California Storm Spurs Flood, Debris Slide Warnings

Join UCLA and UC ANR climate scientist Daniel Swain this evening as he monitors a strong storm over California. The storm's heaviest rain is anticipated during the discussion, along with the highest risk of flooding and debris slides in burn areas.

Get a preview of Swain's comments from his blog post today:

• "Southern California desperately needs more rain this season, and the good news is that this system will certainly bring some. The bad news is that it may fall too hard, too fast, to avoid a substantial risk of debris flows and perhaps flash flooding in, near, and downstream of recent wildfire burn areas in SoCal. This includes both the more recent fires in western/central L.A. County about a month ago, in January, as well as those which occurred during the other major SoCal wildfire outbreak in September (which were most intense/widespread in the mountains of eastern L.A. and western San Bernardino County). While the whole region should see a good soaking, the SoCal mountains will see widespread 3–6 inch totals (locally higher), and that does include most of the major recent fire footprints."
• "Unlike deep-seated landslides, debris flows involve a higher ratio of water to solid material and are therefore more 'mobile.' They also generally form in primarily response to intense short-duration precipitation (as opposed to other sorts of geologic mass movements, regarding which the overall degree of soil saturation and antecedent precipitation is of greater importance). Debris flows, which are already relatively common in the mountains of Southern CA, are more likely in recent wildfire burn areas for several reasons — including removal of vegetation and soil-anchoring root systems; lack of resistance to surface flow/"sheetwash" due to the creation of a "hydrophobic" surface soil layer where intense fire occurred and the overall smoothing of the land surface due to lack of plant cover; and introduction of loose debris (especially fine ash and soil, but also potentially including larger debris including logs and rocks destabilized on steeper slopes)."
• "There will be two 'types' of precipitation during the Thursday storm that could produce debris flow. First, the atmospheric river itself, featuring a very moist airmass being transported by strong winds at lower levels of the atmosphere directly into the "catchers' mitt" of the Transverse Ranges — which will help, via very efficient orographic lifting, to squeeze out heavy rainfall before the cold front arrives — potentially yielding rainfall rates exceeding a half inch per hour in the mountains (enough for debris flows in some areas). Then, as the cold front arrives (and immediately behind it), convective showers and isolated thunderstorms will likely form due to some slight surface-based atmospheric instability and relatively strong dynamic lifting from the front itself. These embedded convective elements along the front could well produce localized rainfall rates up to (or even exceeding) 1 inch per hour — almost certainly enough to generate debris flows if they occur within recent fire footprints. Downpours of this intensity will not occur everywhere, but this is the portion of the storm that is of most concern from a flash flood and debris flow risk in SoCal."

YouTube Office Hours with Daniel Swain

Wednesday, Feb. 12, 5:00 p.m. PT/8:00 p.m. ET