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Hawaiian Islands Synoptic Discussion and Guidance

118
FXHW60 PHFO 140211
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
411 PM HST Sun Oct 13 2019

.SYNOPSIS...
Moderate to fresh trades will focus light passing showers over
windward slopes through Tuesday night. A front dropping southward
toward the state and an upper level disturbance passing overhead,
will weaken the trades and bring a chance for some heavier showers
to the area Wednesday through Thursday. Drier trade wind weather
will resume Saturday.

&&

.DISCUSSION...
A 1022 mb surface high is centered far to the northeast is
driving moderate fresh trade winds over the islands this afternoon.
Upper level ridging over the area is keeping the atmosphere
stable. 00Z soundings show a strong inversion over the area from
6000 to 7000 feet. Radar and visible satellite shows low clouds
and a few passing light showers carrying in with the trade winds
which are focused over the windward slopes. An area of clouds and
showers passing by Oahu late this afternoon and should slide west
to Kauai this evening. Expect the overall weather situation to
continue through Tuesday night with trades pushing showers mainly
over windward and mauka areas.

A more unsettled pattern will develop on Wednesday as a backdoor
cold front pushes into area from the north and with the trades
funneling the associated clouds and showers over the islands
through Thursday. The GFS model has been a little more aggressive
than the ECMWF with this feature bringing more widespread rain to
the islands, however latest model runs are showing the rainfall
hitting the smaller islands and grazing past the Big Island. Both
models show the front weakening to a surface trough as it reaches
the island chain then moving west of the area. The trough will
weaken the trade winds back to moderate breezes on Wednesday, then
strengthen again Thursday into the weekend as it departs west of
the area.

Aloft, an elongated upper low will dig towards the southwest and
cross over the state early Wednesday and will act to enhance the
low level moisture moving in. Thus, there is the potential
(particularly if the GFS is correct) that some of the rainfall
could be on the heavy side.

More typical trade wind weather featuring light windward and
mauka showers will return Friday into the weekend as the upper
trough moves west of the state and the atmosphere stabilizes.


&&

.AVIATION...
A surface high pressure cell well northeast of the state will
allow for moderate to easterly trades to persist this afternoon
and into the evening. Expect scattered showers to affect the
windward coasts and slopes with some isolated activity pushing
over the elevated terrain and onto leeward areas. Partly cloudy
with VFR conditions expect elsewhere.

No AIRMETs are currently in effect.


&&

.MARINE...
Fresh to strong trades associated with high pressure will hold
through Monday, then lower into the light to moderate category
around the midweek time frame as a weakening front approaches the
region from the north. A Small Craft Advisory (SCA) is in place
for the typical windier marine areas around Maui County and the
Big Island through Monday.

The current small south swell will be on a gradual decline Monday
and will be followed by a few smaller pulses through the first
half of the week. For the long range, recent satellite data
showed a large storm-force low (946 mb) well southeast of New
Zealand that has driven seas into the 30 to 40 ft range this past
Saturday. Although the bulk of this energy was focused east of
the islands, a small south swell can't be ruled out locally next
weekend.

Surf along exposed west facing shores may trend up tonight
through Monday as a small westerly swell from Tropical Cyclone
Hagibis fills in. Tiny long-period forerunners in the 22 second
bands are showing up on the nearshore buoys. We should see a
gradual rise Monday into Tuesday before dropping off on Wednesday.
Some of this energy may wrap into some northern exposures as it
fills in.

Surf along north facing shores will remain small through the first
half of the week, then potentially trend up through the second
half as a northwest swell from recurving Hagibis fills in.
Confidence in the details remains low at this point due to the
fast track of Hagibis over the far northwest Pacific in the next
few days.

Rough and choppy surf will continue along east facing shores
through Monday, then trend down into midweek as the trades ease.


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Monday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.


&&

$$


DISCUSSION...Foster
AVIATION...Thomas
MARINE...Kino

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