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Hawaiian Islands Synoptic Discussion and Guidance

895
FXHW60 PHFO 220707
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
830 PM HST Sun Jul 21 2019

.SYNOPSIS...
A trough of low pressure will pass by a few hundred miles to the
south of the islands during the next couple of days, spreading
clouds and showers from southeast to northwest across the island
chain through Tuesday. Drier weather and decreasing cloud cover is
expected by Wednesday. Moderate to locally breezy trades and typical
windward and mauka showers with the occasional leeward spillover for
the smaller islands will then prevail into next weekend.

&&

.DISCUSSION...
Persistent surface high pressure far north coupled with a trough of
low pressure passing south of the islands are bringing moderate to
locally breezy trade winds to the islands this evening into early
Monday morning. There is also an upper level low northwest of the
area. It would be typical trade wind weather for the islands if not
for the upper level feature as well as the low level trough.
Moisture from the passing trough reached the Big Island earlier.
Meanwhile, airmass in the island vicinity is somewhat unstable due
to the present of that upper level low. Hence, thunderstorms did
develop over the Big Island earlier Sunday afternoon. Looks like
much of the unstable weather may just clip the extreme southern Big
Island with a slight chance of thunderstorm and locally heavy rain,
while rest of the islands will see trade wind weather, with some
enhanced showers affecting mainly windward and mountain areas. Winds
will be strong enough to carry some clouds and showers to the lee
areas of the smaller islands through early Monday morning.

This trough of low pressure will move almost parallel to the islands
through Tuesday. Thus, it will stay south of the main Hawaiian
islands. But then, moisture associated with this system will be
spreading northwest. In the mean time, the aforementioned upper
level low will be shifting east but still providing some airmass
instability to the area. Therefore, still expect much of the
unsettled weather will remain south and west of the islands Monday.
The only exception is the Big Island, where atmospheric conditions
will be unstable and moist enough to see a slight chance of
thunderstorms during Monday afternoon and evening. Rest of the
islands may also see an increase in clouds and showers.

The upper level low will be far west enough by Tuesday for more
stable atmospheric conditions to spread from the east, though Kauai
weather may still be under the influence of the low. Latest forecast
solutions have the bulk of the moisture just southwest of Kauai.
Thus unsettled weather may still affect western and southern coastal
waters of Kauai Tuesday. Rest of the islands will see a return of
the typical trade wind weather, with passing low clouds and showers
affecting mainly windward and mountain areas.

Latest GFS indicated a trough may deepen just northwest of the
islands by the middle of the week, with high pressure current far
north of the area moving to the northeast. Forecast solutions also
show high pressure aloft moving to near the islands over later part
of the week. Looks like the islands will see seasonal trade wind
weather for the most part by then, though wind direction may turn
slight southeast due to that possible deepening trough to the
northwest.

&&

.AVIATION...
A high will remain nearly stationary far north of the main Hawaiian
islands as a trough passes southwest of the area. The pressure
gradient between these systems will maintain locally strong east
winds over the area. AIRMET TANGO is in effect for moderate
turbulence below 8,000ft and this AIRMET will likely continue
through Monday.

Clouds and showers that developed over the slopes of the Big Island
and Maui Sunday afternoon will gradually dissipate. The low level
winds will carry scattered to broken low clouds over east sections
of the smaller islands. Isolated MVFR ceilings are possible over
these areas tonight, but we don't expect the ceilings to be
extensive enough to require AIRMET SIERRA for MTN OBSC.

An area of layered clouds and isolated thunderstorms associated with
the trough passing southwest of the area lies just south of the Big
Island of Hawaii. These clouds are expected to remains offshore
overnight, but moisture from this system may fuel heavy showers and
isolated thunderstorms over the Big Island Monday afternoon. AIRMET
SIERRA may be needed for MTN OBSC.

&&

.MARINE...
Fresh to locally strong easterly trades will continue through the
week as high pressure remains north of the area and weak low
pressure passes to the south. Stronger winds will remain across the
typically windier locations from Maui County to the Big Island due
to terrain accelerations. The current Small Craft Advisory through
Monday night will likely be extended into the week.

Heavy showers and a few thunderstorms will remain a possibility
across the waters around South Point of the Big Island overnight
into Monday, then over the leeward waters of the smaller islands
Monday through Tuesday night as the aforementioned area of low
pressure to the south tracks northwestward.

Rough surf will continue along east facing shores due to strong
onshore trades. A slight downward trend is expected for a brief
period Monday through Tuesday as upstream trades relax. A slight
uptick is expected once again Wednesday into Thursday as upstream
trades increase.

Surf along south facing shores will remain small through tonight,
then trend up late Monday through midweek out of the south-southwest
due to a recent system that passed near the Tasman Sea. Heights
should remain below advisory levels as this swell moves through and
peaks Tuesday. A south-southeast long-period swell associated with a
recent compact gale southeast of the Tuamotus will be possible
Thursday night through Friday. Surf will rise along exposed shores
but should remain below advisory levels.

&&

.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...Hui
AVIATION...Donaldson
MARINE...Gibbs

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