Marine Weather and Tide Forecast for Ualapu,e, HI

DISCUSSION
A relatively strong and large 1035 mb surface high is still centered roughly 1200 mi NNW of Kauai. This high has been meandering around in the same general area for the past few days, giving us breezy trades. Meanwhile, a mid- to upper-level low is located on water vapor imagery only about 100 mi N of Oahu.

The GOES-derived CAPE shows a pocket of significant instability associated with this low aloft which is validated by the 12z soundings, but so far there have mainly been only TCU associated with it, due in part to the breezy trades hindering updraft development. The only lightning detected by the GLM or NLDN overnight has actually been from a couple of brief CB that were located 60-100 mi S of Kauai and Oahu, in the downwind convergence zones from the wind shadow of the islands.

Guidance shows this upper low will start to lift slowly NE away from the islands today, gradually taking its pocket of cold air aloft and instability with it. Since the trades are expected to continue breezy, it is very tempting to remove the slight chance thunder from our forecast altogether, but have reluctantly left it in for Kauai for the morning in deference to local guidance and a disorganized band of moisture expected to move through today. We will also leave a slight chance of thunder over the interior slopes of the Big Island for this afternoon, where we have seen a lone thundershower pop up on the slopes of Mauna Loa the last couple of afternoons.

Fine weather is expected for most of the holiday weekend, with any lingering enhancement to incoming trade wind showers fading away on Sat. In fact, models show quite low PW for late May pushing in Sat night, with values maybe even dropping below 1 inch early next week. Although PW increases significantly over the islands toward the middle of the week, forecast soundings show that this PW is largely from abundant high- and mid-level moisture being drawn up by the jet stream from deep convection in the ITCZ far south of the islands, while maintaining a dry and stable boundary layer over us. Thus we are indicating an increase in clouds in the forecast, but not a lot of trade wind rainfall late in the forecast period.

Trade winds should remain breezy over the islands through about Sunday, then start trending downward on Memorial Day as the high gradually shifts south, weakens, and elongates across most of the eastern Pacific becoming a rather weak subtropical ridge by about Wed or Thu when gentle trades are expected, staying locally breezy only in the normal windiest spots.

AVIATION
An upper low near the main Hawaiian Islands will continue to enhance incoming trade wind clouds and SHRA. TEMPO MVFR ceilings and vis can be expected in any SHRA, and can't rule out isolated TSRA mainly near or over Kauai this AM. AIRMET for MTN OBSC may be needed for windward N and E facing slopes. Overall conditions should gradually improve later today with shower coverage and intensity diminishing as the upper low begins to move away. Trade winds will remain breezy for the next few days.

MARINE
No significant changes to the forecast with the morning package.
High pressure off to the distant north-northwest will maintain moderate to locally strong trade winds through the weekend. The high is expected to weaken early next week, with winds near the islands settling back into the moderate to fresh levels. An upper level low lingering near the state today will help to enhance trade wind showers, and could bring the potential for isolated thunderstorms, mainly over Kauai and Oahu waters. A Small Craft Advisory for the windier waters and channels around Maui County and the Big Island has been extended through early Saturday morning.

Surf along south facing shores will gradually build today through Saturday as a long period south-southwest swell is expected to fill in, producing near seasonal average surf through the weekend before declining early next week. Another long-period south- southwest swell is expected to arrive Wednesday into Thursday, bringing more near seasonal average surf to south facing shores.

East shore surf will remain near seasonal average through the weekend as trade winds persist, then gradually decline into midweek as the trades weaken slightly.

Tiny surf will prevail along north facing shores through early next week. A small to moderate medium period northwest swell will arrive late Wednesday into Thursday of next week, bringing above seasonal average surf.

Starting Saturday and lasting into Wednesday, we can expect higher than normal tides, which could lead to some coastal flooding during the peak of high tides in the early afternoon hours. As this elevated tide is expected between the 2 south period swells, it looks like this event will have minimal impact.

HFO WATCHES/WARNINGS/ADVISORIES
Small Craft Advisory until 6 AM HST Saturday for Maalaea Bay- Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters.