February 26, 2024

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Could be 500 Year Flood for Los Angeles

2 min read

A scary discussion from the National Water Center in Tuscaloosa for southern California…we are so lucky to have this facility in our state! We will be talking to Director Mike Gremillion tomorrow night on WeatherBrains!

Area Hydrological Discussion #027 – EXPERIMENTAL
NWS National Water Center – Tuscaloosa, AL
1231 PM CST Sun Feb 04 2024

WHAT: Considerable flash, urban, and small stream flooding
WHERE: Southern California
WHEN: Through this evening

FORECAST RAINFALL AND ANTECEDENT CONDITIONS:
QPF: 2 – 4″, locally higher (HRRR)
QPE: up to 2″ (MRMS past 12 hours)
Rates: up to 1.5/hr (HRRR)
Soils: 60 – 80% (0 – 10 cm RSM, NASA SPoRT)
Streamflows: Much above normal (USGS)

DISCUSSION…
Moderate to heavy rainfall will continue to shift southward,
increasing the chances of flooding impacts across the area of
concern. Light to moderate rainfall through this morning has slowly
increased soil moisture and infiltration capacity will continue to
decrease farther south as the moisture continues to move southward
through the early afternoon, priming the area for increasing flood
threat later today and through tonight. Locally heavy rainfall will
easily convert to overland flow over the region resulting in isolated
considerable flash and urban flooding impacts.

The HRRR-Forced NWM SRF continues to indicate the potential of
Rapid-Onset Flooding (ROF) with scattered probabilities between 50 –
75% and locally higher probabilities. Additionally, MHFM continues to
indicate widespread AEPs around 20%, with an increasing number of
streams falling below 20% suggesting isolated significant higher
streamflows. This increases confidence in the potential for
significant river rises, especially since the SRF does not cover the
entire event yet. Towards the end of this AHD and potentially
overnight, considerable flooding concerns will increase as the HRRR
continues to indicate the potential for 6 hr accumulations up to 6
near Los Angeles, which would be equivalent to a 500 yr + rainfall
event or 0.2% AEP (ATLAS 14). This trend will continue to be watched,
and a subsequent AHD will most likely be needed.

GRAPHICAL REPRESENTATION weather.gov/owp/operations-ahd

Additional National Water Center products are available at
weather.gov/owp/operations

Bill Murray

2024-02-04 19:19:39

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