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Hawaiian Islands Synoptic Discussion and Guidance

982
FXHW60 PHFO 231954
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
954 AM HST Sat Feb 23 2019

.SYNOPSIS...
A surface trough will linger in the vicinity of the Big Island
through the weekend. Coupled with impulses passing high overhead,
locally heavy showers and isolated thunderstorms are likely for
the Big Island and scattered showers for Maui over the rest of the
weekend. High pressure northwest of the islands will maintain a
mostly dry weather pattern for Oahu and Kauai through today. An
uptick in windward showers is possible tonight through Monday as
some of the low level moisture spreads westward from Maui and the
Big Island. The high pressure will strengthen early in the new
work week, with strengthening north to northeast winds, along with
cooler temperatures.

&&

.DISCUSSION...
It is already an active morning with locally heavy showers
occurring along the Hamakua coast of the Big Island. Therefore,
the forecast was updated to include locally heavy showers for the
lower elevation of the Big Island for the rest of today. Also,
conditions are ripe for scattered evening showers, locally heavy
at times, for the lee side of the Big Island through midnight
tonight. We are watching out for Maui, and Molokai, where showers
are already present. We won't rule out another forecast update to
include locally heavy showers for this afternoon, should it trend
toward that direction in the coming hours.

The 2 am HST Hilo sounding show a moist and somewhat unstable air
mass. There is an inversion at 25k feet, that will deter
thunderstorms, but an approaching short wave high aloft, will
likely destroy this high inversion, leading to a more favorable
air mass for thunderstorms this afternoon and evening.

The short wave trough high above, or upper level disturbance, is
a series of two over the weekend. The one approaching the islands
now is forecast to dampen out over the Big Island this evening. A
stronger one is forecast to drop in early Sunday morning, then
gradually move east, exiting the Big Island early Sunday evening.
This second short wave will be strong enough to push the surface
as per both the ECMWF and GFS solutions. Timing is critical, and
what the GFS is saying, is that this second shortwave will clear
things out before the maximum daytime heating Sunday afternoon,
leading to a nicer day (Sunday) than today for Maui and the Big
Island, in particular. The surface trough also gets nudged east
of the Big Island by Sunday evening. The trough finally stalls
some 100 miles off the Big Island coast on Monday, and stays there
through Tuesday.

The remnants or tail end of a front is forecast to reach Kauai
Monday morning, then stall over the central islands Monday evening.
It is a shallow front with cloud tops no higher than 10k feet.
This means the windward and mountain areas will be socked in,
while much clearer conditions prevail on the lee side. Things gets
messy thereafter, Tuesday through Wednesday as another surface
boundary drops in with aided by a vigorous looking upper level
trough, leading to a slight chance of thunderstorms. Also, this
upper level impulse will carry a pocket of very cold temps of
minus 19 degree C at 500 mb over the islands. So, we could have
another round of winter weather for the summits and upper slopes
of the Big Island, and potentially Maui.

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 today
through tonight. The northeast winds strengthen slightly as well
overnight to moderate speed. As the surface trough over the Big
Island gets pushed east of the island Sunday evening, a light to
moderate northeast flow fills and lingers into next week,
including through the next round of potentially unsettled weather.

After this shortwave passes, models suggest that the longwave
pattern near and north of the islands will once again take the
form it has for most of February, with high pressure northwest
of the islands and low pressure to the northeast. This setup
favors a cool and breezy northwest to north wind flow. Shallow
bands of clouds and showers would likely periodically move over
the islands from the north if this large scale pattern were to
hold

&&

.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 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 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.
Small Craft Advisory level winds will be possible during this
time, followed by a decrease in the north winds on Thursday.

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 and seas from the winds,
the northwest swell, and a short period north swell will lead to
rough conditions that will likely require a Small Craft Advisory
for most waters. Though winds will decline on Thursday, seas will
remain elevated.

&&

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

&&

$$

H Lau/Thomas/Wroe

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

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