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Hawaiian Islands Synoptic Discussion and Guidance

514
FXHW60 PHFO 010211
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
411 PM HST TUE MAY 31 2016

.SYNOPSIS...
A somewhat wet trade wind regime will continue for another day or
so in wake of a dissipating front near the Big Island. Moisture
left behind by the front will enhance showers, especially across
windward sections of the state. Moderate trades will strengthen
behind the front, and have already filled in over the smaller
islands. Trades will then turn light and variable by Saturday as
a surface trough approaches the state from the northeast.

&&

.DISCUSSION...
Water vapor imagery shows a weak trough or col over the state,
with a well defined low far to the northeast. At the surface, a
nearly stationary front is located near the Big Island, with a
cloud band extending back across the rest of the state. This front
is dissipating near the islands, but extends to a low far to the
northeast. 00z raobs from Lihue and Hilo show inversions around
9-10kft and precipitable waters around 1.4 inches, which is just
under one standard deviation above normal for this time of year.
Early afternoon MIMIC total precipitable water imagery shows a
band of higher moisture centered over Oahu/Maui County, associated
with the main cloud band behind the surface front. There has been
little movement to the moisture, and only slowly drift southward
is expected in the next day or so.

Greatest rainfall has been occurring across the central islands,
but recently picked up across Kauai during the afternoon. Almost
all windward gages on Oahu received some rain, with a number of
them near or greater than an inch in the past 12 hours. Given the
active cloud band associated with the front and rainfall expected
to continue, have carried categorical rain chances through the
evening for windward sections, with some decrease overnight for
Kauia/Oahu. The Big Island will see the opposite trend, with lower
chances to start the evening and greater ones overnight as the
cloud band settles southward.

Trade winds will fill in behind the front as a new high builds
north of the state. Gusty northeast winds have already picked up
across the smaller islands, and will continue for the next few
days. Forecast soundings/cross sections show inversion heights
briefly lower Wednesday as mid-level heights rebuild slightly...at
least across the western half of the state. Trade showers will
be most active near the Big Island where the old frontal moisture
could hang up. In addition, the high-resolution WRF NMM and ARW
show afternoon convection becoming pretty widespread for the
interior of the Big Island Wednesday afternoon. This seems
reasonable given the amount of lingering moisture avaible to feed
into these showers. With lowering inversion heights and a drier
airmass, the smaller islands will briefly return to a more typical
trade wind weather pattern.

The 12z GFS and ECMWF are somewhat similar with the evolving
pattern late in the week, with the upper low to our northeast
shifting eastward and the surface low mentioned above weakening to
a trough. This trough gets picked up in the low-level
east-northeasterly steering flow and shifts west-southwestward
toward the state. By this point the surface high to our north is
already weakening due to another north Pacific system encroaching
on it, and the surface trough will further weaken winds across the
state late in the week. We should see another light
wind/convective pattern again this weekend, with onshore sea
breezes during the day leading to interior showers in the
afternoon/evening.

Differences emerge between the extended models with the pattern
after this point, with the ECMWF bringing an upper low over the
islands from the east early next week and the GFS keeping an
amplifying trough east of the state. As it stands, there is little
reflection of this low at the surface in the ECMWF, with both
models showing a new high building northwest of the area. Trade
winds should return with the high early next week, but it remains
to be seen how unstable we will be and how much in the way of
trade showers we will receive.

&&

.AVIATION...
A dissipating frontal cloud band will continue to spread clouds
about the state through tonight with gradual clearing on
Wednesday. Til then, MVFR and isolated IFR conditions can be
expected due to some lower ceilings and reduced visibilities
in showers. An AIRMET for mtn obsc is currently in effect for the
entire state. Should see some improvement later tonight from Kauai
to Maui. The frontal cloud band still remains upstream to the
east-northeast of the Big Island. So it could be a not so sunny
day again for the Big Island on Wednesday.

Trade winds have returned to most areas and are up to moderate
speeds at times especially through the channels and around the
mountains. But the high pressure area north of the state that is
generating the trades is expected to weaken Wednesday. So an AIRMET
for low level turbulence is not anticipated at this time.

&&

.MARINE...
A weak slow-moving front currently over northern Big Island will
drift southward over the next couple of days, allowing fresh
northeast winds to the north of the front to spread over the
entire island chain. High resolution models continue to show winds
along the frontal boundary touching small craft advisory strength
at times, though coastal wind observations have not been
supporting winds this strong. We will closely monitor these border
line conditions during the next day or two. Expect a decrease in
winds on Friday and the weekend as high pressure weakens to the
north of the state and a surface trough develops to the northeast.

Surf will remain below advisory levels on all shores this week.
A fading south-southeast swell will be reinforced by another pulse
from a similar direction. This will maintain moderate surf along
south facing shores...with larger sets on southeast facing
coasts...during the next couple of days then drop a notch heading
into the weekend. Small surf will prevail on north and east facing
shores. See the latest Collaborative Nearshore Swell and Wind
Forecast (SRDHFO) for more details.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

Discussion...Bravender
Aviation...Fujii
Marine...Wroe

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



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