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Hawaiian Islands Synoptic Discussion and Guidance

567
FXHW60 PHFO 250630
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
830 PM HST Mon Sep 24 2018

.SYNOPSIS...
Low pressure northwest of Kauai will lift slowly northeast during
the next couple of days, dragging a trough toward the islands.
High moisture levels with southerly winds ahead of the trough will
bring frequent showers to Kauai through Tuesday night, with heavy
rainfall possible. Muggy conditions are expected to continue
across the state through much of the week as the trough slowly
weakens. A more typical trade wind pattern may return by Thursday
night or Friday.

&&

.DISCUSSION...
Numerous showers continue over the coastal waters west and
southwest of Kauai early this evening, however little rainfall has
occurred over Kauai itself thus far. The heaviest rains during the
past few hours have actually occurred over the windward slopes of
Haleakala and windward sections of the Big Island southwest of
Hilo. Satellite imagery shows colder cloud tops over the offshore
waters to the south through west of Kauai, with a line of
thunderstorms located about 180 miles west of Barking Sands at the
moment. Surface analysis depicts a 1005 mb low located about 650
miles northwest of Kauai, with a trough extending southward from
the low. Southeast to south winds ahead of this trough are
bringing a muggy airmass northward over the state, with dewpoints
well into the 70s. Aloft, water vapor imagery also shows a cutoff
low several hundred miles northwest of Kauai, while a ridge lies
over the eastern end of the state. The 00Z Lihue sounding showed a
moist airmass in place, with PW of 1.91 inches, although there
was still a significant inversion based near 6000 feet at that
time. The 00Z Hilo showed rather typical trade wind conditions,
with PW of 1.55 inches and a strong inversion at 7000 feet. MIMIC-
TPW imagery depicts greater than 2 inch PW values moving over
Kauai from the south, with lower PWs of 1.5 to 1.7 inches from
Oahu eastward through the Big Island.

Overnight and Tuesday, high moisture with PW values of 2 inches
or more will continue over Kauai, possibly edging eastward into
the Kauai Channel, as the surface and upper troughs remain stalled
to our west. Models also show some mid-level ascent over Kauai
during this time due to the proximity of the upper trough. Would
expect frequent showers, some heavy, along with a slight chance of
thunderstorms, and the Flash Flood Watch will remain posted for
Kauai and Niihau. The remainder of the state will see a
continuation of southeast/south background flow and a land/sea
breeze pattern, with scattered showers possible over the islands,
especially in the afternoon.

Tuesday night, the high moisture edges slowly eastward into the
Kauai Channel and toward Oahu, while the large-scale ascent
gradually lifts out to the north of the state along with the
upper trough. Some heavy downpours will remain possible over Kauai
during this time, and will have to keep an eye on the eastward
progression of this activity as it may reach Oahu.

Model guidance shows above normal moisture lingering over much of
the state from Wednesday through the remainder of the work week.
Heavy rains will be less likely, as the upper trough flattens out
to our north resulting in the loss of forcing over the islands.
Surface troughing also to our north will maintain light southerly
flow through at least Thursday, therefore expect warm and muggy
weather to continue. Trade winds may resume by Thursday night or
Friday as weak high pressure builds to our north, and a tropical
low pressure area possibly develops to our south. Longer range
guidance shows the tropical low developing further during the
upcoming weekend as it tracks off to the southwest and west of
the islands, with a drier airmass finally returning to the state.

&&

.AVIATION...
A nearly stationary surface trough to the west of Kauai will keep
the low level surface flow weak and out of the southeast. Weak
flow will allow light sea breezes to predominate most areas in
the afternoon and early evening hours and light land breezes in
the late night and early morning hours.

The surface trough will also continue to draw moisture and
instability out of the south. Thickening clouds and persistent
shower activity over Kauai tonight has necessitated the issuance
of AIRMET Sierra for mountain obscuration. Conditions are expected
to persist through at least tomorrow afternoon. Slight chance of
thunderstorms or heavy downpours over Kauai and its adjacent
waters both tonight and tomorrow.

Elsewhere, higher clouds moving in from the east will spread
across the remaining smaller islands by morning and the Big Island
Tuesday afternoon. Localized MVFR conditions are possible in
showers, otherwise VFR conditions will prevail through Tuesday
evening.

&&

.MARINE...
Surface winds will become more southerly beginning tonight as a
trough approaches the western coastal waters. This will result in
increasing showers which may be locally heavy at times, and winds
veering more south to southwest Tuesday into Wednesday for the
western coastal waters. Elsewhere, southeast winds are expected to
persist with some scattered showers through the middle of the
week. A weak front is expected to move across the western coastal
waters late Wednesday into Thursday. High pressure will build
behind it and trade winds will return by the weekend.

A moderate north-northwest swell is expected Friday into the
weekend, but should remain just below advisory levels. Small to
moderate long-period southwest swell is expected later in the
week, which will hold into the weekend before easing.

See the latest Oahu Surf Discussion (SRDHFO) for more details on
surf and swell.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through late Tuesday night for Niihau-Kauai.

&&

$$

DISCUSSION...Jacobson
AVIATION...Bedal
MARINE...Gibbs/Powell

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

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