Tropical Storm Lala has formed over the central Pacific and is strengthening as it moves toward Hawaii, prompting forecasters to issue a hurricane watch for the Big Island ahead of potentially dangerous conditions this weekend.
The storm is located about 700 miles (1,127 km) east-southeast of Hawaii on August 13, with maximum sustained winds of 40 mph (64 km/h), according to the National Hurricane Center.
Lala is moving west, with further strengthening forecast as it approaches the Hawaiian Islands.
The system could be close to hurricane strength near the Big Island, significantly increasing the threat from strong winds. However, rainfall may ultimately become the storm’s most serious hazard.
A hurricane watch means hurricane conditions are possible within the watch area. Such watches are generally issued about 48 hours before tropical-storm-force winds could begin, because preparations become increasingly difficult once those winds arrive.
Rain is expected to begin early Friday
Conditions are expected to begin deteriorating before the center of Lala reaches its closest point to Hawaii.
Rain associated with the storm is forecast to reach the Hawaiian Islands beginning late Friday, with increasingly strong winds and dangerous surf developing as Lala approaches.
Forecast rainfall totals are substantial.
Widespread accumulations of 5 to 10 inches (127 to 254 mm) are possible across the island chain, while isolated locations could receive as much as 20 inches (508 mm).
Those extreme totals would be sufficient to produce serious flash flooding and mudslides, particularly where tropical rain bands repeatedly cross mountainous terrain.
Hawaii’s mountains could dramatically increase the rain
Lala is transporting a deep reservoir of tropical moisture toward Hawaii.
When that moisture reaches the islands, the terrain itself can substantially amplify rainfall.
Moist Pacific air approaching Hawaii’s volcanic mountains is forced upward. As the air rises, it expands and cools, allowing water vapor to condense rapidly into clouds and precipitation.
This process, known as orographic enhancement, means rainfall totals on exposed mountain slopes can become substantially greater than those occurring over the surrounding ocean or even nearby lower elevations.
The effect is particularly important on the Big Island, where Mauna Kea and Mauna Loa rise more than 13,000 feet (3,960 metres) above sea level.
Hawaii’s steep watersheds can respond extremely quickly to torrential tropical rainfall.
Water descending mountain slopes becomes concentrated into streams, valleys and narrow drainage channels. During extreme rainfall, those waterways can rise rapidly with relatively little warning.
Prolonged rain also increases the amount of water contained within steep slopes, raising the possibility of mudslides and rockfalls.
The combination explains why dangerous flooding can develop far from the center of a tropical cyclone.
If isolated rainfall totals approach 20 inches (508 mm), the flooding threat could become particularly serious.
Lala could approach hurricane strength
Lala remains a tropical storm, but the atmospheric and oceanic environment is expected to support additional strengthening.
Maximum sustained winds were around 40 mph (64 km/h) after formation Thursday, barely above the 39 mph (63 km/h) threshold required for tropical-storm classification.
Forecasters expect the circulation to become better organized as it moves west.
If sustained winds reach 74 mph (119 km/h), Lala would become a hurricane.
The hurricane watch for the Big Island reflects the possibility that the system could reach or approach that intensity near Hawaii.
Even without a direct hurricane landfall, strong winds could bring down trees and damage electricity infrastructure, particularly across exposed terrain.
Large swells generated by Lala will also create increasingly dangerous surf and coastal conditions.
Small track changes could also make a big difference
The exact track remains critical. A relatively small shift closer to the Big Island would increase the potential for hurricane-force winds, while a slower approach could prolong the period of tropical rainfall.
This is why Lala’s eventual category should not be considered the only measure of the threat.
A tropical storm carrying an enormous moisture plume can produce destructive flooding even if its strongest winds remain offshore.
The next 24 to 48 hours will therefore be particularly important as forecasters determine how quickly Lala strengthens and how closely its core approaches the Big Island.
Elsewhere in the Pacific, Tropical Storm Hernan also formed Thursday far west of Mexico’s Baja California Peninsula, but it currently poses no threat to land. In the Atlantic, former Tropical Storm Cristobal has weakened into a post-tropical remnant low west of the Azores.
For Hawaii, attention is firmly focused on Lala as the first rain bands approach late Friday and conditions deteriorate into the weekend.
