User:RandomInfinity17/Tropical Storm Namtheun (2021)

Severe Tropical Storm Namtheun was a strong tropical storm that affected the Pacific Northwest and British Columbia as a powerful extratropical cyclone. It was the first of three systems that impacted the Pacific Northwest in late October 2021. The 36th tropical depression, and 19 tropical storm of the 2021 Pacific typhoon season, Namtheun began as an area of convection approximately 333 miles (535 km) southwest of Wake Island on October 8. The JMA recognized it as a tropical depression the next day with the JTWC issuing a Tropical Cyclone Formation Alert (TCFA) at the same time. By midnight, the depression was named Namtheun. Namtheun continued to move west for the next two days before weaking back into a tropical depression due to a subtropical ridge. The storm then started to move to the northeast before strengthening back into a tropical storm on October 13. Namtheun continued to move to the northeast before peaking as a severe tropical storm with winds of 100 km/h on October 16 soon before turning into an extratropical cyclone.

As a tropical cyclone, Namtheun never affected any landmasses. However, the extratropical remnants brought heavy rains and wind gusts of up to 35 mph in the Pacific Northwest and British Columbia. However, only minimal damage was reported.

Meteorological history
On October 8, the Joint Typhoon Warning Center (JTWC) noted an area of convection persisted approximately 289 nmi (535 km; 333 mi) from Wake Island, that had a medium chance a development. The next day, the Japan Meteorological Agency (JMA) recognized the system as a tropical depression. At 14:00 UTC the same day, the JTWC issued a TCFA for the invest as it developed a significant convection around the center. By midnight on October 10, the JMA upgraded it to a tropical storm and named it Namtheun, which comes from the Nam Theun river in Laos. The JTWC soon after upgraded the system to a tropical depression, then a tropical storm at 09:00 UTC. Namtheun managed to maintain its intensity for two days until October 13, when it was downgraded to a tropical depression by the JTWC, as it started moving westwards because of the presence of a subtropical ridge towards the southeast. However, at 15:00 UTC of the next day, the JTWC re-upgraded it to a tropical storm, as it entered over warm sea-surface temperatures which allowed the system to gain it intensity despite the presence of high wind shear. Satellite imagery indicated that the storm exhibited subtropical characteristics. Unexpectedly, Namtheun further intensified into a severe tropical storm according to JMA at 06:00 UTC on October 16, and a Category 1-equivalent typhoon according to JTWC at 09:00 UTC the same day. Satellite imagery depicted that the system had developed an eye-like structure in the core of the storm. The weakening of the vertical wind shear was main reason for the unexpected intensification. A few hours later, the cooling of sea surface temperatures and increasing wind shear caused the storm to weaken further from its peak intensity. Namtheun managed to maintain its intensity as it continued north-northwards before it started its extratropical transition between 00:00 and 15:00 UTC on October 17. For the next several days, the extratropical remnants of Namtheun moved eastward across the North Pacific, before undergoing explosive intensification and developing into a bomb cyclone on October 21, reaching an extratropical peak of 951 millibars (28.1 inHg), while off the coast of the Pacific Northwest. Afterward, the system curved northward and then north-northwestward, while gradually weakening, before being absorbed into another extratropical cyclone from the west, late on October 22.

Preparations, impacts, and notability
Namthuen stayed over open ocean and didn't affect land as a tropical cyclone.

The extratropical remnants of Namthuen was the first of three systems to affect the western coast of Canada and the United States in late October of 2021. The National Weather Service (NWS) issued storm warnings and gale warnings off the coast of the Pacific Northwest on October 21. The storm was expected to bring up to 5 in of rain to the Santa Cruz Mountains and up to 8 in to the North Bay.

The storm carried a category 5 atmospheric river to the region, bringing most rainfall in nine months. Minimal thunderstorms were observed on the Oregon and Washington coasts on October 21. Over 16,000 customers lost power in California alone with another 3,000 in Washington. Along with another powerful system, the rainfall effectively put an end to the state's wildfire season of the year.

Namthuen, along with the other storms in late-October, was unusual for its low pressure. At peak intensity as an extratropical cyclone, it had a pressure of 951 millibars (28.1 inHg). The lowest pressure on record at the time in the area was around 950 millibars (28.05 inHg). However, the following system would become the most intense storm in the region on record at 942 millibars (27.8 inHg).