FXHW60 PHFO 050133 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 333 PM HST Mon Mar 4 2024 .SYNOPSIS... A wet trade wind pattern with isolated thunderstorms and locally heavy rain, especially over windward areas, will prevail through Wednesday night. Breezy trades will further strengthen during mid- week. && .DISCUSSION... The split flow pattern persists across the eastern half of the North Pacific Basin. A tendency for ridging north of the islands within the northern branch of the jet will maintain strong high pressure along and north of 30N through the week which will in turn maintain breezy to strong trade winds through the weekend. At the same time, islands will tend to reside along the axis of the southern branch jet with the western half of the state positioned along its poleward side beneath colder air aloft and the eastern half of the state tending to reside within the favorable right entrance region as upper waves periodically ripple through the area. Taken together, this sets the stage for a few days of wet trade wind weather with potential for moderate to locally heavy rain focused windward and mauka, particularly over the western end of the state tonight and the Big Island and Maui Tuesday and Wednesday. Tonight through Wednesday night. The next wave of moisture approaching the islands is evident on visible imagery where cloud tops in excess of 30kft and isolated thunderstorms have been noted where moisture is deepest. Broad cyclonic flow will exist areawide beneath the upper low while another wave of dynamic forcing sweeps across the state tonight. This should lead to a more even distribution of showers compared to last night with the greatest level of organization expected around Oahu and Kauai. Furthermore, this next wave lifting through the area will bring a reinforcement of cool temperatures aloft to the tune of -15C at 500mb (west) to -12C (east), notably colder than temps at the same level last night. Given that lightning has already been observed at times within the offshore waters, the preponderance of evidence certainly points toward the addition of thunderstorms to the forecast for all windward zones and adjacent nearshore waters through Wednesday night. The addition of thunder to these areas was the primary adjustment to the official forecast. Heavy showers will tend to move quickly due to strong trades helping to mitigate any threat for flash flooding. Instead, expect nuisance type flooding where the heaviest rain occurs. Given that extrapolation of today's satellite trends suggests the heaviest and most organized rainfall will focus over the western half of the state tonight, the flood watch for Maui and the Big Island has been cancelled. Guidance has keyed in on a third and fourth round of moderate to locally heavy rain potential Tuesday night and Wednesday night, both focused mainly over Maui and the Big Island, but still extending west to at least Oahu and probably Kauai. By Wednesday night, modeled 700-500mb layer average specific humidity on the order of 3-4g/kg indicates moisture quality around 2-3x greater over the western end of the state when compared to both last night & tonight while values over the Big Island and Maui remain rather static throughout. The overall synoptic scale setup also remains essentially the same, and the magnitude of heavy rainfall is likewise expected to be similar each night with rates topping out around 1.5"/hr on average. All told, the week will be characterized by a consistent moderate/locally heavy rain threat. The threat will focus over the western end of the state tonight and then the eastern half both Tuesday and Wednesday nights. Finally, web cams indicated substantial icing but little in the way of snow falling on the Big Island summits this morning. In all liklihood this was due to the prominent dry layer centered on the dendrite growth zone (-12C to -18C) in this morning's Hilo sounding. This has in turn allowed any snow falling out of the elevated cloud deck to sublimate before hitting the ground while also sabotaging any potential for dendrites to form efficiently in the first place. Instead, deep trade wind showers washing over the summits deposited several rounds of rain in the sub-freezing environment there. The Winter Storm Warning has therefore been extended through Wednesday night and has been re-focused on icing potential with only mention of passing light snow. Thursday through the weekend. The large scale pattern changes little during this time, but still just enough to shut off the wet trade pattern that will characterize the first half of the week. High clouds will continue to feature prominently as the islands reside along axis of the southern jet, as will breezy to locally strong trade winds. However, global model consensus at this time points toward another upstream wave deepening enough to cause downstream height rises and deep layer subsidence over the islands. This should erode instability and mid-level moisture resulting in a return to a more typical trade wind pattern. && .AVIATION... Moderate to breezy trade winds will persist for the next several days as surface high pressure builds north of the state. A low-level trough moving westward across the islands, along with an upper-level trough and subtropical jet stream will enhance shower activity for all islands, though still favoring windward and mountain areas. Periods of MVFR conditions with low ceilings and visibility in showers are expected mainly over windward airfields, especially overnight tonight, though the heaviest rainfall is expected over windward Big Island slopes. AIRMET Sierra for mountain obscuration is no longer in effect due to improved conditions today. However, increasing rainfall expected tonight will likely prompt a reissuance for most windward areas. AIRMET Tango is in effect for moderate leeward mountain wave turbulence over all islands due to breezy trade winds, and will continue through at least Tuesday. In addition, a portion of the subtropical jet over the state will continue to cause areas of moderate turbulence aloft over the eastern half of the state tonight. && .MARINE... A strong ~1032 mb high will move slowly W to E along 35N the next couple of days, supporting strong to near-gale ENE trade winds. Another high will build N of the islands toward the end of the week, with little change in wind speeds. A Small Craft Advisory is in effect for all waters, and will likely remain in place through the week. A persistent trough aloft will keep the island atmosphere unstable, with briefly heavy, fast-moving showers producing periods of strong wind gusts and low visibility. The potential also exists for cloud-to-surface lightning strikes in isolated thunderstorms, especially over windward waters. The persistent trade winds will generate rough and choppy surf along E facing shores through the week. Gradually diminishing medium- period swells out of the NW and N will continue to trend down through Tuesday. A small, medium-period NW swell is expected Wednesday, followed by a larger, longer period NW swell Friday and Saturday, that will produce peak surf heights near High Surf Advisory levels. && .HFO WATCHES/WARNINGS/ADVISORIES... Winter Storm Warning until 6 AM HST Wednesday for Big Island Summits. Small Craft Advisory until 6 AM HST Thursday for all Hawaiian waters- && $$ DISCUSSION...JVC AVIATION...TS MARINE...Birchard