Cloudy and somewhat cool conditions are expected into Sunday as weak easterly trade winds focus modest shower activity over the central portion of the state tonight then near Kauai and Oahu on Sunday. The atmosphere will become increasingly moist and unstable Monday and Tuesday as a strengthening trough to the west of islands pulls moisture up from the deep tropics. Chances for heavy flooding rainfall and thunderstorms will increase on Tuesday and will remain rather high into Wednesday as the trough slowly moves across the area. A ridge will build over the islands Thursday and Friday, bringing drier conditions.


Cloudy and somewhat cool conditions prevail this afternoon under a gentle to locally moderate easterly trade wind flow. A weak surface high sitting about 500 miles northeast of the islands has brought in a somewhat cool airmass with dew point temps hovering around 60 degrees F and precipitable water values around 0.8 of an inch. Conditions are rather stable as a weak mid level ridge is producing a low level inversion between 4000 and 7500 ft according to the afternoon soundings. These dry and stable conditions are disrupted along a narrow band of low level moisture that has been stalled across Maui County, where only a few tenths of rainfall have fallen along windward slopes. Otherwise, little to no rainfall has been observed. A deepening upper level trough several hundred miles to the west is generating a thick shield of high clouds over and southwest of the state. This, along with the cool airmass in place, has caused afternoon high temps to struggle to get much higher than the mid 70s. Weak easterly trade winds and cloudy skies will prevail tonight into Sunday. Conditions will be somewhat cool with dew points in the lower 60s, but thick high clouds will prevent overnight low temps from dropping more than a few degrees below normal into the low to mid 60s range. Weak mid level ridging is expected to maintain somewhat stable conditions, while the main source for rainfall will be along the diffuse band of low level moisture. This feature will mainly affect windward Oahu and Molokai tonight then lift northward to Kauai on Sunday. As the mid level ridge further erodes and high clouds thin somewhat, there is a chance for some afternoon convection with a heavy shower or two over interior Kauai, but since odds are low, heavy showers have been left out of the forecast for now. The atmosphere will trend increasingly unstable on Monday as the mid to upper level trough edges closer to the islands. High clouds that may have thinned somewhat Sunday and Sunday night will thicken again, and deep tropical moisture will approach the islands from the south. This enhanced moisture may reach the Big Island and windward Maui by afternoon, where heavy showers have been added to the forecast along with increasing chances for snow. Some of this moisture may get caught up in the easterly trade winds and fuel heavy shower development along windward slopes across the rest of the state Monday night. Moist and unstable conditions will prevail Tuesday into Wednesday, raising the possibility of flash flooding as well as winter storm conditions on the high summits of the Big Island. The ECMWF and GFS models show a strong west Pacific jet stream diving into the nearby mid to upper level trough and driving it over the islands. This trough will tap tropical moisture surging from the south to create conditions conducive for heavy flooding rainfall and thunderstorms. Details remain unclear as to which portion of the state is most at risk, with the models hinting at surface low development somewhere over the islands Tuesday night or early Wednesday. Although the freezing level may be somewhat elevated during this time, chances are high that the upper reaches of the Big Island summits could see winter storm conditions with strong winds and significant snowfall. Drier and increasingly stable conditions will develop on Thursday and Friday as the deep trough lifts northeast of the state and low level ridging builds in from the east. This will result in south to southwest surface winds with pockets of showers passing overhead. Another front may approach the state next weekend.


Gentle background east southeasterly flow near the surface will allow for variable winds across much of the state tonight. A weak shower band straddled over Maui County this afternoon will slowly push north up the island chain tonight. Isolated MVFR ceilings and mountain obscurations will be possible near these light showers, but no AIRMET Sierra is in effect for this. A 130 knot subtropical jet over the state will bring moderate turbulence aloft over Kauai and Oahu tonight. AIRMET Tango is posted for these areas. This jet stream is also producing broken to overcast cirrus cloud cover over all islands. As the jet shifts east, the focus of the turbulence will likely shift towards the eastern end of the state Sunday. The band of high clouds is also expected to deepen during the day Sunday and may cause some moderate icing problems near the Big Island.


The long-period northwest swell that caused advisory level surf along north and west facing shores of the smaller islands yesterday through this morning has decreased sufficiently to warrant the cancellation of the High Surf Advisory. The small, short-period north swell from earlier today, will shift out of the north-northeast tonight. A reinforcing modest, longer- period swell from the north-northeast will filter in Sunday night and Monday. This could result in near advisory level surf for east facing shores through Monday night. Elsewhere, a small, long- period south-southwest swell will continue to provide a small bump to the relatively quiet surf along the south shores, before gradually subsiding through the rest of the weekend. An east to west surface ridge near 25N will maintain gentle to locally fresh trade winds through this evening. Winds will weaken and shift out of the southeast starting tonight as a trough deepens northwest of the area. Winds and seas will remain below Small Craft Advisory (SCA) levels for the next several days.

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