Update: 5:15 p.m. ET : The National Weather Service has issued a hurricane warning for the Big Island of Hawaii, as satellite data and information from hurricane hunter research aircraft indicate that Hurricane Iselle strengthened a bit on Wednesday. As of 10 a.m. Hawaii Standard Time, Iselle was about 625 miles east-southeast of Honolulu, Hawaii, moving northwest at 16 miles per hour. Maximum sustained winds had increased to 90 miles per hour, which was up from 85 miles per hour from the previous advisory.According to the National Weather Service, "Hurricane conditions are expected on the Big Island of Hawaii on Thursday," with tropical storm conditions spreading to Maui and Oahu, depending on the storm's track and strength.  Two tropical cyclones, Hurricanes Iselle and Julio, are spinning their way toward the Hawaiian Islands. Iselle is expected to make landfall on the Big Island on Friday morning local time, most likely as a tropical storm with maximum sustained winds between 39 and 73 miles per hour.  Julio is now projected to skirt the islands to the north over the weekend, but may come close enough to bring another round of uncharacteristically wet and winds weather to the state.  A parade of storms are lined up across the tropical Pacific Ocean.  As of Wednesday at 5 p.m. ET, Iselle had maximum sustained winds of 90 miles per hour, making it a Category One hurricane. This was significantly weaker than its peak intensity as a Category Four storm just a few days ago, but weakening was expected as the storm has encountered a more hostile environment, with cooler ocean waters and stronger winds aloft, both of which tend to disrupt such storms.  Tropical storm warnings are in effect for the Big Island, including Hilo, while a tropical storm watch is out for Oahu, including the city of Honolulu. A flash flood watch was also in effect for all of the Hawaiian Islands until Saturday morning local time.  Forecast track of Hurricane Iselle as of August 6, 2014.  Hurricanes are not typical occurrences in Hawaii, as the islands are typically protected by ocean currents that bring cooler waters to the surface to the east of the islands, causing storms approaching from that direction to sputter out as they approach. In addition, there tends to be greater wind shear near the islands, which occurs when winds blow in different directions or at different speeds, or both, with height. Wind shear can disrupt tropical cyclones by eroding the tall thunderstorms that wrap around the storm center.  The last hurricane to make landfall in Hawaii was Hurricane Iniki in 1992. That storm, which was a Category Four storm on the Saff Simpson Scale, caused extensive damage on the western-most island of Kauai. No tropical storms or hurricanes have made landfall since then.  Since the beginning of reliable hurricane records, just seven storms of tropical storm intensity or greater have made landfall in Hawaii. The prospect of two striking there in the same week is extraordinary, and it may have something to do with the sea surface temperatures and atmospheric circulation patterns across the tropical Pacific.  Much of the water in the tropical Pacific Ocean is significantly warmer than average right now, from the western Pacific near Indonesia to the west coast of Mexico.  This is in part related to natural climate variability, such as the development of an El Niño event, but some portion of the above average water temperatures may also be due to global warming.  Pacific Ocean sea surface temperature anomalies.  This sea surface temperature pattern, together with the related air circulation patterns in the atmosphere, are promoting the development of tropical storms and hurricanes (and in the western Pacific, typhoons, which are the same type of storm just named differently). On Wednesday, four named storms were lined up like planes approaching busy Honolulu International Airport, including Typhoon Halong, which could unleash deadly mudslides in Japan when it makes landfall on Saturday (local time).  Only 7 tropical cyclones of TS+ intensity have hit HI, now 2 in 3 DAYS?! #iselle #julio @capitalweather pic.twitter.com/YUpRwBCLJi  — Brian McNoldy (@BMcNoldy) August 5, 2014  The National Weather Service in Honolulu warns residents that Iselle could bring "heavy rains... high surf and strong winds." The latest forecasts give Hilo, Hawaii a 63% chance of seeing low-end tropical storm force sustained winds from Iselle, and a smaller chance — 23% — of experiencing high-end tropical storm force winds, which would be sustained at 57 miles per hour.  In its ALL-CAPS style, the NWS said in a statement issued on Wednesday:  TROPICAL STORM CONDITIONS COULD REACH THE BIG ISLAND ON THURSDAY AND MAUI COUNTY LATE ON THURSDAY. IF ISELLE REMAINS ON THE FORECAST TRACK...TROPICAL STORM CONDITIONS COULD AFFECT THE OAHU AND KAUAI COUNTY THROUGH FRIDAY. HIGH SURF IS EXPECTED TO REACH THE ISLANDS AHEAD OF THE HEAVY RAINS AND STRONG WINDS. THE HIGH SURF MAY BRING COASTAL FLOODING...PARTICULARLY WHEN COMBINED WITH AFTERNOON ASTRONOMICAL HIGH TIDES.  Forecasters emphasized that there is still some uncertainty regarding the track and strength of the storm when it approaches the islands:  HURRICANE CONDITIONS ARE EXPECTED ON THE BIG ISLAND OF HAWAII ON THURSDAY. TROPICAL STORM CONDITIONS ARE EXPECTED TO SPREAD TO MAUI COUNTY THURSDAY NIGHT AND POSSIBLY TO OAHU ON FRIDAY. SWELLS GENERATED BY ISELLE ARE EXPECTED TO REACH THE MAIN HAWAIIAN ISLANDS TODAY...POSSIBLY BECOMING DAMAGING ALONG SOME COASTLINES STARTING LATE TONIGHT OR THURSDAY. HIGH SURF IS EXPECTED TO REACH THE ISLANDS AHEAD OF THE HEAVY RAINS AND STRONG WINDS. THE HIGHSURF MAY BRING COASTAL FLOODING...PARTICULARLY WHEN COMBINED WITH AFTERNOON ASTRONOMICAL HIGH TIDES...  IT IS VITAL THAT YOU DO NOT FOCUS ON THE EXACT FORECAST TRACK. FORECAST MOVEMENT...DIRECTION...AND SPEED ARE ONLY ESTIMATES. EVEN SMALL ERRORS IN THE FORECAST TRACK CAN MEAN MAJOR DIFFERENCES IN WHERE THE WORST CONDITIONS WILL OCCUR. DAMAGING EFFECTS CAN EXTEND FAR FROM THE CENTER.  It is possible that the high mountains on the Big Island, which include the 13,796-foot Mauna Kea and 13,679-foot Mauna Loa, will significantly weaken the storm if it passes directly over that area, thereby lessening impacts in Oahu and on other islands.  Computer model projections for Hurricane Julio, from the GFS Model on August 6, 2014.  "We've never had a direct hit (from a tropical storm on the mountains) before," National Weather Service meteorologist Eric Lau told the Honolulu Star Advertiser. "We just don't know."  Computer model projections for Hurricane Julio show that it will follow close behind Iselle, but that it may miss a direct hit on the islands, and instead pass to the north. If it is close enough, though, it would still bring strong winds, heavy rains and high surf to many areas, particularly the north-facing sides of the islands.  While Hawaii's hurricane history is relatively sparse, concerns have been raised that global warming will make the ocean waters — that have protected the area from tropical trouble for so long — more conducive to maintaining storm intensity all the way to the state.  Hurricane Iselle moving toward Hawaii on August 6, 2014.  A study published in May 2013 in the journal Nature Climate Change found that environmental conditions will become more favorable for tropical cyclones in and around the Hawaiian islands between 2075 and 2099. The study showed that warming temperatures in the central tropical Pacific would help alter weather patterns, and shift storm tracks northward.  Simulations of a hurricane striking Honolulu have shown that the low-lying area, which is full of strategic infrastructure including the state's main oil refinery, busiest airport and largest military bases, is extremely vulnerable to storm surge flooding.  Have something to add to this story? 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Hawaii, a state that tends to be protected from tropical storms and hurricanes, is bracing for two such storms in the same week -- an unprecedented event.