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Hawaiian Islands Synoptic Discussion and Guidance

779
FXHW60 PHFO 241308
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
308 AM HST Mon Jun 24 2019

.SYNOPSIS...
A wet pattern is unfolding across the western end of the state due
to an out of season upper low and surface front positioned to the
west-northwest. Warm and humid conditions with increasing rain
chances are anticipated through Tuesday 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. Outside a few spotty
showers over Kauai and Oahu (accumulations less than .25" through
2 AM HST), mostly quiet conditions have prevailed. The bulk of the
heavy showers and storms have remained positioned a couple of
hundred miles south-southwest of 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 tonight
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...
A moist and unstable airmass is expected to linger in the area,
and will bring more unsettled weather with possible thunderstorms
and heavy rain mainly to Kauai today. Conditions have improved
over Kauai early this morning such that the AIRMET Sierra for
mountain obscuration has been canceled for now. Mainly VFR
conditions are expected for rest of the islands today, through
brief MVFR ceiling may affect the sites due to low clouds,
especially later today as convective clouds develop over the
islands. AIRMET SIERRA for mountain obscuration will be posted
again if conditions arise.

Southerly synoptic winds will persist across much of the islands
today, with Big Island seeing more easterly winds, due to the
present of a trough near the area. Early morning land breezes will
turn to sea breezes by the afternoon.

&&

.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 tonight 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 this evening through Wednesday afternoon
for Niihau-Kauai-Oahu.

&&

$$

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

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