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Hawaiian Islands Synoptic Discussion and Guidance

183
FXHW60 PHFO 201316
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
316 AM HST Tue Apr 20 2021

.SYNOPSIS...
Scattered showers capable of producing locally heavy rain and
possibly a thunderstorm are expected today. Light and variable
winds will continue into mid-week followed by a return to moderate
to breezy trades this weekend.

&&

.DISCUSSION...

Weakly convergent southwest flow is supporting scattered showers
around Kauai while dry weather and increasing high clouds prevail
over the smaller islands. Some increase in shower coverage as far
south as Oahu will be possible through the morning, but otherwise
conditions should remain fairly static for the next several
hours.

The morning sounding out of Lihue shows PWATs have increased
to 1.38", which is in line with expectations. The inversion has
also risen by a couple thousand feet, to around 9kft, compared to
yesterday afternoon. And the deep mid-level dry layer has eroded
considerably, but is still stout enough to keep a lid on
convection this morning. As a result, showers have been small and
have produced only modest rainfall, the highest reading as of 2am
HST being 0.25" atop Mount Waialeale. Forecast soundings indicate
this dry layer eroding by mid-morning which will ease capping and
allow convection to grow in vertical extent. Overall, the official
forecast reasoning remains unchanged. The advection of warm and
moist air within a deep layer of confluent southwesterly flow will
contribute to modest forcing for ascent through the day while a
fairly dynamic 60kt jet will provide favorable upper-level support
by this afternoon. This will support an increasing coverage of
showers capable of producing locally heavy rainfall, particularly
over Kauai and Oahu. With that said, convection is expected to
remain unremarkable thanks to a combination of paltry mid-level
lapse rates, weak forcing, and limited surface-based instability
due to increasing high clouds. The vigor of the sea breeze
circulation will likewise be reduced by the increase in high
clouds, so prospects for thunder appear quite limited. The lone
exception will be leeward Big Island where high clouds will hold
off until later today yielding a greater opportunity for afternoon
destablization. A return to stable conditions can be expected by
late tonight as mid-level subsidence overspreads the area from
northwest to southeast.

A building mid-level ridge over the central Pacific will cause
moderate to breezy trades to return by the end of the week. Model
consensus is improving that a mid-level trough moving southward
along the periphery of the building ridge will move over or just
east of the area Friday and Saturday. This will bring a reduction
in stability and a trend toward wetter trades.


&&

.AVIATION...

The cold front far northwest of the Hawaiian Islands continues
its slow march eastward. Light variable winds will shift to the
south today as the front edges closer to Kauai. Current satellite
imagery reveals increasing layered clouds across the western
third of the state spreading eastward.

Radar indicates minimal shower activity, but showers are expected
increase in both coverage and intensity across Kauai by morning.
These prefrontal showers should then spread across Oahu by noon
and the remainder of the smaller islands by evening.

On the Big Island, afternoon sea breezes will encourage cumulus
buildups over the island's interior. Precipitable water values
are sufficient to produce locally heavy downpours that could
linger through the evening hours.

The air mass over the state is expected to become increasingly
unstable. Slight chance of thunderstorms over Kauai, Oahu, and
Interior Big Island today.

No AIRMETs in effect at this time. However, mountain obscuration
will likely become an issue for Kauai later this morning.


&&

.MARINE...

A front approaching from the northwest will keep moderate east-
southeast winds in place over the eastern islands and light and
variable winds over the western end of the state through
Wednesday. A few heavy showers and even a thunderstorm will be
possible through this evening over the Kauai and Oahu waters as
an upper level disturbance moves through. Moderate trades are
expected to return from east to west across the state Wednesday
night and Thursday as high pressure builds north of the islands.
The trades are forecast to strengthen into the fresh to strong
range Friday through the weekend, and Small Craft Advisories will
likely be needed for some marine zones at that time.

Surf along north facing shores will remain small through
Tuesday. A new short period north-northwest swell will then give
north shore surf a boost during the middle of the week, with a
slow decline expected Friday through the weekend. Head high to
slightly over head sets can be expected along north facing shores
as this swell peaks, but surf will remain well below advisory
thresholds.

East shore surf will remain small through the next 7 days,
although we should see a slow trend upward Thursday into the
weekend as the trades over and upstream of the islands strengthen.

Surf along south facing shores will remain small during the next
7 days, with mainly background south and south-southwest swell
energy moving through.


&&

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

&&

$$


DISCUSSION...JVC
AVIATION...Bedal
MARINE...M Ballard

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