000 FXHW60 PHFO 242013 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 1013 AM HST Tue Apr 24 2018 .SYNOPSIS... Strong high pressure northeast of the state will maintain breezy trades and a stable regime of mainly windward and mauka rainfall through tonight. Trades will gradually weaken on Wednesday and Thursday, then shift out of the southeast on Friday ahead of a front. The front will affect the western end of the state on Friday then move to the eastern end of the island chain on Saturday, likely bringing wet and unsettled weather to portions of the state. && .DISCUSSION... A somewhat typical spring trade wind pattern is in place. A 1034 mb surface high located 1,300 miles north-northeast of the state is maintaining a breezy and gusty trade wind flow. Ridging aloft is producing stable conditions, as overnight soundings and recent aircraft data show an inversion between 4,500 and 6,000 ft. Precipitable water (PW) ranges between 0.9 to 1.1 inches on the soundings, which is just below April normal, and random pockets of moisture observed in the trade wind flow contain PW values slightly above April normal. One such area of moisture is affecting windward areas from the Big Island to Oahu this morning, bringing up to a half inch of rainfall to the wettest windward slopes in the last three hours and less than one tenth of an inch to most windward areas during this time. Only minor changes are expected during the next couple of days. The breezy trades will hold through tonight then gradually decline Wednesday and Thursday as the surface high to the northeast weakens. The mid level ridge will weaken but remain strong enough to maintain somewhat stable conditions, with the inversion slowly weakening and rising as an upper level trough moves in from the northwest. The current pocket of moisture in the trade wind flow should clear the state by evening, and the GFS and ECMWF are showing another area of low level moisture moving in on Thursday along with increasing high clouds. A marked shift in the weather pattern will begin Thursday night as a deep low forms roughly 800 miles north of the state and generates a cold front about 250 miles north and northwest of Kauai. Trades will rapidly decrease and shift out of the southeast as the front develops and advances toward the islands. The GFS and ECMWF show the area of low level moisture lingering along windward areas, and although the atmosphere does not appear to be destabilizing rapidly, we will have to watch out for the potential of heavy showers, mainly over the western half of the island chain. The front will likely affect the western half of the state on Friday and Friday night. The GFS and ECMWF depict this feature as somewhat poorly defined initially, then merging with a surface trough forming in the pocket of low level moisture pooled near the islands. The supporting upper level low will likely be closest to the state late Friday, and the GFS and ECMWF depict the islands near the entrance region of a forming jet stream aloft. This points to the potential for localized heavy rainfall along the eastward-moving front, and given ongoing model differences, the greatest probability for this rainfall looks to be from Kauai to Molokai during this time. Uncertainty increases heading into the weekend, though there is high potential for wet and unsettled weather over portions of the state. The GFS and ECMWF have yet to show a consistent depiction of the front during this time, though the latest runs suggest that the feature will stall near the Big Island late Saturday. PW values in excess of 1.75 inches are expected along the stalled front as deep southeasterly flow draws up deep moisture from the tropics, and flooding rainfall is possible for most likely the eastern half of the state. To the west of the front, a drier and more stable northerly flow will fill in. And while uncertainty in the frontal position remains, Kauai and Oahu have the greatest chance of experiencing drier conditions as early as Saturday. && .AVIATION... Fresh to strong trade winds will carry in an area of low clouds and showers to the islands today. The clouds and showers will focus along windward and mountain areas from the Big Island to Oahu, with some showers tracking to leeward sides of the smaller islands at times. Brief mountain obscurations will be likely across windward areas today, with AIRMET Sierra for tempo mountain obscurations likely. AIRMET Tango is in effect through today over and west of the mountains on all islands due to the strong trade winds. && .MARINE... A Small Craft Advisory is currently posted through tonight for all Hawaiian Coastal Waters due to strong high pressure far north of the area. Winds are expected to decrease a bit Wednesday and Thursday as the high weakens. The current small northwest swell will continue to lower today through Wednesday. The first in a series of north northwest swells is expected to arrive on Thursday, peak on Friday, then lower gradually on Saturday. A rather strong low pressure system is forecast to develop north of the area this weekend. Surf heights later this weekend and on into early next week will be highly dependent on the location of the low center and the fetch orientation aimed at the islands. Current guidance suggests advisory level surf from this low but confidence in surf heights remains rather low since much will depend on the strength of the low and the fetch orientation. Easterly trade winds will continue to produce short period choppy surf along east facing shores through Thursday with a downward trend expected later in the week and on through the weekend. Small, mainly background south swells are expected through Saturday with a slightly larger south swell expected Sunday and Monday. && .HFO WATCHES/WARNINGS/ADVISORIES... Small Craft Advisory until 6 AM HST Wednesday for all Hawaiian waters. && $$ Discussion...Wroe Aviation...Wroe Marine...Burke