| Current Conditions | ||
| Temp |  | 0.4 C |
| RH | | 96 % |
| Wind | | SW 15 mph |
| Road | | Open (4x4) |
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Hawaiian Islands Synoptic Discussion and Guidance 588 FXHW60 PHFO 190144 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 344 PM HST Thu Oct 18 2018 .SYNOPSIS... Gentle to locally moderate trade winds will persist through the next couple of days. Showers will remain focused across windward slopes, while instability near the Big Island will likely trigger leeward and interior showers and a few thunderstorms through this afternoon. An upper level trough slowly passing overhead from west to east on Friday through Sunday will produce unstable conditions and bring a risk of thunderstorms to much of the island chain. Trade winds will rebuild on Sunday and early next week, though unstable conditions may linger over portions of the state. && .DISCUSSION... A gentle to locally moderate trade wind flow remains in place, along with moderately unstable conditions around the Big Island. A slow-moving front sitting less than 500 miles north of Kauai has weakened the local pressure gradient and resulting trade winds. A nearly stationary mid- to upper-level trough remains overhead, sending high clouds over the eastern half of the state and producing some instability. While low level ridging continues to be strong enough to maintain modest stability over the western half of the state, a stubborn pocket of moisture with precipitable water values of 1.75 to 2 inches is contributing to instability around the Big Island. This will continue fuel a diminishing amount of showers and thunderstorms over interior Big Island through the remainder of the afternoon. The rest of the state will experience mainly windward showers through the night. On Friday, trades will remain somewhat weak, and unstable conditions will develop over much of the state. A shortwave aloft, currently located about 500 miles northwest of Kauai, will drop into the mid- to upper-level trough already in place over the islands. As the shortwave trough passes over the western end of the state, 500 mb temps will approach -10C, but a lack of deep moisture will likely inhibit thunderstorms from Kauai to Maui. These islands will see continued windward rainfall with some afternoon showers over leeward terrain. Lingering moisture around the Big Island should trigger another round of afternoon thunderstorms. Gentle to moderate trades and unstable conditions will hold over most the state Friday night and Saturday. The latest runs of the GFS and ECMWF are showing some differences in the handling of the mid- to upper- level trough. The GFS keeps the feature moving eastward, while the ECMWF suggests that it will stall over the central islands on Saturday. Expect unstable conditions to persist. The trades will be strong enough to produce showers over windward slopes. Thunderstorms could pop up anywhere during this time, mainly over interior terrain during the day and windward terrain at any time. With 0-6 km wind shear climbing to around 30 kt and ample instability, strong thunderstorms will be possible over central and eastern portions of the island chain. Building trade winds and decreased instability are expected on Sunday. The GFS and ECMWF continue to differ on the upper level features, so there is some uncertainty. The GFS pushes the now cut-off mid- to upper-level low east of the Big Island, while the ECMWF drifts the gradually weakening feature just southwest of the state. The GFS solution would suggest lingering instability over the Big Island and typical trade wind weather elsewhere. The ECMWF solution points toward decreasing chances of heavy showers and thunderstorms over most islands. For now, the forecast leans heavily toward the GFS. By Monday, trade winds will strengthen to locally breezy levels. Uncertainty in the location of the low aloft remains, with the ECMWF pointing toward a wetter and less stable pattern. Both the GFS and ECMWF hint at deeper tropical moisture moving up from the southeast Tuesday and Wednesday and potentially affecting the southern end of the island chain. && .AVIATION... Light winds, combined with some instability over the Big Island, have produced a few towering cumulus and thunderstorms this afternoon. IFR conditions are possible in and near thunderstorms, with some improvement expected after sunset. AIRMET SIERRA for mountain obscuration was expanded to include all of the Big Island this afternoon with the cloud buildup, but expect that to be trimmed back, and possibly eliminated overnight. Clouds have also increased over Maui, so AIRMET SIERRA for mountain obscuration was also expanded this afternoon to over the whole island. Conditions should improve over Maui overnight. Winds will continue to weaken over the next 24 hours, and thunderstorms are possible again tomorrow afternoon on the Big Island. With the lighter winds, we could see additional clouds and showers over all the islands tomorrow afternoon, which could prompt additional AIRMETs for mountain obscuration. Otherwise, VFR conditions are expected. && .MARINE... Trade winds are forecast to remain weak through Friday night. The trades may gradually strengthen from Saturday into next week. The current forecast does not have Small Craft Advisory (SCA) conditions over any of the coastal waters or channels through Sunday night. SCA conditions may develop across the typically windy waters adjacent to the islands of Maui County and the Big Island early next week. Unstable atmospheric conditions are forecast to develop over parts of the state beginning Friday night. This may cause locally heavy downpours, and allow thunderstorms to form across some areas this weekend. Mariners should monitor future weather forecasts as this situation evolves. The current north northwest swell will subside through this weekend. The weak trades will cause surf to slowly decrease along most east facing shores through Friday. However, some east facing shores with a northerly exposure may continue to see small breakers from the north northwest swell energy during the next couple of days. A new long period south swell is expected to Friday and peak this weekend. Based on the latest guidance, surf will likely reach the advisory criteria along south facing shores starting Saturday. There is also a possibility surf may approach the High Surf Warning threshold along south facing shores this weekend, before the swell begins to gradually decline early next week. See the latest Collaborative Nearshore Swell and Wind Forecast for Oahu (SRDHFO), which was updated Wednesday afternoon, for more details on swells and surf. && .HFO WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Wroe AVIATION...M Ballard MARINE...Foster Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office |
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