Natural Hazards Weekly — 2026-04-19
TITLE: Natural Hazards Weekly — April 19, 2026: Alaska Winter Storms, Typhoon Sinlaku Declarations, Quiet Seismic Week
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Seismic
Seismic activity across the U.S. and territories was notably subdued over the reporting window. No M4+ events were recorded in the latest USGS feed; the strongest event captured was a M3.28 offshore the U.S. Virgin Islands, 76 km NNE of Cruz Bay, at a depth of 8.2 km (event `pr71513928`, 2026-04-19 02:07 UTC). No tsunami threat was issued.
Other notable low-level activity:
- M2.82 — 25 km W of Petrolia, CA (`nc75347412`), depth 16.4 km, offshore Mendocino triple junction region — an area worth continued monitoring despite the modest magnitude.
- Swarm-like clustering near Ocotillo Wells, CA — two events (M1.04 `ci41444704`, M1.34 `ci41444696`) within ~6 minutes, depths 7–9 km.
- Paired micro-events NE of Julian, CA (`ci41444728`, `ci41444712`) at ~6 km depth.
- Routine background seismicity continued at The Geysers, CA, Hawaii (Naalehu), Mina, NV, and south-central Alaska.
Desk take
Nothing in this window meets public-impact thresholds. Monitor the Ocotillo Wells and Julian clusters for any escalation in subsequent pulls; both sit within Salton Trough / Peninsular Ranges fault systems.
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Severe Weather
Lead: Alaska winter storm complex
A significant late-season winter storm is the dominant hazard, with three Winter Storm Warnings issued by NWS Fairbanks at 6:42 PM AKDT Saturday, valid through Monday, April 20:
- Yukon Delta Coast / Lower Yukon River — 4–7" additional heavy snow, gusts to 40 mph, visibilities possibly under ¼ mile. Warning in effect until 10 AM AKDT Monday.
- Lower Yukon & Innoko Valleys; Upper Kuskokwim Valley — 5–10" total accumulation through 4 PM AKDT Monday.
- Middle Yukon Valley — 5–10" accumulation, effective 1 AM Sunday through 4 PM AKDT Monday.
A companion Winter Weather Advisory covers the Bering Strait Coast, Shishmaref, and Diomede through 4 PM AKDT Monday for blowing/drifting snow with gusts to 45 mph. Travel impacts and visibility reductions are the principal concerns across western and interior Alaska.
Marine hazards — Gulf and Great Lakes
A broad Small Craft Advisory regime is in place across the northern Gulf of America, issued by NWS New Orleans at 9:45 PM CDT Saturday:
- NE winds 15–25 kt, gusts to 30 kt; seas building to 5–8 ft on outer waters (20–60 NM zones off Port Fourchon, SW Pass, Stake Island) — valid through 1 AM CDT Tuesday, April 21.
- Inner waters (Terrebonne Bay, Barataria Bay, Lake Pontchartrain, Mississippi Sound, Chandeleur Sound, Breton Sound, Lake Borgne, Lake Maurepas) under SCA through noon–7 PM CDT Monday.
On the Great Lakes, short-duration SCAs on southern Lake Michigan (Calumet Harbor IL to Michigan City IN; Michigan City to St. Joseph MI) expired/expire early Sunday morning with W–NW winds to 25 kt and waves to 5 ft.
Housekeeping
One NWS test message (Montgomery, `KEEPALIVE-54844`) is non-operational and disregarded.
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Federal Declarations
Lead: Typhoon Sinlaku — Pacific territories
FEMA issued Emergency Declarations (EM) on April 11, 2026 for Typhoon Sinlaku impacts across the Marianas and Guam:
- Guam (EM-3644) — territory-wide.
- CNMI (EM-3645) — covering Saipan, Tinian, Rota, Northern Islands, and the territory-wide designation.
Incident begin date is April 11; response operations should be assumed active. This is the most consequential federal action in the reporting window and warrants continued situational awareness for logistics, power, and water-system recovery in the Marianas.
Montana — December 2025 winter storm cluster
Three Major Disaster Declarations (DR) were approved April 7, 2026 for December 2025 events:
- DR-4902 — Severe Winter Storm and Straight-Line Winds (Dec 17–18, 2025): Fort Peck Indian Reservation, Blaine County, Carbon County.
- DR-4901 — Severe Storms and Flooding (Dec 9–11, 2025): Lincoln County, MT.
These are retrospective declarations unlocking Public Assistance for the listed jurisdictions.
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Watch items for next cycle
- Ocotillo Wells / Julian, CA micro-seismicity — watch for clustering escalation.
- Alaska winter storm — verify accumulation totals and any advisory upgrades through Monday afternoon.
- Gulf marine conditions — SCAs extend into Tuesday; reassess for upgrade to Gale Warning if gusts strengthen.
- Typhoon Sinlaku recovery — anticipate additional CNMI/Guam amendments or transition to DR status.
*— Natural Hazards Weekly desk*