Typhoon season is picking up in the Pacific, and right on schedule Japan is sandwiched between a pair of tropical storms. Tropical Storm In-fa is dumping copious rainfall in China and inducing significant flooding near Shanghai, lashing a country that days earlier dealt with deadly and historic flooding. A tropical storm named Nepartak, meanwhile, is sweeping northwest toward northern Japan.

Neither storm is a typhoon, though In-fa spent several days as a typhoon north of Taiwan last week. The pair of storms is already helping to drag deep tropical moisture northward, spelling rounds of heavy afternoon downpours in central and northern Japan.

Officials at the 2021 Summer Olympics have already taken to rescheduling rowing, but are optimistic that other events may be feasibly executed as planned. The Japanese Meteorological Agency has hoisted high wave and storm surge advisories for the eastern coastline of Honshu, Japan’s main island. No alerts are currently in effect for Tokyo, although a few brief downpours are possible there into Tuesday before the bulk of the storm’s rain shifts to the north.

As of 5 a.m. Eastern time, or 6 p.m. Japanese time Monday, Nepartak was a tropical storm with winds of 40 mph. It was located a little more than 300 miles east of the Boso Peninsula, which encompasses Chiba Prefecture, or the apex of the archipelago nation’s bend.

The storm was moving northwest at 20 mph, and it slated to make landfall in northern Honshu early Wednesday local time.

Nepartak isn’t overly impressive on satellite imagery. The Japanese weather satellite Himawari-8, which peers downward at the Pacific from 22,241 above Earth, revealed Nepartak to be lopsided and asymmetrical. The counterclockwise circulation of In-fa, the system moving ashore Zhejiang Province in China, has helped spill south a relatively less humid air mass that is being ingested into the western limb of Nepartak.


Thus far, that’s prevented the strengthening of Nepartak, and will likely curb its intensification and it is not forecast to become a typhoon. In fact, the coldest satellite-derived cloud top temperatures, signifying the tallest clouds, were not present near within the cloud shield associated with Nepartak, but rather the trailing frontal system well to its south.

The biggest impact from Nepartak will be heavy rainfall, focused mainly north of Tokyo and close to the coast. The storm is swirling north extreme tropical moisture, with PWATs, or precipitable water indices, in excess of 3 inches. That means that every column of atmosphere contains enough water to drop three inches of rainfall – and that’s an air mass that will be continually replenished as a conveyor belt of moisture feeds into the system.

The first batch of torrential downpours was arriving Monday along the east coast of Honshu, with individual thunderstorm cells containing rainfall rates over 2 inches per hour. The northern half of Japan will see isolated to widely-scattered rains through Wednesday. The majority of locations should see 1 to 2 inches of rainfall with localized totals of 3 inches.

Winds could gust near 35 mph at the coast as the tropical storm makes landfall Wednesday. Near shore waves should stack up 10 to 15 feet tall, with 30 footers well offshore.

After any showers from Nepartak depart Tokyo Tuesday, heat and oppressive levels of humidity will be the primary weather story at the Olympics. Clouds and rain from Nepartak will actually help keep high temperatures down on Tuesday, with highs in the low-to-mid 80s but they rebound to near 90 Wednesday into the weekend.

Due to climate change, July and August average temperatures have warmed by 2.7 degrees in Tokyo since 1964, the last time the Olympics were held there, according to Climate Central, a science communications firm.

Olympic officials had originally considered holding the games in October when temperatures would be marginally cooler. That would run the risk of the competitions coinciding with peak typhoon season. Super Typhoon Hagibis forced the cancellation of the Rugby World Cup in Tokyo during October of 2019 while spelling widespread flooding along the Izu Peninsula; some places saw three months’ worth of rainfall in 48 hours as totals up to 3 feet were realized. Typhoon Bualoi brought more devastating flooding two weeks later.