-drawn analysis, because of per-sonnel limitations and limitations in the avail-ability of data.
Common Errors in Frontal Analysis
Some of the more common errors in frontalanalysis are listed below:
1. Inconsistent displacements from previouspositions
2. Cold fronts improperly designated as warmfronts and vice versa
3. Too many fronts, particularly secondaryfronts
4. Isobars too sharply kinked at fronts orkinked improperly toward low pressure
5. Frontal patterns in the horizontal whichhave an impossible three-dimensional structure
6. Use of unrepresentative data (particu-larly temperature) in locating fronts
7. Dropping of fronts in areas of sparse or no reports without designating frontolysis on preceding chart or charts
8. Ignoring past history and not continuingto indicate fronts that may appear weak or nonex-istent due to daytime heating
9. Not using available satellite data to aid inthe frontal placement
NOTE: Before you proceed to the next lesson("Finalizing the Analysis"), obtain a copy of the latest practical training publications:
Cold Front Analysis
Warm Front Analysis
Occluded Front Analysis
These practical training publications are excellent training devices that will provide you with hands-on training in frontal analysis.