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Hawaiian Islands Synoptic Discussion and Guidance

780
FXHW60 PHFO 310145
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
345 PM HST SAT JUL 30 2016

.SYNOPSIS...
Light to moderate trade winds will become a little stronger by the
middle of next week, with trade winds potentially diminishing again
next weekend. The trades will periodically carry areas of moisture
over the islands, with clouds and showers most prevalent over
windward slopes and coasts during nights and mornings. A nearby low
aloft will keep showers somewhat active the next couple of days,
with a few short-lived heavy showers. Although the low will move
away from the islands by the middle of the week, enhanced moisture
from the remnants of dissipated Hurricane Frank may fuel additional
showers Thursday and Friday.

&&

.DISCUSSION...
A relatively strong 1033 mb high pressure cell will prevail far NE
of the islands the next couple of days before shifting W and
strengthening to near 1038 mb during the middle and second half of
next week. A low aloft now centered about 675 miles NE of the
islands will keep winds over the islands lighter than one might
expect with a high of this strength to the NE, as it reducing the
local pressure gradient. It is also weakening the subsidence
inversion, and to a lesser degree, the upper-levels of the island
atmosphere, as seen in afternoon soundings. While an elevated
inversion based near 9 kft was observed in the PHTO sounding, the
PHLI sounding detected a nearly isothermal layer around 7 kft, with
the potential for surface-based parcels to ascend to near 15 kft due
to weak instability.

Satellite and radar data show that shower coverage has diminished
since this morning, but the typical nocturnal enhancement of the
incoming cloud field is expected, resulting in increased windward
showers as the night progresses. Additionally, showery cloud
clusters currently immediately upstream of the Big Island and about
200 miles NE of the smaller islands will be traversing the area over
the next 24 hours or so, aiding to create a somewhat wet trade wind
weather pattern. Some of the showers will have small 40-50 dBZ
cores, resulting in pockets of briefly heavy showers, especially
over Kauai and Oahu, due to the proximity to the low aloft.

The slow-moving low aloft is expected to weaken and move
sufficiently far from the islands around Tuesday and Wednesday to
allow the low-level gradient to tighten, and trade wind speeds to
increase. While the departure of the low would typically lead us to
expect a trend toward a drier trade wind weather pattern, especially
with a mid-level ridge forecast to develop overhead, global models
are indicating that moisture associated with the remnants of now-
dissipated Hurricane Frank may arrive on the trade wind flow
Thursday and Friday, leading to increased showers. Another potential
moisture source could become embedded in the trade wind flow to fuel
more showers next weekend, but confidence is low with respect to
that portion of the forecast.

&&

.AVIATION...
The persistent trade wind flow will produce periods of MVFR ceilings
and VIS in shra along windward slopes, with a poorly organized band
of moisture moving over the island chain from the east overnight.
VFR conditions will prevail over leeward areas, except for MVFR
ceilings lingering over leeward Big Island slopes through the
evening. Radar VAD wind profiles show that trade winds are mainly 20
kt or less in the low levels with models showing a decrease
overnight. No AIRMETs are in effect, though we will monitor the
potential for areas of mountain obscuration overnight.

&&

.MARINE...
Winds and seas are expected to remain below small-craft-advisory
(SCA) levels for the next few days as a low aloft weakens the
pressure gradient over the area. An ASCAT pass sampled Big Island
waters this morning, and found max winds on the order of 20 kt,
although land-based observations indicated locally breezy conditions
this afternoon. As the low moves N around the middle of next week,
the pressure gradient will tighten and trade wind speeds will
increase. Winds may become sufficiently strong at that time to
support the issuance of a SCA for the windier marine zones around
Maui and the Big Island.

There will be a series of small long-period S swells through the
next week, but surf will remain below advisory levels. Trade winds
will drive short-period wind waves toward the islands from the E,
producing moderate and below-advisory surf along E facing shores
through the forecast period.

&&

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

&&

$$

DISCUSSION/MARINE...Birchard
AVIATION...Wroe

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



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