|50||N or S||90 miles N of 0 deg C at 850||150|
|150||N||along the track of the low||150|
|50-150||N||100-200 miles N of 0 deg C at 850||150|
|50-150||N||50-75 miles N of 0 deg C at 850||0-75|
Types of Patterns: A. Closed low at 700mb (at standard 60m height intervals). B. Weakness area (light wind area where a split in the westerlies has occurred, and there is obviously a vorticity maximum). C. Open troughs, without a clear vorticity maximum. A. Closed low For current and 24 hour prognostic charts overlay (12-hourly position): a. Positions of the center point, inner, and outer closed contours of the 700mb low. b. 850mb zero and -2 degree C isotherms Determine frozen precipitation in the ensuing 24 hours as follows: - The Eastern extent determined by current position of the 700mb ridge line downstream starting time determined by progged positions of this ridge line. - The Northern extent of the snowfall usually lies along or just south of the path of the northernmost points on the outer 700mb closed contour. - The Southern limit of steady precipitation (snow or rain) normally lies along or just north of the southern-most point on the path traveled by the inner closed contour of the 700mb low. - Snow ends west of the 700mb low (cut-off points) in the southern quadrant with the passage of the N-S center line through the 700mb low and in the northern quadrants with the passage of the outermost closed 700mb contour. - Extent and intensity of precipitation: *Southernmost band of frozen precipitation occurs as light snow or mixed types in the area lying between the zero and -2 850mb isotherms. *Rain, or snow quickly changing to rain will occur in the area south of the zero 850mb isotherm as the 700mb low approaches *Partition the remaining precipitation north and east of the low into a light snow band (along the northern edge, roughly as wide as the light snow band between the 0 and -2 isotherms) and a moderate to heavy region between the two light bands. B. Weakness Area The northern limit of the snowfall is at the point where the split in the 700mb flow occurs. The associated moderate to heavy snow band, if it is present, is narrower in width and of shorter duration than that for closed 700mb lows. C. Open Troughs Similar to a closed low, with the following exceptions: - The snow band is usually narrow (100-150 miles) but wide variations are common. - Any moderate to heavy snow that does occur will be of short duration and is confined to a limited area. - The probability of moderate to heavy snow decreases as the overall precipitation area increases. - Rapid changes in precipitation pattern may take place if a closed low forms at some point in the 700mb trough.
Location of snow:
Type of precipitation:
see the temperature section under forecast ingredients.
Cloudiness and precipitation increase under warm advection and decrease under cold advection at 200mb. One need not to worry about moisture in developing storms. If the dynamics are strong, moisture will find its way into the storm. Initially dry conditions serve only to delay the precipitation a few hours. Cold advection in mid-troposphere (700mb) indicates downward motion and warm advection upward motion. use this to adjust the forecast derived from the 200mb chart. Warm air at 200mb reflects cold air at 500mb. If the storm is not strong enough to affect the 200mb temps, heavy snow usually doesn't occur. Expect continued digging if the cold pocket at 200mb is southeast of 500mb low; expect recurvature if the cold pocket at 200mb is northeast of 500mb low. The vorticity maximum at 500mb tends to move toward 200mb cold pockets. If the 200mb cold pocket develops northeast of a 500mb low, the low will recurve in 24 hours. The 200mb warm pocket usually coincides with 500mb vorticity maximum, especially in well-developed systems. The direction of movement of a vorticity maximum at t = 12 to t = 36 hours will be parallel to a line connecting 200mb warm and cold pockets at t = 0. If the 200mb warm pocket is ill-defined or shows an elongated warm sector then the 500mb vorticity maximum is used as the 200mb warm pocket. Limitations: Do not use for large scale cyclonic flow over North America with short waves moving through or cutoff lows in SW US that have been stationary for 24 hours. For winter storms only (mid october - march 10). Don't use near lakes, hills, mountains.