- A surface ridge northwest of Hawaii will bring weak trade winds to western islands this week, while lighter winds and localized breezes affect eastern areas due to a lingering trough near the Big Island. Abundant moisture will provide cloudy and showery weather at times. - Through the week, above normal moisture and weak winds will persist across the state due to the trough and moisture drifting south, bringing clouds and showers. Gentler trades may return over the weekend as the trough weakens. - A significant south swell is expected to arrive Saturday and peak Sunday-Monday, potentially reaching advisory levels for south shores. Otherwise surf will remain below levels through the next several days.

1. Aviation
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

2. 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

3. 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

4. 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
http://www.prh.noaa.gov/hnl/pages/AFD.php