Weather Alert in Oregon
Red Flag Warning issued August 25 at 9:06PM PDT until August 26 at 9:00PM PDT by NWS Portland OR
AREAS AFFECTED: Northern Willamette National Forest; Southern Willamette National Forest
DESCRIPTION: The National Weather Service in Portland has issued a Red Flag Warning for thunderstorms and high instability, which is in effect from 9 AM to 9 PM PDT Tuesday. * AFFECTED AREA...In Oregon, Fire Weather Zone 689 Northern Willamette National Forest and Fire Weather Zone 690 Southern Willamette National Forest. * THUNDERSTORMS...Scattered thunderstorms with critically dry fuels. * OUTFLOW WINDS...Gusts up to 40 mph may travel outward up to 25 miles from thunderstorm cores. * IMPACTS...Thunderstorm outflow winds may result in unanticipated erratic fire spread. High instability may result in fire growth. Frequent lightning and critically dry fuels may result in fire starts.
INSTRUCTION: A Red Flag Warning means that critical fire weather conditions are either occurring now, or will shortly. A combination of the above conditions can contribute to extreme fire behavior.
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Weather Topic: What is Precipitation?
Home - Education - Precipitation - Precipitation
Next Topic: Rain
Precipitation can refer to many different forms of water that
may fall from clouds. Precipitation occurs after a cloud has become saturated to
the point where its water particles are more dense than the air below the cloud.
In most cases, precipitation will reach the ground, but it is not uncommon for
precipitation to evaporate before it reaches the earth's surface.
When precipitation evaporates before it contacts the ground it is called Virga.
Graupel, hail, sleet, rain, drizzle, and snow are forms of precipitation, but fog
and mist are not considered precipitation because the water vapor which
constitutes them isn't dense enough to fall to the ground.
Next Topic: Rain
Weather Topic: What are Shelf Clouds?
Home - Education - Cloud Types - Shelf Clouds
Next Topic: Sleet
A shelf cloud is similar to a wall cloud, but forms at the front
of a storm cloud, instead of at the rear, where wall clouds form.
A shelf cloud is caused by a series of events set into motion by the advancing
storm; first, cool air settles along the ground where precipitation has just fallen.
As the cool air is brought in, the warmer air is displaced, and rises above it,
because it is less dense. When the warmer air reaches the bottom of the storm cloud,
it begins to cool again, and the resulting condensation is a visible shelf cloud.
Next Topic: Sleet
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