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Hawaiian Islands Synoptic Discussion and Guidance

436
FXHW60 PHFO 150644 CCA
AFDHFO

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
844 PM HST Thu Nov 14 2019

.SYNOPSIS...
A light wind regime will continue as a cold front approaches Kauai
from the northwest and a low level trough moves into the Big
Island. Expect daytime sea breezes and overnight land breezes for
all islands with increasing shower activity over Kauai and the
Big Island by Friday afternoon. Clouds and showers moving in with
these two systems will spread to portions of Oahu and Maui County
on Saturday. Expect wet unsettled weather to linger into the
middle of next week with moderate trade winds.

&&

.DISCUSSION...
The satellite imagery this evening shows a moderately strong low
level trough in the easterlies (also known as a Screaming Eagle)
located roughly 200 miles southeast of the Big Island. Another
low pressure system, a cold front, is located roughly 550 miles
northwest of Kauai. These two low pressure systems will converge
over the state starting on Friday, with enhanced clouds and
showers reaching the Big Island and Kauai by Friday afternoon.

A trend towards wet and unsettled weather arrives on Friday as
two low pressure systems begin to converge over the Hawaiian
Islands, a trough southeast of the Big Island, and a cold front
northwest of Kauai. Deeper moisture and instability will move in
with two bands of Precipitable Water (PW) ranging from 1.5 to 2.0
inches. Weak southerly winds will develop over Kauai with
prefrontal cloud bands favoring the southern and western slopes
of the island. Big Island showers will start along the windward
slopes then spread into interior and leeward sections from
Friday afternoon into the overnight hours.

Unstable wet weather will spread into Oahu and Maui County from
Saturday afternoon into Sunday with numerous rain showers
forecast statewide by Sunday morning. Moderate trade winds also
return to the region through the weekend. Higher rainfall amounts
will favor windward and mountain slopes on Sunday due to
increasing trade wind speeds. Enough instability aloft will
produce periods of heavy rainfall and a few thunderstorms.
Persistent and slow moving heavy rain bands may develop,
increasing the threat for localized flooding.

By Sunday night cold air aloft will produce an upper level low
over the state. Colder temperatures aloft will keep unsettled
conditions with wet trades in the forecast through Tuesday.
Low chances for a few thunderstorms remain in the forecast mix,
much will depend upon the strength and location of this upper
level low.

&&

.AVIATION...
A light wind regime will continue across the island chain over the
next 24 hours, with overnight land breezes and daytime sea breezes
expected statewide. The airmass will remain dry and stable, with
only a few isolated showers possible near the coast at night and
over the island interiors during the afternoon into the early
evening hours.

No AIRMETs are currently in effect and none are anticipated
overnight.

&&

.MARINE...
A front approaching Kauai from the northwest will cause the winds
to shift to a more southerly direction through early Friday
morning. The winds will also weaken during the day on Friday
before northeast winds start to develop Saturday. Easterly trade
winds will gradually strengthen, and may approach Small Craft
Advisory (SCA) levels early next week over the typically windy
waters adjacent to the islands of Maui County and the Big Island.

The current north-northwest swell appears to be 1 to 3 feet
higher than the Wave Watch III Model guidance according to recent
observations from buoys 51001, 51101, and 51000, as well as the
Waimea Bay buoy. Therefore, we have adjusted the swell and surf
forecasts accordingly. A High Surf Advisory (HSA) remains in
effect through Friday afternoon for north facing shores of the
islands. Surf will continue to build this evening, peak later
tonight, and then gradually lower on Friday. In addition to the
large surf, the combined seas will remain elevated into Friday. As
a result, a SCA has been added to all waters with an exposure to
the large north-northwest swell, including the Kauai and Kaiwi
Channels

As the current north-northwest swell fades from Friday night
through Saturday, an even larger west-northwest swell is expected
to arrive late Friday night. This will cause surf to again reach
the HSA criteria along most north and west facing shores of the
smaller islands starting Saturday. This swell may be large enough
to approach the High Surf Warning thresholds along some shorelines
from Saturday night into Sunday. In addition, surf may reach the
HSA criteria along west facing shores of the Big Island starting
Saturday night. Combined seas are again forecast to reach the SCA
criteria over waters exposed to this northwest swell this weekend.

Small southerly swells will persist through this weekend. Surf
will also remain small along east facing shores into early next
week due to the light winds over and upstream of the state. The
surf will increase slightly along east facing shores by late
Monday or Tuesday as high pressure builds in to our north, and
the trades strengthen.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Friday for Niihau-Kauai
Windward-Kauai Leeward-Oahu North Shore-Oahu Koolau-Olomana-
Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala-Big Island North and East.

Small Craft Advisory until 6 PM HST Friday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Channel-Oahu Windward Waters-
Kaiwi Channel-Maui County Windward Waters-Big Island Windward
Waters.

&&

$$

DISCUSSION...Bohlin
AVIATION...Jelsema
MARINE...Houston

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



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