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Hawaiian Islands Synoptic Discussion and Guidance

576
FXHW60 PHFO 210650
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
850 PM HST Fri Sep 20 2019

.SYNOPSIS...
Moderate to breezy trade winds will continue through Saturday, then
trend down into the light and variable range Saturday night through
Sunday. Light winds combined with increasing moisture and
instability will translate to increasing rainfall chances across
the state. Although some drier air is expected to fill over the
Big Island and Maui County by Tuesday, the wet pattern is forecast
to hold over Kauai and Oahu through midweek.

&&

.DISCUSSION...
Water vapor imagery showed a shortwave trough digging southward
across the the island chain with plenty of deep tropical moisture
pooling northward to its east. Some of this middle- to upper-level
moisture has translated to cloudy conditions across the Big Island.
Although some mid-level dry air is advancing southward over the
western end of the state in the wake of the shortwave trough, a
sufficient amount of instability in place is supporting scattered
moderate to heavy showers developing and quickly moving through
from east to west. Most of these showers are focusing over
windward locations, with some spilling into leeward areas
periodically. The latest rainfall summary showed accumulations
ranging from a quarter to near half of an inch over the past six
hours (through 7 pm HST).

The short-term guidance (through Monday) remains in good
agreement and depicts a wet pattern evolving across the region as
the aforementioned upper trough continues to dig southeastward and
carves out a broad upper low in the vicinity of the islands. This
upper low is forecast to drift westward across the state through
the weekend. A surface trough associated with it, currently well
east-southeast of the state, will move through the area late
Saturday through Sunday. A combination of this upper low and
surface trough drawing deep tropical moisture northward will
translate to increasing rainfall chances. This potential will be
realized as early as late Saturday over the eastern end of the
island chain, then across the entire state Sunday through Monday.
A combination of deep tropical moisture (PWs potentially
exceeding 2 inches) and a sufficient amount of instability
associated with the upper low will support localized heavy
showers. Although a few thunderstorms can't be ruled out, the
anticipated instability (cold pool aloft) and deep moisture being
out of phase will likely be a limiting factor. Shower coverage
will continue to focus over windward areas through Saturday, then
shift over interior locations Sunday through Monday as the trades
diminish.

Guidance remains in decent agreement through the first half of next
week and shows the previously discussed upper low lifting northward
and away from the state. A plume of deep tropical moisture to its
east is forecast to setup and stall over Kauai and Oahu, where
two inch PWs will likely hold through midweek. This moisture
combined with light east-southeast low-level flow will keep the
rain chances up. Rain chances will likely taper off for the Big
Island and Maui County as much drier air moves Tuesday through
midweek.

&&

.AVIATION...
High pressure far to the northeast of Hawaii will maintain light
to locally moderate east to northeast winds across the forecast
areas through Saturday evening. Low level clouds and scattered
showers will favor windward slopes and coasts through tonight and
tomorrow. Isolated MVFR ceilings and visibilities are to be
expected in passing showers, otherwise VFR conditions will prevail
all areas.

An upper level trough will linger over the state through
twenty-four hours and beyond. A band of layered middle and high
clouds associated with the upper trough is currently blanketing
the Big Island. Conditions are expected to persist overnight then
spread westward over Maui County on Saturday. The trough is
producing light to moderate turbulence aloft over the eastern half
of the state and light icing has been reported in the layered
clouds. AIRMET TANGO is in effect for moderate turbulence. Light
icing is noted in the AIRMET bulletin.

Beginning Saturday afternoon, light sea breezes along leeward
coasts and slopes will encourage widespread shower activity over
leeward slope. The trough aloft will further encourage such
convective development. Expect locally heavy showers across all
island interiors on Saturday with isolated thunderstorms possible
over Interior Big Island.

&&

.MARINE...
High pressure far north-northeast of the state will maintain fresh
to locally strong northeast trades through Saturday. A Small Craft
Advisory remains in effect for most of the typical windy areas around
Maui and the Big Island through Saturday afternoon. Beginning Saturday
night, a trough passing just south of the state will bring increasing
showers and a decrease to light to moderate wind speeds out of the
east-southeast. Sea and land breezes will be possible on Sunday
especially along the leeward coasts. Models are currently indicating
fresh east to east-southeast winds returning on Monday, and a possible
front developing northwest of the state later in the week.

A series of small swells from the southwest, west-southwest and
south are expected through the weekend. A small northwest swell
is expected late Saturday into early next week. In the longer
term, a prolonged south swell from a storm south of New Zealand
could arrive around the middle of next week and potentially bring
advisory level surf by the end of the week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Saturday for Pailolo Channel-
Alenuihaha Channel-Big Island Leeward Waters-Big Island
Southeast Waters.

&&

$$

DISCUSSION...Gibbs
AVIATION...Bedal
MARINE...TS

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