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SNOW
Snow storms are fairly common in the Inland Northwest from late autumn through
spring. Wintry weather can be a combination of snow, rain or freezing precipitation.
If strong winds accompany the precipitation, hazardous conditions can result
due to blowing and drifting snow or bitter cold wind chill factors.
Snow forms in sub-freezing
clouds as supercooled water droplets transform directly into ice crystals. These
ice crystals keep their hexagonal shape as they grow larger and heavier, and
finally fall to the surface. As long as surface temperatures remain below freezing,
snow will begin to accumulate on the ground.
The NWS issues Snow Advisories
for the Columbia Basin and the valleys for 2-3 inches of snow in 12 hours or
3-5 inches of snow in 24 hours. In the east slopes of the Cascades, snow advisories
are issued for 6-11 inches of snow in 12 hours or 6-17 inches of snow in 24
hours.
The NWS has different criteria
for heavy snow for different areas of the Inland Northwest. A heavy snow warning
is required if 4+ inches of snow in 12 hours or 6+ inches of snow in 24 are
expected in the Columbia Basin and the valleys. For the Northeast Mountains,
the Panhandle mountains and the Okanogan Highlands, 8+ inches of snow in 12
hours or 12+ inches of snow in 24 hours prompts a heavy snow warning. A heavy
snow warning for the east slopes of the Cascades requires 12+ inches of snow
in 12 hours or 18+ inches of snow in 24 hours. In the Blue Mountains, 12 + inches
of snow in 24 hours is needed for a heavy snow warning.
Blizzard conditions exist when sustained winds of 50 mph or more with considerable
falling and drifting snow, causing visibilities to drop to near 1/4 mile for
at least 3 hours. Though there are no temperature requirement, temperatures
that fall below 20 degrees F under these conditions can lead to life-threatening,
sub-zero wind chill readings.
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