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Winter Weather Forecasting
Loaded Gun Chasing
website of storm chaser Mikey Gribble
-This page is my notes on winter weather forecasting. The vast majority of this information is from
Tim Vasquez's Weather Forecasting Handbook, which is an exceptional book if you are in the early
stages of learning how to forecast.
Rules of Thumb (Vasquez)
- solids that pass through a warm layer depth of at least 600ft. will melt partially
- solids that pass through a warm layer depth of 1,200ft. will melt completely
- liquid that passes through a cold layer depth of 800ft. will freeze
- liquid that refreezes will always form a frozen droplet. Snow is not formed in this manner

Snow Through a Warm Layer (Vasquez)
(this section is referring to layers of warm air near the surface that snow falls through)
- warm air layer > 1,200ft. = most likely rain
warm air layer of 900ft. = 50% chance snow 50% rain
warm air layer of 700ft. = 70% chance snow
warm air layer of 300ft. = 90% chance snow
- the freezing level often lowers 500-1,000ft. during the first 1-2 hours after the precipitation begins
falling and may rise again to its original level 3 hours after the layer becomes saturated
- be wary of evaporational cooling in the lower dry levels
- wet bulb temperature is usually about halfway between the temperature and the dewpoint
- it has been shown that 0.38 inches of liquid equivalent precipitation can contribute 8 degrees F of
cooling towards an air parcel's wet bulb temperature
Ice Multiplication (Vasquez)
(also known as riming or accretion)
- when temperatures in a column above a station have fallen below -10 degrees C, this indicates that
ice multiplication will cease and measurable snowfall amounts will taper off

Aggregation (Vasquez)
- (quote from Weather Forecasting Handbook) When only one type of ice particle (dendrite column
etc) is in a cloud, the particles tend to fall at similar speeds. However when there are multiple types
of ice particles, fall speeds are varied and the chance of ice crystal collision is greatly enhanced. This
results in growth of the ice crystal, turning it from an ice crystal into a snowflake and making it vastly
larger by the time it reaches the ground. The best way to produce multiple types of ice particles (and
to produce snowflakes) is to have a broad range of temperatures within the saturated layer
(preferably the full range from 0-20 degrees C). Furthermore, it has been found that an isothermal
layer at about -5 to 0 degrees C extending through 1,000ft. or more can further boost aggregation
of ice crystals, resulting in very large snowflakes. This is because snowflakes tend to be "sticky" at
these warmer temperatures".
Rain Snow Line
A quick way to guesstimate the rain snow change over line when forecasting is to use the 540dam
line. It will typically be labeled on surface charts produced by models. This line is usually pretty close
to the transition from rain to snow.