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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 271356

Area Forecast Discussion
National Weather Service Honolulu HI
356 AM HST Thu Apr 27 2017

Trade winds will ease and shift to the southeast today and tonight
as a late season cold front approaches the state, with spotty
afternoon and evening showers possible. On Friday, the front will
bring increasing clouds and rain to Kauai, while prefrontal
showers, some heavy, develop across the rest of the island chain.
The front is expected to move through Oahu Friday night, then
stall over Maui County or near the northern Big Island over the
weekend. Wet and unstable conditions are expected over much of the
state through the weekend and into early next week.


Scattered showers have increased across the waters north of Oahu
and east of Kauai during the last few hours, as well as over the
waters east of the Big Island. Some of these showers are moving
onshore into windward sections of those islands. Mainly dry
conditions prevail elsewhere across the state on this early
Thursday morning. Trade winds are beginning to weaken, as a cold
front moves eastward through the Northwestern Hawaiian Islands and
the surface ridge north of the state weakens. Aloft, a narrow mid
level ridge is located over the state, between an upper level low
several hundred miles southeast of the Big Island and a large
mid/upper level trough moving east from the international
dateline. The 12Z soundings reflect a dry and stable atmosphere
remaining in place, with PW values ranging from 0.92 to 1.11
inches, and strong inversions based near 7000 feet. MIMIC-TPW
satellite imagery shows a small pocket of slightly higher moisture
north of Oahu, associated with the scattered showers there, and a
larger area of moisture about 50 miles east of the Big Island.
These areas of enhanced moisture are moving westward in the trade
wind flow.

For today, passing trade showers over windward areas this morning
will give way to a more convective pattern this afternoon, as
trade winds continue to weaken and veer to the southeast. The
weakening background flow will allow widespread seabreezes in the
afternoon over much of the state, while trade winds hang on across
windward parts of Maui and the Big Island. A deep mid/upper level
trough driving the surface front will also advance eastward
during this time, displacing the mid level ridge initially over
the islands. This will cause the inversion to begin to weaken and
lift, and this decreased stability combined with marginal moisture
availability and a low level trigger provided by the expected
seabreeze regime could allow for some spotty heavier showers to
develop during the afternoon, especially over leeward and interior

Latest guidance remains consistent with earlier runs in bringing
the surface cold front to Kauai Friday afternoon, with increasing
clouds and showers accompanying the front. Models are depicting a
shallow front with the majority of rainfall on Kauai falling over
the northern slopes and coast into Friday night. The atmosphere
will become more unstable as the upper trough brings colder
temperatures over the state. Prefrontal convergence and the
unstable atmosphere will bring an increase in shower activity over
the rest of the state with some pockets of heavier showers

The shallow front is forecast to reach Oahu Friday night, then
stall over eastern Maui County or the northern Big Island by
Saturday afternoon. Meanwhile, a very strong upper level low for
this time of year will be cutting off within the deep upper trough
and dropping southward toward the islands, bringing increasing
instability over the area. This will lead to the potential for
heavy, possibly flooding, rainfall over portions of the state
this weekend, and continuing into early next week. The latest GFS
and ECMWF continue to show varying solutions on how quickly this
upper low will move southward over the islands, with the EC
bringing the core of the low over Oahu Sunday afternoon while the
GFS is a full day slower, bringing the core of the low near Oahu
Monday afternoon. Timing and position differences with the mid
level low have persisted for the last several model runs, which
causes us to have rather low confidence in these details at this
point. The models do agree on a lower level convergence band
forming along the old frontal boundary located someone near Maui
County or the Big Island Saturday night through Monday, with this
feature becoming the focus for heavy rain. In addition,
thunderstorms will be likely near the core of the upper low due to
very cold temperatures aloft, but it is not yet possible to pin
down the exact timing or location for the thunderstorm threat. For
now, we have introduced a slight chance of thunderstorms for the
entire state for Sunday and Sunday night, and for all islands
except Kauai on Monday.

Deepening moisture and cold temperatures aloft, combined with
strengthening winds associated with the mid level trough, could
produce wintry conditions on the Big Island summits starting
during the weekend and continuing through Monday.

Overall, there appears to be a substantial threat for heavy rains
and thunderstorms during the weekend and into early next week,
with the greatest threat seeming to focus over the eastern half of
the state at this point, but it is likely going to take another
few model cycles before we can have more confidence in the timing
and location details. The forecast will continue to be refined
during the next couple of days.

Gentle trade winds and much more stable weather should return by
the middle of next week, as the upper trough weakens and moves
east of the area.


High pressure northeast of the state will continue to lose its
influence over the area today and tonight, as a front approaches
from the northwest. This will lead to a steady decrease in the
trade winds, with daytime sea breezes and overnight land breezes
expected in many areas. Showers will favor windward areas through
mid morning, with some brief MVFR cigs/vsbys possible in SHRA.
Coverage of showers should increase across the interior of the
islands this afternoon and evening, with some reduced cigs/vsbys
possible once again. Otherwise, predominantly VFR conditions are
expected through 12Z Friday.

AIRMET Sierra is in effect for mountain obscuration across
windward sections of Kauai. Conditions are expected to improve
here later this morning.


A ridge of high pressure located 400 nm north of Kauai, is
forecast to move east today due to a front advancing eastward.
Our trade winds will be turning light and variable today as a
north to south oriented trough forms over the central islands
ahead of the advancing front. The trades have weakened enough
to where the earlier small craft advisory (SCA) for the
Alenuihaha Channel can be taken down.

The front is forecast to reach the NW offshore waters tonight
and Kauai Friday afternoon, then reach Oahu Friday night, before
stalling over Maui County on Saturday. Breezy NE winds following
the front may require the issuance of an SCA over the western
coastal waters. The front is forecast to inch slowly eastward to
the Big Island late in the weekend with the NE winds gradually
weakening to moderate speeds. The arrival of the front will be
enhanced by a strong upper level trough resulting in an unsettled
weather period including a chance of thunderstorms over the
surrounding waters through next Monday.

Swell-wise, the currently NW swell will continue to subside. A medium
size NW swell with a 8 second period arrives Friday night and
peaks Saturday. A similar size swell from the north follows Sunday
night and peaks Monday. A small NW swell with a 14 second period
is on tap for Tuesday night and Wednesday.

For the south facing shores, there will be a series of south swells
during the forecast period. The first of two is due to arrive early
Friday morning. Surf from this swell could be large enough to warrant
a high surf advisory for the south facing shores. A gradual
lowering will follow beginning early Saturday morning. Another
pulse is due in Monday afternoon but the core of this swell may
miss Oahu.

Small surf can be expected along the east facing shores over the
weekend as the trades take a breather.






Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office