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Hawaiian Islands Synoptic Discussion and Guidance

205
FXHW60 PHFO 250640
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
840 PM 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 beginning tonight as this upper low begins
to drift toward the islands. Although the threat for heavy rain and
storms will focus over the western end of the state through Tuesday,
a gradual eastward shift down the island chain will become a
possibility by midweek. Winds will remain light out of the south to
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" PW values) being drawn northward
over the state (greatest moisture western end of the state). In
addition to the upper low, a shortwave trough depicted on recent
water vapor imagery was shown diving southeastward toward the
islands this evening. Upper heights have responded and begun to
lower, which are triggering heavy showers and storms along a north
to south oriented confluent boundary located around/over Kauai this
evening (mostly over the adjacent coastal waters of Kauai). Expect
this trend to continue overnight, with the greatest potential for
heavy showers and storms over Kauai and its surrounding waters.

The latest short-term guidance remains in decent agreement through
midweek and continues to advertise the wet pattern holding across
the western end of the state (Kauai and Oahu) through Tuesday. A
gradual eastward shift down the island chain into Maui County will
become a possibility Tuesday through Wednesday. Periods of heavy
showers and storms will be the main concern, especially where the
confluent boundaries setup and the rain becomes focused for any
duration. Although guidance continues to indicate 2-4 inch total
accumulations by Thursday, localized higher amount will be
possible, which could translate to flash flooding.

Later in the week through the weekend, model and ensemble guidance
shows a drying trend with a return of a more typical summer trade
wind pattern spreading westward over the state.

&&

.AVIATION...
An upper level low approaching the Hawaiian Islands from the
northwest will produce unstable weather conditions across the
state. Expect low cloud ceilings and visibilities with periods of
MVFR conditions and brief periods of IFR conditions across the
western half of the state. Thunderstorms also remain in the
forecast for the western islands through Wednesday.

VFR conditions will prevail across Maui and the Big Island
airfields through the middle of the week, however brief periods of
MVFR and IFR conditions are possible with this event.

AIRMET Sierra is currently in effect for Kauai, Oahu and Molokai
for mountain obscurations above 2,000ft. Periods of low cloud
ceilings may expand to Lanai, Maui, and the Big Island by
Wednesday.

&&

.MARINE...
Unsettled weather over waters around Kauai and Oahu will last
through Thursday and potentially spread east to include Maui
county waters later Tuesday into Wednesday. Threats to mariners
include locally erratic wind and reduced visibility in areas of
heavy showers and thunderstorms, as well as the potential for
cloud-to-surface lightning strikes and waterspouts. This system
is not expected to bring impactful weather to Big Island waters.

Outside areas of convection, expect moderate to locally fresh
east to southeast winds over Big Island waters, while the
remainder of the waters will see light to moderate southeast to
south winds that may occasionally shift southwest. South winds may
briefly become fresh to strong near Kauai later in the week. East
trade winds will build over the area 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 a long-period south swell arriving
next weekend is expected to bring advisory-level surf to south
facing shores. A small short-period northeast swell will linger
for another day or so before diminishing, while building trade
winds next weekend will bring increasingly choppy surf to east
facing shores.

&&

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

&&

$$

DISCUSSION...Gibbs
AVIATION...Bohlin
MARINE...Powell

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



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