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Hawaiian Islands Synoptic Discussion and Guidance

763
FXHW60 PHFO 240223 CCA
AFDHFO

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
423 PM HST Sat Feb 23 2019

.SYNOPSIS...
A surface trough will linger in the vicinity of the Big Island
through most of Sunday. Coupled with impulses passing high
overhead, locally heavy showers and isolated thunderstorms are
likely for the Big Island and scattered showers for Maui through
tonight. Drier conditions and less showers on tap for Sunday. High
pressure northwest of the islands will maintain a mostly dry
weather pattern for Oahu and Kauai through this evening, with a
slight uptick in windward showers later tonight. The remnants of
an old front will settle over the islands from the west between
Monday and Tuesday. These showers will then be enhanced by another
passing upper level impulse resulting in showery weather Tuesday
through Wednesday. The weather clears up after Wednesday, replaced
by a drier, cooler, more stable air mass along with breezy north
and northeast winds.

&&

.DISCUSSION...
It has been an active shower day for mainly the northern part of
the Big Island, leading to a series of Flood Advisories.
These spotty but heavy showers are still ongoing, with activity
now spilling onto the lee side.

Another upper level impulse is rapidly approaching the islands
from the west this afternoon. This will maintain the unsettled
weather across the Big Island overnight. The impulse may also
increase the showers across the remaining islands, but primarily
Maui and Molokai. The impulse is forecast to exit the Big Island
tomorrow afternoon. This means a much quieter Sunday afternoon for
especially the Big Island. Little was done on the grids for the
tonight and Sunday periods,except increase the shower chances
across Maui County for tonight and Sunday morning.

As for the surface winds, the prevailing wind flow will be a
light northeast breeze across the smaller islands and light and
variable over and in immediate vicinity of the Big Island tonight
into Sunday morning. The surface trough is forecast to move east
of the Big Island SUnday afternoon with a light to moderate wind
flow filling in.

Then, on Monday morning, the remnants of an old front reaches
Kauai, then pushes down to the central islands between Monday
night. The moisture at this point will be shallow, no higher than
10k feet, meaning the windward side will be cloudy and showery,
and much nicer on the lee side. Then, a vigorous look upper level
impulse follows in on Tuesday that will flare up the moisture
already in place, making for a showery and messy situation mainly
over the central islands and Big Island. The upper level impulse
moves east of the islands Wednesday night, and this will lead to a
period of some nice dry weather along with a cool and breezy
north and northeast winds flow that may last through Friday.

&&

.AVIATION...
A surface trough near the Big Island will keep winds light
across the region today with land breezes developing in a weak
northerly background flow. Expect SHRA with occasional +SHRA,
associated with the feature to affect Maui and the Big Island
through the remainder of this afternoon. Lowered vis and mountain
obscuration will cause MVFR conditions at times. The rest of the
islands may have isol low clouds, especially inland and over the
mountain ridges and slopes, but generally VFR conditions.

AIRMET SIERRA remains in effect for north to southeast sections
of Maui and the Big Island due to mountain obscuration from low
clouds and SHRA above 2500 ft.

&&

.MARINE...
Gentle to locally moderate northeast winds will prevail over most
of the island chain into Monday. A weak surface high passing
north of the state is driving the winds, while a surface trough
stretching across the Big Island is leading to variable winds to
the south and east of the Big Island. The trough will drift east
slowly, but otherwise, little change is expected through the
weekend, with thunderstorms possible around the Big Island into
Sunday morning.

Winds will increase through the first half of the work week.
Winds will begin to strengthen late Monday or Monday night as a
dissipating front sags southward over the islands and another
surface high passes to the north. Winds will shift out of the
north and build to fresh to possibly strong levels late Tuesday
and Wednesday as a diffuse front moves down the island chain and a
surface low develops several hundred miles to the northeast of
the state. Small Craft Advisory level winds are possible during
this time, followed by a decrease in the north winds Thursday and
Friday.

Small surf is expected on all shores this weekend as an east
swell fades and tiny northwest swell holds well below winter time
average. A pulse of moderate northwest swell is expected on
Monday, followed by a large, long period northwest swell that will
build on Tuesday and peak on Wednesday and early Thursday. This
large swell will produce surf around High Surf Warning levels for
north and west facing shores. In addition, the combination of
fresh to potentially strong north winds, associated wind waves,
the northwest swell, and a short period north swell will lead to
rough conditions that will likely require a Small Craft Advisory
(SCA) for most waters and a possibly a High Surf Advisory (HSA)
for east facing shores. Though winds will decline on Thursday,
seas will remain elevated. Winds and seas should decrease
significantly on Friday, and the SCA will likely be dropped.
However, HSA level surf may persist along east facing shores
exposed to the declining north swell.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$


H Lau/Thomas/Wroe

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