Figure 4-11.-Closed cells in a subtropical high. MIDDLE CLOUDS IN RELATION TO THE JETSTREAM Jets indicate as much individuality with respect to associated   weather   as   do   fronts.   Because   of   the individuality  of  jetstreams,  and  also  because  of  the individuality   of    each    situation    with    respect    to humidity   distribution   and   lower   level   circulation patterns,   statistically   stated   relationships   become somewhat vague and are of little value in forecasting. SHORT-RANGE EXTRAPOLATION LEARNING   OBJECTIVES:    Compare   short-range extrapolation techniques for the movement of frontal systems and associated weather. The  purpose  of  this  section  is  to  outline  several methods  that  are  particularly  suited  to  preparing forecasts   for   periods   of   6    hours    or    less.    The techniques   presented   are   based   on   extrapolation. Extrapolation is the estimating of the future value of some variable based on past values. Extrapolation is one of the most powerful short-range forecasting tools available to the forecaster. NEPHANALYSIS Nephanalysis   may   be   defined   as   any   form   of analysis of the field of cloud cover and/or type. Cloud observations received in synoptic codes permit only a highly generalized description of the actual structure of the cloud systems. Few forecasters make full use of the cloud reports plotted  on  their  surface  charts,  and  often,  the  first consideration in nephanalysis is to survey what cloud information  is  transmitted,  and  to  make  sure  that everything pertinent is plotted. For very short-range forecast, the  charts  at  6-,  12-,  and  24-hour  intervals are apt to be insufficient for use of the extrapolation techniques    explained    in    this    chapter.    Either nephanalysis or surface charts should then be plotted at   the   intermediate   times   from   3-hourly   synoptic reports or even from hourly sequences. An integrated system  of  forecasting  ceiling,  visibility,  cloud  cover, and precipitation should be considered simultaneously,   as   these   elements   are   physically dependent upon the same synoptic processes. With  present-day  satellite  capabilities,  it  is  rare that  a  nephanalysis  would  be  manually  performed. Instead,  the  surface  analysis  and  satellite  imagery will be used together. FRONTAL PRECIPITATION For  short-range  forecasting,  the  question  is  often not whether there will be any precipitation, but when will it begin or end. This   problem   is   well   suited   to   extrapolation methods.   For   short-range   forecasting,   the   use   of hourly   nephanalyses   often   serve   to   “pickup”   new precipitation  areas  forming  upstream  in  sufficient time to alert a downstream area. Also, the thickening and lowering of middle cloud decks generally indicate where an outbreak of precipitation may soon occur. Forecasting   the   Movement   of   Precipitation Areas by Isochrones The    areas    of    continuous,    intermittent,    and showery precipitation can be outlined on a large-scale 3-hourly or hourly synoptic chart in a manner similar to  the  customary  shading  of  precipitation  areas  on ordinary  synoptic  surface  weather  maps.  Different types of lines, shading, or symbols can distinguish the various  types  of  precipitation.  Isochrones  of  several hourly past positions of the lines of particular interest can then be added to the chart, and extrapolations for several hours 4-9


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