000 FXHW60 PHFO 240645 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 845 PM HST Sun Sep 23 2018 .SYNOPSIS... An area of low pressure northwest of Kauai will lift northeastward over the next couple days, dragging a weakening front toward the islands. Deep tropical moisture and showers associated with the front will overspread the western end of the state Monday night and Tuesday, then linger in the vicinity of Kauai through the middle of the week as the front stalls out and slowly dissipates. Hot and muggy conditions can be expected across much of the island chain as winds shift around to the south and southeast ahead of the front. A more typical trade wind weather pattern is expected to return Friday through next weekend as high pressure builds to the north of the islands. && .DISCUSSION... Currently at the surface, a 1005 mb low is located around 750 miles northwest of Kauai, with a front extending southeastward to near 24N 163W, then south-southwestward to near 14N 170W. Meanwhile, a large 1032 mb high is centered around 1950 miles northeast of Honolulu. Light to moderate trade winds continue to prevail across the eastern end of the state this evening, while light southeasterly boundary layer flow has taken hold over Kauai. Infrared satellite imagery shows mostly cloudy skies in windward areas from Oahu to the Big Island, with partly cloudy conditions in most leeward locales. Some more extensive cloud cover is seen over Kauai where some mid and high cloud is streaming in overhead. Radar imagery shows scattered showers over windward areas from Oahu to the Big Island, with a few isolated leeward showers beginning to wind down with the loss of daytime heating. Meanwhile over Kauai, scattered showers are streaming into southern and eastern shores due to the southeasterly boundary layer flow. Main short term concerns revolve around the timing and progression of deep tropical moisture into the islands, as well as the potential for some heavy rainfall over the western end of the state. A closed upper level low several hundred miles northwest of Kauai will shift slowly eastward tonight through Monday night, allowing the weak front west of Kauai to inch closer to the western end of the island chain. The upper level low will then open up and lift northeastward Tuesday through mid week as another highly amplified upper level trough digs well to the northwest of the state. This will likely halt the eastward progression of the front somewhere near or just west of Kauai Tuesday into Tuesday night, with the front or its remnant moisture then lingering over this area through Thursday and possibly into Thursday night. Both the GFS and ECMWF then indicate that high pressure will build north of the state Friday through the weekend, allowing a more typical trade wind weather pattern to overspread the island chain. Of note, both the GFS and ECMWF show a tropical cyclone south of the state next weekend, so this will need to be monitored closely in the coming days. As for forecast details, the GFS and ECMWF have come into better agreement with the timing and progression of deep layer moisture associated with the front west of the state moving into the western islands during the early to middle part of the week. The deepest tropical moisture with precipitable water values around or in excess of 2 inches appears to get held up over Kauai, with PW values dropping off to near normal or slightly above normal levels to the east. As a result, we will continue to advertise the highest rain chances over Kauai through much of the work week, with more of a convective shower regime in a sea breeze driven environment from Oahu eastward to the Big Island. There is some indication that heavy rainfall and even isolated thunderstorms could affect Kauai, particularly Monday night through Tuesday night. Will take a close look at the latest model guidance as it continues to come in this evening and make a decision on the need for a Flash Flood Watch for the island of Kauai with the morning forecast package. By Wednesday, the best large scale forcing for ascent will have lifted far enough northeast of the state that we should be left in a convective mainly sea breeze driven shower regime statewide, with these conditions continuing through Thursday. More typical trade wind weather featuring windward and mauka showers is then expected Friday through next weekend. && .AVIATION... Surface trough just west of the islands will continue to bring synoptic southeast winds to much of the areas. As the wind flow parallels the orientation of the island chain, winds will become light enough over the islands to allow land breezes to develop. Passing low clouds and showers will remain mostly confined to the windward and mauka areas through early Monday morning, with isolated MVFR ceiling possible. Some clearing is expected over the interior areas as land breezes develop overnight. Otherwise, VFR conditions will prevail. No AIRMET is anticipated through early Monday morning. && .MARINE... An area of low pressure far northwest of Kauai will continue to move slowly northeast and east through the middle of the week. In response, surface winds will be light to moderate and out of the southeast through Monday. A weakening front associated with the low is expected to approach the coastal waters surrounding Kauai Monday night into Tuesday. This will result in increasing showers which may be locally heavy at times, and winds veering more south to southwest Tuesday into Wednesday for the western coastal waters. Elsewhere, southeast winds are expected to persist with some scattered showers through the middle of the week. High pressure is expected to build across the area Thursday into Friday leading to the return of trade winds. Surf will remain below advisory levels along all shores through the week. A small to moderate north-northwest swell is expected Thursday into Friday. Small, long-period southwest and south swells will give surf along south facing shores a small boost throughout the week, but remain well below advisory level. && .HFO WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Jelsema AVIATION...Hui MARINE...Kino