| Current Conditions | ||
| Temp |  | -4.0 C |
| RH | | 40 % |
| Wind | | NE 0 mph |
| Road | | Closed |
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Hawaiian Islands Synoptic Discussion and Guidance 282 FXHW60 PHFO 260635 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 835 PM HST Thu Feb 25 2021 .SYNOPSIS... Strong and gusty trade winds will persist across the region into early next week due to a series of strong surface highs to the north and northeast of the islands. Rainfall will remain focused over windward slopes, but an upper level disturbance will give an added boost to these trade showers. The disturbance may even kick off a few thunderstorms Friday and through the weekend over the eastern end of the island chain. && .DISCUSSION... An exceptionally strong and broad 1047 mb surface high is centered nearly 1600 miles north of the islands this evening. It is the main source of of the strong and gusty easterly trade winds. A Wind Advisory remains posted for portions of the Big Island and Maui County through Friday. There is a small handful of sensors registering sustained winds of around 30 mph and gusts to around 40 miles per hour, across Maui County and the Big Island. The trade showers have been limited area-wide, but if any, they are favoring the windward and mountain areas. Showers have been absent across leeward Big Island this afternoon and evening, but there is a small shower or two located off the Kona coast. Satellite imagery shows a field of widely dispersed showers mostly upwind of Maui and the windward coast of the Big Island from Upolu Point to the Puna district. Do expect these trade showers to pick up a little more as night progresses. Looking ahead into the weekend, the trade winds are expected to ease off slightly starting late Friday through Saturday. This temporary lull comes about as a new high starts moving in from the west to replace the surface high currently located 1600 miles north- northeast of the islands, which will be moving southeastward and weakening. The new high is not as strong as the previous but it is still rather strong at 1043 mb at a point far north of the islands Sunday night. The high then eases southward and weakens to 1037 mb by Monday night. Under this scenario, trade winds are likely to regain very wind status by Saturday night, leading to another Wind Advisory for Maui County and northern Big Island. More details are forthcoming. The forecast becomes more challenging as an upper level low with a pool of cold air of minus 13 to minus 14 degree C at 18k feet sinks over the islands from the north in the next 24 hours, but favoring the eastern part of the island chain. It is about minus 11 degree C currently at 18k feet area-wide. The air mass over the islands is not all that very stable now, and the approaching upper low and cold air will only destabilize air mass further more in the next 24 hours. So we are anticipating an uptick in heavier trade showers, area-wide. The strong and breezy trade winds will help move along most of these showers, meaning the lee side of the smaller islands will get their share of showers. We anticipated the wind- sheltered leeward Big Island to be an active Friday afternoon with an outside chance of a thunderstorms. The potential for a thunderstorm spreads over to the rest of the Big Island and windward Maui on Saturday, and persisting through Sunday. The area with the highest risk of sustained heavy rainfall will likely be windward Big Island, though a Flash Flood Watch does not appear warranted at this time. The upper low gives way to a trough by Monday, and this cold pool of minus 14 warms slightly to minus 12 degree C. Both the GFS and old ECMWF suggest things to improve across the smaller islands Sunday night into Monday, but keeping the Big Island unstable. It should be noted at this point of Monday, the GFS differs from the old ECMWF run, where the GFS maintain a windy trade wind flow, while the EC lighten the trades. && .AVIATION... Strong easterly trade winds will continue tonight and tomorrow as an impressive surface high remains rooted roughly 1600 miles northeast of the state. As a result, AIRMET Tango remains in effect to account for low level mechanical turbulence for areas over and immediately south through west of mountains. Additionally, since strong winds are forecast to persist through tomorrow, AIRMET Sierra remains in effect to account for surface winds greater than 30kts in the Alenuihaha Channel and south of the Big Island. Low level wind shear will also continue to be a concern at some area airports. Models indicate MVFR clouds and showers will be possible overnight and during the early morning hours for some locations along east- facing sections of the islands. This appears most likely at Lihue and Hilo, but will continue to monitor trends and update as needed for other east-facing locations. Otherwise, VFR conditions should prevail during the day tomorrow. && .MARINE... A very strong high northeast of the Hawaiian islands and a trough west of the area are creating a tight pressure gradient that is producing strong east winds. A Gale Warning is in effect for the Alenuihaha Channel and the Big Island Leeward Waters through Friday evening. A Small Craft Advisory (SCA) is in effect for the rest of the Hawaiian Coastal Waters through the weekend. Winds will subside a bit Friday night as the high weakens. Winds are likely to strengthen again over the weekend as a new high build north of the area. Winds could reach gale force again Sunday night or Monday. A long fetch of strong northeast and east trade winds is producing elevated seas. A High Surf Advisory in effect for exposed east facing shores will remain up for the next several days. Surf elsewhere will be below advisory levels. Several long-period northwest swells are expected over the next week. These swells will peak tonight into Friday, Saturday night into Sunday, and Wednesday into Thursday. The mid-week swell is expected to be the largest of the three. Some trade wind swell will wrap into N and S facing shores. A small swell from the southern hemisphere is also expected the next couple of days. && .HFO WATCHES/WARNINGS/ADVISORIES... High Surf Advisory until 6 AM HST Monday for Kauai Windward-Oahu Koolau-Olomana-Molokai Windward-Maui Windward West-Windward Haleakala-South Big Island-Big Island North and East. Wind Advisory until 6 PM HST Friday for Molokai-Kahoolawe-Maui Windward West-Maui Leeward West-Maui Central Valley-Windward Haleakala-Leeward Haleakala-South Big Island-Big Island North and East-Kohala-Big Island Interior. Small Craft Advisory from 6 PM Friday to 6 PM HST Sunday for Big Island Leeward Waters. Gale Warning until 6 PM HST Friday for Alenuihaha Channel-Big Island Leeward Waters. Small Craft Advisory until 6 PM HST Sunday for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel- Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui County Windward Waters-Maui County Leeward Waters-Maalaea Bay- Pailolo Channel-Big Island Windward Waters-Big Island Southeast Waters. && $$ H Lau/Vaughan/Kinel/Donaldson Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office |
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