Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 11/12/17


Area Forecast Discussion
National Weather Service Honolulu HI
330 PM HST Sat Nov 11 2017 .SYNOPSIS... An area of moisture moving across the islands this weekeend will interact with an upper level disturbance pushing over the state to bring wet weather to many locales. The weather will start to improve from east to west on Sunday as the moisture area moves away from the islands, with more settled trade wind weather expected for most of next week. A strong surface high developing north of the islands next week will strengthen the trades back to breezy levels for most area by Wednesday. && .DISCUSSION... A pocket of low level moisture (with CIMSS MIMIC precipitable water values around 1.75" or so) is pushing across the islands this afternoon. Meanwhile, a weak but digging mid-level shortwave is approaching the islands from the NW, bringing cooler air aloft. The combination of these two features is destabilizing the airmass and enhancing showers from the Kauai Channel to down past the Big Island late this afternoon. A few embedded thundershowers have also occurred today, mainly near Hilo and Kaneohe. So far, Kauai has remained fairly quiet, but more active showers are approaching from the east. The surface pressure gradient over the islands is fairly slack, keeping mostly gentle trades in the background flow around the islands. The pocket of low level moisture is expected to continue to move rather slowly westward across the state tonight, with enhanced showers reaching Kauai early this evening. As the upper trough continues to dig over the islands, expecting some locally heavy showers along with a few thundershowers. The background trade wind flow should limit the chances for excessive rainfall, but very heavy rainfall rates in the more intense cells could lead to some localized problems. The axis of deeper moisture should begin to push west of the islands overnight, and expecting a gradual decrease in coverage and intensity late in the night for most areas. Although there should be fewer trade-wind showers on Sunday and Sunday night, rather chilly 500 mb temperatures from -10C to -12C should provide some oomph to whatever showers do ride in. Primary focus should return to windward and mauka sections, but any heavier showers could briefly make their way over to adjacent leeward areas over the smaller islands. The unstable air and lingering moisture will probably lead to some locally heavy afternoon showers or thundershowers over the lee slopes of the Big Island on Sunday. Monday into Tuesday should be relatively quiet days with drier, more stable air as the trough axis weakens and shifts to the E of the islands. More typical trade wind weather is expected. The remnants of an old front will likely drop south over the islands Tuesday night, along with another shot of cooler air aloft, and this should lead to a minor increase in showers Tuesday night into Wednesday. The trades will increase back to breezy levels as this boundary pushes through, as strong high pressure far to the N drops S. A weak cold front could bring another band of showers through late in the week, followed by cooler and drier air. Although the models are in fairly good agreement with this feature, there are some timing differences and this is pretty far out in time. && .AVIATION... A moist and unstable trade wind flow will persist over the state tonight. An area of enhanced low level moisture covers all islands this afternoon, though the deepest moisture has yet to fill in on Kauai. An approaching mid to upper level trough has essentially wiped out the inversion, according to the afternoon soundings and aircraft data, and has triggered a few thunderstorms around the Big Island and Oahu. AIRMET Sierra for mountain obscuration will remain in effect for all islands through the evening and likely through the night for widespread MVFR conditions over mountains and along windward slopes. VFR conditions will mostly prevail over leeward sections of the smaller islands tonight, though brief periods of MVFR conditions will occur as showers periodically push over the terrain. In addition, an isolated thunderstorm cannot be ruled out anywhere. Unstable, though improving conditions are expected on Sunday. The area of low level moisture will push west of the state, likely decreasing low cloud cover. However, the mid to upper level trough will be overhead, providing unstable conditions that could trigger a heavy shower or two. && .MARINE... The combination of enhanced moisture and an upper level disturbance moving over the state will lead to locally heavy showers and the slight chance of thunderstorms through the night. Improving conditions, from east to west are expected tomorrow as drier air moves in. The High Surf Advisory has been cancelled. The declining north- northeast swell will continue to subside through tomorrow. A west-northwest swell is forecast to fill in Monday night and Tuesday, before peaking Tuesday night. The forecast has the swell peaking near 5 feet, 14-15 sec. However, due to the more westerly nature of the swell, Kauai will provide a fair amount of blocking to the other islands. Another north-northeast swell is forecast to peak midweek, near 9 feet, 10-11 sec at buoy 51000. This swell will likely bring advisory, to potentially warning level, surf to the east facing shores again. Advisory level surf is also a possibility for north facing shores as well. A series of small, long period south to southwest swells will move through the islands through the up coming week, producing small bumps in surf along south facing shores. Winds and seas should stay below the Small Craft Advisory (SCA) threshold through the beginning part of the week. High pressure building north of the state should return locally breezy winds by Wednesday, producing SCA conditions through the typically windier waters around the Big Island and Maui County. Seas may also reach the SCA threshold from the new north-northeast swell. && .HFO WATCHES/WARNINGS/ADVISORIES... None. && $$ R Ballard/Wroe/EATON