370 FXHW60 PHFO 182003 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 1003 AM HST Mon Nov 18 2019 .SYNOPSIS... An unsettled weather pattern will persist across the island chain through Tuesday, with locally heavy rainfall and thunderstorms possible statewide. The airmass will stabilize by Wednesday, but a showery wet trade wind pattern is expected to hold through the weekend. Breezy to windy conditions are expected Thursday through Saturday. && .DISCUSSION... Water vapor imagery shows an upper low centered about 80 miles northeast of Lihue and slowly tracking westward this morning. The 500 mb temperature on the 12Z Lihue sounding showed -10.5 C while the GFS depicted the coldest air at -14.5 C within the core. Precipitable water (PW) was a near normal 1.3 inches at Lihue with a bit higher values across the eastern portion of the state with 1.7 inches at Hilo. Meanwhile, at the surface, a 1029 mb high centered around 1250 miles north northeast of Hawaii is driving moderate trade winds across the island chain. A broad surface trough is located just over the islands which is disrupting the trade wind flow slightly. Visible satellite and radar show some high clouds associated with the upper low passing over Kauai and Oahu this morning. A few heavier showers have popped up across the northeastern end of Kauai as well as some thunderstorms a couple of hundred miles south of the island chain. Elsewhere, partly cloudy skies prevail this morning. Daytime heating over the islands, as well as the unstable atmosphere and dynamic forcing from the upper low, will contribute to the potential for heavy rain and scattered thunderstorms to develop today. Moisture parameters look better across the eastern portion of the state, but the higher instability and forcing will persist westward. Potentially, any part of the state could see heavy rain or thunderstorms, but coverage will not likely be widespread as the trade winds will be working against this development. A Flash Flood Watch remains in effect through 6 pm HST today. The upper low will continue to linger near Hawaii as it slowly weakens into Tuesday while the low level moisture hangs around. Thus, we will still have a slight chance for thunderstorms and heavy rain in the forecast for tonight and Tuesday. By Wednesday, the upper low will move northeast of the state with more stability returning. Trade winds will also strengthen on Wednesday as the surface trough moves west of the state and a new high pressure system north of the state builds in. The trades will keep windward and mauka showers in the forecast through the end of the week. The trade winds will become strong by Friday into the weekend as the high moves northeast of Hawaii and strengthens. Leeward sides of the smaller islands will likely get passing trade wind showers at times as the winds strengthen. && .AVIATION... The potential for heavy showers and a few thunderstorms will remain in the forecast today due to an upper low over the area. Although clouds and showers will be most active over north and northeast facing slopes with moderate to breezy trades in place, some will manage to spill over into leeward areas periodically. Trades will gradually trend down later today through tonight to light and variable for Oahu and Kauai by Tuesday morning. MVFR conditions are possible in and around SHRA/TSRA. AIRMET Sierra for mountain obscuration remains in effect over north and east sections of Kauai due to low clouds and showers, but may also be needed elsewhere as conditions evolve through the day. && .MARINE... Moderate to locally strong trade winds are being supported by a surface high passing well N of the islands, but a well-developed closed low aloft is disrupting the flow somewhat, as are the thunderstorms and heavy showers forming in response to the low's instability. As the low tracks generally W, a weak low-level trough is expected to sharpen slightly along the longitude of Kauai (160W), leading to a reduced ESE flow beginning tonight, while continuing to bring the potential for thunderstorm development. A Small Craft Advisory (SCA) posted through today for the windier zones around Maui and the Big Island should be allowed to expire as this occurs. Strengthening trade winds are expected by the middle of the week as the trough dissipates, and a new high pressure cell builds N of the islands. The low aloft will also weaken, and the potential for thunderstorms will diminish after Tuesday. Winds may become locally strong, potentially reaching gale force in some of the waters around Maui and the Big Island by the end of the week. Surf along all shores will remain below High Surf Advisory (HSA) levels the next day or two, but a new long-period NW swell arriving Wednesday will require a HSA for exposed shores of most islands through Thursday. If peak swell heights are larger than WaveWatch guidance (which is pretty much the norm recently), then surf could approach the 25 foot threshold for a High Surf Warning. Another NW swell arriving next weekend may also require a HSA. In the meantime, a new NW swell will build tonight and peak Tuesday, with peak surf remaining below advisory levels. Strong trade winds will likely drive an increase in short-period wind waves Friday into the weekend, leading to high surf along E facing shores. No other significant swells are expected. && .HFO WATCHES/WARNINGS/ADVISORIES... Flash Flood Watch until 6 PM HST this evening for all Hawaii islands. Small Craft Advisory until 6 PM HST this evening for Maalaea Bay- Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters. && $$ DISCUSSION...Foster AVIATION...TS MARINE...Birchard