Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxhw60 phfo 201931 
afdhfo

Area forecast discussion
National Weather Service Honolulu HI
931 am HST Tue Nov 20 2018

Synopsis...
a moderately strong high pressure system passing north of the
islands will bring a boost in the trade winds through Wednesday,
resulting in locally strong speeds. As the high moves to northeast
of the islands during the second half of the week, the trades
will weaken to light to moderate speeds Friday. Winds are expected
to become light and variable over the upcoming weekend. Under this
trade wind flow, expect some brief passing showers, favoring the
windward and mountain areas. An increase in moisture is expected
Wednesday night and Thursday, and this moisture may linger into
the weekend, fueling increased showers.

&&

Discussion...
a surface high was analyzed at 1031 mb, about 1000 miles north of
Kauai this morning, which is a couple of millibars weaker than
earlier GFS runs. It is forecast to weaken to 1028 mb while moving
to the southeast during the next 24 hours. The high will continue to
weaken while moving farther away from the islands through
Thursday. We are predicting a boost in the trades in the next 18
hours across the area. But given the high is 1031 mb and
weakening, it is not all favorable for Wind Advisory for the
typically windy places of Maui and Big Island counties tonight and
Wednesday. There will be isolated areas but not widespread enough
to warrant a Wind Advisory. In fact, about all of the high
resolution models have backed off for an even windier situation
for tonight and Wednesday. The high may peaked overnight, and is
now on a downward trend.

Latest satellite imagery shows an area of broken low clouds with
embedded showers, aimed at primarily Oahu and Maui counties. A
certain percentage of these clouds will be reduced due to daytime
mixing, but it will likely retain a chance of showers for
especially the windward and mountain areas for these two counties.
It looks like these clouds will have a hard time reaching Kauai in
one piece, thus the forecast was adjusted for Kauai, for nicer,
drier Tuesday. We do expect this broken area of clouds to fill in
late this afternoon, and give windward Kauai a small boost in
showers tonight.

Both the GFS and European model (ecmwf) suggest patches of shower bearing clouds
affecting the islands before the remnants of a frontal band
settles over US Wednesday night and Thursday. This moisture
lingers over part of the island chain into Friday, while the low
level winds become light and variable over land and light
easterlies over the surrounding waters. It should be noted there
is a difference between the ec and GFS models low level winds.
This solution is a blend between these two models. At the upper
levels, a NE to SW trough is noted west of Kauai by both models, but
with the GFS being closer to Kauai. The models should be coming to
a closer agreement in the next 24 hours, so the wind scenario for
at least Friday Onward is subject to adjustment.

We will be watching this scenario closely under this pattern, for
it tends to potentially bring some heavy rain issues to part of
windward Oahu. Thus far, the models are indicating a mid level
ridge over the islands, with an inversion 8 to 10k feet. The upper
trough appears not to be a very strong or vigorous one, so we
should be fine. The two models agree on the upper trough moving
down the island chain on Saturday, but differs on the timing with
the GFS being the more progressive and faster one.



&&

Aviation...
a strong high pressure system in the central Pacific will
maintaining breezy to locally strong trade winds across the
Hawaiian islands through Wednesday. Expect isolated to scattered
showers favoring windward and mountain areas for the next few
days.

Strong trade winds will continue to cause low-Level Mountain wave
turbulence over and immediately south through west of mountains
on all islands below 080 through Wednesday. Airmet Tango remains
in effect for moderate mountain wave turbulence over and immediately
south through west of all islands, and surface winds greater than
30 kts over the channels and waters south of The Big Island. No
other airmets in effect or expected.

&&

Marine...
strong trades will continue across the Hawaiian waters through
Wednesday due to strong high pressure positioned north of the
state. Gales can't be ruled out across the Alenuihaha Channel and
waters south of The Big Island tonight through Wednesday, although
the latest model runs are showing slightly weaker winds now for
tonight. Seas are expected to still respond and build to the
advisory level (10 ft) over the exposed waters. A gale watch will
continue to be in effect for the Alenuihaha Channel and waters
south of The Big Island beginning tonight and will be reassessed
this afternoon. Elsewhere, a Small Craft Advisory is in effect.

Winds and seas will quickly trend back down through the second
half of the week and into the upcoming weekend as the area of high
pressure weakens and shifts eastward in response to a cold front
passing to the north. Confidence begins to lower through the
second half of the weekend into next week as the pattern shifts
across the northern Pacific to a more progressive setup. The GFS
solution indicates the ridge axis will remain overhead locally,
which would keep more of a light and variable pattern in place.
The ECMWF, however, shows this ridge remaining positioned farther
north, which would favor light to moderate trades holding.

The current small northwest swell will peak today and decrease
Wednesday through Thursday. Strong trades will bring an increase
in rough and choppy surf along east facing shores starting tonight
and may bring advisory level surf Wednesday through Thursday.
Surf should slightly lower Friday into the weekend as the trades
weaken. Small surf will continue along south facing shores with
mainly background southern Pacific pulses moving through.

For the extended, guidance is reflecting a large and broad gale-to
storm-force low setting up across the far northwest Pacific in
response to the aforementioned pattern shift Thursday night
through the weekend. Seas are forecast to climb into the 40-50 ft
range within the associated fetch region focused at the islands
within the 300-320 directional band. Although there remains some
noticeable differences between the various solutions as this
feature evolves through this time, all depict the first
significant northwest swell reaching the islands Sunday night
through early next week. Surf will likely exceed warning levels
Monday. Specifics will come later in the week as this scenario
evolves and confidence increases.

&&

Hfo watches/warnings/advisories...
gale watch from this evening through Wednesday afternoon for
Alenuihaha Channel-Big Island leeward waters-Big Island southeast
waters.

Small Craft Advisory until 6 PM HST this evening for Alenuihaha
Channel-Big Island leeward waters-Big Island southeast waters.

Small Craft Advisory until 5 PM HST Wednesday for Kauai northwest
waters-Kauai windward waters-Kauai leeward waters-Kauai Channel-Oahu
windward waters-Oahu leeward waters-Kaiwi Channel-Maui County
windward waters-Maui County leeward waters-Maalaea Bay-Pailolo
Channel-Big Island windward waters.

National Weather Service Glossary of Abbreviations