IAP 12.310
January 2008
Significant U.S. Weather Events
Santa Ana Winds (Jan 16-17, 2008)
Real-Time
Data:
http://www.wrh.noaa.gov/mesowest/mwmap.php?map=la
(Wind gusts in red)
A strong downslope wind event in Southern California, called the
Santa Ana winds, is forecasted to build through the day on Jan 16th
and peak early on Jan 17th (see figure below). During a Santa Ana,
the winds blow offshore from the desert to the ocean and can gain
terrific speeds as the winds pass through canyons and over mountain
ranges. It is not uncommon during a strong Santa Ana event for wind
gusts to exceed 100mph in some locales. Additionally, since the air
starts out at relatively high altitudes and travels generally
downhill toward the coast, it warms as the air descends and
compresses. Since the moisture content of the air is low to begin
with and remains constant, the relative humidity must subsequently
decrease. By the time the air reaches the coast, it is often warm
and very dry. This is why during the dry season, Santa Ana winds are
feared due to the extreme conflagrative conditions they provide.
High resolution Weather and Research Forecast (WRF)
model forecast for Jan 17, 2008 at 00Z. Vectors show the wind
velocity at 750m and the relative humidity is shaded. Note the
strong winds and low relative humidity over Southern California
associated with the forecasted strong Santa Ana event.
As high pressure builds over the Great Basin of the intermountain
West, a strong pressure gradient is achieved over Southern
California. As you now know, a strong pressure gradient implies
strong winds. However, there is something peculiar about these winds
in that they mostly blow across isobars, i.e. down the pressure
gradient. This is partially due to frictional effects, but since the
timescale of these winds is short compared to the rotation period of
the Earth, the Coriolis force plays a secondary role.
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