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Hawaiian Islands Synoptic Discussion and Guidance

721
FXHW60 PHFO 171404 CCA
AFDHFO

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
404 AM HST Sun Jan 17 2021

.SYNOPSIS...
A wet pattern is on tap for the islands during the first half of
the coming week. This comes about as a front moves down the island
chain this afternoon and tonight, then stalling around Maui and
the Big Island between Monday and Tuesday. The front, or trough,
is then slated to drift back to Oahu and Kauai between Tuesday
afternoon and Wednesday. The front will have good support aloft
from an upper level disturbance, resulting in heavy showers and
isolated thunderstorms. On top of the rain, strong northeast
trade winds will follow in behind the front. Moderate to strong
easterly trade winds return to the region during the second half
of the week with a bit of a wet flavor.

&&

.DISCUSSION...
A change in the weather is forthcoming. A cold front with strong
trade winds and somewhat cooler temperatures behind it will move
down the main Hawaiian Islands, starting with Kauai this afternoon,
Oahu early this evening, Maui County during the rest of tonight/early
Monday morning. The front is expected to come to a rest around
Maui and the Alenuihaha Channel early Monday morning. Both the EC
and GFS does not have the front crossing the Big Island. A couple
of facts, the new ECMWF is about 3 hours slower than the GFS for
the first 12 hours. The GFS is also drier once the front clears
Kauai this evening. The current forecast package has the threat of
thunderstorms ending over Kauai by midnight tonight in line with
the GFS. Although rather windy, Kauai should be fine Monday through
the first half of Tuesday.

Between Monday and Tuesday, Maui County and the Big Island will
most likely get the brunt of the weather as upper level trough
approaches Kauai slowly from the northwest, adding some extra
energy to an already moist and unstable air mass. As noted in the
evening discussion, there is a pool of cold air of minus 15
degrees centigrade at 18k feet. This temperature is a departure of
about 8 degrees of the norm. Through Monday night, Oahu will be
at the fringe of this weather. In the wake of the front, strong
and gusty northeast trade winds will settle in, possibly strong
enough to warrant a Wind Advisory for most, if not all of the
smaller islands.

The upper trough stalls over Kauai, then evolves to a low southwest
of Kauai on Tuesday. At the surface, the front weakens into a
trough that moves west, back across Oahu and Kauai Tuesday
afternoon and Tuesday night, giving these two islands another
round of wet and unstable weather. By Wednesday afternoon, the
surface trough will be west of Kauai. But the air mass over the
islands remains unstable with plenty of low level moisture,
leading to a showery trade wind regime that may persist through
the second half of the work week. Thunderstorms are not part of
the forecast during this stretch, but a second look will be taken
in the coming days.

A Flash Flood Watch has been issued for Oahu and Maui County
starting at 6 pm HST tonight. We left Kauai out of this watch
since the front is expected to move through the island at about 15
to 20 mph before slowing down after reaching Oahu. This is a
pretty fast speed, so a Flood Advisory will suffice to this
situation. We also did not include the Big Island in the watch as
well. Models are hinting at Monday night at the earliest for
weather to turn sour there.

I have backed off on the cooler temps. Although it will be
cooler, the strong trade winds, the clouds and showers will keep
the overnight low from reaching the minimum dew point values, that
is low 60's on Oahu, and upper 50's on Kauai. We are looking at
overnight lows through Tuesday night in the 64 to 70 range.

Radar shows some hit and miss showers present on all but Oahu.
An isolated shower or two is still present at Kawaihae Big Island.
These showers can quick form, and quickly dissipate. This is the
pattern for today before the front arrives.

&&

.AVIATION...
Latest observations indicate some expansion of shower coverage is
underway within modestly convergent southwest flow that has
emerged well ahead of an approaching cold front. The bulk of this
activity is confined to Kauai and adjacent waters but will
gradually shift east to include Oahu through the daytime hours
when diurnal trends will act as a damper on both coverage and
intensity. Prevailing VFR is therefore forecast for Oahu through
Big Island for the balance of the daylight hours.

For Kauai, the probability of MVFR will begin increasing over
Kauai by late this morning as low-level moisture increases.
Issuance of AIRMET Sierra for tempo mountain obscuration is likely
by this evening. The arrival of the cold front is expected for
Kauai around 02z/4pm HST and around 06z/8pm HST for Oahu and will
be marked by a greater coverage of showers, some locally heavy,
followed by abrupt shift to gusty northerly winds.

&&

.MARINE...
A High Surf Warning for exposed N and W facing shores has been
extended through Monday night, despite surf having already diminished
below warning levels on Kauai, while also likely dropping below
warning levels by tonight elsewhere. This is because another
powerful NW swell building tonight into Monday will once again
bring warning-level surf, with peak surf heights expected to be
slightly lower than they were this most recent swell. Very strong
N to NE winds will destroy any chances of quality surf along N
facing shores with this new swell however.

A High Surf Advisory for exposed N and W facing shores of the Big
Island was also extended through Monday night. Noteworthy that
the PacIOOS nearshore Hilo buoy has recently reported combined
seas near 14 feet with a dominant period near 16 seconds. If the
full brunt of these swells were actually reaching the shoreline,
we would expect surf heights to be well above warning-levels, but
to the best of our knowledge, most of this swell energy is not
realized at the coast with this particular swell direction.
However, due to the potential for significant surges within Hilo
and Kahului harbors today, a Marine Weather Statement is posted.

A Small Craft Advisory (SCA) remains in effect for all waters
except Maalaea Bay through today, as the NW swell continues to
produce seas greater than 10 feet. Seas will diminish slowly
today, but will be ramping up again tonight and Monday, normally
requiring an extension of the SCA. However, in this case a Gale
Watch is posted for waters from the Alenuihaha Channel westward
starting tonight as very strong N to NE winds are expected to
develop after the passage of a cold front that is rapidly
approaching from the NW, producing frequent gusts greater than
gale force. We will be monitoring wind observations from NDBC
buoys 51001/51101 NW of Kauai to determine the need to upgrade to
a Gale Warning later today.

As the front approaches today, weakly-convergent, light to
moderate S to SW winds will lead to the development of some pre-
frontal showers through the day. The front will enter Kauai waters
this afternoon, Oahu waters this evening, before stalling over
the central portion of the island chain early Monday, bringing
some heavy showers and the potential for thunderstorms. A developing
sharp trough aloft on Monday and Tuesday will likely invigorate
the lingering moisture through Wednesday as it drifts westward,
keeping the potential for heavy showers and thunderstorms.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch from this evening through Tuesday afternoon
for Oahu-Molokai-Lanai-Kahoolawe-Maui.

High Surf Warning until 6 AM HST Tuesday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Olomana-Molokai-Maui Windward West-Maui Central Valley-
Windward Haleakala.

High Surf Advisory until 6 AM HST Tuesday for Kona-Big Island
North and East-Kohala.

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

Gale Watch from this evening through late Monday night for Kauai
Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-
Kauai Channel-Oahu Windward Waters-Oahu Leeward Waters-Kaiwi
Channel-Maui County Windward Waters-Maui County Leeward Waters-
Maalaea Bay-Pailolo Channel-Alenuihaha Channel.


&&

$$

H Lau/JVC/Birchard

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