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Issued: Apr 22, 2019 3:47 AM HST

Synopsis
Moderate to locally breezy trade winds will continue through the work week, bringing periods of clouds and showers to windward areas, while leeward areas remain mostly dry. The trades will begin to ease a bit on Friday, with a further decrease over the weekend allowing for the development of sea and land breezes. This will favor showers developing over interior and mountain areas during the day, and in windward areas near the coast at night.

Discussion
Currently at the surface, a 1032 mb high is centered around 1550 miles northeast of Honolulu, with the ridge axis extending west- southwestward to a location around 500 miles north of Kauai. The resulting pressure gradient is driving moderate to locally breezy trade winds across the island chain early this morning. The airmass remains dry and stable, with precipitable water (PW) values ranging from 1.0 to 1.2 inches and inversion heights hovering around 6 to 7 kft based on the morning PHTO and PHLI soundings. Infrared satellite imagery shows clear to partly cloudy skies in most areas, with some patches of more extensive cloud cover in some windward locales. Radar imagery shows a few light showers moving into windward areas, with mainly rain free conditions in leeward sections. Main focus revolves around the potential for some unsettled weather over the weekend.

Quiet trade wind weather is expected to prevail through much of the week, with a ridge of high pressure holding in place to the north of the state. Moderate to locally breezy trades should persist through the work week, although a slight easing of the trades is expected on Friday as an approaching front weakens the ridge to the north of the islands. The airmass will remain dry and stable with PW values holding in the 0.9 to 1.3 inch range, while inversion heights hover between 5 and 8 kft. As a result, we should see a continuation of the dry trade wind weather through the work week, with rainfall amounts remaining on the lighter side and confined primarily to windward and mauka areas, particularly at night and during the early morning hours. An isolated shower or two may also reach leeward areas of the smaller islands during nights and early mornings, with generally dry conditions each afternoon. Leeward sections of the Big Island may see a few showers develop each afternoon/evening, with mainly dry conditions late at night and during the morning.

Model solutions continue to exhibit some differences over the weekend, and this is where forecast confidence decreases. Both the GFS and ECWMF do however show a deep upper level trough developing to the northwest of the state and digging southeastward towards islands during the weekend. This results in a further erosion of the ridge to the north of the state as a surface cold front edges closer to the area from the northwest, with surface troughing developing in advance of the front and in the vicinity of the island chain. This is expected to shift the background boundary layer flow around to the southeast and south over the weekend, with surface winds becoming light and variable allowing sea and land breezes to develop in many areas. Additionally, the low level inversion will erode as the upper level trough moves into the area, and a modest increase in PW values to above normal levels is expected as the boundary layer winds shift southerly. Overall, we should see a more convectively driven weather pattern over the weekend, with showers favoring interior and mountain areas during the afternoon and early evening hours and windward and coastal areas favored under a background east-southeasterly to southeasterly boundary layer flow. There is the potential that some of the shower activity could be locally heavy and a thunderstorm can't be ruled out at this time. Given the differences in the models and being so far out in the forecast period, will hold off on adding any mention of thunder to the forecast until details become more clear.

Aviation
A ridge of high pressure will remain far northeast of Hawaii and drive fresh trade winds over the next 24 to 36 hours. AIRMET Tango remains in effect for moderate turbulence below 6000 feet over and downwind of the terrain due to the trade winds. These conditions are expected to hold in place through today, possibly longer.

Trade winds will carry in periodic clouds and showers that will focus over windward and mauka sections. Brief MVFR ceilings and visibilities, and mountain obscurations may occur within the showers.

Marine
A ridge north of the state will maintain fresh to locally strong trade winds through Thursday. A front is expected to stall north of the state Friday into the weekend and cause the ridge of high pressure to weaken. This will result in a decrease of winds Friday through the weekend and may cause sea breezes to develop near coastal areas by Saturday afternoon. The current Small Craft Advisory (SCA) is in effect for the typical windy areas around Maui and the Big Island through Wednesday.

Surf along all shores is expected to remain below advisory levels for this week. A few small west-northwest and north-northwest swells will provide small surf along north and west facing shores throughout the week. A series of small south swells will provide small surf starting today and will likely continue through most of the week and into the weekend. Local PacIOOS/CDIP buoy at Barbers Point is starting to show an uptick in energy across the 16 to 18 second bands this morning. This swell should continue to fill in through the day today and will likely peak tonight and slowly drop over the next couple of days. Several reinforcing south swells are expected towards the latter half of the week. Surf along east facing shores will hold steady in the small to moderate range through Thursday due to the breezy to locally strong trades.

Watches/Warnings/Advisories
Small Craft Advisory until 6 PM HST Wednesday for Maalaea Bay, Pailolo Channel, Alenuihaha Channel, Big Island Leeward Waters, Big Island Southeast Waters.


DISCUSSION...Jelsema

AVIATION...Powell

MARINE...Kino