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Hawaiian Islands Synoptic Discussion and Guidance

317
FXHW60 PHFO 191945
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
945 AM HST Fri Jul 19 2019

.SYNOPSIS...
Gusty trade winds will ease slightly through tonight, with moderate
to locally breezy trade winds then persisting into next week. Clouds
and showers will favor windward and mountain area, especially during
nights and mornings. Most leeward areas will remain dry, except
leeward Big Island, where a few afternoon and evening showers can be
expected.

&&

.DISCUSSION...
Trade winds will ease to moderate levels through tonight as high
pressure to the distant N weakens slightly. Moderate trade winds
will then continue into early next week, although locally breezy
conditions can be still be expected. The trade winds will deliver
low clouds and showers to windward areas that will occasionally
spread leeward, especially when showery cloud clusters move through.
Anticipating the arrival of such clusters is generally a low skill
endeavor beyond the short term, thus a climatological trade wind
forecast dominates into next week, with showers most active during
nights and mornings.

A trough/convergence band to the S and SE of the islands has become
active over the past couple of days, and models have been
inconsistent in depicting various weak lows/disturbances briefly
emerging before dissipating, and little significant organization is
currently noted. Models generally agree that the axis of convection
will shift slowly northward through early next week before
weakening. GFS indicates a disturbance developing near 153W this
weekend, and then tracking toward the NW. An associated slug of
moisture moves toward (and over some of) the islands from the S/SE
from Sunday through Tuesday, while latest 12Z ECMWF guidance
continues to keep most of the associated moisture S of the island
chain. The latest official forecast represents a blend of the two
models, indicating the bulk of the moisture passing over coastal
waters just S of the islands early next week. Another disturbance E
of 140W may inject moisture into the trade wind flow that arrives
around the middle of next week, but confidence in that is close to
nil.

In the upper levels, a low aloft centered N of Kauai is supplying
instability above about 13000-15000 feet, but a strong subsidence
inversion based near 8000 feet is not allowing surface based parcels
to tap into this instability. The low and associated trough may
bring periods of high clouds into next week before a ridge aloft
develops over the area by the middle of the week.

&&

.AVIATION...
A high will remain nearly stationary far northeast of the main
Hawaiian islands and maintain locally strong east winds over the
area through Saturday. AIRMET TANGO is in effect for moderate
turbulence below 7,000ft and this AIRMET will likely continue into
Saturday.

Broken low clouds cover a large area around the main Hawaiian
Islands. Most of these clouds are relatively shallow stratocumulus,
but an area of more-showery cumulus has been moving west across the
islands. The back edge of the showery clouds is near Oahu. AIRMET
SIERRA for MTN OBSC is in effect for Oahu and Kauai. The area of
showery low clouds should clear the islands this afternoon and we
expect to drop the AIRMET then.

&&

.MARINE...
Small Craft Advisory (SCA) conditions will continue across all
coastal waters into this afternoon, with fresh to strong trade winds
across the area. Winds will begin to trend down this evening as the
high pressure to the north weakens, leaving SCA winds through the
typically windier waters around the Big Island and Maui County.
Further weakening is possible the beginning of next week, with a
chance of winds falling below the SCA threshold for all waters.

East facing shores will continue to see rough and choppy surf just
below advisory levels in response to the trade wind swell today. As
the winds ease this weekend, the surf will drop slightly. Some small
background south and southwest swells are expected through the
weekend. An increase in surf along south facing shores is possible
early next week as a couple small longer period southwest swells
arrive in the islands.

See the latest Oahu Surf Discussion (SRDHFO) for additional details
on surf and swell.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Sunday for Maalaea Bay-Pailolo
Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island
Southeast Waters.

Small Craft Advisory until 6 PM HST this evening for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-Oahu
Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui County
Windward Waters-Maui County Leeward Waters-Big Island Windward
Waters.

&&

$$

DISCUSSION...Birchard
AVIATION...Donaldson
MARINE...Eaton

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



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