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Hawaiian Islands Synoptic Discussion and Guidance

814
FXHW60 PHFO 211359
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
359 AM HST Fri Jul 21 2017

.SYNOPSIS...
A surface ridge of high pressure far to our north will keep
breezy trades blowing through today. The trades will ease over
the weekend as weakening Fernanda approaches the waters to our
northeast. An upper level trough is expected to form close to
the islands today, bringing with it a period of unsettled weather
through early next week.

&&

.DISCUSSION...
Not a whole lot of change with the forecast package. An unsettled
showery pattern is likely over the next several days as an upper
level trough set up shop near the main Hawaiian Islands in the
next 24 hours.

The forecast track of weakening Tropical Storm Fernanda will
likely dictate the weather between Sunday night and Tuesday. We
anticipate Fernanda to become a remnant low as it passes north of
the island early next week. It is vague phrase in "passing north".
Should the remnant low stay close to the main Hawaiian Islands,
say within 200 miles of the islands, the trades will become light
especially on Monday. The ECMWF solution has the injection of
additional deeper tropical moisture to move through the main
islands between Sunday night and Tuesday morning. A drier trade
wind air mass follows closely behind with stronger trades. As the
deep tropical moisture moves through the island chain, we
anticipate a showery pattern with spotty heavy showers.

But even before Sunday night, we are likely to be in a unsettled
air mass as the upper trough develops close to the main Hawaiian
Islands by Saturday. A low cloud field running well ahead of
Fernanda, is gradually spreading westward from the Big Island this
hour, and will be the main source of moisture for the showery
pattern through Saturday. The low level inversion weakens
allowing moisture currently between 8k and 12k feet to gradually
rise to between 10k over Kauai to 18k feet over the Big Island
this afternoon. Depth of the low level moisture rises further to
15 to 20k ft by Saturday afternoon, and this is not the deep
tropical moisture associated with Fernanda. Flood threat potential
is small at this point as the trades will be blowing at moderate
speed. As of Saturday afternoon, Fernanda is expected to be a
remnant low, about 420 east- northeast of the Big island. The air
mass should be unstable enough on Saturday afternoon to favor a
thunderstorm or two over the higher terrains of the Big Island. It
is not in the forecast now, but is likely by the end of today
should the models continue to show it.

The showery pattern continues between Sunday and Tuesday as the
upper trough interact on the deep tropical moisture, resulting in
spotty heavy showers. Lighter winds on Monday may accentuate the
situation especially the afternoon hours with the higher chance of
locally heavy showers.

As noted above, we anticipate a drier, less humid air mass, and
stronger trades to return by Tuesday night.

&&

.AVIATION...
A high pressure ridge north of Hawaii will keep moderate to breezy
northeast trade winds in the forecast with moderate low level
turbulence. An upper level trough currently moving into the Big
Island will continue westward across the southern islands this
morning bringing scattered to numerous showers over the Big
Island, Maui and Lanai. Periods of MVFR conditions in moderate to
heavy showers are more likely over northeastern windward slopes.
Expect isolated to scattered showers elsewhere.

AIRMET Sierra in effect for Tempo mountain obscuration along
northeastern sections of the Big Island. AIRMET Sierra may expand
to windward sections of Maui later this morning. AIRMET Tango in
effect for moderate low level turbulence over mountains and south
through west sections of all islands.

&&

.MARINE...
Locally strong trade winds will continue to produce Small Craft
Advisory (SCA) conditions through early Saturday morning for the
typically windy waters adjacent to the islands of Maui County and
the Big Island.

The latest forecast for Tropical Storm Fernanda issued by the
Central Pacific Hurricane Center at 11 pm HST Thursday continues
to indicate the tropical cyclone will weaken to a remnant low that
may eventually move to a position northeast of the Big Island
Sunday afternoon. As this area of lower pressure approaches the
region, expect the trade winds to weaken this weekend. The
background flow will likely become much lighter early next week as
the remnants of Fernanda pass by to the north of the islands from
late Sunday into early Tuesday.

A High Surf Advisory remains in effect for the east facing shores
of the Big Island and Maui through early Saturday morning due to
a mid- to long-period east swell associated with a captured fetch
from when Fernanda was a major hurricane. Wave model guidance
indicates reinforcing east swell energy from Fernanda arriving
along the east facing shores later today and tonight. Note that as
this reinforcing swell continues to spread westward to the
remainder of the smaller islands, a High Surf Advisory will also
likely be needed for east facing shores of Molokai, Oahu and
Kauai starting Saturday or Sunday.

The largest high tides of the month (king tides) will occur each
afternoon during the next few days. Note that these abnormally
high tides combined with wave run up due to the east swells
produced by Fernanda may result in coastal flooding in some areas,
especially from today into Monday.

Elsewhere, no significant swells are expected. Small south swells
will continue to produce small background surf along south facing
shores into next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Saturday for east facing shores
of Maui and the Big Island.

Small Craft Advisory until 6 AM HST Saturday for Maalaea Bay,
Pailolo and Alenuihaha Channels, and waters south of the
Big Island.

&&

$$

Discussion...Lau
Aviation...Bohlin
Marine...Houston

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



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