Photo by Charley Knoderer

Procedure Descriptions:

Snowfall relative to Upper Air Features

Cook Method

Garcia Method

Magic Chart

Satellite Techniques

LEMO Technique

1. Synoptic Climatologies


Goree and Younkin 1966: Used 50 cases 1963-65 (including 50 developing or occluding which reached severe intensity). Good old fashioned study. Results are based on hand analysis using storm relative coordinates.

They found the most favorable location for the occurrence of heavy snow to be:
• 6.5-7 degrees latitude downstream of the 500mb vorticity maximum and 2.5 degrees to the left of its track during the following 12 hours.
• 5 degrees latitude along and 2.5 degrees to the left of the path of the surface low.
• Near the thickness ridge within the contour interval 5310 to 5370 gpm.


Browne & Younkin (1970): Used 81 heavy snow occurrences between 1965-67. Related heavy snow to the 850mb low center

• The 850mb isotherm that bisected heavy snowfall averaged -5 degrees C.
• In 94% of the cases, this initial bisecting isotherm was in the range -10-0C.
• The highest probability of heavy snow lies 90nmi to the left of the track of the 850mb low center.



2. Koontz "Heavy Snow Forecasting Aids"



• With an 850-700mb thickness of 1540-1555 m and 1000-850mb thickness less than 1300 m plus a surface temperature less than 0C, forecast moderate/ heavy freezing precipitation.
• The heaviest snow occurs along the 1000-500mb thickness ridge between 5310-5370m.
• The axis of heavy snow is 75-100nm to the left of the 2840 m thickness contour, provided the 850mb low is within 50nm of the 2840 m line. The heavy snow band is usually 150 nm wide.
• The farther south the 850mb low is from the 2840m line, the closer the heavy snow axis will be to the 2840 m line. The width decreases to less than 75 nm as well.
  
  
  
  
• Vorticity,
  
  
  
• Miscellaneous Rules of Thumb



• Using 850mb Features


If the 850mb low is:
______(A) miles ______(B) of the 2840m 1000-700mb thickness contour (approximately 0 deg C at 850), then heavy snow will fall ______(C) and the band will be ______(D) miles wide.



A	B	C	D
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



The South edge of heavy snow is usually 25 miles N of 0degC at 850mb. Moderate snow begins no more than 300 miles ahead of the surface low, or if snow begins more than 300 miles downstream of the low center, add 1" for each additional 100 miles beyond 300 miles to the storm total (this is most useful when a weak vorticity maximum moves ahead of the main one). If the 850mb temperature at the south edge of the snow band is -6degC to -10degC, subtract 30% from the expected storm totals. The average speed and direction of the 850mb low is to 68 degrees at 26 knots


• Using 700mb Features




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. 

• The strength of the low level system is reflected in the thermal pattern at 200mb. For a strong system, expect warm air (-40C to -45C) in the troughs and cold air (-65C) in ridges. May see temperatures in -50s for weaker systems.
  
  
• Average snow in inches from t=0 (present) to t=24 is 1/2 of the maximum "indicated warm advection" (in degrees C) at 200mb. Warm advection is defined as the difference between the warmest temperature at 200mb and the coldest temperature along the height contours within a distance of 15 degrees latitude (840nm) upstream of the forecast area.
  
  
  • If warm advection extends less than 6 degrees latitude (360nm) upstream from the forecast area, the precipitation is of short duration.
    
    
  • If cold advection is observed within 8 degrees latitude upstream of the forecast area at 700mb, divide the temperature difference by 4 instead of 2 to get the snowfall.

Location of snow:

• The maximum snowfall between t=0 to t=24 occurs near the coldest 200mb temperature downstream of the warmest temperature at t=0 (provided it is cold enough for snow).
  
  
• Snow bands are almost always parallel to the t=0 200mb contours or cross at a small angle from higher to lower values. The width of the snow band is frequently about the same as the distance [120m] between adjacent [height] contours on the 200mb chart.
  
  
• The southwest end of the heavy snowband is just downstream from "maximum warm advection" [warmest 200mb air] and the northeast end should be near the eastern side of the area of coldest 200mb temperature, or limited to 14 degrees latitude downstream from warmest 200mb air.

Type of precipitation:
see the temperature section under forecast ingredients.

Additional notes:

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.

Garcia Method