categories, it will be advected with about 50 percent of the wind component normal to the isotherm. Note in all three  cases  above,  the  movement  is  related  to  the contour pattern. l  Cold  Advection  area.  Advect  the  point  with approximately 75 to 80 percent of the wind component normal to it. In the case of a slow moving, closed low at the 850-hPa level, the 0°C isotherm will move eastward with respect to the closed low as cold air is advected around the low. MOVEMENT   OF   THE   850-hPa   0°C WET-BULB ISOTHERM.— The wet-bulb temperature can be forecast by the above procedures and rules.  Remember  that  the  wet-bulb  temperature  is dependent  upon  dewpoint  as  well  as  the  temperature. The dewpoint will be advected with the winds at nearly the  full  velocity,  whereas  the  temperature  under nonsaturated  conditions  moves  slower.  The  following observations with respect to the 0°C wet-bulb isotherm after saturation is reached may help: . The 0°C wet-bulb isotherm does not move far offshore in the Gulf and the Atlantic because of upward vertical motion in the cold air over the warmer water. . If the 0°C wet-bulb isotherm lies in a ribbon of closely packed isotherms, movement is slow. . Extrapolation works well on troughs and ridges. After the forecast of the surface and 850-hPa level temperature and dewpoint values are made, you are ready  to  convert  these  values  to  their  respective wet-bulb temperature. The following procedures are recommended l Use figure 4-23, views (A) and (B), to compute the wet-bulb temperatures for the 1,000- and 850-hPa levels, respectively. (The surface chart may be used for the  1,000-hPa  level.)  Admittedly,  the  wet-bulb temperatures at just these two levels do not give a complete picture of the actual distribution of moisture and temperature, and error is introduced when values are changing rapidly, but these are values the forecaster can  work  with  and  predict  with  reasonable  accuracy. . Refer to figure 4-24. From the surface wet-bulb temperature  at  the  bottom,  go  up  vertically  until  you intersect  the  computed  850-hPa  level  wet-bulb temperature to the left. This intersection indicates the form of precipitation that can be expected. A necessary assumption for use of this graph is that the wet-bulb temperatures at these two levels can be predicted with reasonable accuracy. Known  factors  affecting  the wet-bulb temperature at any particular station should be carefully  considered  before  entering  the  graph.  Some of the known factors are elevation, proximity to warm bodies of water, known layers of warm air above or below the 850-hPa level, etc. Area “A” on the graph calls for a rain forecast, area “B” for a freezing rain forecast, and area “C” for a snow forecast. Area “D” is not so clear cut because it is an overlap portion of the graph;  however,  wet  snow  or  rain  and  snow  mixed predominate  in  this  area.  Sleet  occurring  by  itself  for more than 1 or 2 hours is rare, and should be forecast with  caution. FORECASTING THE AREA OF MAXIMUM  SNOWFALL The intent of this section is to introduce the patterns associated  with  maximum  snowfall  and  to  present techniques  for  predicting  the  areas  where  snowstorms are likely to appear. Synoptic Types There  are  four  distinct  types  of  synoptic  patterns with associated maximum snow area. BLIZZARD   TYPE.—   The   synoptic   situation features an occluding low. In the majority of cases, the “wrapped around” high pressure and ridges are present. The track of the low is north of 40°N, and its speed, which  initially  may  be  average  or  about  25  knots, decreases  into  the  slow  category  during  the  occluding process. In practically all cases, a cold closed low at the 500-hPa level is present and captures the surface low in 24 to 36 hours. The area of maximum snowfall lies to the left of the track. At any particular position, the area is located from due north to west of the low-pressure center. When this type occurs on the east coast with its large temperature contrast  and  high  moisture  availability,  heavy  snowfall may occur. The western edge of the maximum area is limited by the 700-hPa level trough, or low center, and the end of all snow occurs with the passage of the 500-hPa level trough or low center. MAJOR   STORM   AND   NONOCCLUDING LOWS.—  The  synoptic  situation  consists  of  a nonoccluding wave-type low. The track of the low or wave is south of 40° latitude, and its speed is at least the average of 25 knots, often falling into the fast-moving category. The upper-air picture is one of fast-moving troughs,  generally  open,  but  on  occasion  could  have  a 4-27