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Hawaiian Islands Synoptic Discussion and Guidance

218
FXHW60 PHFO 211351 CCA
AFDHFO

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
351 AM HST Sat Sep 21 2019

.SYNOPSIS...
Increasing moisture and instability will translate to a wet
pattern through the weekend. Shower coverage will continue to
focus over windward areas today, then become more widespread
Sunday through Monday as the trades diminish. Although some drier
air is expected to fill in over the Big Island and Maui County by
Tuesday, the wet pattern is forecast to linger over Kauai and Oahu
through midweek.

&&

.DISCUSSION...
Water vapor imagery showed an upper low drifting westward across
the region with plenty of deep tropical moisture pooling
northward to its east. Instability associated with this upper
feature has been triggering isolated heavy showers and even a
thunderstorm (near the Kona coast) overnight. 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 2 am HST).

The short-term guidance (through Monday) remains in good
agreement and depicts a wet pattern evolving across the region as
the aforementioned upper low continues westward across the state.
A surface trough associated with it, currently well east-
southeast of the state, will move through the area later today
through Sunday. A combination of this upper low and surface trough
drawing deep tropical moisture northward into the area will
translate to increasing rainfall chances. This potential will be
realized as early as this evening over the eastern end of the
island chain, then across the entire state Sunday through Monday.
As this scenario evolves, a Flash Flood Watch may be needed in
later packages. In addition to the potential for heavy rainfall, a
few thunderstorms will remain a possibility through the weekend
as the upper low remains over the region. Shower coverage will
continue to focus over windward areas today, then become more
widespread Sunday through Monday.

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 drier air moves in 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 this evening. Low level clouds and scattered
showers will thus continue to favor windward slopes and coasts.

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. The upper level cloudiness is expected to spread
westward over Maui County by afternoon and across the remainder of
the state by late evening. Light icing is expected abv FL180 in
the layered clouds.

The trough aloft is acting to destabilize the air mass over the
state, especially the eastern half. Isolated thunderstorms across
leeward waters this morning are expected to spread statewide by
late afternoon. Shower activity will intensify after sunrise over
island interiors. Locally heavy downpours expected across
interior Big Island this afternoon and evening.

&&

.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 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 are 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 this 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 next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST this evening for Maalaea Bay-
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