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Hawaiian Islands Synoptic Discussion and Guidance

718
FXHW60 PHFO 162009
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1009 AM HST Mon Oct 16 2017

.SYNOPSIS...
High pressure north of Hawaii will keep a windy trade wind
pattern in place through much of the upcoming work week, with the
trades trending down slightly Friday and weakening further over
the weekend. Clouds and showers will favor windward and mauka
areas through the period, but showers will reach leeward areas
from time to time as well due to the strength of the trades.

&&

.DISCUSSION...
A 1030 mb surface high centered around 1050 miles north of
Honolulu, is driving windy trades across the island chain this
morning. Visible satellite imagery shows partly to mostly cloudy
skies, with cloud cover most prevalent over windward and mauka
sections. Passing scattered showers continue to be carried into
windward areas with the trades, with isolated showers reaching
leeward sides of the smaller islands.

High pressure north of the state will slowly move southwest over
the next couple of days, with a re-enforcing high building in
behind it midweek. The high will then move southeast and weaken
Friday, and especially over the weekend as a cold front
approaches from the northwest. As a result, a tight pressure
gradient will remain in place over Hawaii, and will persist the
windy trades through Thursday. A Wind Advisory remains in effect
for the entire state with the exception of the Haleakala and Big
Island summits through 10 pm this evening. The Wind Advisory may
be need to be extended for some areas of the state. Trades will
taper off on Friday, and then weaken further over the weekend.

As for sensible weather details, fairly typical trade wind weather
is expected through the period, with clouds and showers favoring
windward and mauka areas, with an occasional shower drifting
leeward from time to time due to the strong trades. Showers
coverage will be greatest at night, tapering off during the
daytime.

The latest GFS solution indicates that some tropical moisture may
try to lift northward into the islands by the end of the upcoming
weekend, but the ECMWF solution keeps the deeper moisture south
of the state. Will keep the forecast featuring typical trade wind
weather through next weekend and await better model agreement
before deviating away from climatology.

&&

.AVIATION...
Strong and gusty NE tradewind will continue today, and through
much of the week. AIRMET TANGO for low level turbulence is posted
across the state. AIRMET TANGO is also posted for strong surface
winds greater than 30 KT across the state, with most likely
areas over mountain ridges and through valleys and channels.

Scattered showers will continue to move over with the trades. Most
clouds and showers will focus along the windward and mauka areas;
strong trade winds will carry a few showers to leeward areas.
Periods of MVFR CIGS/VIS should be expected in passing showers.
Otherwise, VFR conditions will prevail.

&&

.MARINE...
Very strong and gusty trade winds will continue through much of
this week due to a nearly-stationary, strong surface high N of
the main Hawaiian Islands. This high will be replaced by another
strong on Wednesday night, continuing the windy conditions for an
additional day or so. Gale warnings are posted for the Pailolo
and Alenuihaha channels Thursday afternoon, while a Small Craft
Advisory remains in effect for the remaining Hawaiian waters
through the same time. Trades should begin on Friday as the high
starts to weaken and move off to the east.

The strong to gale-force trade winds are also causing very rough,
choppy, and dangerous seas around the islands. Surf is also
elevated along E facing shores of all islands, with a high surf
advisory in effect through Wednesday afternoon. This advisory
will likely be extended.

A long-period S swell will peak today. The latest swell observation
from the Pearl Harbor buoy is around 4 and a half feet. A High
Surf Advisory is in effect for the S facing shores of all islands
through Monday. The next S swell will be a small one, slated for
the upcoming weekend.

A 3 foot NNW swell has peaked and is on its way down with the
period becoming shorter. The next NNW swell is slated to arrive
Thursday night, building to a peak of around 6 feet early Friday
with a long period of 16 to 19 seconds. Expect moderate size surf
from this swell, but remaining below advisory level. A somewhat larger
NW swell is due in Sunday night and Monday of next week, with the
surf potentially approaching high surf advisory levels along the
N and W facing shores.

The Oahu Surf Discussion (SRDHFO) has detailed information regarding
the swell sources.

&&

.FIRE WEATHER...
Strong trade winds coupled with a drier airmass pushing over the state
will allow minimum relative humidity to drop into the 40 to 45 percent
range. These conditions create potential for a fire to ignite easier
than usual and to spread quickly if they do. The windy and relatively
dry conditions will continue through the end of the work week
before the trade winds weaken. The Keetch-Byram Drought Index, or
KBDI, is at 572 today (and would need to be at 600 for Red Flag
Warning criteria). Keep in mind that we will be close to these
conditions each afternoon.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Wind Advisory until 10 PM HST tonight for all portions of the
state except for the Big Island summits and Haleakala summit.

High Surf Advisory until 6 PM HST today for south facing shores
of Niihau, Kauai, Oahu, Molokai, Lanai, Kahoolawe and the Big
Island.

High Surf Advisory until 6 PM HST Wednesday for east facing shores
of Kauai, Oahu, Molokai, Maui and the Big Island.

Gale Warning until 6 AM HST Thursday for Pailolo Channel-
Alenuihaha Channel.

Small Craft Advisory until 6 AM HST Thursday for all remaining
coastal waters.

&&

$$

DISCUSSION/FIRE WEATHER...Foster
AVIATION...Eaton
MARINE...TS

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

Current Conditions
Temp		4.3 C
RH		8 %
Wind		NNE 34 mph
Road		Open (4x4)