CHAPTER 3 FORECASTING SURFACE SYSTEMS

CHAPTER 3 FORECASTING SURFACE SYSTEMS With the upper air prognosis completed, the next step is to construct the surface prognostic chart Since more data is available for the surface chart, and this chart is chiefly the one on which you, the Aerographer’s Mate, will base your forecast, you should carefully construct the prognosis of this chart to give the most accurate picture possible for the ensuing period. The surface prognosis may be constructed for periods up to 72 hours, but normally the period is 36 hours or less. In local terminal forecasting, the period may range from 1 to 6 hours. Construction of the surface prognosis consists of the following three main tasks. 1. Progging the formation, dissipation, movement, and intensity of pressure systems. 2. Progging the formation, dissipation, movement, and intensity of fronts. 3. Progging the pressure pattern; that is, the isobaric configuration and gradient. From an accurate forecast of the foregoing features, you should be able to forecast the weather phenomena to be expected over the area of interest for the forecast period. FORECASTING THE FORMATION OF NEW PRESSURE SYSTEMS LEARNING OBJECTIVES Recognize features on satellite imagery and upper air charts conducive to the formation of new pressure systems. The central problem of surface prognosis is to predict the formation of new low-pressure centers. This problem is so interrelated to the deepening of lows, that both problems are considered simultaneously when and where applicable. This problem mainly evolves into two categories. One is the distribution of fronts and air masses in the low troposphere, and the other is the velocity distribution in the middle and high troposphere. The rules applicable to these two conditions are discussed when and where appropriate. For the principal indications of cyclogenesis, frontogenesis, and windflow at upper levels, refer to the AG2 TRAMAN, volume 1. The use of hand drawn analyses and prognostic charts in forecasting the development of new pressure systems is in many cases too time-consuming. In most instances, the forecaster will generally rely on satellite imagery or computer drawn prognostic charts. SATELLITE IMAGERY — THE FORMATION OF NEW PRESSURE SYSTEMS To most effectively use satellite imagery, the forecaster must be thoroughly familiar with imagery interpretation. Also, the forecaster must be able to associate these images with the corresponding surface phenomena. The texts, Satellite Imagery Interpretation in Synoptic and Mesoscale Meteorology, NAVEDTRA 40950, and A Workbook on Tropical Cloud Systems Observed in Satellite Imagery, volume 1, NAVEDTRA 40970, contain useful information for the forecaster on the subject of satellite interpretation. The Naval Technical Training Unit at Keesler AFB, Mississippi, also offers the supplemental 2-week course, Weather Satellite Systems and Photo Interpretation (SAT INTERP). The widespread cloud patterns produced by cyclonic disturbances represent the combined effect of active condensation from upward vertical motion and horizontal advection of clouds. Storm dynamics restrict the production of clouds to those areas within a storm where extensive upward vertical motion or active convection is taking place. For disturbances in their early stages, upward vertical motion is the predominant factor that controls cloud distribution. The comma-shaped cloud formation that precedes an upper tropospheric vorticity maximum is an example. Here, the clouds are closely related to the upward motion produced by positive vorticity advection (PVA). In 3-1