textproduct: Honolulu

SYNOPSIS

Breezy east-northeast trade winds will gradually veer to the east and then east-southeast this weekend into the upcoming week as Hurricane Lowell approaches from the southwest. Lowell is forecast to pass west of Kauai's coastal waters as a category 2 or 3 Hurricane and a Tropical Storm watch is in effect for Kauai and Niihau. Impacts such as high surf and heavy rain are forecast to extend hundreds of miles from the center, potentially impacting any of the islands.

DISCUSSION

Radar and satellite imagery this afternoon shows a wetter that normal summertime trade wind pattern with scattered to numerous showers over windward and mountain areas. Increasing moisture upstream is also evident and will contribute to increasing showers this evening into tomorrow. Weather conditions then deteriorate Sunday evening into the upcoming week.

Hurricane Lowell is currently sitting about 620 statute miles southwest of Kauai moving west-northwest at 4 kt as a category 4 (130 mph maximum sustained winds). Lowell is forecast to continue its northerly track and move slightly northeast tomorrow through Monday. It's forward speed will increase over the next 2 days and it will make its closest approach to the main Hawaiian Islands Monday night as a category 2 to 3 hurricane. Lowell is forecast to pass west of Kauai's coastal waters, and will bring several hazards to the state. The main hazards are expected to be flash flooding, strong southerly winds, and high surf. A Tropical Storm watch is in effect for Kauai and Niihau and its associated waters.

The greatest risk for flooding at this time appears to be Sunday evening through Tuesday morning, and could potentially linger into Wednesday, as deep tropical moisture associated with Lowell is drawn up from the south. The environment will begin destabilizing Sunday afternoon as CAPE values and moisture depth increase. The winds will also veer to the east- southeast and increase starting Sunday evening. As of this forecast package, the islands of Kauai and Niihau are expected to receive the highest rain totals, however the threat of heavy rainfall and thunderstorms will be present anywhere in the state, and flooding could occur as a result. Heavy showers and thunderstorms that develop will likely be disorganized. Therefore everyone in the state should be prepared for flooding. A flood watch is in effect statewide from Sunday evening through Tuesday.

Winds will increase and veer out of the east-southeast by Sunday night into Monday morning with Kauai County having the strongest winds. Winds will also increase out of the southeast for the rest of the islands to a lesser extent, and could be amplified by island terrain.

The threat of short-lived, fast-moving waterspouts will be present mainly near Kauai and Niihau as Lowell makes its closest approach to the islands. These waterspouts would likely be embedded in squally rainbands that would be tornadic if they come ashore. This is due to the islands proximity to Lowell's east side, where low level shear and instability will increase as the center of circulation passes west of Kauai. On Monday, 0-1 km shear increases to 20-25 kt or more on Kauai and Niihau, and 0-1 km SRH increases for a time to 200-300 m2/s2 in the RRFS. The threat appears quite a bit less for the other islands, though non- zero in any downwind convergence zones peeling off the islands. This threat should decrease quickly early Tuesday morning.

For surf and other marine impacts, please see the marine section of the discussion below.

Any deviation in track or intensity could also change how these hazards effect the islands.

High pressure is forecast to build back in Thursday through Saturday, bringing in drier and more stable conditions. Long range models then indicate a tropical wave potentially moving over the islands next Sunday into Monday, although confidence is very low this far out.

AVIATION

Moisture within a moderate to locally fresh easterly trade wind flow will produce MVFR conditions over most windward slopes through tonight, while VFR will prevail leeward. AIRMET Sierra for mountain obscuration now covers north through east facing areas from Kauai to Maui, and Big Island may need to be added by evening. AIRMET Tango remains in effect for low-level mod turb over and south through west of all island mountains, but this AIRMET may be dropped as early as this evening as winds drop slightly and the inversion weakens.

Expect winds to decrease only slightly and veer more easterly on Sunday. The atmosphere will slowly destabilize through the day, and deepening moisture along the far periphery of Hurricane Lowell may overspread the state and lead to isolated heavy showers and thunderstorms in the afternoon.

MARINE

A surface high north of the Hawaiian Islands and will slowly drift east through the weekend. Currently, Hurricane Lowell is around 540 nm south-southwest of Kauai as it moves to the west- northwest. The trade winds have continued to gradually ease through the day, and therefore the Small Craft Advisory (SCA) has been cancelled. Lowell is expected to turn more northward on Sunday, which will cause winds across the coastal waters to veer to the southeast to south with the strongest winds across the western end of the state. A Tropical Storm watch is in effect for Kauai and Niihau and its associated waters.

Based on the latest advisory from the National Hurricane Center, hurricane conditions are expected in the offshore waters west of 160W by Sunday night and lingering through Tuesday as it tracks north. While confidence is increasing in the track forecast of the cyclone, any slight deviation in the track to the west or east from the current predicted path will have significant differences in impact for the coastal waters around Kauai County. As for Tropical Storm Karina, it is expected to weaken and dissipate as it moves northeast of the state.

Southerly swell generated by Hurricane Lowell will keep surf along south facing shores at High Surf Advisory criteria through at least Sunday. As Lowell begins to turn more north, its swell will gradually veer from south to southwest and begin a rapid upward trend. Very large surf exceeding warning criteria appears increasingly likely for south and west facing shores by Monday. Coinciding also with monthly high tides, this will create the potential for impacts to harbor entrances, channels, as well as vulnerable coastal properties and infrastructure including roadways. Surf will then rapidly decrease on Tuesday after the hurricane passes north of the island chain. A pulse of northwesterly swell could then bring a minor bump to north shore surf on Monday night into Tuesday and fade through Thursday.

East facing shores will see continued advisory level surf from a large, medium-period east swell generated by Tropical Storm Karina to the east of the islands. This swell will gradually fade early next week as Karina weakens. Late next week, surf along east facing shores could be bolstered by some medium-period swell generated by Hurricane Marie in the East Pacific. Otherwise, expect choppy conditions from easterly trade wind swell through the week.

FIRE WEATHER

Fire weather risk will remain low through the forecast period due to increasing moisture associated with Hurricane Lowell. The temperatures inversion heights are currently near 8,000 ft, and are expected to become elevated during the next few days.

HYDROLOGY

Heavy rain looks likely Sunday evening through potentially Wednesday, depending on how long the moisture lingers in the area. Storm total rainfall estimates range from 2 to 8 inches statewide. Locally higher amounts (around 12+ inches) are possible for Kauai due to its proximity to Lowell. It is important to note that heavy showers or thunderstorms could develop anywhere, bringing intense rain rates and significantly raising storm total rainfall amounts. A flood watch is in effect statewide from Sunday evening through Tuesday.

HFO WATCHES/WARNINGS/ADVISORIES

High Surf Advisory until 6 AM HST Tuesday for Niihau-Kauai Leeward-Waianae Coast-Olomana-Kahoolawe-Maui Leeward West-Kona- Kauai East-Kauai South-East Honolulu-Honolulu Metro-Ewa Plain- Koolau Windward-Molokai Windward-Molokai Southeast-Molokai West- Molokai Leeward South-Lanai Leeward-Lanai South-Maui Central Valley South-Windward Haleakala-Kipahulu-South Maui/Upcountry- South Haleakala-Big Island South-Big Island Southeast-Big Island East-Big Island North.

Tropical Storm Watch for Niihau-Kauai-

Flood Watch from Sunday evening through Tuesday afternoon for all Hawaii islands-

Tropical Storm Watch for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel.