| Tropical storm (SSHS) | |
|---|---|
| Satellite image of Tropical Storm Ignacio | |
| Formed | August 17, 1997 |
| Dissipated | August 19, 1997 |
| Highest winds | 1-minute sustained: 40 mph (65 km/h) |
| Lowest pressure | 1005 mbar (hPa; 29.68 inHg) |
| Fatalities | None reported |
| Areas affected | California, Pacific Northwest |
| Part of the 1997 Pacific hurricane season | |
Tropical Storm Ignacio was a weak tropical cyclone that existed during August 1997. The 12th tropical cyclone and 9th named storm of the 1997 Pacific hurricane season, Ignacio formed west of Mexico from a weather disturbance embedded within a more broad system. It was classified as a tropical depression on August 17, and although it quickly intensified into a tropical storm, harsher environmental conditions caused it to begin weakening as the cyclone moved northward. On August 19, Ignacio transitioned into an extratropical cyclone as it approached California, where its remnants produced unprecedented August rainfall, numerous traffic accidents, and 78,000 power outages.
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Tropical Storm Ignacio originated in a broad weather system west of Mexico during the middle of August 1997. The specific precursor to Ignacio developed on August 16, well to the southwest of the Baja California Peninsula, among multiple other disturbances. Later that day, banding features formed and convection began turning counterclockwise around an area of low pressure in the middle levels of the atmosphere. It continued to become better organized, and at 0000 UTC on August 17, the National Hurricane Center classified it a tropical depression. At the time, the storm was situated approximately 520 mi (840 km) southwest of Cabo San Lucas, outside of the area in which tropical cyclones usually form in the Eastern Pacific ocean basin.[1]
The depression moved northwestward at 11 to 17 mph (18 to 27 km/h) under steering currents from a mid- to upper-level low pressure system offshore of California and an associated trough. A burst of deep convection formed near the cyclone's center early on August 17, and its estimated maximum sustained winds subsequently increased to 40 mph (64 km/h). As such, it was upgraded to Tropical Storm Ignacio at 1200 UTC. At the same time, the storm reached its lowest recorded barometric pressure of 1005 millibars. However, due to increasingly colder waters and the presence of inhibiting southwesterly wind shear, it immediately stopped intensifying. Ignacio maintained tropical storm status for a short time, and early on August 18, it weakened back into a depression.[1]
During its weakening stage, deep convection existed only in the northwestern quadrant of the circulation. On August 19, cold cloud tops reappeared, although this brief revival was attributed to non-tropical baroclinic processes. At 1200 UTC, the depression was declared extratropical, and it continued northward until dissipating near California a day later. Moisture from the system persisted as it passed through the state and the Pacific Northwest, ultimately merging with a large extratropical cyclone involving the remnants of Hurricane Guillermo.[1]
As a tropical cyclone, Ignacio had little or no effect on land. However, its remnants produced unprecedented precipitation in parts of coastal California.[1] The rainfall reached record levels in some areas,[2] amounting to as much as 2.17 in (55 mm). San Francisco received about 1 in (25 mm) of precipitation,[1] leading to its wettest August on record, and the first time it had ever experienced measurable rainfall on August 19.[3] In contrast, the area usually receives only a trace of moisture during August.[4] The unusual rainfall triggered isolated flash flooding and a debris flow which was estimated to be up to 7 ft (2.1 m) deep.[5] Zinfandel grape crops were disturbed by the heavy rainfall from Ignacio, and again from Hurricane Linda in September.[6]
Several weather-related traffic accidents occurred, at least one of which was described as "major". Two crashes on Interstate 80 forced the closure of four lanes for 30 minutes. It is unclear how many injuries resulted from the various incidents.[4] Thunderstorms associated with the storm system left about 78,000 electric customers in Sonoma County without power after lightning struck a power substation, although the outages lasted less than an hour.[7] Lightning also knocked three workers unconscious in a vineyard near Lodi,[8] and they were in good condition after being hospitalized.[4] Rainfall extended as far north as the Northwestern United States,[3] ending an unusually long, 35-day dry spell at Astoria, Oregon.[9] It remains the most significant rainfall event from a tropical cyclone in Washington and Oregon.[10] Due to the lack of severe damage, the name Ignacio was not retired following its usage. It was reused during the 2009 season, and remains on the rotating list of names.
