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Temp2.4 C
RH8 %
WindNE 22 mph
RoadOpen (4x4)
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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 220136

Area Forecast Discussion
National Weather Service Honolulu HI
336 PM HST Sun Apr 21 2019

Moderate to locally breezy trade winds will continue through
Thursday, bringing periods of clouds and showers to windward areas,
while leeward areas remain mostly dry. Lighter winds are in the
forecast beginning Friday, and continuing into next weekend, which
may also bring an increase in moisture.


A surface high will remain nearly stationary far NE of the islands
through Tuesday before weakening and moving NE on Wednesday. A new
high pressure cell will pass far N of the area around the same time,
and the pressure gradient over the islands will change little, with
moderate to locally breezy ENE trade winds persisting through
Thursday. As this new high moves into the NE Pacific by the end of
the week, its associated ridge will be shunted S toward the islands
by a deep-layer low that develops to the NW. In response, E winds
will weaken Friday and shift to the SE or S next weekend, bringing
an increase in low-level moisture.

A mid-level ridge will be near or over the islands through the
upcoming week, then shift E by next weekend as height falls over the
islands are supported by the development of the low to the NW. A
subsidence inversion based between 5000 and 7000 feet will change
little as the ridge persists, thus maintaining a stable island
atmosphere. With a slight increase in PWAT to near 1.1" (up from
0.8") expected this week, the inversion will be sufficiently high to
allow trade wind showers to occasionally dampen windward areas, most
likely during nights and mornings. Little in the way of leeward
rainfall is expected, except for leeward Big Island, where a few
afternoon and evening showers will be possible.

The increased moisture combined with the height falls next weekend
could bring an increase in shower coverage and intensity, but
confidence is low. Model solutions diverge around next Sunday, with
ECMWF and FV3 bringing a destabilizing cold impulse aloft and
surface front over the islands, while the operational GFS indicates
weaker versions of both.

In the short term, low cloud coverage immediately upstream of most
islands is minimal, except for an area of broken to overcast stable
and showery low clouds that is approaching windward Big Island/Maui
from the E. These clouds are expected to fuel an increase in showers
there later tonight, with nocturnal cooling allowing low clouds
approaching the smaller islands to fuel a few brief windward showers
as well.


A ridge of high pressure will remain far northeast of Hawaii and
drive fresh trade winds over the next few days. AIRMET Tango remains
in effect for moderate turbulence below 060 over and downwind of the
terrain due to the trade winds. These conditions are expected to
hold in place into Monday, possibly longer.

Trade winds will carry in periodic clouds and showers that will
focus over windward and mauka sections. Brief MVFR ceilings and
visibilities, and mountain obscurations may occur within the showers.


A ridge north of the state will maintain breezy to occasionally
strong trade winds through Thursday. However, a front passing well
north of the state has weakened the ridge and caused the winds to
ease slightly across select windward zones and channels. Therefore,
the Small Craft Advisory (SCA) is now in effect for only the
typically windy areas around Maui and the Big Island, extending at
least through Tuesday.

Surf along all shores is expected to remain below advisory levels
for this week. A small west-northwest swell will provide small surf
along north and west facing shores for the first half of the week. A
series of small south swells will provide small surf starting Monday
and will likely continue through most of the week and into the
weekend. Surf along east facing shores will hold steady in the small
to moderate range through Thursday due to the breezy to locally
strong trades.


Small Craft Advisory until 6 PM HST Tuesday for Maalaea Bay-Pailolo
Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island
Southeast Waters.




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