Current Conditions
Temp3.3 C
RH14 %
WindNE 11 mph
RoadOpen (4x4)
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Hawaiian Islands Synoptic Discussion and Guidance

529
FXHW60 PHFO 280654
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
854 PM HST Mon Sep 27 2021

.SYNOPSIS...

Breezy trades will decrease to moderate Wednesday and Thursday and
will potentially become light by early next week. A typical trade
wind pattern with showers focused over windward and mauka areas
will prevail during this time.

&&

.DISCUSSION...

Locally strong trades have materialized in choice windier locations
this evening in response to a weak trough axis that is advancing
through the island chain at press time. The increased northeasterly
winds, which have gusted between 30 and 40 mph at Lanai, Kohala, and
Kuaokala, will be short-lived as the surface trough shifts west of
the area during the next few hours. In addition, a narrow corridor
of stronger dynamic forcing embedded within the southern periphery
of the closed low that is just north of the islands helped elicit
some deeper convection over the lower slopes of Leeward big Island
today. Aided by healthy component of onshore flow, the rain was
heavy enough / 3.12" at the Honaunau gauge / to prompt a flood
advisory which has since been allowed to expire. Pockets of moderate
or locally heavy rain will continue to percolate over downslope
areas, tending to organize nearer the land breeze boundary as the
evening wears on.

Breezy trades will persist for one more day, carrying the next batch
of moisture presently centered around 150W toward the Big Island
tomorrow afternoon and the smaller islands tomorrow evening. As a
result, a typical nocturnal uptick in showers over windward and
mauka areas can be expected. As the core of the low drifts directly
overhead during Wednesday and Thursday, the local pressure gradient
will diminish causing trades to decrease into the moderate category.
The occasional shortwave rotating around the parent low and the
elevated inversion may allow for brief upticks in trade wind showers
during this time. Also of note, veered mid-level flow will carry
increased moisture toward the Big Island by Thursday. The deeper
moisture and weakly divergent flow aloft will favor a greater
coverage of afternoon clouds showers over upslope sections of the
Big Island, including the summits, during the latter half of the
week. T-storm potential over the BI slopes late this week appears
marginal but will be worth monitoring. Marginal 700-500MB lapse
rates of around 6.5C/km can be steepened by diurnal heating of the
higher elevations, but that potential will be entirely conditional
on the extent of cloud cover that develops earlier in the day.

For the latter half of the weekend into early next week, longwave
ridge amplification centered near the Dateline is modeled to force
surface high pressure southward over the central Pacific, the lead
edge representing a shallow cold front that will potentially reach
the 30N-25N range on Sunday night. Should this materialize, trades
would become light and potentially give way to a land/sea breeze
pattern to start next week.


&&

.AVIATION...

Surface high pressure N of the islands will move ESE over the next
24 hours or so, with breezy to locally strong ENE gradually easing
in response. AIRMET Tango for low level turbulence over and
downwind of the terrain remains posted, but it may be dropped on
Tuesday.

The trade winds will deliver stable and showery low clouds that
will primarily focus over windward coasts and slopes, producing
isolated to tempo MVFR VIS/CIG in SHRA. Coverage may increase at
times to warrant AIRMET Sierra for mountain obscuration, but not
at the current time. VFR conditions will prevail over most leeward
areas, with the exception of some afternoon and evening clouds
and showers on the leeward slopes of the Big Island producing
isolated MVFR conditions.


&&

.MARINE...

High pressure far north of the state will begin to weaken Tuesday
and gradually shift eastward. This will cause the pressure
gradient to relax over the state, resulting in the trade winds
gradually easing Tuesday into Wednesday. Winds are expected to
drop below Small Craft Advisory (SCA) thresholds in the typically
windy zones around Maui County and the Big Island during this
time. A new high will build north of the state, bringing the
return of moderate to breezy trade winds towards the end of the
week. However, trades will likely weaken over the weekend as a
front passes north of the state.

A small reinforcing north-northwest swell is expected to fill in
late tonight into Tuesday followed by a small short-period north
swell towards the middle of the week. Another small north-
northwest swell is possible towards the end of the week. Surf
along south facing shores will gradually decline over the next few
days, with only background swells expected throughout this week.
A small Tasman swell from the southwest is possible Thursday into
Friday. Surf along east facing shores will remain relatively small
due to the lack of strong trade winds.

&&

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


&&

$$


DISCUSSION...JVC
AVIATION...Birchard
MARINE...Kinel

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