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Hawaiian Islands Synoptic Discussion and Guidance

414
FXHW60 PHFO 262007
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1007 AM HST Wed Jun 26 2019

.SYNOPSIS...
An area of low pressure northwest of the islands will support a wet
and unsettled weather pattern over Kauai county and Oahu through
tonight, with rain, heavy at times, and a chance of thunderstorms.
Some areas of Maui county may see some wet weather today as
well, while the Big Island will be mostly dry. As the low moves
away Thursday and Friday, drier conditions will move in from the
east, and a somewhat less humid, more seasonable trade wind weather
pattern will prevail over most areas by the weekend.

&&

.DISCUSSION...
Moist and unstable conditions will persist over the western end of
the state today. An unseasonably strong and broad area of low
pressure remains parked northwest of the state and continues to
draw an exceptionally moist air mass over Kauai and Oahu, as
evidenced by the 2.24 inches of PW in the overnight Kauai
sounding. Heavy rainfall was focused across Oahu overnight and
reached as far east as Molokai and Lanai before dissipating. The
short term focus has shifted back to Kauai, where the most
persistent shower activity has been occurring in the past couple
of hours. Though Kauai will have the greatest chance for heavy
showers and thunderstorms today, expected afternoon showers and
thunderstorms will maintain a flood threat for Oahu. For Maui
County and the Big Island, a somewhat unstable east-southeast flow
will produce showers across east facing terrain and spotty
afternoon showers over interior sections, especially on Molokai
and Maui.

The moist and unstable conditions will shift westward tonight.
The GFS and ECMWF models show a surface low developing roughly 175
miles west of Kauai and generating an area of deep moisture
convergence over and just west of Kauai. We will take a close look
at this today, and an extension of the Flash Flood Watch
(currently in effect for Kauai and Oahu) will likely need to be
extended for Kauai tonight, and possibly into Thursday if the deep
moisture lingers near the Garden Isle. Across the rest of the
state, rainfall chances will gradually diminish under a humid
southeasterly flow.

For Thursday and Friday, easterly trade winds will gradually build
in from east to west as the surface low lifts away from Kauai.
Wetter conditions will likely hang on for Kauai as the area of
deep moisture creeps slowly westward. As a result, we cannot rule
out heavy showers flaring in the moist southeasterly flow.
Elsewhere, building easterly trade winds will focus rainfall
across windward slopes, with spotty showers developing each
afternoon over leeward and interior sections.

A more stable easterly trade wind flow is expected for the weekend
and into early next week.

&&

.AVIATION...
A low pressure trough west of Kauai, combined with abundant
tropical moisture, will lead to another day of widespread showers
and isolated thunderstorms for mainly Oahu and Kauai. The
heaviest activity may drop visibility under 3 SM and/or lower
ceilings below 1500 feet. The islands further east will
experience isolated to scattered showers, but without the
lightning threat. As a high pressure builds to northeast of the
area today, background flow will become southeasterly.

&&

.MARINE...
A surface trough west of Kauai combined with abundant deep
tropical moisture will keep unsettled weather over most of the
waters surrounding Kauai, Niihau, and Oahu, through this
afternoon. The potential for heavy rainfall and thunderstorms will
continue mainly around the waters of Kauai and Niihau through
Thursday. Threats to mariners from the heavy downpours and
thunderstorms include locally erratic and gusty winds, reduced
visibilities, and cloud-to-surface lightning strikes.

Background moderate east to southeast flow will likely persist
near the Big Island for the next couple of days. Southerly winds
are expected to strengthen today near Kauai and Niihau and with
winds already picking up this morning, we have issued a Small
Craft Advisory for the windward, northwest and leeward waters of
Kauai through today. The remainder of the coastal waters from
Oahu to Maui will have light to locally moderate southeast to
south winds into Thursday. Beyond Thursday, there are still some
uncertainties in the wind forecast due to the disagreement about
the placement of the surface trough to the west of the state in
some of the forecast guidance. We currently have easterly trade
winds in the forecast for this weekend.

Surf along all shores is expected to remain well below the High
Surf Advisory (HSA) criteria through Friday. The current small
short-period northeast swell will linger through this afternoon
before diminishing tonight. Small background surf will continue
along south facing shores through Friday. However, a new long-
period south-southwest swell arriving late Friday night is
expected to cause surf heights to reach the HSA threshold along
south facing shores from Saturday night into Monday. Assuming the
trade winds strengthen this weekend, short-period choppy surf may
cause a modest increase in surf heights along east facing shores
from Saturday into early next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch until 4 PM HST this afternoon for Niihau-Kauai-
Oahu.

Small Craft Advisory until 6 PM HST this evening for Kauai
Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters.

&&

$$

DISCUSSION...Wroe
AVIATION...JT
MARINE...Kino

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

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