Current Conditions | ||
Temp | ![]() | -2.7 C |
RH | ![]() | 99 % |
Wind | ![]() | N 2 mph |
Road | ![]() | Closed |
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Hawaiian Islands Synoptic Discussion and Guidance 996 FXHW60 PHFO 270322 CCA AFDHFO Area Forecast Discussion...CORRECTED National Weather Service Honolulu HI 522 PM HST Fri Feb 26 2021 .SYNOPSIS... Strong and gusty trade winds will continue into early next week, while a trough aloft keeps the island atmosphere unstable. Clouds and showers will focus over windward areas, with some occasional heavy downpours. Increasing moisture near Maui and the Big Island will bring a more widespread threat of heavy windward showers, and a slight chance of thunderstorms. && .DISCUSSION... Very strong (1046 mb) surface high pressure to the distant NE will gradually weaken the next couple of days, leading to a short-term reduction in trade wind speeds. A new strong high (near 1044 mb) will build far N of the islands Saturday night and Sunday, leading to an increase in wind speeds Sunday and Monday. Overall, strong and gusty trade winds will persist over the islands through early next week, potentially easing around mid-week as a large low pressure in the NE Pacific pushes the high westward, leading to a relaxation of the local pressure gradient. A Wind Advisory that was in effect for parts of the state has been cancelled, but the increased winds Sunday and Monday could warrant a re-issuance. The island atmosphere will remain unstable into next week, due to the presence of a persistent trough aloft, and its associated cold pool. Moisture riding in on the trade winds will be enhanced, leading to higher than normal cloud tops (and larger-than-normal rain drops), delivering heavier windward showers than might otherwise be expected with trade winds in place. While these showers will be progressive and will not linger over any one area for a prolonged period, high-resolution and global model guidance anticipates their repeated development over the windward slopes of Maui and the Big Island over the next couple of days. This persistent rainfall is expected to lead to increasing ground saturation and rising levels in streams and rivers, with additional rainfall potentially resulting in flash flooding as the weekend progresses. A Flash Flood Watch (FFA) is in effect for windward portions of Maui and the Big Island. An expansion of the FFA to windward portions of the other islands is not anticipated as guidance indicates the deepest moisture (and instability) over the Big Island occasionally spreading to windward portions of Maui. The cold temperatures aloft and deeper-than-normal moisture means that the Big Island Summits will receive periods of wintry weather, with the primary threats being freezing rain leading to icy roadways, although snow showers will mix in as well. A Winter Weather Advisory is in effect through Monday morning, and may need to be extended as the trough aloft lingers well into next week. && .AVIATION... Strong easterly trade winds will continue for the next several days, sustained by an area of very strong high pressure located north- northeast of the main Hawaiian Islands. AIRMET Tango remains in effect for low-level mechanical turbulence over and immediately south through west of mountains. Additionally, since especially strong winds are forecast to persist in the Alenuihaha Channel and south of the Big Island, AIRMET Tango is also in effect for surface winds greater than 30 kt. Low-level wind shear could also be a concern at a few locations into this evening, especially PHOG. As an upper-level low moves into the area into tonight, an uptick in shower activity is expected, particularly over eastern sections of the islands. MVFR conditions are likely in heavier showers, especially for the Big Island and Maui tonight through Saturday, though other islands could be included as well. There are currently no AIRMETs for mountain obscuration, but are likely tonight as showers increase further. Isolated thunderstorms are also possible on leeward slopes of the Big Island this afternoon. Otherwise, VFR conditions will prevail throughout the period. Instability from the upper low will generate a chance for thunderstorms over most of the Big Island and parts of Maui Saturday and Saturday night. && .MARINE... Large, strong high pressure located off the mainland west coast has and will continue to maintain a tight enough pressure gradient across the eastern Pacific to produce strong trades over the local Hawaiian Island waters. An early week eastern migration of a secondary northern Pacific high will keep a taut pressure gradient back across the islands and sustain strong trades through Tuesday. A mid week trough of low pressure developing over the Northeast Pacific will push northerly high pressure southeastward. This will slightly weaken the late week northeast to southwest pressure gradient and responding trades will fall back to more moderate magnitudes. Rough sea conditions will remain elevated under persistent strong trades over the next several days. The most recent ASCAT pass verified that trades were locally strong through the windier inter- island channels with only occasional gusts to gale force within the Alenuihaha Channel. Thus, to account for these ongoing widespread strong trades and high seas, an all coastal water Small Craft Advisory is still in effect through the weekend. Satellite-derived winds verify a large belt of strong to near gale trades focused over and upstream of the state. This very long fetch of robust trades will sustain rough, elevated seas and high surf along most east-facing shorelines. Some of this trade wind swell will also wrap around into adjacent northern and southern exposures. The High Surf Advisory will therefore remain in effect the next several days for the east-facing shorelines of Kauai, Oahu, Molokai, Maui and the Big Island. As current northwest swells fade through the day, a backing northwest swell will fill in this weekend and peak near or slightly above head high surf late Saturday into early Sunday. The next scheduled northwest swell will be larger than this weekend's and peak surf to near advisory levels along northern and western exposed shores late Wednesday into early Thursday. Small, Southern Hemispheric background swell will keep small southern- facing shore surf around through early next week. && .HFO WATCHES/WARNINGS/ADVISORIES... High Surf Advisory until 6 AM HST Monday for East facing shores of Kauai Oahu Molokai Maui and the Big Island. Flash Flood Watch through Sunday afternoon for Maui Windward West- Windward Haleakala-Haleakala Summit-South Big Island-Big Island North and East-Big Island Interior. Winter Weather Advisory until noon HST Monday for Big Island Summits. Small Craft Advisory until 6 PM HST Sunday for all Hawaiian waters. && $$ DISCUSSION...Birchard AVIATION...TS MARINE...Blood Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office |
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