Current Conditions | ||
Temp | ![]() | 7.5 C |
RH | ![]() | 28 % |
Wind | ![]() | NNE 17 mph |
Road | ![]() | Open (4x4) |
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Hawaiian Islands Synoptic Discussion and Guidance 023 FXHW60 PHFO 232003 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 1003 AM HST Mon Apr 23 2018 .SYNOPSIS... High pressure passing north of the islands today will bring locally windy trades over the state through Tuesday. Areas of moisture riding in on the trades will bring increased clouds and showers to some windward and mauka areas at times this week. Trade wind speeds will gradually decline during the latter half of the week as low pressure develops to our north. A cold front will start to push down the chain later Friday and Friday night, and may stall with wet weather over the islands this weekend. && .DISCUSSION... A surface high will pass about 900 mi N of the islands today, boosting trade winds to locally windy speeds through Tue. The high will stall roughly 1200 mi NNE of the islands through midweek, far enough away to allow the trades to moderate just a bit. A surface low will develop roughly 850 mi to the N of the islands Thu night, disrupting the trades and leading to land breezes. The low will sharpen a cold front near Kauai on Friday. Unfortunately, the models have been trending much slower with this front over the last day or so (a trend started by the 18z GFS run on Sunday), now showing it hanging up near Maui and the Big Island on Sat and leaving it there through the weekend. The brand new 12z ECMWF run is also in this camp. This will probably allow locally breezy N or NW winds to overspread the northern main islands, with lighter winds elsewhere through the remainder of the weekend. Trade wind rainfall will increase a bit today as an area of clouds and showers moves across Maui and the Big Island. Another area of clouds and showers riding in on the trades will push across the islands from E to W late tonight and Tue, followed by yet another for Thu before the low level flow begins to veer ahead of the next front. A slower front stalling over the islands has significant impacts for the weekend forecast. For now, the models are trending toward a rather ominous solution with very deep tropical moisture (precipitable water in excess of 2.1 inches) intersecting the frontal boundary. There will be more than sufficient cold air aloft and a low level focusing mechanism to generate excessive rainfall and heavy thunderstorms. The latest guidance suggests the highest risk extends from about Oahu to the Big Island. However, model placement and trends this far out (despite good agreement) are often suspect, and it's very possible that the models could switch gears once again, with the front in a completely different place by the weekend. It seems prudent for the afternoon package to raise PoPs for the southern half of the chain, and continue to carefully monitor guidance trends. && .AVIATION... Expect mainly VFR conditions through the forecast period with scattered showers along north and east slopes of all islands. Moderate to strong trade winds will produce low level mechanical turbulence over and downstream of the mountains. An increase in windward showers is expected tonight as enhanced moisture pushes into the windward slopes. && .MARINE... Fresh to strong trade winds will pick up slightly tonight and hold into Tuesday as strong high pressure currently far north of the state moves off to the northeast. A Small Craft Advisory (SCA) currently covers most waters from Oahu to the Big Island and expands to all Hawaiian waters tonight and Tuesday. Winds will gradually ease on Wednesday and Thursday as the ridge north of the state is weakened. The SCA will likely be trimmed back to the usual windy areas around the Big Island and Maui, then be dropped completely on Friday as a front moves over the state from the northwest. Uncertainty in the position of the front means low confidence in the wind forecast for the weekend. A pair of moderate northwest swells are due during the next several days, with considerable uncertainty for the weekend. The current swell is peaking at 4 to 6 ft at 14 to 15 seconds according to NOAA and PacIOOS buoys. This swell will drop to 3 to 4 ft at 12 seconds on Tuesday, then fade on Wednesday. A similarly sized swell will build on Thursday and peak on Friday. Uncertainty increases in the swell forecast for the weekend. The GFS and ECMWF are showing a gale low forming north of the state between 30N and 40N, but the models lack agreement in the exact position and strength of the low. There is the potential for a large north-northwest swell to produce surf well above advisory levels. Rough, moderate surf will hold along east facing shores during the next few days. The Mokapu and Hilo PacIOOS buoys currently show short-period, trade wind generated seas of 6 to 7 ft at 7 to 9 seconds. Seas should increase slightly tonight and Tuesday, but resulting surf along east facing shores should remain below the advisory level. East shore surf will gradually decline Thursday and Friday. Small to occasionally moderate surf is expected along south facing shores this week. Inconsistent southwest swell will persist during the next few of days. A moderate south-southwest swell is expected this weekend into early next week. && .HFO WATCHES/WARNINGS/ADVISORIES... Small Craft Advisory until 6 AM HST Wednesday for Oahu Leeward Waters-Kaiwi Channel-Maui County Windward Waters-Maui County Leeward Waters-Maalaea Bay-Pailolo Channel-Alenuihaha Channel- Big Island Leeward Waters-Big Island Southeast Waters. Small Craft Advisory from 6 PM this evening to 6 AM HST Wednesday for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-Oahu Windward Waters-Big Island Windward Waters. && $$ R Ballard/Morrison/Wroe Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office |
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