Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 240634
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
834 PM HST Fri Feb 23 2018

.SYNOPSIS...
Moist and somewhat unstable conditions will persist across
portions of the state through the weekend. East to southeast
winds will focus rainfall over windward and southeast slopes. The
greatest threat for heavy showers and thunderstorms will be over
the Big Island tonight, then shift to the western end of the
island chain on Sunday. Drier conditions will push back over the
state early next week as potentially strong east to east-
southeast winds develop across the entire state.

&&

.DISCUSSION...
The large scale weather picture continues to show an upper level
closed low roughly 900 miles northwest of Kauai and a strong 1040
mb high center far to the northeast of the state. The interaction
between these two features continues to draw tropical moisture
northward over the Hawaiian Islands keeping us in moist and
moderately unstable low level environment with a weak high level
subsidence inversion setting up around 10000 to 12000 foot level.
This stable inversion layer aloft has helped to limit shower
activity today to the windward slopes of each island. Chances for
heavy showers will linger over the Big Island overnight where
stronger instability exists in a deeper moisture layer. A Flash
Flood Watch remains in effect for the Big Island until 6 am
Saturday morning. A Winter Storm Warning also remains in effect
for the Big Island Summits with snow packed icy roads are expected
for elevations above 12500 feet through Saturday morning.

The interaction between the ridge and a surface trough west of
Kauai has increased winds over portions of the Big Island. A wind
advisory was issued today for sustained winds forecast to range
from 15 to 30 mph with locally higher gusts up to 50 mph. This
advisory remains in effect for the Big Island through Saturday
afternoon.

In the short term, we continue to see the upper low drifting
slowly northwest away from the state as the strong high pressure
ridge gradually builds in from the northeast. Shower activity on
Saturday will focus mainly over the windward slopes of each island
with higher chances for heavier showers developing along the
eastern slopes of Maui and the Big Island. On Sunday forecast
guidance continues to show a shift in shower activity towards the
west with increasing rainfall chances over Kauai and portions of
Oahu as an upper level trough sweeps across the western edge of
the state.

A drier trend develops next week as the strong ridge builds in
over the islands. Enough moisture and instability will linger over
the windward sections of the Big Island for continued showers,
however the rest of the state will see improvement trends from
Monday to Tuesday. Stronger winds are expected on Tuesday
afternoon and Wednesday where additional wind advisories may be
needed as wind speeds ramp up across the state.

&&

.AVIATION...
High pressure northeast of the state and a trough of low pressure
to the west, will keep a gusty east to southeast flow in place
across the island chain through Saturday. A moist and unstable
airmass will remain over the island chain, particularly the E and
SE areas of the Big Island, where widespread MVFR and isolated
IFR conditions are expected. Across the smaller islands isolated
MVFR conditions will be possible mainly in windward areas.

AIRMET SIERRA remains in place for mountain obscuration across
E to SE facing slopes of the Big Island and NE thru SE facing
sections of east Maui. No other AIRMETs are in effect.

&&

.MARINE...
An unstable airmass will continue the possibility for isolated
thunderstorms over all coastal waters through Saturday, before
shifting the focus over the western half through Sunday.

A Gale Warning remains posted for waters around the Big Island
overnight due to the strong winds observed in the scatterometer
pass earlier today. Still expect the winds there to drop a bit
Saturday back to Small Craft Advisory (SCA) levels. Strong high
pressure far to the north-northeast will maintain breezy east to
southeast winds through the weekend, before an uptick in winds
during the first half of the new work week. An SCA remains posted
for most waters as a result of the winds and the resultant wave
heights. Its possible the SCA will need to be expanded next week
as the winds increase, and additional gale force winds are
possible.

A long and rather broad fetch of easterly winds on the
south side of the high will continue to produce advisory level
surf along east facing shores through the weekend and on into
early next week. Latest guidance from the Wavewatch model
indicates surf could reach warning levels as early as Monday night
or Tuesday. This combined with high astronomical tides could
cause some coastal inundation issues along some low lying areas
exposed to east.

The current small northwest swell will lower gradually tonight
through Saturday. Otherwise surf along north and west facing
shores will be well below normal for this time of year during the
Sunday through Thursday time period. A series of small, mainly
background south swells can be expected through the weekend and on
into the middle of next week.

See the latest Oahu Surf Discussion (SRDHFO) for additional
details on surf and swell.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Monday for Kauai Windward-Oahu
Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-South Big Island-Big Island North and East.

Flash Flood Watch until 6 AM HST Saturday for Kona-South Big
Island-Big Island North and East-Kohala-Big Island Interior.

Wind Advisory until 6 PM HST Saturday for Kona-South Big Island-
Big Island North and East-Kohala-Big Island Interior.

Winter Storm Warning until 6 AM HST Saturday for Big Island
Summits.

Small Craft Advisory from 6 AM Saturday to 6 AM HST Monday for
Alenuihaha Channel-Big Island Windward Waters-Big Island Leeward
Waters-Big Island Southeast Waters.

Gale Warning until 6 AM HST Saturday for Alenuihaha Channel-Big
Island Windward Waters-Big Island Leeward Waters-Big Island
Southeast Waters.

Small Craft Advisory until 6 AM HST Monday for Kauai Northwest
Waters-Kauai Windward Waters-Oahu Windward Waters-Kaiwi Channel-
Maui County Windward Waters.

&&

$$

DISCUSSION...Bohlin
AVIATION...Jelsema
MARINE...M Ballard


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