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Hawaiian Islands Synoptic Discussion and Guidance

370
FXHW60 PHFO 181335
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
335 AM HST Tue Jun 18 2019

.SYNOPSIS...
Light to moderate east-southeast winds will strengthen and
shift to a more typical easterly direction later today, and then
continue through Thursday. Showers will be focused over windward
slopes and coasts, though we could see some clouds and a few showers
over leeward areas this afternoon. Winds will decrease and shift to
the southeast Friday into the weekend, leading to spotty showers and
muggy conditions.

&&

.DISCUSSION...
Trade winds are veered toward the ESE, mainly over the W end of the
island chain, due to surface high pressure centered far NE of the
area, and a persistent trough to the NW. As a developing low along
this trough axis tracks NE away from the area, the high will build
westward, and slightly stronger E winds will build over all islands
later today into tonight. MOS guidance responds to this by bringing
slightly lower dew points (65-67F vice 70-72F) over the islands by
Thursday as the air mass dries somewhat, and the trade flow takes
on a more typical ENE direction. This will be short-lived however,
with lighter SE winds developing Friday, and then continuing through
the weekend, and potentially into next week. This will occur as
another low forms along the trough axis, thus forcing the ridge
extending SW from the high over the islands. This will lead to light
S to SE winds over Kauai and Oahu, and light to moderate E winds
near the Big Island. Long range forecast features the ridge moving
back toward the N in about a week, leading to strengthening E trade
winds.

Clouds and showers will continue to favor windward slopes and
coasts through Thursday, though, like yesterday, afternoon sea
breezes may trigger a few showers over leeward areas during the
afternoon hours again today. Morning soundings show elevated PWAT
near 1.5", and a somewhat unstable profile, so a few showers could
be briefly heavy, especially over Kauai and Oahu where moisture is
maximized. Trade winds should be sufficiently strong to preclude
significant leeward convection Wednesday and Thursday, and
brief windward showers will be most likely during nights and
mornings as the air mass dries and stabilizes.

The weekend weather regime introduces increased uncertainty as to
where clouds and showers will occur (and when), due to the veered
wind flow. Generally speaking, a large closed low aloft is expected
to prevail to the distant NW of the area, supporting the
persistent trough and surface low. Instability associated with this
feature is not expected to spread over the islands, and a ridge
aloft to the SE of the Big Island should help keep some semblance of
stability over the E end of the state, with moisture and instability
maximized over waters W of Kauai. Thus, we are expecting mainly
brief windward showers over the Big Island, while the other islands
will see spotty showers - some forming in the afternoons over
interior areas due to daytime heating and sea breeze convergence,
others forming due to orographic lift, and others forming within
eddies and cloud plumes peeling off other islands. Speaking of
which, a difficult-to-anticipate overnight cloud plume emanating
from Molokai was responsible for fueling showers over portions of
southern Oahu through much of the night.

&&

.AVIATION...
The background east-southeast flow is keeping most of the smaller
islands in a light wind regime early this morning. There are low
clouds and showers moving over some windward sections of the
island chain, especially the Big Island, Oahu and eastern Maui.
There have also been some low clouds and light showers moving
across leeward sections of Oahu. Brief MVFR conditions will
likely persist over some windward areas through mid-morning.

The trade winds are forecast to strengthen and become more
easterly later today. This will keep the main coverage of low
clouds and showers over windward areas. However, there may be
local sea breezes over some leeward sections, which might produce
local cloud build ups and brief showers from late this morning
into the afternoon.

AIRMET Sierra remains in effect for mountain obscuration over
windward sections of Oahu. The trade winds are expected to
strengthen later today and tonight, which will increase the
possibility of mechanical turbulence developing leeward of the
higher terrain on the islands. If so, we may need to issue AIRMET
Tango later today or tonight.

&&

.MARINE...
The combination of a trough to the west and high pressure far
northeast of the state will continue moderate to locally strong
east to east-southeast winds through midweek. Winds have diminished
slightly overnight, with the latest ASCAT pass not depicting any
25 kt winds over windward Big Island and Maui. Therefore, the Small
Craft Advisory (SCA) has been cancelled for those waters, but remains
in effect for the Alenuihaha Channel and waters south of the Big
Island. Winds are expected to gradually weaken and veer southeast
Thursday into the weekend as the trough to the west strengthens.

A slight bump in south shore surf is expected today into Wednesday
as a new small long-period swell fills in. Only minimal surf is
expected towards the latter half of the week.

A small pulse of northwest swell is expected today. Otherwise, only
background energy out of the northwest is expected through rest of
the week. Surf along east facing shores will begin to rise this week
as trade winds become more established, especially for the east and
central isles late in the week. A short-period, northeast swell is
possible over the weekend.

&&

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

&&

$$

DISCUSSION...Birchard
AVIATION...Houston
MARINE...TS

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

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