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Hawaiian Islands Synoptic Discussion and Guidance

596
FXHW60 PHFO 160206
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
406 PM HST Sun Sep 15 2019

.SYNOPSIS...
High pressure will persist far north of the islands. A low aloft
and associated surface trough will make the atmosphere unstable
through this evening bringing a chance for heavy rain and
thunderstorms. East southeast winds will strengthen slightly and
the atmosphere will become drier and more stable this week.

&&

.DISCUSSION...
At the surface, a 1030 mb high is centered about 1400 miles north
of Hawaii this afternoon. This fairly strong high is driving
moderate to locally east southeast winds over Hawaii, however the
wind flow is being somewhat disrupted by a both a weak front and a
trough between the high and our islands. The trough just north,
along with a tropical disturbance far south of the islands, is
inducing a slightly south of east wind direction this afternoon.
Partial blocking by the Big Island is further reducing wind speeds
for some areas of the smaller islands.

Water vapor imagery shows an mid to upper low centered about 400
miles north of Hawaii that will gradually be lifting northwest Monday.
Southwest winds aloft along the eastern flank of the low is
currently drawing scattered to broken midlevel clouds across the
Big Island as shown on visible satellite. 00Z sounding shows near
average precipitable water value of 1.43 inches at Lihue (Hilo is
a bit higher with the midlevel cloud contribution). The sounding
shows a very weak, almost nonexistent inversion with about 550
J/kg mixed layer CAPE. Dew points are mainly in the low 70s at the
sea level observation stations.

On radar, the Kona coast and the northwest section of the Big
Island is also seeing some afternoon downpours from the daytime
sea breezes and instability. Similarly, leeward and interior
sections of the smaller islands are seeing some heavy showers
this afternoon. These showers will likely last into the evening
hours and then dissipate. A slight chance for thunderstorms is
also in the forecast through the early evening hours.

As the upper low and surface trough both move away on Monday, the
atmosphere will become more dry and stable. Trade wind flow will
build in across the Big Island, but winds will veer east southeast
towards the western end of the island chain as a tropical
disturbance sets up west of the state. Aside from the wind
direction shift, this potential area of showers and thunderstorms
will remain west of the state. Across Hawaii, lighter showers
will focus over mainly windward areas during nights and mornings.
Leeward areas should be clear at night with partly cloudy skies
during the day.

Starting Friday, the forecast becomes rather murky with big
differences among the computer models. the models generally agree
that the atmosphere will become more moist, so an increase in
rainfall is expected.


&&

.AVIATION...
The air mass over the state remains unstable. Locally heavy
downpours and isolated thunderstorms are expected over island
interiors through early evening and possibly beyond. Shower activity
will persist overnight, but lessen in intensity.

Isolated MVFR conditions are to be expected in showers, otherwise
VFR conditions will prevail across the forecast area. No AIRMETs
are in effect or anticipated this time.

Trade winds are expected to gradually strengthen over the next
twenty-four hours. Clouds and scattered showers will become
focused along windward slopes and coasts on Monday.


&&

.MARINE...
Big island blocking keeps winds lighter across leeward waters of
the smaller islands. Winds are stronger across exposed windward
waters, especially around Maui County and the Big Island. A Small
Craft Advisory (SCA) remains in effect for waters around Maui and
the Big Island through at least late Monday afternoon. This SCA
may need to be extended. Trades are expected to drop off a notch
on Thursday.

A series of small swells from the southwest, south, and southeast
is expected this week. A prolonged southwest swell expected on
Tuesday will last through most of the week. A southeast swell is
slated to arrive Wednesday, followed by a south swell on Friday.

Small surf is expected along north, east and west facing shores
through the new week. The trade wind boost will slightly raise
the surf along east facing shores. Expect a small east swell from
Tropical Cyclone Kiko as early as Wednesday.


&&

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


&&

$$


DISCUSSION...Foster
AVIATION...Bedal
MARINE...Powell

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