Cold Wave A  forecast  of  a  cold  wave  gives  warning  of  an impending  severe  change  to  much  colder  temperatures. In the United States, it is defined as a net temperature drop  of  20°F  or  more  in  24  hours  to  a  prescribed minimum that varies with geographical location and time of the year. Some of the prerequisites for a cold wave  over  the  United  States  are  continental  Polar,  or Arctic  air  with  temperatures  below  average  over  west central Canada, movement of a low eastward from the Continental  Divide  that  ushers  in  the  cold  wave,  and large pressure tendencies on the order of 3 to 4 hPa occurring behind the cold front. Aloft, a ridge of high pressure develops over the western portion of the United States or just off the coast. An increase in intensity of the   southwesterly   flow   over   the   eastern   Pacific frequently  precedes  the  intensifying  of  the  ridge. Frequently, retrogression of the long waves takes place. In  any  case,  strong  northerly  to  northwesterly  flow  is established aloft and sets the continental Polar or Arctic air mass in motion. When two polar outbreaks rapidly follow one another, the second outbreak usually moves faster  and  overspreads  the  Central  States.  It  also penetrates farther southward than the first cold wave. In such  cases,  the  resistance  of  the  southerly  winds  ahead of the second front is shallow. At middle and upper levels,  winds  remain  west  to  northwest,  and  the  long wave trough is situated near 80° west. Most  cold  waves  do  not  persist.  Temperatures moderate  after  about  48  hours.  Sometimes,  however, the upper ridge over the western portion of the United States and the trough over the eastern portion are quasi-stationary, and a large supply of very cold air remains in Canada. Then, we experience successive outbreaks   with   northwest   steering   that   hold temperatures well below normal for as long as 2 weeks. Heat Wave In the summer months, heat wave forecasts furnish a  warning  that  very  unpleasant  conditions  are impending.  The  definition  of  a  heat  wave  varies  from place to place. For example, in the Chicago area, a heat wave is said to exist when the temperature rise above 90°F on 3 successive days. In addition, there are many summer  days  that  do  not  quite  reach  this  requirement, but are highly unpleasant because of humidity. Heat  waves  develop  over  the  Midwestern  and eastern  portions  of  the  United  States  when  along  wave trough stagnates over the Rockies or the Plains states, and along wave ridge lies over or just off the east coast. The belt of westerlies are centered far to the north in Canada.  At  the  surface  we  observe  a  sluggish  and poorly  organized  low-pressure  system  over  the  Great Plains or Rocky Mountains. Pressure usually is above normal over the South Atlantic, and frequently the Middle  Atlantic  states.  An  exception  occurs  when  the amplitude of the long wave pattern aloft becomes very great.  Then,  several  anticyclonic  centers  may  develop in the eastern ridge, both at upper levels and at the surface. Frequently, they are seen first at 500 hPa. Between these highs we see formation of east-west shear lines situated in the vicinity of 38° to 40°N. North of this line winds blow from the northeast and bring cool air from the Hudson Bay into the northern part of the United  States.  A  general  heat  wave  continues  until  the long wave train begins to move. SUMMARY In  this  chapter  we  discussed  condensation  and precipitation   producing   processes.   Following   a discussion  on  condensation  and  precipitation  producing processes,  we  then  covered  condensation  and precipitation  dissipation  processes.  Forecasting  of frontal   clouds   and   weather   was   then   discussed, including   the   topics   of   frontal   cloudiness   and precipitation, air mass cloudiness and precipitation, vertical motion and weather, vorticity and precipitation, and middle clouds in relation to the jetstream. We then covered short-range extrapolation techniques, which included use of the nephanalysis, frontal precipitation, lowering of ceilings in continuous rain areas, the trend chart as an aid, and the time-liner as an aid. A discussion of  cloud  layer  analysis  and  forecasting  was  then presented  along  with  the  importance  of  RAOB  use  in cloud analysis and identification, the humidity field, a 500-hPa level analysis of the dewpoint depression, a three-dimensional   analysis   of   the   moist   layer, precipitation  and  clouds,  and  cirrus  indications.  A discussion  of  the  prediction  of  snow  versus  rain followed.  Topics  presented  were  geographical  and seasonal considerations, the physical nature of the problem,  general  synoptic  considerations,  forecasting techniques, and areas of maximum snowfall. The last topics  of  discussion  were  factors  affecting  temperature, and the forecasting of temperatures during special situations. 4-32