CHINOOK AND UPSLOPE EVENTS
METEOROLOGIST JEFF HABY
Chinook and upslope events can result in dramatic changes in weather in a short amount of time. A Chinook event has some similarities
to a dryline passage such as the dewpoint dropping and the wind coming from a higher elevation source. A Chinook is associated with
a strong downsloping wind. Air that sinks will decrease in relative humidity and increase in temperature. This can occur when air
in high elevation regions such as over the mountains, sinks down into the surrounding lower elevation regions. The effect can be
more pronounced when the wind speed is strong since there will be a fast and widespread replacement of the cold air with warmer
and drier air. These Chinook or downslope episodes can dramatically warm the temperature, quickly melt snow away that is on the
ground and dry the air. This can be welcome air in the winter since the air with a Chinook can replace very cold air that was
in place. This can make winter feel like spring for a day or so. The downsloping air can result in temperatures increasing by 10s
of degrees in a short time period.
An upslope event has the opposite impact. Rising air cools, helps produce clouds/precipitation and moistens the air. An upslope event
occurs when a persistent wind flows from lower elevation regions toward higher elevation regions (such as a mountain range). An
upslope event can bring flooding rains in the warm season and very heavy snows in the cool season. A slow moving low pressure
system that is advecting in moisture can produce many hours of favorable upslope conditions and thus heavy amounts of precipitation.
A weather forecaster looks for the number of hours upslope conditions will be in place (via wind direction), the moisture
associated with the air and how quickly the air is rising in order to assess the significance of the upslope event.