We’re back to reality after a nice warm-up earlier this week.
The only record broken during the “hot” spell on Tuesday, was in Wenatchee where the mercury rose to 84 degrees. Cold weather has returned, with overnight lows dipping below freezing once again. Average lows during this time of the year are in the upper 30s, but the average date for the last frost is not until around May 4 for this area. Believe it or not, frosty temperatures have occurred as late as mid-June, though it is extremely rare.
This time of the year, winter storms are pretty much behind us, and it’s still a bit early for much thunderstorm activity. One weather hazard that starts to make news this time of year is flooding, especially during periods of unseasonably warm temperatures. Rapid rises in area rivers can occur as spring rains are coupled with snow melt.
Not all floods are the same. River flooding occurs over a period of days, and the National Weather Service provides flood stage information as well as river crest forecasts, which give ample warning to those with interests along flood plains. Flood conditions can last for many days or even weeks, as folks in North Dakota along the Red River Valley can attest.
In contrast to river flooding, is flash flooding, which can occur within a few hours of a heavy rain event, and be over just hours after it starts. Flash flooding results in rapid rises in water along a stream, river, or even over normally dry land, and is extremely dangerous.
The causes of flash floods include heavy thunderstorms, ice jams, snow melt, and even dam or levee failures. Urban areas can be at risk when heavy rains overwhelm storm drains. Mountainous terrain can prove deadly with rapidly descending water and mudslides.
According to Mike Vescio, meteorologist in charge at the National Weather Service in Pendleton, Ore., one of the worst flash flood events in American history occurred back in June 1903 in the town of Heppner, in north-central Oregon. Heavy rains produced raging waters in Willow Creek, which swept through the town, destroying it and claiming 250 lives.
Since 1986, more than 700 floods and flash floods have been reported across Washington, Idaho and Oregon. Most fatalities due to flash flooding are automobile related. It only takes a few inches of water to sweep away a car, and it is difficult to gauge the depth of water when it covers the roadway. It’s even possible that the road has been washed away underneath. If you have Internet access, a dramatic picture of what can be lurking under a water covered road, can be found at www.wrh.noaa.gov/ pqr/seasonal/springmon.php.
When heavy rains cause small streams to swell or flooding in low lying areas, but dangerous “flash flooding” is not expected, the National Weather Service will issue an urban and small stream flood advisory, to inform the public of potentially hazardous conditions.