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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 211952

Area Forecast Discussion
National Weather Service Honolulu HI
952 AM HST Sat Sep 21 2019

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.


Currently at the surface, a trough of low pressure is located
around 250 miles east-southeast of the Big Island, while a 1026 mb
high is centered around 1250 miles north-northeast of Honolulu.
The resulting gradient is producing moderate trade winds across
the island chain this morning. Meanwhile aloft, a closed low is
located over the Oahu and Maui County. Visible satellite imagery
shows partly to mostly cloudy skies across the state, with cloud
cover more prevalent over windward locales. Radar imagery shows
scattered to numerous showers moving into windward areas, with
quite a bit of leeward spillover. Main short term concerns over
the next few days revolve around the potential for heavy rainfall
and thunderstorms.

Today through Tuesday,
Model solutions are in good agreement the next several days,
showing the upper level low retrograding westward, dragging a
trailing surface trough and plume of deep tropical moisture with
2.0 to 2.5 inch precipitable water values westward and through
the island chain. This is expected to result in an unsettled
period of weather across the state, with the potential for
producing locally heavy rainfall and thunderstorms at times.
Showers are expected to favor windward areas, although leeward
areas will see a fair amount of afternoon and early evening
shower activity due to the unstable airmass in place. The plume of
deep tropical moisture is expected to arrive over the eastern end
of the state tonight, then spread westward over Maui County and
Oahu on Sunday and into Kauai Sunday night. Most of the state will
be in the soupy airmass on Monday, with some drier air beginning
to work in from the east, particularly across the Big Island an
Maui County, Monday night and Tuesday. For now the most unstable
airmass associated with the upper low and deeper tropical moisture
associated with the trailing surface trough don't appear to align
in a manner consistent with a widespread flash flood threat. That
said, given the copious amount of deep tropical moisture moving
through, we will continue to monitor the potential for flash
flooding closely as the pattern evolves.

Tuesday night through Friday,
There are some conflicting signals in the guidance in the longer
range periods, with the ECMWF suggesting the surface trough and
trailing plume of deep tropical moisture will shift just west of
the state Tuesday night, although it will remain in close
proximity to Kauai. The GFS on the otherhand, lingers the surface
trough and deep tropical moisture over Kauai through Thursday,
before bringing in a drier trade wind pattern for the end of the
work week. For now will keep the forecast more wet across the
western islands through Thursday until details become more clear.


A moist and somewhat unstable trade wind flow will persist through
tonight. High pressure far northeast of the state will maintain
gusty trades while an upper level low centered over the central
islands drifts westward. The unstable trade wind flow is producing
a rather active shower pattern, leading to periods of MVFR
ceilings and visibility along windward slopes in passing moderate
to heavy showers. AIRMET Sierra is currently in effect for parts
of Maui and the Big Island, but expect that AIRMET to be dropped
shortly. Clouds and spotty showers, and perhaps a thunderstorm
will develop over interior and leeward sections today, mainly on
the Big Island where localized MVFR conditions are possible. Over
other leeward areas, expect brief, though rather heavy, showers to
produce isolated MVFR conditions.

A band of layered middle and high clouds associated with the
upper low is expected to spread northwestward over the state late
tonight and Sunday. AIRMET Zulu for light icing will be taken
down shortly but should be needed again as the high clouds thicken


High pressure far north-northeast of the state will maintain fresh
to locally strong northeast trades through today. A Small Craft
Advisory (SCA) remains in effect for the typical windy areas
around Maui and the Big Island through this afternoon. Beginning
tonight, 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
with winds increasing to borderline SCA levels near the windy
areas around Maui County and the Big Island by Monday afternoon. A
front developing northwest of the state Monday into Tuesday,
should lead to light to moderate east-southeast winds by middle of
next week.

A series of small swells from the southwest, and south are
expected through this weekend and into early next week. A small
northwest swell is expected to fill in later tonight and peak
Sunday and lower through Monday. 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. Additionally, a few small northwest
swells will be possible during the second half of the week.


Small Craft Advisory until 6 PM HST this evening for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.




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

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It was last modified on: Wed, Dec 17 2014 - 1841 UTC
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