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Menu of Text Products for the Hawaiian Islands and the Tropical Pacific/Atlantic Oceans: Select a text product: Hawaiian Islands Synoptic Discussion and Guidance Hawaiian Islands 7-day Zone Forecast Terminal Aerodome Foreasts (TAFS) Area Forecast for Hawaii and Adjacent Coastal Waters AIRMETs for IFR Conditions, Turbulence, and Icing Oahu Surf Observation and Forecast/Coastal Forecast for Hawaiian Waters Offshore Forecast for Hawaiian Waters North Pacific High Seas Forecast South Pacific High Seas Forecast Hawaiian Islands Automated Surface Observation System Hourly Weather Roundup Hawaiian Islands Wind Data Hawaiian Islands Rainfall Summary Daily Hawaii Cooperative Observations

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Hawaiian Islands Synoptic Discussion and Guidance 955 FXHW60 PHFO 251402 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 402 AM HST Wed Apr 25 2018 .SYNOPSIS... Strong high pressure far north-northeast of the state will maintain breezy trades, with windward and mauka showers. The trades will gradually weaken from tonight through early Thursday, then shift out of the southeast by late Thursday ahead of a front. The front will affect the western end of the state Friday and Friday night, then move to the eastern end of the island chain on Saturday. A band of clouds and showers associated with the front will likely bring wet and unsettled weather to portions of the state starting late Thursday or Friday, and continuing this weekend. && .DISCUSSION... A 1034 mb surface high located about 1,330 miles north-northeast of Honolulu is maintaining a tight pressure gradient across the state early this morning. As a result, the breezy and gusty low- level trade wind flow persists across the island chain. Aloft, a mid-level ridge west of the state is providing northwesterly flow high above the island chain, with significant upper-level subsidence. This is maintaining stable atmospheric conditions over the region. The early morning soundings from Kauai and the Big Island indicate the low-level trade wind inversion is between 7 and 8 thousand feet. The latest MIMIC precipitable water (PW) values are 1.0-1.1 inches, with isolated pockets of up to 1.3 inches, in the vicinity of the state. Note that the PW values just upstream of the state are 1 inch or less. Satellite imagery and radar reflectivity data show broken low clouds with scattered trade showers streaming into windward Maui and the Big Island. Elsewhere, the coverage of low clouds and showers has decreased dramatically overnight compared with Tuesday evening. Based on the latest forecast guidance, the surface high will remain far north-northeast of the area. However, since it will be gradually weakening, the pressure gradient will relax slightly starting later today or tonight. This will likely bring trade wind speeds down a notch by this evening. The mid-level ridge will also remain in place, which will keep the stable atmospheric conditions across the region. The slightly drier conditions upstream of the islands will continue to spread westward. This will likely reduce the coverage of low clouds and showers over many areas later today compared with Tuesday. Expect some changes to our present trade wind weather pattern to begin taking shape by Thursday. The GFS and ECMWF forecast models continue to show the mid-level ridge will weaken due to an upper- level trough digging down from the northwest toward the state on Thursday. This will result in some destabilization of the atmosphere, especially by late Thursday. The trough aloft will also likely cause the development of a deep surface low about 800 miles north of the islands. This feature will drag a cold front down toward Kauai late Thursday night, which will cause the trade winds to weaken, and eventually shift to a southeast direction. The models also depict an area of slightly enhanced low-level moisture moving in from the east by Thursday. Assuming this pocket of increased moisture arrives in the islands, expect an increase in low clouds and showers, especially over windward areas. The models also continue to hint that a surface trough may develop near the western islands. This feature may become the focus for prefrontal showers Thursday night and Friday morning, especially over Kauai and Oahu. We will have to monitor this situation, since there is a potential for heavy downpours, mainly over the western half of the island chain. On Friday, the winds will continue to shift to a south to southwest direction ahead of the approaching front. The front is forecast to reach Kauai Friday afternoon, and eventually push down across Oahu to Maui County from Friday evening through early Saturday. At the same, the upper-level trough is forecast to pass over the area, while the islands will be near the entrance region of a developing jet stream aloft. Assuming these features evolve in this manner, it would greatly increase the likelihood of heavy rainfall, and possibly localized flooding, ahead of and along the front as it moves down the western end of the island chain, especially from Kauai to Molokai. The potential for wet and unsettled weather shifts to the central and eastern islands as the front continues to push eastward this weekend. The forecast models continue to depict differing scenarios for the front once it reaches Maui and the Big Island. The front may stall near the Big Island, or eventually push east of the state. If it stalls, there is a good chance enhanced moisture will be drawn up over the eastern islands. This could lead to flooding rainfall for the eastern end of the state starting some time this weekend. The latest guidance is leaning toward the front eventually pushing just east of the Big Island. We will keep conditions wet in the forecast through Sunday over the eastern end of the state, but there is a chance things could improve, especially over Maui County, later this weekend. To the west of the front, a drier and more stable northerly flow will fill in. Therefore, while there is still some uncertainty in where the front will move to, Kauai and Oahu have the greatest chance of experiencing drier conditions starting as early as mid- day Saturday. Heading into early next week, a return to typical trade wind weather appears to be unlikely. The upper-level trough will remain over the region, while a surface low is forecast to meander several hundred miles north of the state. As a result, surface winds will likely remain relatively light. In addition, the models are hinting that the remnant moisture from the frontal band may push back westward over the state some time early next week. If so, expect a wet weather pattern to return to portions of the state. && .AVIATION... Breezy trade winds will persist and only gradually weaken later in the day as high pressure remains far north-northeast of the islands. VFR will predominate, but MVFR conditions will mainly affect windward and mauka sections from time to time as the trades carry low clouds, some with embedded showers, through the state. An AIRMET for mountain obscuration continues in effect over Kauai, though will likely be cancelled this morning. With the fresh to occasionally strong trades, the AIRMET for TEMPO low level turbulence to the lee of the mountains also remains posted. && .MARINE... A 1034 mb surface high far NNE of the islands is producing moderate to locally strong trade winds over the area. A Small Craft Advisory (SCA) is posted for the windier marine zones around Maui and the Big Island for today and tonight. The high will weaken further as it moves southwestward through Thursday, causing the trades to become light and variable on Thursday night, and the cancellation of the SCA. A low is forecast to form rapidly far N of the islands Thursday night and Friday, with a front reaching Kauai early Friday evening. The front stalls near the Big Island Saturday afternoon, and lingers there for the rest of the weekend. Initially, right behind the front, northerly winds may briefly reach SCA criteria with some of the marine zones. The low N of the islands will be generating a sloppy swell toward the islands this weekend into next week, with associated seas rising above 10 feet in most zones. But before this swell arrives, a moderate 5 foot NW swell with a period of around 15 seconds, is due to arrive late Thursday, peaking early Friday. This swell is slated to be reinforced by a series of potential large NNW swells, generated by the low N of the islands late in the week. The amount of swell/surf that arrives in the islands will depend on the evolution of the low and associated fetch, and current indications are that a fairly significant swell will produce high- end advisory-level surf along exposed N and W facing shores, with the peak of the swell around Sunday. There is some uncertainty with this swell, and the forecast will likely need adjustments as details become clearer. The weather pattern in the S hemisphere has become conducive for S swells to arrive in the islands, and a series of relatively small SW to S swells is expected over the next week or so. Lastly, the rough and choppy surf along E facing shores brought on by the breezy trades, will start easing off after today, along with the diminishing trades. && .HFO WATCHES/WARNINGS/ADVISORIES... Small Craft Advisory until 6 AM HST Thursday for Maalaea Bay- Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters. && $$ Houston Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office

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