| Current Conditions | ||
| Temp |  | 0.2 C |
| RH | | 100 % |
| Wind | | SW 49 mph |
| Road | | Closed |
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Hawaiian Islands Synoptic Discussion and Guidance 787 FXHW60 PHFO 220153 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 353 PM HST Thu Feb 21 2019 .SYNOPSIS... Generally light winds are expected across much of the area for the next couple days. Fairly dry conditions will also remain over the western half of the state. A surface trough lingering near the Big Island will enhance clouds and showers over Maui and the Big Island into the weekend. High pressure systems moving in from the northwest may strengthen the winds early next week. && .DISCUSSION... Water vapor imagery shows an upper level ridge just east of the state with a shortwave trough passing by to our north. At the surface, a trough is located just east of the Big Island and a weak low is a few hundred miles west-northwest of Kauai, with a very weak pressure gradient between them across the islands. The 00z sounding from Lihue shows a solid inversion just below 6kft and a precipitable water of 0.80 inches (which is about a standard deviation below normal for this time of year). The Hilo sounding showed a weaker low-level inversion and a more pronounced one near 13kft, and a PW of 1.46 inches (which is about a standard deviation above normal). MIMIC total precipitable water imagery captured this gradient across the islands, with higher values above 2 inches just offshore south and east of the Big Island that have made only slight progress westward during the day. With the light gradient in place across the state, onshore sea breezes were pretty widespread today. Cloud buildups have mainly affected interior and northeast sections of the islands, with little in the way of rainfall from them. The afternoon clouds are most noticeable across Kauai/Oahu away from the more widespread mid-level clouds farther east. The sea breezes will give way to offshore land breezes this evening, leading to some clearing overnight, at least across the western islands. Skies will remain at least partly cloudy with some showers in the east, where there is greater moisture closer to the surface trough. The surface trough near the Big Island will drift westward slowly through Friday, spreading additional low-level moisture across the Big Island and eventually Maui County as well. Rainfall chances are greatest on Friday, then start to taper off Saturday into Sunday. With the wind shifting more southeast near the Big Island, highest rainfall amounts will be focused across windward and southeast sections. An upper level trough moving through the area may enhance lift, and with the marginally unstable airmass and abundant moisture we could see embedded heavy rain showers. However, the upper trough and surface feature will be weakening with time and don't appear to provide a strong focus for convection. At this point we're not expecting widespread flash flooding and therefore do not anticipate a flash flood watch, although localized heavy rain is possible. Over the weekend, the surface trough will weaken and shift eastward as a new high builds in from the northwest of the state. Deeper moisture and some showers will linger longest across the Big Island, but conditions there should start to stabilize by Sunday night/Monday. This first high will be somewhat brief, dissipating early next week as a front passes by far north of the state. Another high will build in behind the weakening front, bringing an increase to north/northeast winds and cooler/drier conditions by the middle of next week. && .AVIATION... A surface trough near the Big Island will slowly track westward tonight resulting in an increase in showers over the Big Island tonight and Maui on Friday. Mountain obscurations due to these clouds and showers may become more extensive tonight and require an AIRMET later on. No AIRMETs are currently in effect at this time. A mix of mid and high clouds continue to stream across the central and eastern portion of the state. Low clouds over the islands are due to the afternoon sea breezes will provide brief mountain obscurations late this afternoon, which should begin to dissipate after sunset as land breezes take over. && .MARINE... A trough near the Big Island of Hawaii will maintain light to gentle variable winds over most Hawaiian waters and gentle to moderate southeast winds over waters east of the Big Island. Winds are forecast to remain below Small Craft Advisory (SCA) speeds through at least early next week. The trough will slowly weaken and shift east as high pressure builds far northwest of the main Hawaiian Islands. Light to moderate north winds will develop starting Saturday night. Winds will strengthen and shift out of the northeast by the middle of next week. Winds could reach SCA speeds over some areas at that time. Surf will remain below the advisory threshold along all shores through the weekend. A new northwest swell is expected to produce surf near advisory levels Monday night. A very large northwest swell may produce warning level surf by the middle of next week. This swell is also expected to boost wave heights above the 10 foot SCA threshold. && .HFO WATCHES/WARNINGS/ADVISORIES... Wind Advisory until midnight HST tonight for Big Island Summits. && $$ DISCUSSION...Bravender AVIATION...Foster MARINE...Donaldson Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office |
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