Current Conditions
Temp0.9 C
RH14 %
WindESE 7 mph
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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 030633

Area Forecast Discussion
National Weather Service Honolulu HI
833 PM HST Wed Dec 2 2020

The light trade winds will be gaining strength Thursday and
Thursday night as a new surface high passes north of the islands
Friday. As the high passes rapidly north of the islands, it will
generate a brief period of moderate to locally strong trades
across the main Hawaiian Islands between Friday and Saturday
morning. Trade winds will then give way to light and variable
winds late in the weekend as a front approaches the islands from
the northwest. Trade winds return on Monday as the front stalls
and dissipates north of Kauai.



The forecast package was updated, for lower POPs and less clouds
for windward areas of Maui and the Big Island tonight. All
islands, especially the larger ones, can expect fair skies except
for an isolated shower or two for the windward and mountain areas

Light trade winds at the moment will be gaining speed on Thursday
and Thursday night as a new 1022 mb surface high approaches the
main Hawaiian Islands from the northwest. The boost of moderate to
locally strong trade winds will only last mainly between Friday
and late Friday night. With the high racing eastward, the trades
will be down to moderate speed and limited to around the Big
Island by Saturday afternoon. The rest of the island chain will
likely see the trades turn sea breeze by Saturday afternoon,
especially those islands down wind of Maui. A nighttime land
breeze and a daytime sea breeze pattern is expected to continue
into Sunday. This change is due to a front approaching the islands
from the northwest. Up to this point, both the old ECMWF and new
GFS agree on this scenario.

On Sunday night, the front stalls and starts to dissipate roughly
200 miles northwest of Kauai. Light trades are expected to ease
back in Sunday night and gradually strengthen to moderate to locally
strong by Monday night. Some of this moisture from the front may
make it to Kauai Monday before the trade winds pushes it west,
away from Kauai and the rest of the main Hawaiian Islands. This
new batch of trade winds will be supported by a 1028 mb surface
high. Another front approaching the islands from the northwest
will diminish the trades over the western half of the island chain
briefly on Wednesday. This front is expected to stall well west of
Kauai, meaning moderate to breezy trades continues into the second
half of next week. Both the ECMWF and GFS concur on this scenario
as well.

A couple of additional points. It should be noted that a narrow
band of moisture, belonging to the remnants of an old front, will
encroach on Kauai Friday. It is not expected to make it to the
Garden Isle at this time.

Forecast models are homing in on an area of clouds and showers
currently 550 miles east and southeast of the Big Island. Models
are saying this area of moisture will be moving westward with the
trades, and although the bulk of the moisture will be passing south
of the islands Thursday night through Friday night, some of it
may skirt the Big Island and Maui. Perhaps in other words, a slight
boost in trade showers during this period of moderate to locally
strong trades. Furthermore, this clump of moisture will aided by a
westward moving upper level trough with slightly cool temps that
may favor a thunderstorm or two for the higher elevations of the
Big Island over the weekend.



Mostly clear skies and weak trade wind flow are the result of
high pressure passing north of the state. The lack of significant
broad scale winds will allow local wind effects to drive local
weather. Expect light land breezes to prevail most areas
overnight. Light sea breezes and isolated inland showers will
return Thursday afternoon and persist into early evening. Slightly
stronger trade winds will return Thursday night--bringing patchy
clouds and scattered showers to windward slopes and coasts.

No AIRMETs in effect or anticipated at this time. VFR conditions
are expected to prevail all areas through at least Thursday
afternoon. Mountain obscuration could become an issue along
east facing slopes Thursday night.


The large northwest swell currently impacting the Hawaiian waters
has peaked and quickly trending down based on the early evening
observations at the offshore and nearshore PacIOOS Waimea and
Hilo buoys. Despite the downward trend, the Small Craft Advisory
due to 10 ft seas and the High Surf Warning for exposed north and
west facing shores of the smaller islands will remain in effect
overnight and potentially through the first half of the day
Thursday before lowering. Surf along exposed coasts could briefly
dip below the advisory levels Friday, then trend back up slightly
and hover around the advisory levels over the weekend as a new
northwest source arrives. This new source will be from a wide
swath of strong- to low-end gales that has setup from the far
northwestern Pacific to around 700 NM northwest of the islands
today within the 300-320 degree band relative to the area.

The active and progressive pattern across the North Pacific
will continue, featuring a series of fronts passing the state to
the north. Guidance takes the developing gale currently off the
coast of Japan quickly eastward toward the Date Line Friday, then
northeastward into the Gulf of Alaska over the weekend. Although
not as aggressive as earlier model cycles, a compact, hurricane-
force low is shown by the time it reaches the Date Line Friday.
ECMWF-Wave and WAVEWATCH III guidance reflect this and depict seas
climbing into the 35-40 ft range focused at the island waters
around or just over 1000 NM Friday. Long-period forerunners should
arrive locally late Sunday, then peak early Monday driving surf
heights along exposed north and west facing shores well above
warning levels (XL category) and seas above the 10 ft advisory
threshold. Surf associated with this episode should dip below the
advisory levels by Wednesday afternoon of next week.

This long stretch of advisory- to warning-level surf discussed
above will only exacerbate the ongoing coastal erosion issues -
especially with water levels running higher than the predicted
each day. Minor overwash onto vulnerable low-lying roads occurred
early this morning by daybreak around the high tide and that
potential will also be in place Sunday night into Monday around
the peak of the aforementioned next event.

Surf along east facing shores will remain small due to lack of trade
winds locally and upstream. Fresh to locally strong trades will
briefly return Friday into the weekend, then trend down once again
Sunday into next week due to the ridge weakening as another cold
front passes to the north.

Surf along south facing shores will remain small with mainly a
mix of small, short-period southeast and background south-
southwest energy moving through.


High Surf Warning until 6 PM HST Thursday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Molokai-Maui Windward West-Maui Central Valley-Windward

High Surf Advisory until 6 PM HST Thursday for Kona.

Small Craft Advisory until 6 PM HST Thursday for all Hawaiian
waters except Maalaea Bay-



H Lau/Bedal/Gibbs

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