What to expect the last week of January and first week of February
If we thought we would escape some harsher winter weather this year, unfortunately we will not be able to completely avoid it. According to the medium range forecasts released by the National Weather Service, in the last half of January the west coast will be seeing some wild weather from California to Alaska. It will likely include temperatures slightly below normal and some powerful storms bringing significant rainfall to our area.
There is even the possibility of snow or freezing rain as some of the arctic air that has plagued Siberia (-90° F) and the Eastern United States take a glancing blow at the Western US. The west has been enjoying a fairly mild winter to date with temperatures above normal and precipitation near normal. This weather does not follow the La Niña pattern initially forecast and has actually been more typical of an El Niño winter.
There is still the problem with forecasting even the medium range outlooks because the past patterns are not able to accurately predict the current climate trends. Putting that aside for a moment, there is strong agreement between several models that we are in store for some nasty weather in the coming couple weeks so it’s best to be prepared in the event flooding occurs.
What to Expect & Where
For Thurston County, we may see some high groundwater and some stream or river flooding. We currently have above average groundwater, lake levels, and streamflows that have remained above average since the wet, prolonged winter of 2016-2017. Not significantly higher than average but higher nonetheless. A large storm or a series of storms could set the stage for flooding within the next couple weeks.
Here are in Thurston County:
- The western third from Grays Harbor County line throughout the Black Hills and the Willapa Hills in the southwest part of the County. The Black River and Black Lake are often affected by flooding originating in the Black Hills and other western tributaries. The mid part of the Chehalis River also receives a significant portion of its flow from the weather systems that affect the Willapa Hills.
- The next areas that are affected by large rainfall events are the central lowlands between the Black Hills and the Cascade foothills. This area includes the I-5 and Highway 99 corridors where the majority of the population of Thurston County resides. In this region, we have multiple flood prone creeks and streams that can flood if there is a period of high precipitation. These locations are also where our more susceptible lakes and groundwater flooding concerns arise. In particular, Lake St Clair and the Salmon Creek Basin high groundwater areas.
- Lake St Clair is likely to see very high water levels if we experience the forecast 4-6 inches of rain within the next two weeks. Salmon Creek Basin is also quite likely to experience groundwater flooding in some of the more flood prone lower elevation.
- The next area of concern is the Cascade foothills where the headwaters of the Nisqually River and the Deschutes River begin their journey to Puget Sound. This is also where the Chehalis River and its tributaries, such as the Skookumchuck River, get much of their water from as well.
- Landslides are more likely. During high and prolonged rainfall events there is also a greater risk for landslides in many areas of the County. The most prone areas are where the slopes are greatest such as the Black Hills and the Cascade foothills but landslides can also occur in less obvious areas under the right conditions. In Thurston County we have such a variable geologic setting that susceptible soils are widespread throughout our area. But in general the Black Hills, the Cascade foothills including areas around Tenino and Bucoda and the Willapa Hills (Michigan Hill areas). These areas combine susceptible soils, steep slopes and high rainfall – all the ingredients to start the ground sliding.
Why Flooding is Likely in Thurston County
The areas and the possible problems listed above are a result of how our county is situated geographically in the Puget Sound Basin. We are located between the Olympics (Labeled ‘Coast Range’ below) and the Cascade Range, which causes the predominant weather patterns to get focused between these regions. Any gaps or higher elevation areas get more precipitation resulting in increased potential for flooding and the increased likelihood of landslides.
Another unique feature of Thurston County occurs in the Chehalis River valley known as the Chehalis Gap. This is a feature where the Chehalis River has eroded through the higher hills that make up the southern Black Hills and the Willapa Hills. This gap allows Pacific moisture to spill into the southern part of Thurston County without having to go over any higher elevation terrain. Because the storm can avoid going over high hills, it is not forced to drop its precipitation due to cooling.
Remember, as humid clouds have to rise over hills or mountains they bunch up and get cooler the higher up they have to climb to get over an obstacle such as the Black Hills. This increase in elevation cools the clouds and causes a lot of moisture to be dropped on the windward side of mountains. Think of Eastern Washington’s dramatically different climate compared to Western Washington. Eighty to 90 percent of the moisture from Pacific storms is lost over the westerns slopes of the Cascades and Olympics as the moist air is forced up over the mountains, leaving eastern Washington as dry as the Arizona desert!
Another unique feature of Thurston County occurs in the Chehalis River valley known as the Chehalis Gap. This is a feature where the Chehalis River has eroded through the higher hills that make up the southern Black Hills and the Willapa Hills. This gap allows Pacific moisture to spill into the southern part of Thurston County without having to go over any higher elevation terrain. Because the storm can avoid going over high hills, it is not forced to drop its precipitation due to cooling.
Remember, as humid clouds have to rise over hills or mountains they bunch up and get cooler the higher up they have to climb to get over an obstacle such as the Black Hills. This increase in elevation cools the clouds and causes a lot of moisture to be dropped on the windward side of mountains. Think of Eastern Washington’s dramatically different climate compared to Western Washington. Eighty to 90 percent of the moisture from Pacific storms is lost over the westerns slopes of the Cascades and Olympics as the moist air is forced up over the mountains, leaving eastern Washington as dry as the Arizona desert!
This same effect occurs even with much smaller hills and mountains causing rain shadows (dry areas) and rain focusing (wet areas) around hills and valleys. The Chehalis Gap allows the Pacific moisture to sneak up the Interstate 5 corridor further without having to lose its moisture so it can penetrate further inland. This is why areas from Littlerock to Tenino and Grand Mound typically see 25 percent higher rainfall than further north in the I-5 corridor in the Olympia area.
Be Prepared: Twenty Inches More Rain than Normal is Possible
Because Thurston County is situated in such a unique nexus of topography, geology and Pacific air currents, we have the potential to go from a typical wet winter (average 52 in/yr.) to a flooding winter (> 70 in/yr.) very rapidly depending on the strength and location of storm systems approaching the west coast.
These conditions combined with high water levels from previous years can quickly lead to flood conditions. Looking at the short term two-week forecast, we may have the right conditions in place for some flooding to occur. As these conditions can occur rapidly it is always best to be prepared for floods especially if you live in the flood prone areas of Thurston County. Visit our website and sign up to receive flood alerts from Thurston County.
People who build in the 100-year flood plain or in high groundwater areas are especially vulnerable (see if your Thurston County property is in a flood hazard area). Most new homes and structures will be required under Thurston County Codes to be elevated at least two feet above the predicted 100-year flood elevation as determined for each flood area by the Federal Emergency Management Agency (FEMA). The reasoning behind elevating structures is to keep them above the determined high water flood elevation based on land surveys and flood plain models. The intent is to minimize or eliminate inundation of homes businesses, roads and utilities and protect lives and property. Even with the codes in place, it is still possible that flooding can occur in severe cases and the best protection if you live in a flood zone is to have flood insurance.
Flood Insurance Premiums are Lower in Thurston County
Thurston County has worked successfully to get flood insurance rates as low as possible through the Community Rating System (CRS) offered through FEMA.
The CRS recognizes and encourages community floodplain management activities that exceed the minimum NFIP standards. Depending upon the level of participation, flood insurance premium rates for policyholders can be reduced up to 45%. Besides the benefit of reduced insurance rates, CRS floodplain management activities enhance public safety, reduce damages to property and public infrastructure, avoid economic disruption and losses, reduce human suffering, and protect the environment. Learn more on the County’s website.
Technical assistance on designing and implementing some activities is available at no charge. Participating in the CRS provides an incentive to maintaining and improving a community’s floodplain management program over the years. Implementing some CRS activities can help projects qualify for certain other Federal assistance programs. If you live in a flood plain you can contact Thurston County’s Floodplain Manager, Tim Rubert, for details. Email Tim at rubert@co.thurston.wa.us or call him at 360-867-2123. You can also learn more online at FEMA’s website.
Climate Matters 