SynopsisLocally breezy trade winds will continue through next week, becoming stronger for the next few days due to a building high to our north. Passing low clouds and showers carried by the trades will affect mainly windward and mountain areas. An upper level trough will move over the state this weekend, resulting in heavier showers and wet trade wind weather into early next week.
DiscussionWater vapor imagery shows a weak mid-level ridge over the state, with a trough to the northwest. At the surface, high pressure is located far north-northwest of the state, with a front in between around 34N. 12z soundings from Hilo and Lihue show inversions of 7-8kft and precipitable waters of 1.1-1.2 inches, which is drier than normal for this time of year. Early morning MIMIC total precipitable water imagery shows the drier air across the state, with a band of moisture south of the Big Island and across leeward offshore waters. There is also a pocket of moisture with values near 1.4 inches over the windward offshore waters east of Oahu.
The moisture associated with an old front has cleared south of the state. Some rain gages reported rainfall amounts up to an inch over the past 12 hours, with less over the last 3-6 hours as conditions started to dry out. Clouds remain banked across windward sections, with more typical trade wind showers expected for today. The additional moisture noted upstream will spread across the islands later today/tonight, enhancing trade showers once again. Forecast soundings/cross sections show a minor surge in moisture depth, but with the inversion remaining in place. We will also see a surge in trade wind speeds as well. The high to the north-northwest will shift to the southeast and weaken slightly. However, the front between us and the high will dissipate, bringing us more under the influence of the high and lead to the stronger wind speeds.
Heading into the weekend, the upper trough currently to the northwest will amplify and dig across the state this weekend, then linger nearby through Monday. Falling mid-level temperatures will help to erode the inversion and destabilize the atmosphere. Showers will be more active this weekend, and with the strong trade winds they will more frequently spread leeward as well.
The GFS and ECMWF are pretty similar with the overall pattern near the islands through the forecast period. By Tuesday, a strong mid-level ridge builds across the state from the west, reestablishing the inversion and returning us to a more typical trade wind weather pattern. Through a series of surface highs and fronts passing far north of the state, the subtropical ridge gradually shifts southward through next week. Late in the week is may approach the far northern offshore waters, with trade wind speeds across the islands dropping to more moderate levels.
AviationVFR conditions prevail at all TAF sites and these conditions are expected to prevail through tonight. There may be some brief MVFR conditions over windward slopes as moisture riding in on the trades interacts with island terrain.
No AIRMETS are currently in effect, however there may be some local low level turbulence over and south through west of mountain ranges due to the trade winds.
MarineWith the expected increase in wind speeds, the current small craft advisory already expands to include all coastal waters tonight. Based on the recent consistency as to trade wind speeds, we have extended the advisory through the weekend as well.
A series of small to moderate north and northwest swells will fill in over the weekend and again toward the middle of next week, but surf heights are expected to remain below advisory levels. A long period east swell is possible this weekend from the distant Hurricane Seymour in the Eastern Pacific. Furthermore, east facing shores will see an increase in rough and choppy surf under the strengthening trades, possibly approaching advisory level in the next few days.
HFO Watches/Warnings/AdvisoriesSmall Craft Advisory until 6 PM HST Saturday for Oahu Windward Waters, Oahu Leeward Waters, Kaiwi Channel, Maalaea Bay, Pailolo Channel, Alenuihaha Channel, Big Island Leeward Waters, Big Island Southeast Waters.
Small Craft Advisory from 6 PM this evening to 6 PM HST Saturday for Kauai Northwest Waters, Kauai Windward Waters, Kauai Leeward Waters, Kauai Channel, Maui County Windward Waters, Maui County Leeward Waters, Big Island Windward Waters.