National Flood Awareness Week
By Mike McLane, Senior Service Hydrologist
By Mike McLane, Senior Service Hydrologist
March 12-16th has been designated National Flood Awareness Week. The purpose of this campaign is to increase public awareness of the many ways floods can occur, the hazards associated with floods, and actions that can be taken to save life and property.
Here in the United States flooding causes more damage than any other severe weather re-lated hazard, averaging around $5 billion a year. No region is immune, with flooding being documented in all 50 states and U.S. Territories.
Flooding can occur year round. For many states the highest frequency of flooding occurs during the spring snowmelt season. Rapid melt of snow, often accompanied by warm spring rains, often results in significant flooding of rivers and streams. The floods that oc-curred along the Mississippi, Ohio, and other rivers in the spring of 2011 were unprece-dented in their magnitude, with new record flood levels being set at many locations.
The figure below, from the National Operational Hydrologic Remote Sensing Center (NOHRSC) shows snow depth across the nation as of January 18th, 2012. Although there are some areas with good snowpack, most of the nation has little or no snow at this time. There are still a few weeks for snow to accumulate before the peak melt season, though as it currently stands it appears the risk of spring flooding for most areas will be less this year than in 2011.
Here in the desert southwest spring is usually one of the driest times of the year. Although there is often snow in the higher elevations of the state, spring snow melt is not a signifi-cant contributor to river flooding. Melting snow pack can, however, help to increase pre-cious water supply storage in area reservoirs.
Flooding in Arizona, by contrast, normally occurs during the convective or monsoon sea-son. The monsoon season runs from June 15th to September 30th, though convective, heavyrain producing storms, can occur outside of this period. The graphic below depicts the per-centage of annual precipitation that falls dur-ing the months of June, July, August, and Sep-tember.
In the lower deserts of east-central Arizona 30 to 40 percent of annual precipitation falls dur-ing these four months. Monsoonal flow usually hasn't established itself until the end of June and often is winding down by early September. The bulk of the precipitation that falls during the monsoon season, therefore falls during the two months of July and August.
The monsoon has a greater impact on precipi-tation in extreme southeast Arizona, where greater than 50 percent of the annual totalfalls during this period.
High rainfall amounts and high rainfall rates can result in flooding. Flooding that occurs in the desert southwest is usually classified as "flash" flooding, and not river flooding. Flash flooding is defined as a flood which occurs within six hours of a rain event, or after a dam or levee failure, or following a sudden release of water held by an ice or debris jam. In a flash flood creeks, streams, normally dry washes, and many roadways that cross these drainages can become quickly inundated with flood waters. Flash floods occur quickly and can catch people unprepared.
The National Weather Service issues Flash Flood Warnings when flash flooding is occurring or forecast to occur. You will, however, not always have a warning that these deadly, sud-den floods are coming. If you live in areas prone to flash floods it is important to stay alert for flood waters and stay informed when convective storms threaten by monitoring NOAA Weather Radio or your favorite media source.
Should you encounter a flooded roadway, do not drive into flood waters. It takes very little flowing water to sweep a vehicle off the road. Either wait until water subsides or Turn Around – DON'T DROWN! If that isn't enough incentive, Arizona also has a "Stupid Motor-ist Law" (Section 28-910 of the Arizona Revised Statutes) which states that any motorist who becomes stranded after driving around barricades to enter a flooded stretch of road-way may be charged for the cost of his/her rescue.
For more information on National Flood Safety Awareness Week and to obtain additional flood safety information go to the National Weather Service Flood Safety Web Page at:
http://www.nws.noaa.gov/floodsafety/index.shtml
Source: http://www.wrh.noaa.gov/psr/pns/Feb_2012_Newsletter.pdf
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