Punahou, HI Weather

Synopsis

Northeasterly trade winds should gradually weaken Sunday into Monday. Brief passing showers will drift into the islands on the trade winds, favoring windward and mountain areas during the overnight to early morning hours. A significant change to our weather pattern is expected early next week. Deep tropical moisture will bring the potential for heavy rain and thunderstorms across the state starting Monday night and persisting through at least Thursday.

Discussion

Radar this morning shows scattered showers along windward and mauka locations. Satellite imagery shows a large closed low about 900 miles to our northeast, a ridge to our northwest and another to our east-southeast. At the surface, a low is centered beneath the upper level low, with a ridge to either side.

Monday afternoon the low to our north will drift slowly southeast, both aloft and at the surface. At the same time, a low, CP94 and a new surface low will form to our west- southwest. A trough will connect these lows...the one that drifted into a position to our northeast along with the two to our west- southwest. Southerly winds will bring deep tropical moisture northward as this trough moves north toward and then through the state Monday night through Thursday. PWATs will get up over 2 inches, and max out around 2.5 inches.

Heavy rain and thunderstorms will be possible through this period over most of the state, but models are slow to converge on areas of maximum concern. A Hydrologic Statement describing the concern for flooding was issued yesterday afternoon. A Flood Watch may be issued if models continue to show the potential for heavy rain. Upper level winds may be just strong enough to enable the development of organized convection.

Winds will remain generally easterly through Tuesday, but after that there is uncertainty. Some models show centers of low pressure forming along the trough as it moves over the state. The location and timing of these lows depicted by various models, are quite different and therefore may result in wind variations in both speed and direction by island. This makes for a very low- confidence wind forecast. Regardless of exact timing/location of any additional lows that may form, it does not appear that strong winds will impact the state with this overall system. However, showers and any thunderstorms will have the potential for brief gusty winds.

Aviation

Moderate trades through the period. Low cigs and SHRA should primarily impact windward and mauka locations with limited spillover into leeward areas. MVFR conds possible in SHRA otherwise VFR prevails.

AIRMET Sierra is in effect for mtn obsc for Oahu to the Big Island. Conds should improve in the mid morning hours.

Marine

A surface low far northeast of the Hawaiian Islands will continue to dig southward today and stall roughly 800 nm northeast of the state by Monday, keeping the subtropical ridge just north of the area rather weak. In response, trade winds will remain gentle to moderate today, then decrease further to gentle by Monday. A low pressure trough will then approach the area from the southwest late Monday and bring heavy showers with embedded thunderstorms to the waters through much of the work week. Winds will remain in the gentle to locally moderate range.

A medium period north-northwest swell will slowly ease through Monday, with surf reaching near or slightly above seasonal averages. By late Monday, a long- period northwest swell will arrive and help generate moderate surf along north- and west- facing shores through Wednesday. A small north- northeast swell should arrive during the second half of this week.

A series of small overlapping south-southwest swells will keep surf along south-facing shores to around seasonal averages through early this week. A larger south-southwest swell may push surf slightly above seasonal average by mid to late this week.

Surf along most east-facing shores will be small through early next week, but areas exposed to wrapping northerly swells will experience periods of moderate surf through Monday. East-facing shores should also see a bump in surf if the north-northeast swell materializes during the latter half of this week.