San Mateo County, CA

Supports B1-a Hazards

14 of the 18 National Risk Index hazards apply here; omission evidence is shown only where NRI rates a hazard "Not Applicable", no federal declaration names it and Storm Events has no record of it since 1996.

Highest expected annual loss (NRI, modeled): Earthquake, Inland Flooding and Coastal Flooding.

Also check locally (not covered by NRI): dam failure (incl. upstream dams), levee failure, flash flood as its own subtype, ice jams, debris flows, erosion (riverine and coastal), land subsidence and sinkholes and expansive soils.

Check your state hazard mitigation plan's hazard list for this county; explain any hazard you drop.

Supports B1-b Location

• Avalanche, Earthquake, Hurricane, Landslide, Tornado, Tsunami, Volcanic Activity and Wildfire: Census tract rating counts from the National Risk Index tract table (see B1-f). Tract maps and place names are not in this packet.
• Coastal Flooding and Inland Flooding: The FEMA Flood Insurance Rate Map (FIRM) is the required location source for flooding. See the FEMA Flood Map Service Center (https://msc.fema.gov) for this county.
• Cold Wave, Drought, Hail, Heat Wave, Ice Storm, Lightning, Strong Wind and Winter Weather: Affects the whole planning area; national data do not distinguish locations within the county. Confirm locally.

Supports B1-c Extent

Storm Events records for 14 of 20 hazard groups, with the scale, observed range and worst recorded event for each; the others say local data required.

• Hail: 0.25 in hail, 2024-03-30; same size also recorded on 2017-01-22, 2017-01-23, 2017-04-08, 2017-09-11, 2018-02-22, 2018-03-17, 2019-01-20, 2020-01-16
• Thunderstorm Wind: 84 kt (97 mph) measured gust, Thunderstorm Wind, 2024-12-14
• Tornado: F1 tornado, 2005-03-20, 3 mi within this county, 30 yd wide, $800,000 property; $0 crops (2005 dollars)

Supports B1-d Previous occurrences

24 federal declaration rows for this area (18 counted incidents), each with type, dates and county Public Assistance dollars.

New since the last plan (approved 2021-12-09): DR-4699-CA, EM-3592-CA, DR-4683-CA and EM-3591-CA.

10 statewide declarations listed separately.

Hazards with no recorded events are stated as such (3 of 18).

Storm Events history: 387 records (notable ones in the PDF, all of them in the Excel file).

Supports B1-e Probability

FEMA annualized frequency (modeled) for each hazard, with a descriptor from a scale defined in the packet.

• Highly likely: Landslide and Lightning
• Likely: Coastal Flooding, Heat Wave, Inland Flooding, Wildfire and Winter Weather
• Possible: Cold Wave, Hail, Strong Wind, Tornado and Tsunami

Historical frequency by intensity band (tornado, hail, wind): 9 bands from Storm Events.

Supports B1-f Variation across the planning area

Range of census tract risk scores within the area for 18 hazards (173 tracts; rated tracts only).

21 member jurisdictions: plan clock, repeat-flood count, Public Assistance dollars and event records naming the place (starter, confirm locally).

Supports B2-a, B2-b Assets and impacts

Local data required

County-level starter. The plan must describe impacts for each participant and its own assets.

Storm Events damage since 2021-12-09: $40 in county-coded records (37 records); $120 reported for the zone (37 zone records). Data through 2026-06-30; nominal dollars.

Checklist of what to gather locally: Critical facilities and lifelines per hazard; A problem statement per participant per hazard; Changes in development in hazard-prone areas since the last approval, or a statement of none (E1-a); Future assets.

Supports B2-a Context

Population 763,661; building value $174,411,578,394 and agriculture value $102,262,383 (total, not at risk).

National Risk Index rating Relatively High; social vulnerability Relatively Low; community resilience Very High.

Supports B2-c Repetitive loss

Repetitive-loss properties, FMA definition (44 CFR 77.2): 1 repetitive loss, 6 severe repetitive loss. NFIP insurance definition: 47 and 6.

Mitigated 0; insured 21; NFIP data as of 2026-08-02. Counts by occupancy and for each member community are in the packet.