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Hawaiian Islands Synoptic Discussion and Guidance

673
FXHW60 PHFO 180146
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
346 PM HST Tue May 17 2022

.SYNOPSIS...

Southerly winds will focus clouds showers over leeward zones for
the next several days. Late tonight through Wednesday night, heavy
rain will bring the potential for flash flooding to Kauai. A few
strong thunderstorms will also be possible. For the remaining
islands, coverage and duration of any heavier showers is expected
to be limited.

&&

.DISCUSSION...

This evening...Stable and dry conditions prevail over the islands at
press time courtesy of moderate stability supported by a deep layer
of dry air in the mid-levels and a weak temperature inversion.
Inland convergence favoring interior cloud/shower development never
really materialized today owing to the stout southerly gradient
which overwhelmed the sea breeze. Low topped clouds and a few
showers will continue to track northwestward, focusing over leeward
zones, with little consequence through this evening.

Tonight...Attention then turns to the extratropical cyclone
presently positioned near 35N/165W, the southern extent of which
will amplify tonight through tomorrow as an upper jet digs southward
along its western flank. Already, satellite imagery west of Kauai
has shown an impressive corridor of broad confluent southerly flow
in advance of the attendant cold front. Southerly flow, around 10-
15kts per the latest ASCAT, has allowed organized banding of showers
to persist through today's unfavorable diurnal period while also
drawing north PWATs of 2 inches or greater. As the eastern periphery
of this deepening band of moisture encroaches on Kauai during the
latter half of tonight, eroding stability will favor an increasing
coverage of showers that will focus over the south-facing slopes of
Kauai. The probability for heavy showers will steadily increase
through the night. For leeward areas of the other islands, a few
showers will be possible, but they will be comparatively low in
coverage and intensity.

Wednesday and Wednesday night...The aforementioned trough
amplification will commence as the jet digs southward. Even with
lead time now inside 24 hours, guidance continues to trend stronger
with this feature, including the latest 18z run of the GFS. As the
trough axis pivots through the western end of the state on
Wednesday, it will be accompanied by appreciable deep layer forcing
and outstanding upper divergence centered over Kauai. The peak
forcing period will be centered in the approximate 9-hr window
spanning 11am-8pm HST Wed. During this time, the upstream band of
moisture will be forced eastward over Kauai. Deep saturation in the
presence of PWATs around 2", increasing forcing for ascent, and
breezy southerly flow will set the stage for a period of heavy
rainfall and high rain rates over the island of Kauai. Although
convection will tend to focus over the south-facing slopes, the
flash flood threat will encompass the entire island, particularly
during the time period outlined above. Several inches of rain will
be possible on Wednesday with rises of several feet possible on
island rivers and streams. The Flood Watch is therefore in good
shape and remains in effect with no adjustments.

As the trough has steadily trended stronger in the guidance, so too
has the associated wind field. One standout feature of this system
is going to be its deep and increasingly veered wind field. Although
convective instability remains a question mark, the parameter space
certainly appears ripe for a few severe or borderline severe
showers/thunderstorms over/around Kauai. A quick look at forecast
soundings reveals a transition from straight low-level hodographs
midday Wednesday transitioning to larger cyclonically curved
hodographs by Wed evening. Either could be potentially supportive of
an isolated severe wind (60 mph) threat given sufficiently vigorous
updrafts, especially in the presence of terrain effects. By evening,
any deeper updrafts would stand a good chance of acquiring some
rotation leading to a tick upward in the probability of waterspouts.
The warm temperature profile and limited instability will limit hail
potential. Given the high degree of uncertainty regarding convective
instability, the greatest potential for strong/severe convection
may likewise focus in the 11am-8pm HST time window.

As the trough lifts northeast, moisture will be pushed further east
over Oahu Wednesday night. Diminishing forcing and the negating
effects of the shallow remnant stable layer will most likely keep
shower intensity in check, though a few locally heavy showers can
still be expected. For Kauai, the departure of the upper trough axis
will mark a transition to more pure orographic forcing. Given that
the ample supply of deep moisture will remain in place through the
night, the heavy rain may continue through Wednesday night. It is
not out of the question that the Flood Watch would need to be
extended into the first half of Thursday.

Thursday through the weekend...Southerly flow weakens considerably
Thursday into Friday as the low lifts northeast. Meanwhile, the
remnant stable layer only gets deeper with eastward extent, limiting
the degree of expected shower coverage over the eastern end of the
state. Nonetheless, a sneaky heavy rain threat may yet materialize
on Friday and Saturday as the gradient diminishes and gives rise to
a sea breeze pattern. With the remnant moisture band still lingering
over the state, any afternoon showers over island interiors would
have the potential to produce heavy rainfall.

&&

.AVIATION...

Low-level winds will persist from the south over the west end of
the island chain and from the southeast elsewhere. Showers are
continuing to develop and move over Kauai this afternoon. Low
clouds and showers are expected to increase over Kauai and Oahu
tonight as southerly winds and moisture trend up. Thunderstorms
are also a possibility west of Kauai tonight and from Oahu
westward Wednesday.

No AIRMETs are currently in effect. Mountain obscuration is
possible later this evening and overnight, especially for south-
facing slopes of Kauai and Oahu. AIRMET Tango for low-level
mechanical turbulence is possible later tonight as well as
southerly winds increase over and around Kauai.


&&

.MARINE...

Some changes to the forecast with the afternoon package, but the
general flavor remains the same. Winds haven't increased much near
Kauai, but some of the high resolution models like the FV3 and ARW
indicate winds will reach Small Craft Advisory (SCA) levels by
morning. Have issued the SCA starting tonight for the increase in
winds. Lighter winds remain over the remaining waters.

The increase in southerly winds is due to an approaching cold
front which is expected to stall out northwest of the islands.
This front will also be bringing an increase in low level
moisture, while its associated upper level feature will help to
provide instability. Thunderstorms that develop late tonight
through tomorrow could be sufficiently strong enough to warrant
Special Marine Warnings.

This front will also bring increasing southerly winds to the
offshore waters to the northwest and north of Kauai. Gale force
winds are expected in the high seas just north of the offshore
waters tonight through Thursday, and haven't completely ruled out
gales reaching the offshore waters. With most of the instability
with the system remaining just northwest of the coastal waters,
the offshore waters could see some strong thunderstorms tonight
through Thursday.

The current south swell will keep surf elevated through the
second half of the week as a similar reinforcement arrives
Thursday. In addition to the long- period energy moving through
that is typical for this time of year, southerly winds picking up
through midweek plus the added rainfall moving in will add to the
mix with short period choppy conditions.

Although surf along east facing shores will lower with the loss
of the trades, some residual easterly swell from upstream should
produce small surf into midweek before completely dropping off.

Surf along north facing shores will remain small through midweek,
then trend up Thursday as a small, medium-period northwest swell
arrives. A return of summer-like conditions is expected over the
weekend as this source moves out.


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flood Watch from Wednesday morning through late Wednesday night
for Niihau-Kauai-

Small Craft Advisory until 6 AM HST Thursday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters.


&&

$$


DISCUSSION...JVC
AVIATION...TS
MARINE...M Ballard

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