Weather Alert in Oregon
Red Flag Warning issued August 26 at 7:54AM PDT until August 26 at 9:00PM PDT by NWS Portland OR
AREAS AFFECTED: Northern Willamette National Forest; Southern Willamette National Forest
DESCRIPTION: * AFFECTED AREA...In Oregon, Fire Weather Zone 689 Northern Willamette National Forest and Fire Weather Zone 690 Southern Willamette National Forest. * THUNDERSTORMS...Isolated to scattered thunderstorms. Around a 25% chance of thunderstorms. 10-20% chance of wetting rains in passing storms. * DRY AND UNSTABLE...Unstable conditions with elevated mixing heights may promote pyroconvective activity. * OUTFLOW WINDS...Instantaneous gusts as high as 40 mph. Gusts at these speeds can be observed tens of miles away from the core of the thunderstorm. * IMPACTS...Conditions may be favorable for rapid fire spread which may threaten life and property. 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 Rain?
Home - Education - Precipitation - Rain
Next Topic: Shelf Clouds
Precipitation in the form of water droplets is called rain.
Rain generally has a tendency to fall with less intensity over a greater period
of time, and when rainfall is more severe it is usually less sustained.
Rain is the most common form of precipitation and happens with greater frequency
depending on the season and regional influences. Cities have been shown to have
an observable effect on rainfall, due to an effect called the urban heat island.
Compared to upwind, monthly rainfall between twenty and forty miles downwind of
cities is 30% greater.
Next Topic: Shelf Clouds
Weather Topic: What is Sleet?
Home - Education - Precipitation - Sleet
Next Topic: Snow
Sleet is a form of precipitation in which small ice pellets are the primary
components. These ice pellets are smaller and more translucent than hailstones,
and harder than graupel. Sleet is caused by specific atmospheric conditions and
therefore typically doesn't last for extended periods of time.
The condition which leads to sleet formation requires a warmer body of air to be
wedged in between two sub-freezing bodies of air. When snow falls through a warmer
layer of air it melts, and as it falls through the next sub-freezing body of air
it freezes again, forming ice pellets known as sleet. In some cases, water
droplets don't have time to freeze before reaching the surface and the result is
freezing rain.
Next Topic: Snow
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