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Hawaiian Islands Synoptic Discussion and Guidance

972
FXHW60 PHFO 240637
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
837 PM HST Sun Jun 23 2019

.SYNOPSIS...
A wet pattern is unfolding across the western end of the state due
to an out of season broad upper low and surface front positioned to
the west-northwest. Warm and humid conditions with increasing rain
chances are anticipated through the first half of the week as this
upper low begins to drift toward the islands. Although the main
threat for heavy rain will initially focus over the western end
of the state, a gradual eastward shift down the island chain will
become a possibility by midweek. Winds will remain light out of
the southeast, which will allow the land and sea breeze regime to
continue. A drying trend along with a return of a more typical
trade wind pattern is not anticipated until late Thursday through
the upcoming weekend.

&&

.DISCUSSION...
The latest surface analysis showed a front extending southward from
an area low pressure several hundred miles west of Kauai. Plenty of
moisture continues to pool northward over the region from the deep
tropics due to a combination of this front/SFC low and an anomalous
upper low centered to the northwest. Recent satellite-derived
precipitable water (PW) imagery reflected this and showed a broad
area of above average moisture (2.25" PW values) being drawn
northward over the state (greatest moisture western end of the
state). The low-level flow remains light out of the southeast over
the islands, which has translated to warm and humid conditions, with
dewpoints hovering in the low to mid 70s. The large shield of light
to moderate rain with a few embedded heavier showers and storms
advancing eastward across the Kauai Channel and farther south this
evening has begun to dissipate or collapse as it approaches Oahu.
Isolated to scattered showers, however, have started to develop and
track northward over Kauai. Expected this trend to continue
overnight, with the best rain chances holding over Kauai.

The latest short-term guidance remains in decent agreement through
the first half of the week and continues to advertise a wet pattern
unfolding across the western end of the state. The broad upper low
that has been parked several hundred miles northwest of the islands
drawing deep moisture northward over the area is forecast to begin
drifting toward the state. Deep tropical moisture (above normal PWs;
2-2.3") in place combined with lowering upper heights and increasing
instability will support an increasing threat for heavy rain and
even a few thunderstorms over Kauai and Oahu, especially Monday
night through Wednesday. In addition to the wet pattern, light winds
giving way to a land and sea breeze regime combined with dewpoints
lingering in the low 70s will keep the humid conditions in place.
Although confidence remains low this far out, the threat for heavy
rain may shift eastward toward Maui County by Wednesday. For the Big
Island, a more typical land and sea breeze regime should hold with
mainly scattered afternoon showers and potentially a storm or two.

Extended model and ensemble guidance continues to advertise a
drying trend with a return of a more typical summer trade wind
pattern spreading westward over the state late Thursday into
Friday. This pattern should hold into early next week.

&&

.AVIATION...
Wet weather will continue to affect Kauai as a moisture band
associated with a front moved into the area. Latest radar as well
as lightning data did indicate much of the thunderstorms in that
island vicinity has weakened, though with the moist and unstable
airmass expected to linger in the area, more unsettled weather
with heavy rain and thunderstorms may develop through rest of
the evening into early Monday morning. AIRMET Sierra for mountain
obscuration continues over Kauai due to clouds and showers.

Mainly VFR conditions are expected for rest of the islands,
through brief MVFR ceiling may affect the site due to low clouds
through early Monday. AIRMET Sierra for mountain obscuration is
posted over Oahu and the Big Island, but with the expected
improved conditions as daytime convection subsides, the AIRMET
will mostly likely be canceled.

Rather weak southerly synoptic winds will persist across much of
the islands through Monday, with Big Island seeing more easterly
winds, due to the present of a frontal band near Kauai. Expect
land breezes to develop overnight over the islands, then turning
into sea breezes during the day on Monday.

&&

.MARINE...
Heavy showers and thunderstorms are increasingly likely,
especially Tuesday through Thursday, as a slow-moving convergence
band develops over the area. Threats to mariners include locally
gusty wind and reduced visibility in areas of heavy
showers/thunderstorms, as well as dangerous cloud-to-surface
lightning strikes, and the potential for waterspouts.

The threat of unsettled weather will initially be greatest over
waters around Kauai, spreading to Oahu waters from Monday night
into Tuesday, potentially spreading east to Maui county waters
later Tuesday into Wednesday. A closed low aloft may develop
northwest of Kauai, keeping unsettled weather over (at least)
waters around Kauai and Oahu through Thursday before the low
gradually moves away Friday into Saturday.

Expect moderate to locally fresh east to southeast winds near the
Big Island, while the remainder of the islands receive light to
moderate southeast to south winds. However, periods of locally
fresh south to southwest winds will be possible around Kauai.
Seasonably strong trade winds will build over the area from the
east Friday and Saturday. Until that time, winds and seas are
expected to be below Small Craft Advisory criteria.

Surf along all shores is expected to be below High Surf Advisory
levels through the week, but odds are increasing that an advisory-
level south swell will arrive next weekend. A small short-period
northeast swell will linger for the next couple of days before
diminishing, while building trade winds next weekend will bring
increasingly choppy surf to east facing shores.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch from Monday evening through Wednesday
afternoon for Niihau-Kauai-Oahu.

&&

$$

DISCUSSION...Gibbs
AVIATION...Hui
MARINE...Powell

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