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Hawaiian Islands Synoptic Discussion and Guidance

441
FXHW60 PHFO 290203
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
403 PM HST Tue Jan 28 2020

.SYNOPSIS...
Stable and rather dry conditions will persist through Thursday,
featuring light and variable winds across the western half of the
state and east to southeast winds with light windward showers on
the Big Island. A front will stall near Kauai on Friday, bringing
wet and unsettled conditions across the western end of the state
into Saturday as trade winds rebuild. Trades will veer
southeasterly again early next week.

&&

.DISCUSSION...
A stable and rather dry conditions persist. With the subtropical
ridge sitting within 200 miles north of Kauai, easterly trade
winds across the Big Island quickly veer southeasterly, placing
the western half of the island chain in the wind shadow of the
Big Island and allowing sea breezes to prevail this afternoon. A
mid level ridge overhead is maintaining very stable conditions
and a strong inversion, while precipitable water around the state
remains below January normal. Aside from modest showers along
windward Big Island slopes, scant rainfall has been recorded
during the past 24 hours, and little is expected through the
remainder of the day.

Stable and rather dry conditions will dominate through Thursday.
An approaching front will push the subtropical ridge over the
islands by late Wednesday, further weakening and veering the
background flow and allowing land and sea breezes to dominate. The
mid level ridge will remain in place overhead and maintain stable
conditions. As a result, expect light windward showers to favor
Big Island and east Maui, and the rest of the state will be under
a stable land and sea breeze pattern, featuring afternoon clouds
and a few light showers followed by clearing at night.

Wet and unsettled weather will likely develop across the western
end of the state late Thursday night and Friday and may last
through Saturday. A front will drop down from the northwest and
stall near Kauai, while surface high pressure passing north of the
front ushers in building trade winds. An upper level trough will
pass over the islands on Friday and could create enough
instability to trigger heavy showers. The GFS and ECMWF had been
trending weaker with the upper level trough during the past few
runs, but the 12Z ECMWF run showed the feature cutting off into
an upper level low on top of the state Friday night and Saturday.
Due to continuity, we are placing greater weight in the GFS, which
lifts the upper trough out before it cuts off on Saturday. The
forecast calls for wetter conditions to favor windward areas of
Kauai and Oahu with moderate, mainly windward showers elsewhere.
Uncertainty remains somewhat high.

Uncertainty lingers into Sunday, hinging upon the above mentioned
upper level low. The GFS shows a building mid level ridge
bringing increasingly stable conditions and a drier trade wind
flow, and the ECMWF lingers the upper level over the state,
leaving open the possibility of unstable conditions. For now, the
forecast represents a blend of the models, showing a somewhat wet
trade wind pattern.

Heading into next week, expect trades to veer southeasterly again
with drier conditions.

&&

.AVIATION...
A surface ridge of high pressure will continue to hold north of
the state this afternoon and generate light easterly flow. Sea
breezes are expected to dwindle this evening and be replaced by
land breezes overnight.

Scattered low clouds and isolated light showers will be mainly
limited to windward areas of Maui and the Big Island.

&&

.MARINE...
A nearly stationary ridge just north of the area will keep winds
light and variable across Kauai and Oahu, and moderate to locally
fresh out of the east to southeast over Maui County and the Big
Island through most of the week. An approaching front from the NW
will shift the ridge over the island chain by Thursday, weakening
winds around Maui County and Big Island and generating unsettled
weather. Northeast fresh to locally strong trades will fill in
behind the front by the end of the week.

A dynamic weather pattern across the Pacific will keep surf
elevated along north and west facing shores beginning tonight.
The next northwest swell has reached Buoy 51001 which came in at
7 feet 16 seconds and is anticipated to reach Kauai by midnight.
Thus, a High Surf Advisory has been issued for north and west
facing shores of the smaller islands beginning midnight tonight
through Wednesday night. A Small Craft Advisory has also been
issued for waters exposed to the swell for seas above 10 feet.
Surf will remain elevated near advisory level Thursday. Another
slightly larger, northwest to north-northwest, swell is expected
to fill in Thursday night and peak Friday at solid advisory
levels, then slowly decline through the weekend as swell shifts to
a more northerly direction.

Surf along exposed east facing shores will remain rather small
through the period due to the lack of persistent trades locally
and upstream across the eastern Pacific. An increase in surf is
possible over the weekend as trades build behind the front.

Surf along south facing shores will remain small through the week.
However, background south swells should be enough to keep things
from going flat.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory from midnight tonight to 6 AM HST Thursday
for north and west facing shores of Niihau, Kauai, Oahu, and
Molokai and for north facing shores of Maui.

Small Craft Advisory from midnight tonight to 6 AM HST Thursday
for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward
Waters-Oahu Windward Waters-Maui County Windward Waters-Big
Island Windward Waters.

&&

$$

DISCUSSION...Wroe
AVIATION...Thomas
MARINE...Almanza/TS

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



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