Guillermo weakened overnight into a tropical storm, but it is still expected to bring heavy rain, possible flooding and high winds to the islands.

The storm had sustained maximum winds of 70 mph with higher gusts at 5 a.m. Monday, just below hurricane strength.  

At 5 a.m. Monday Guillermo . was centered 620 miles east-southeast of Hilo and 835 miles east-southeast of Honolulu. The storm was moving west-northwest at 10 mph.

Forecasters with the Central Pacific Hurricane Center expect Guillermo to be a weakening tropical storm Monday through Thursday as it passes just north of the islands. The latest forecasted track represents a slight shift north, but the entire state still remains within the forecast’s so-called “cone of uncertainty.”

A tropical storm watch will likely be posted Monday for the Hawaiian islands. Watches are posted about 48 hours ahead of a major storm’s arrival, and a warning is posted about 24 hours before a possible landfall.

“It is still too soon to determine with certainty which islands are most likely to experience the greatest impacts from Guillermo. It is also important to note that significant impacts can extend well away from the center,” forecasters said.

Wind shear tearing at the storm began to weaken it and slow its progress Sunday, but it is still expected to have winds greater than 40 mph when it passes close to Hawaii later this week.

Rain and wind ahead of the storm’s center should begin to affect Hawaii island late Tuesday night into Wednesday morning, said Matthew Foster, a meteorologist with the Honolulu office of the National Weather Service.

“We are anticipating heavy rainfall and winds,” Foster said. High surf could also threaten lives and property.

The winds will likely be from the northeast, so windward and mauka areas will see most of the rain and wind. But leeward sections will also see some effects.

The damage from Guillermo may not be as severe as Tropical Storm Iselle, which hit the Big Island on August 8, 2014, the eve of the primary election, with sustained winds of 60 mph and higher gusts. The storm downed trees and power lines blocking roads and leaving thousands of people without electricity for days. The primary election for two precincts was postponed for six days until roads could be cleared and power restored to polling places.

The current forecast for Guillermo calls for winds of about 50 mph on Tuesday, decreasing to 45 mph on Wednesday.

But Foster said he “wouldn’t be surprised” to see similar wind damage with downed trees and branches and wind damage to weak structures and roofs. Rain could be heavy enough to cause flooding.

With the impact of Iselle still fresh in their memories, residents of Pahoa on Hawaii island, were keeping close tabs on Guillermo’s progress.

“Everyone is on edge,” said Michelle Williams, a clerk at Da Store on Kahakai Boulevard. “The last time, people were more casual, thinking, ‘We never get hit.’ Now you can see that more people are paying attention. They’re cutting their shrubs and anything else that might fly around. They’re real nervous.”

Williams said Da Store increased its supply of bottled water, toilet paper and other items that tend to disappear from shelves during an emergency.

Yet residents’ concerns about the storm have not yet translated to a run on emergency supplies. For example, it was business as usual at Longs Drugs in Pahoa on Sunday, with no noticeable uptick in sales.

The hurricane has already prompted a high-surf advisory for the eastern shores of Oahu, Hawaii island, Kauai and Maui from noon Sunday through 6 p.m. Thursday. Forecasters warned that the surf on those shores will become “life-threatening” on Monday. 

 “It’s important for everyone to take advantage of the lead time we have ahead of this storm to make their preparations. Tropical cyclones bring a triple threat of wind, heavy rains and battering surf and we need to prepare for any impacts that may occur,” Maui County Emergency Management Officer Anna Foust said in a news release. “Secure or move inside any loose objects that may become airborne. Clean out gutters and other drainage ways that may become clogged with debris and exacerbate flooding. Stock up on your emergency kit supplies and be prepared for possible power outages.” 

Moderate to strong tradewinds will continue through Monday, then weaken by Tuesday as Guillermo disrupts the flow, the National Weather Service said. Showers will increase as remnant moisture associated with former Tropical Depression 8E moves over the islands Sunday, with best chances of rain over windward and southeastern portions of Hawaii island. 

The forecast for this week will be highly dependent on the eventual track of Guillermo, the agency said, but rain can be expected across the islands Tuesday night through Thursday night. A small craft advisory remains in effect for the waters south of Maui and Hawaii island until 6 p.m. Sunday, with east winds of 23 to 29 mph and seas of 7 to 11 feet. 

A crew from the Air Force Reserve’s 53rd Weather Reconnaissance Squadron — known as the Hurricane Hunters — flew through the storm Sunday and data from the flights “has been critical in locating the center, determining the current intensity and adjusting the wind radii (of Hurricane Guillermo),” forecasters said. 

Flying aboard Lockheed-Martin WC-130J aircraft specially designed to withstand the rigors of hurricane conditions. 

A NOAA Gulfstream IV plane flew through the storm on its way to Hawaii Sunday afternoon.

The Hurricane Hunters planned another mission into the storm Sunday night. The Air Force and NOAA planes will alternate missions into Guillermo as the storm approaches the islands.

In May, the Central Pacific Hurricane Center said there is a 70 percent chance of a busier-than- usual hurricane season, with five to eight storms. The normal season has four or five storms. 

Driving the activity in large measure is a persistent El Nino, associated with higher-than-normal sea surface temperatures in the eastern tropical Pacific. 

The hurricane season in the North Pacific lasts from June 1 to Nov. 30. Two of Hawaii’s most destructive hurricanes, Iwa in 1982 and Iniki in 1992, came late in the season.