A high-risk flood watch has been issued over the whole region of Southern California, classified as one among the highest levels of severe weather warnings within the recent past years. The NOAA/WPC has indicated that high-risk flood days make up less than 4% of annual days, representing more than 80% of total flood damage and 36% of total flood-related flood deaths within the United States. This week, high rain, wind, and wet snow across the Sierra are forecasted resulting from the steady onslaught of atmospheric rivers, threats to flash flooding, landslides, debris flow, especially within the burned scar regions.
1. Recognizing Threats from Atmospheric Rivers
“Atmospheric rivers are the names given to the narrow channels of heavy moisture. These channels sometimes trace back thousands of kilometers from the tropics. The atmospheric rivers in the modern world include the so-called ‘Pineapple Express.’ This river gets replenished with warm and moist air from the central Pacific. According to a report by the US Geological Survey, ‘An intense atmospheric river has the capability of carrying seven to fifteen times the mean flow of the Mississippi River in a day.’ On the West Coast of North America, atmospheric rivers account for thirty to fifty percent of the yearly precipitation.”
2. Timing and Intensity of the Storms
The initial severe storm makes its landing in Northern California Tuesday night and also covers Southern California Wednesday morning. Rainfall amounts could possibly see rates of 1 inch per hour in the mountains, and even 1.5 inches per hour in some areas. After the lull on Wednesday evening, another severe storm from the Pacific brings rains and strong winds to the area on Christmas Day and then snow in the Sierra services. Coastal and valley areas are poised to see rain amounts of 4 to 8 inches towards the end of the week, and 8 to 12 inches for the foothills and mountains.
3. Areas at Greatest Risk
Regions where there is an area of highest threat are mountainous regions in Santa Barbara, Ventura, L.A., and San Bernardino Counties, as well as regions close to the L.A. Basin area. Burn scar regions left by the fires in “Eaton Fire” and “Palisades Fire” are also expected to be threatened, given the soil composition, which does not allow for penetration, meaning that January rains rush by immediately in the form of “mudslides.” These regions have evacuations in place, where deputies canvass individual structure doors in their door-to-door process.
4. Secondary Hazards – Landslides and Delayed Mud
The saturated soil may lead to landslides or debris flows several weeks after the end of the rainfall. According to the National Weather Service, it is the prolonged saturation of the soil caused by the series of rainfall occurrences that lead to the potential of slope failure, in spite of the clear atmosphere. The case in Laguna Beach in 2005 in the Bluebird Canyon landslide illustrates that the saturated soil created by the winter rainfall several months ago has effects.
5. Wind and Power Outages
These storms will be accompanied by strong south winds, with gusts of 40-60 mph in topographical areas, though isolated gusts may reach as high as 80 mph in ideal locations. Gusts of 55 mph may also be realized within the LA Basin. The saturated soil levels combined with strong winds may result in downed trees, power lines, and power interruptions.
6. Expansion in the Desert Southwest
The flood threat does not remain within California. There shall be heavy rain affecting the Desert Southwest, including Las Vegas and possibly Phoenix, with flood watches issued. There shall be urban flash flooding within typical dry washes and arroyos surrounding the Vegas Valley.
7. Psychological Preparedness and Safety Procedures
Weather can heighten levels of anxiety, particularly during the holiday season where many people travel. However, in order to be safe, one must focus on those areas that can be controlled. Mayor of the City of Los Angeles, Karen Bass, stated that people must be careful in the event that they travel. This shall be achieved in a collective manner where the city is preparing to ease the effects. The National Weather Service urges people to be ready for an evacuation if the need is there. Do not drive in floodwater.
8. The Larger Weather Pattern
In order “The juxtaposition of the high-pressure ridge over the Bering Sea and the trough in the Northeast Pacific is resulting in atmospheric rivers being directly steered into California, extending into Southern California in many instances,” said meteorologists, who further added, “La Niña’s influences on the jet stream are likely contributing to where these storms make landfall, leading to one of the wetter months in recent times in Southern California.”
9. Needed: Himalayan Power
The threat that these storms also convey is balanced by the snow that these storms bring much needed relief to the Sierra Nevada region, especially since it is well behind schedule with respect to its snow packs. The feet of snow that would be accumulating by the end of Friday would ensure that the refillment levels for the water sources would be well ensured. As far as Californians are concerned, they would have to be prepared for unexpected weather changes. The only validation that a high-risk region has been declared is that the dangers associated with the weather event warrant taking precautionary measures.
