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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 Flash Flood Statement Special Messages 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

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Hawaiian Islands Synoptic Discussion and Guidance 193 FXHW60 PHFO 020632 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 832 PM HST Wed Apr 1 2020 .SYNOPSIS... Deep low pressure moving in from far northwest of the state on Thursday and Friday will cause trade winds to veer out of the southeast, leading to a somewhat unstable land and sea breeze regime that will produce spotty showers, briefly heavy at times, over each island in the afternoon to early evening hours. Winds will turn more southerly on Friday and the weekend, likely drawing up deeper moisture over the islands and fueling more shower activity. A convergent cloud band with the low will develop over the state starting Friday night keeping unsettled weather in the long range forecast through Tuesday. && .DISCUSSION... The satellite picture this evening shows an upper level low east of the main Hawaiian Islands and a low level trough moving westward through Kauai producing a slight enhancement for showers over Kauai County. On the radar imagery conditions appear fairly stable with passing isolated showers outside of the showers near Kauai. A strong low pressure system roughly 1300 miles north- northwest of Kauai will approach the islands over the next few days, potentially delivering additional wet weather impacts to the state on Saturday and Sunday. A light east to southeast wind regime will continue for the next several days as a low pressure area north of the island chain continues to displace the high pressure ridge that normally lingers north of the region. Expect land and sea breezes to continue over all islands through Friday. Sea breeze convergence over each island will cause clouds and showers to develop during the day favoring afternoon to early evening hours. Overnight divergent land breezes will bring improving weather conditions over each island will diminishing cloud cover. Short range forecast guidance through Friday shows the subsidence inversion heights around the 7000 to 8000 foot level, enough for scattered shower activity, however the southeasterly background wind flow may limit shower production along eastern slopes as clouds tend to move more parallel to the island chain. The weather pattern changes this weekend as a large low pressure system drifting in from the north becomes cut off from the mid latitude steering flow. This low will slow down and become stationary roughly 900 miles northwest of Kauai on Friday. Initially a cold front out ahead of the low will approach the islands before losing forward motion as it weakens. A low level convergent cloud band will form over the islands, between the southerly and southeasterly winds, out ahead of the diminishing frontal boundary. Winds over the western half of the state will become light southerlies bringing up deep and unstable tropical moisture from the south by Friday afternoon onward. Shower activity will trend higher from Friday into the weekend especially over the western half of the state and along the southeastern slopes of the Big Island from Kau to Puna. Additional moister and instability within this convergent band may lead to heavy showers at times with isolated thunderstorms. Medium range model guidance between the American (GFS) and European (ECMWF) is in good agreement through Sunday. Longer range model guidance diverges a bit from Monday onward with the GFS showing a drying trend from Monday into Wednesday as the southern tail of this convergence band drifts east, while the ECMWF model keeping the convergent band with enhanced showers lingering over the islands through the middle of next week. Predicting where the tail of this cloud boundary will end up this far out remains challenging for the long range precipitation forecast. && .AVIATION... Mostly a nighttime land breeze, daytime sea breeze scenario into Thursday. Instability and light trades will keep the chance for isolated showers tonight, especially over Kauai and on windward coasts and slopes. Mountain obscuration should be brief and passing but will issue AIRMET Sierra for mountain obscuration if necessary. Thursday afternoon inland areas again could see showers, especially on the Big Island with sea breeze activity, some of which could be heavy. No AIRMETS are in effect at this time. && .MARINE... A surface high far northeast of the area is maintaining easterly trade winds across Hawaiian waters, but a trough over the Kauai Channel will keep the winds gentle to locally fresh though Thursday. A low south of the Aleutians is forecast to move south and approach the area, causing winds to veer out of the southeast tomorrow. Winds will weaken over the western part of the coastal and offshore waters but winds may strengthen over eastern waters, especially where the southeast flow accelerates around the Big Island and Maui. Winds could even reach Small Craft Advisory speeds over the Big Island windward waters and Big Island leeward waters off South Cape. A convergence band associated with the low will likely move over the northwest offshore waters Saturday. Winds will veer out of the south and a chance of thunderstorms will develop. The current small northwest swell will continue subsiding through Friday night. A new swell building Saturday will produce moderate surf along north and west shores over the weekend, but is expected to remain well below the advisory threshold. Swells from the southeast Pacific are partly blocked by the islands of French Polynesia, but the current swell is coming through a narrow swell window. This swell will maintain elevated surf on the south shores through tomorrow, but surf will remain below the advisory threshold. Surf along east shores will remain up through midweek due to an upstream fetch, despite the trades relaxing locally, then subside by the end of the week. && .HFO WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Bohlin AVIATION...Chevalier MARINE...Donaldson Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office

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