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Hawaiian Islands Synoptic Discussion and Guidance

087
FXHW60 PHFO 210703
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
903 PM HST Fri Oct 20 2017

.SYNOPSIS...
Passing showers will continue into Saturday, as the trade winds
remain strong and gusty. The clouds and showers will stay focused
over windward and mauka areas with some brief showers venturing
into the leeward areas of the smaller islands. The trades will
collapse late Saturday, becoming light and variable with daytime
sea breezes on Sunday. An approaching front will bring on
southerly winds ahead of it along with an increase in vog and
moisture.

&&

.DISCUSSION...
The weakening of the trades has begun, and will continue through
Saturday. Winds will still be rather strong Saturday, but not as
strong and gusty as in the past few days. The surface high
generating the trades is considered not to be very strong with a
1026 mb center, but the pressure gradient across the area is
still rather tight. This gradient should loosen up even more
Saturday afternoon.

Change is forthcoming after Saturday. A front is forecast to
advance toward the islands from the northwest over the weekend,
with the ridge of high pressure running ahead of it. This ridge is
expected to be 200 miles north of Kauai Sunday morning, then over
Maui County Monday morning. It is not a sharp east to west oriented
ridge, but a rounded one that will bring on a southerly wind flow
across all of the main Hawaiian Islands. The front is supported
by by both the GFS and ECMWF, to reach Kauai Monday evening, Maui
County Tuesday morning, and pushed just east of the Big Island by
mid morning Wednesday.

The bulk of the weather is slated to occur well ahead of the
front. Models are in good agreement in bringing in a low level
trough with some deep tropical moisture to the Big Island Sunday
afternoon and evening. This moisture will then spread to other
islands Sunday night into Monday, along with the southerly winds.
The bulk of this moisture is expected to reside across Maui
County and the Big island with Oahu at the fringe. The front will
be aided by a rather vigorous upper level trough although the bulk
of the dynamics will remain north of the islands. With this said,
locally heavy showers are possible for Maui County and the Big
Island between Monday and Tuesday, along with a slight chance of
a thunderstorm. For Oahu, the heavier weather is most likely
for Monday night.

As the front approaches Kauai, southerly winds will become rather
strong on Monday that may warrant a wind advisory for Kauai. We
will see how this transpire. It will be monitored closely as the
event nears. As noted, the front cuts through Maui County Tuesday
morning, then on to the Big Island Tuesday afternoon and night.
In the wake of the front, a north wind will bring in a cooler and
drier airmass. On Thursday, the front remains stalled just a few
hundred miles east of the Big Island.

In the mean time, satellite imagery shows a layer of broken low
clouds streaming through Kauai and Oahu, along with the northeast
winds. These clouds will likely be directed away from these two
islands later tonight or Saturday as the trades turn a bit more
easterly. Molokai is in the clear thus far, while some broken low
clouds are and will be affecting the windward shores of Maui and
the Big Island's northeast facing coast throughout tonight. There
is an isolated shower located just off Kailua Kona town now, and
expect them to hang around the coastline through midnight before
calling it quits.

The wind advisory for parts of the Big Island, Lanai, and
Kahoolawe, has been extended to 4 am HST Saturday.


&&

.AVIATION...
Strong and gusty trade winds will continue to support AIRMET
Tango overnight for low level turbulence downwind of the
mountains on all islands. In addition, AIRMET Tango is also in
effect for upper levels over the islands due to speed sheer
between FL260 and FL340.

VFR will predominate, as usual. Brief, to occasionally TEMPO,
MVFR in passing trade showers will mainly affect windward and
mauka sections over all isles through the overnight hours.

&&

.MARINE...
A dynamic (but seasonable) wind, wave and weather pattern is
expected over the next several days as high pressure N of the
area is replaced by a low that sends a front through the island
chain early next week. After the front, light to moderate N winds
are possible by the middle of next week, but forecast confidence
is lower than normal.

Strong to near gale force trade winds currently prevail, but will
be diminishing over the next 24-48 hours as high pressure N of
the islands shifts E. A Small Craft Advisory (SCA) remains posted
for all waters through tonight, but winds and seas are expected
to diminish below SCA criteria in most zones on Saturday, and in
the remaining zones by Saturday night. The SCA will be extended
in time for these zones with the evening forecast update. As winds
and seas diminish, rough surf along E facing shores is expected
to diminish below High Surf Advisory criteria by early Saturday.

Although short-period wind waves will be easing, building NNW and
S swells Sunday and Monday are expected to keep combined seas
elevated, but below the SCA criteria of 10 feet. A larger,
shorter-period NNW swell building on Tuesday will likely lead to
SCA in all zones as seas exceed 10 feet once again. The first NNW
swell will produce surf near the advisory threshold along exposed
N facing shores, and the second swell could produce high surf as
well. Advisory-level surf along S facing shores is possible
Sunday and Monday, with the swell generated by a distant low in
the S Pacific. A small long-period W swell is possible next week,
generated by large and slow-moving Typhoon Lan in the W Pacific.
See the recently updated Oahu Surf Discussion (SRDHFO) for
details on the swell sources affecting the islands.

&&

.FIRE WEATHER...
Some relief expected for parched dry areas of the leeward
districts of the smaller islands as night progresses. Relative
humidity will increase some, although the trades will remain
rather stiff and gusty for most of the night. We have noticed a
few sprinkles or light showers reaching the leeward areas of Oahu
and Kauai in particular. We are expecting a tad fewer trade
showers and not-as-strong trades for all islands Saturday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Saturday for east facing shores
of most islands.

Wind Advisory until 4 AM HST Saturday for Lanai, Kahoolawe,
Big Island North and East Kohala.

Small Craft Advisory until 6 AM HST Saturday for all Hawaiian
waters.


&&

$$

Discussion/Fire Weather Lau
Aviation Kinel
Marine Birchard

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