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Temp3.1 C
RH21 %
WindWNW 18 mph
RoadOpen (4x4)
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Hawaiian Islands Synoptic Discussion and Guidance

075
FXHW60 PHFO 310634
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
834 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...
Scattered showers embedded in moderate trade wind flow are moving
across the coastal waters and into windward areas of the state
this evening. Most of the showers are on the light side, but with
isolated tops up to 15-20 thousand feet there are likely a few
brief heavier downpours occurring. Rain gauge data shows Kauai as
the wettest island during the past 12 hours, with almost all
windward locations receiving some rain. Elsewhere, 12 hour
rainfall totals are mostly less than a quarter inch, with many
locations remaining dry. Surface analysis depicts a 1033 mb high
centered about 1600 miles north of Honolulu. The distance of this
high from the islands along with a low pressure trough aloft to
our north has lessened trade winds somewhat over the state.
MIMIC-TPW satellite imagery shows a pocket of relatively drier
air passing over and west of the state currently, but an area of
higher moisture is moving in from the east and will reach the
islands overnight. This area of increased moisture is evident on
infrared satellite imagery as showery clusters of cumulus just to
the northeast and east of the coastal waters early this evening.
The 00Z soundings sampled the drier airmass in place over the
state at that time, showing slightly below normal PWATS and
typical inversions ranging from 6 to 8 thousand feet.

Overnight, expect showers to become increasingly frequent and
active, especially over windward areas, with some showers spilling
over into leeward sections of the smaller islands. The existing
forecast reflects very high rain chances over windward areas after
midnight, which appears to be reasonable. No forecast updates are
planned for this evening.

A somewhat wet gentle to moderate trade wind pattern is forecast
to continue through the rest of the weekend and into early next
week, as a weak mid/upper level trough lingers north of the state
and pockets of enhanced moisture continue to move through from
east to west. The upper trough should move away off to the north
around midweek, with a mid level ridge building in from the east.
The North Pacific surface high is also forecast to strengthen
around that time, which should lead to increasing trade winds over
the islands. We would typically expect this combination of factors
to produce a drier trade wind pattern, but models continue to show
enhanced moisture associated with the remnants of dissipated
Hurricane Frank moving across the state Thursday into Friday,
which would likely cause trade wind showers to increase. Overall,
expect rather damp conditions to continue over windward areas
through the next several days, while leeward areas see passing
mostly light showers, primarily during the night and morning
hours. Typical afternoon/evening clouds and widely scattered
showers will be present along the Kona coast and slopes of the Big
Island.

&&

.AVIATION...
Clouds and showers will move in from the east tonight and bring
periods of MVFR ceilings and visibility. Initially, the windward
Big Island will be affected this evening, but the clouds and
showers are expected to fill in for western end of the state
later tonight. AIRMET Sierra has been issued for the windward
section of the Big Island for TEMPO mountain obscuration. No
AIRMETS are in effect elsewhere, but will be monitoring for
additional areas that could be included in the AIRMET.

&&

.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. The Saturday morning ASCAT pass
found max winds near 20 kt around the Big Island, although land-
based mesonet observations indicate locally breezy conditions
continuing this evening around parts of Maui County. As the low
aloft moves north 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 south swells through
the next several days, but the resulting surf will remain below
advisory levels. Trade winds will drive short-period wind waves
toward the islands from the east, producing moderate and below-
advisory level surf along east facing shores through the forecast
period.

&&

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

&&

$$

DISCUSSION/MARINE...Jacobson
AVIATION...Foster

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