Hawaii island and Maui County are under a tropical storm watch, and other islands may be included by Tuesday morning as Tropical Storm Guillermo continues on course to pass just north of the state, forecasters said.

A watch means that tropical storm conditions are likely within 48 hours. 

Although projected to continue weakening, Guillermo is still capable of bringing heavy rain, and the potential for flooding and strong winds to the islands starting late Tuesday night through Thursday.

Guillermo had sustained maximum winds of 70 mph with higher gusts at 5 a.m. Tuesday, according to the Central Pacific Hurricane Center. Tropical storm-force winds extend 175 miles from the center. At 5 a.m. the storm was centered 430 miles east of Hilo and 620 miles east of Honolulu and was moving northwest at 12 mph.

“Although Tropical Storm Guillermo shows signs of weakening, we strongly encourage residents and visitors to prepare themselves and their families for potential severe weather,” said Doug Mayne, administrator of Hawaii Emergency Management. “There is some uncertainty associated with weather forecasting, so we need to plan for the worst and hope for the best.”

The National Weather Service issued a flood potential outlook, citing the possibility of heavy rain and flooding throughout the islands Wednesday and Thursday. How much rain and wind is still uncertain.

“While the forecast track keeps the center of Guillermo northeast of the main Hawaiian islands, a significant deviation to the left of the track would have the potential to bring tropical storm conditions to the Big Island or Maui County,” hurricane center forecasters said Monday afternoon. “When accounting for the size of the system, there remains enough uncertainty to warrant issuance of a tropical storm watch for Hawaii and Maui counties. Other counties may be added later as needed.”

Most of the heavy rains and winds are north of the center. So if it follows the current track, the state could avoid the worst of it.

The 5 p.m. forecast track moved even further north, away from Hawaii, from the previous forecast, with all islands now just outside of the so-called “cone of uncertainty.”

The margin of error for the track 48-hours ahead of the storm is about 85 miles in each direction, so the storm could still pass directly over the islands, bringing tropical storm force winds and heavy rains.

The weather service Monday afternoon cited high seas and strong winds in a tropical storm warning for Hawaiian offshore waters beyond 40 nautical miles out to 240 nautical miles including the portion of the Papahanaumokuakea Marine National Monument east of French Frigate Shoals.

Even if the storm passes north of the islands, Guillermo is about 350 miles wide and Hawaii will still see some rain, possibly heavy rain Wednesday and Thursday.

Wind shear tearing at the storm continues to slowly weaken it, but Guillermo should still have winds greater than 40 mph when it passes just north of the islands or over the islands Wednesday into Friday.

A tropical storm warning was posted Monday at 12:30 p.m. for offshore waters from 40 to 240 nautical miles out. Northeast winds up to 23 mph and seas up to 9 feet were expected Monday afternoon, officials said.

“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.

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.

A high-surf advisory for the eastern shores of Oahu, Hawaii island, Kauai and Maui is posted through 6 p.m. Tuesday. Forecasters warned that the surf– expected to be 10 to 14 feet through Tuesday — on those shores could be “life-threatening” on Monday. 

As Guillermo weakens, a new storm system is developing in the Eastern Pacific and is expected to become a tropical depression later in the week as it moves toward the Central Pacific, according to the National Hurricane Center in Miami. The developing storm is about 1,000 miles southwest of Baja California.

Guillermo’s 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 60 mph on Tuesday, decreasing to 50 mph on Wednesday and 45 mph Thursday.

Foster said if the storm passes close or over to the islands, he “wouldn’t be surprised” to see similar wind damage from Guillermo 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.

“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.” 

Tradewinds are expected to gradually decrease as Guillermo disrupts the flow ahead of its arrival, the National Weather Service said. 

Crews from the Air Force Reserve’s 53rd Weather Reconnaissance Squadron — known as the Hurricane Hunters — have been flying through the storm 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. 

A NOAA Gulfstream IV plane is also taking measurements in the storm.

The Hurricane Hunter and NOAA planes are alternating 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.

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The Hawaii Emergency Management Agency offers these safety and preparedness recommendations in advance of Tropical Storm Guillermo:

>> Do not use candles during a blackout or power outage due to risk of fire.

>> Use 9-1-1 only to report life-threatening emergencies.

>> Make sure to monitor local broadcasters and/or sign up for local notification systems if you live in a flood prone area.

>> Do not attempt to cross fast moving water, especially if you are unsure of the depth.

>> Follow the Department of Education online at http://www.hawaiipublicschools.org for the latest information on possible school closures. Make sure you know the closure notification procedure if your children attend a private or charter school.

>> Check with local officials and the Department of Land and Natural Resources (DLNR) if you plan to hike or camp next week. Parks and other trails may be closed depending on the weather forecast. Visit the DLNR website at http://dlnr.hawaii.gov/blog/category/news/ for the latest closure information.

>> Visitors should download and read the Hawaii Tourism Authority’s Travel Safety Brochure at http://www.travelsmarthawaii.com.