Current Conditions
Temp8.2 C
RH35 %
WindW 12 mph
RoadOpen (4x4)
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Hawaiian Islands Synoptic Discussion and Guidance

899
FXHW60 PHFO 271959
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
959 AM HST Sat Nov 27 2021

.SYNOPSIS...

Light winds and mostly dry weather will continue through the first
half of Sunday. Trades will then quickly strengthen, potentially
becoming locally strong by Sunday night as showers increase over
windward and mauka areas. Occasional trade wind showers can be
expected until winds once again diminish during the middle of next
week.

&&

.DISCUSSION...

A corridor of weakly convergent boundary layer flow is in place from
the Kauai channel to Oahu extending northeast. A result of easterly
flow impinging on very light winds within a col, this area of weak
low-level convergence will remain a focus for showers during the
next 24 hours as it drifts westward. Around sunrise this morning,
showers blossomed within this area, mainly over Oahu, and the
forecast was updated accordingly. The latest radar imagery shows
showers already diminishing in coverage and it has become clear
that most of this morning's uptick in activity was a consequence
of a gravity wave rolling through the area from the northwest.
Nonetheless, the larger scale support for scattered showers will
remain in place over Windward Oahu into this afternoon before
shifting toward windward sections of Kauai overnight.

The next to 36 to 48 hours will feature a transition away from the
quiet and dry weather pattern that has dominated over the state
recently. This transition will occur as energetic mid-latitude
flow causes a closed low to develop and rapidly deepen near 150W
Sunday into Sunday night. The response at the surface will be
fairly impressive as a sharp trough develops east of the islands
during that same time period. The subsequent tightening of the
surface pressure gradient will result in trade winds quickly
ramping up Sunday into late Sunday evening, then persisting in
some fashion through Monday. Although the daytime hours on Sunday
will again remain on the drier side of normal, the development of
locally strong trades by early Sunday night will bring with it a
greater coverage of trade wind showers. Frequent leeward
penetration of showers can likewise be expected courtesy of the
weakening inversion and stronger trades. As the upper low becomes
established in the vicinity of the eastern islands, the
aforementioned surface trough will begin to weaken as it pivots
westward through the forecast area on Monday night. This could
bring another quick hit of briefly stronger trades and robust
trade wind showers.

The latest modeling suggests that the deepest moisture will
remain east of islands as it drawn northward during the middle of
next week. Thus, as the gradient weakens and trades diminish
beneath the lingering upper low during the mid-week period, the
forecast focus will likely shift toward the Big Island where
steepening mid-level lapse rates will result greater potential for
a few afternoon thunderstorms over the interior slopes. At the
same time, freezing levels will likely fall below summit level
and, conditional on the exact location of the upper low, wind
headlines may also be play for the Big Island summits.

EC/GFS/GEM paint the same general picture of an elevated threat
for more active weather in the extended range as renewed
troughing potentially develops near or west of the islands during
the latter half of next week.


&&

.AVIATION...

AIRMET Sierra is posted for mountain obscurations over windward
sides of Oahu and Molokai due to some passing showers this
morning. Elsewhere, VFR conditions prevail with light background
winds. Sea breezes may bring some cloud build ups with isolated
showers over the island interiors, particularly on Kauai and the
Big Island where sunny skies prevail over most areas.

&&

.MARINE...

Trade winds will be on the lighter side through tonight as a
weakening frontal boundary passes by to our north. An area of high
pressure will then pass from west to east north of the area
Sunday through Tuesday producing moderate to locally windy trade
winds across the area. A Small Craft Advisory will likely be
required for some areas especially the typically windy waters
around Maui County and the Big Island. Winds will weaken Tuesday
night and Wednesday as a low level trough begins to move in from
the east. The weather may become more unsettled as well especially
over the eastern waters.

The current northwest swell will continue to lower through
tonight. A reinforcing northwest swell will fill in on Sunday,
peak Sunday night, then lower gradually Monday and Tuesday. Also,
a small short period north swell will also move through the area
Monday night and Tuesday likely causing conditions to be a bit
disorganized. Another reinforcing northwest swell is then expected
Wednesday and Thursday. Advisory level surf is not expected with
any of these swells. A larger but rather short period north
northwest swell may arrive around Friday.

Surf along south facing shores will remain very small throughout
the forecast period with only background swell energy from the
Southern Hemisphere moving through the area. Some short period
choppy surf is possible later in the week as winds turn southerly
out ahead of a front. Short period choppy surf for east facing
shores will gradually build beginning Sunday as trade winds
increase over and upwind of the state. A downward trend is then
expected beginning Wednesday as trade winds slowly diminish.

&&

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

&&

$$


DISCUSSION...JVC
AVIATION...Foster
MARINE...Burke

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