Area Forecast Discussion for Hawaii

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Issued: May 27, 2015 8:52 PM HST

Synopsis
A surface ridge building northwest of the state will bring weak trade winds across the western islands through the rest of the work week, but winds will remain light enough to allow localized land and sea breezes. A persistent trough lingering near the Big Island will maintain even lighter winds with widespread sea breezes over the eastern half of the state. Abundant moisture lingering around the state will provide a rather cloudy and showery weather pattern at times, particularly each afternoon and early evening. Trade winds should increase somewhat across the state during the weekend, but lingering moisture will keep showers active especially across windward and mauka areas.

Discussion
A highly anomalous weather pattern prevails over the north Pacific ocean, with a large vertically stacked low pressure area at the surface and aloft sitting near the position where the northeast Pacific surface high usually resides at this time of year. A complex surface trough extends south and west from this low to near the Big Island, with a persistent small-scale circulation within this trough evident in satellite/radar data just to the south of the Big Island. Abundant cloudiness associated with the trough continues to blanket the Big Island and most of Maui county. Further to the west, a ridge extending eastward from high pressure centered out near the international dateline is providing weak trade wind type flow over the western end of the state. Mimic-tpw satellite imagery depicts a large area of enhanced moisture around and east of the Big Island, with another smaller pocket of enhanced moisture near and west of Kauai. A somewhat drier airmass prevails in between, but estimated pwats are above normal even in that area. The 00Z Hilo sounding reflected a deep moist layer up to a high 11kft inversion and well above normal pwat of 1.51 inches. The 00Z Lihue sounding found a drier airmass in place at that time, with the inversion base of 7kft and pwat of 1.16 inches both near normal values.

Overnight, cloudy conditions appear likely to continue across Maui and the Big Island, due to the lingering low pressure trough in that area. Kauai will probably remain quite cloudy as well with another area of moisture lingering there. Oahu should see improving conditions during the night as developing land breezes push clouds and showers mostly offshore. Will adjust sky cover and pops for some areas this evening, but no major forecast changes are planned.

Only subtle changes are forecast in the pattern through the remainder of the work week. Model consensus indicates that generally above normal moisture with weak/high inversions will linger across the state through the period, due to a combination of the trough to our east and moisture drifting southward from a dissipated front. The deep low to our NE will shift slowly eastward and weaken, with a weak sfc trough slowly filling near the Big Island, while high pressure will remain far to our WNW near the dateline. The resulting pressure gradient will be stronger near kauai/oahu, and weaker across the eastern half of the state. Sea breezes and land breezes will be more localized near kauai/oahu than farther east, but will still contribute to island-scale effects. Enhanced convergence along the leeward sea breezes will contribute to interior/leeward clouds and showers during the afternoons and early evenings, while the weak trades will produce some windward showers as well especially over Kauai and Oahu. More widespread cloudiness and showers may linger over the Big Island through Thursday due to the proximity of the sfc trough, but conditions should slowly improve toward the end of the week as the trough gradually weakens.

During the weekend and into early next week, the trough to our NE and E will finally weaken and move further away, allowing a more zonal flow pattern aloft to develop over the central Pacific. A sfc high building to our north will bring gentle to moderate trades back across the entire state. A somewhat more typical trade wind weather pattern should return with clouds and showers focused mainly across windward/mauka areas, but continued above normal moisture and high inversions may allow for more showers and clouds leeward and interior along local sea breezes than would usually be the case.

The latest available GFS and ECMWF runs continue to diverge somewhat by the middle of next week. The GFS brings another mid/upper level trough and sfc front south of 30°N, weakening and veering the flow over Hawaii, although the front itself remains well north of the state. The ECMWF keeps these mid-latitude features weaker and further north, allowing for a stronger subtropical sfc ridge to our north and maintaining moderate trade wind flow over the islands. Model trends and early June climatology appear to favor the ec solution at this time.

Aviation
A trough of low pressure over the Big Island continues to produce widespread cloudiness and scattered showers there. Areas of MVFR and local IFR will remain possible across windward sections overnight, and airmet sierra for mountain obscuration is posted from Upolu Point to PHTO to Volcano. Elsewhere, a weak pressure pattern is in place, which will allow sea and land breezes to dominate the weather pattern overnight and Thursday. Cannot rule out some brief MVFR cigs/vsbys in SHRA, but predominantly VFR conditions are expected.

Marine
The current south swell will continue to subside, though a series of smaller south swells will maintain small to moderate surf along south facing shores through the rest of the week. Latest wavewatch guidance continues to show a significant south swell arriving Saturday night, then peaking late Sunday into Monday, with resulting surf likely reaching advisory levels for south facing shores. This swell was produced by a strong gale low east of New Zealand with a long fetch aimed toward Hawaii a few days ago. Will monitor observations from the aunuu buoy near american samoa as this swell will be peaking at that location Thursday and Thursday night.

Otherwise, surf will remain well below advisory levels through the next several days with only small swells expected.

Watches/Warnings/Advisories
None.


JACOBSON