Area Forecast Discussion
Issued by NWS Honolulu, HI

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000
FXHW60 PHFO 181955
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
955 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 east southeast across the
western end of the island chain due to surface high pressure
centered far northeast and and a persistent trough to the northwest.
As a developing low along this trough axis tracks northeast away
from the area, the ridge will build in south of this feature and
push in slightly stronger east winds 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 east northeast direction. This will be short-lived however,
with lighter southeast winds developing Friday, and then
continuing into next week. The light wind regime will occur as
another, stronger low forms along the trough axis to the northwest
which will force the ridge on top the islands. More specifically,
light south to southeast winds are expected over Kauai and Oahu,
and light to moderate east winds near the Big Island.

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 1.6 inches, and a somewhat unstable profile, so a few
showers could be briefly heavy, especially over Kauai and Oahu
where moisture is maximized. Radar shows heavy showers passing
well south of Kauai this morning, but indicative of the unstable
environment. 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 northwest 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 southeast of the Big Island should help keep
some semblance of stability over the eastern end of the state,
with moisture and instability maximized over waters west of
Kauai. Thus, we are expecting mainly brief windward showers over
the Big Island, while the other islands will see spotty showers
with 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. Otherwise, conditions will be warm and
muggy beginning Friday through at least the first half of next
week. Long range forecast features the ridge moving back toward
the north around Thursday of next week, leading to strengthening
trade winds.


&&

.AVIATION...
A 1034 mb surface high pressure cell resides far northeast of the
Hawaiian Islands. This feature will act to generate light to
moderate easterly trades across our region for the remainder of
today. As is typical with this type of flow pattern, most shower
activity will be limited to the windward slopes. However,
isolated activity could spill over portions of the interior or
leeward sides during the afternoon. Expect similar conditions to
exist tonight.

No AIRMETS are currently in effect.


&&

.MARINE...
A surface high will remain far northeast of the area though the
week. The pressure gradient southwest of the high will maintain
locally locally strong trade winds over the coastal waters through
mid-week. A Small Craft Advisory is in effect for the Pailolo and
Alenuihaha Channels and waters south of the Big Island through
tomorrow. Winds will weaken and shift out of the southeast during
the second half of the week as a trough deepens west of the
offshore waters. In the coastal waters, the southeast flow will be
partly blocked by the Big Island so winds will be light and
variable around the smaller islands.

The already-small surf today will fade even more through tomorrow.
Surf will remain well below the advisory threshold along all
shores through the coming weekend, but there will be a small boost
in surf along east facing shores as a short-period, northeast
well builds over the weekend.


&&

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


&&

$$


DISCUSSION...Foster
AVIATION...Thomas
MARINE...Donaldson


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