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Temp2.9 C
RH7 %
WindNNE 1 mph
RoadOpen (4x4)
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Hawaiian Islands Synoptic Discussion and Guidance

677
FXHW60 PHFO 262000
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1000 AM HST Wed Feb 26 2020

.SYNOPSIS...
A stable, breezy trade wind flow will continue to focus modest
rainfall along windward slopes through Thursday. High pressure
will strengthen northeast of the state on Friday, possibly
boosting trades into the windy range through the weekend. A low
aloft will drift overhead during this time, leading to an increase
in showers, some possibly heavy. Wet conditions will likely
persist into early next week as trades decline and potentially
shift out of the southeast.

&&

.DISCUSSION...
A stable, breezy, and gusty trade wind flow remains in place. A
surface ridge sitting about 600 miles north of Kauai extends from
a 1034 mb high centered off of the northern California coast,
resulting in breezy trades over the islands. A broad ridge aloft
is maintaining stable conditions with a solid low level inversion
near 7000 ft. Precipitable water is an inch or less (slightly
below seasonal normal), leading to shallow showers within the
trade wind flow. These showers dropped modest rainfall (less than
1/4 of an inch) across windward slopes overnight, while the few
showers that briefly spread to leeward areas produced little to no
accumulation. Dew points are holding in the upper 50s to lower
60s, which is maintaining a somewhat cool feel under the breezy
trades.

Expect little change through tomorrow. The surface ridge north of
the state will break down as another, strengthening high moves in
from the western Pacific. Trade winds should be more or less
unchanged during this transition. The ridge aloft will begin to
erode as a mid to upper level low far east of the state deepens,
but stable conditions will hold with modest rainfall remaining
focused over windward slopes.

On Friday through the weekend, there will be a possibility of
stronger trade winds. The above mentioned surface high will
strengthen to around 1040 mb as it becomes parked about 1300 to
1500 miles north- northeast of the state. A high of this strength
and at that position would normally ensure that trades would ramp
up significantly. However, there is some uncertainty, as the mid
to upper level low currently east of the state approaches on
Friday and drifts overhead during the weekend. This feature could
slightly weaken the local surface pressure gradient, keeping the
strongest trade winds to the northeast of the islands. At this
time, it still appears that we will see an increase in trades,
possibly requiring a Wind Advisory.

There is greater confidence that there will be an increase in
shower activity late Friday through the weekend. As the mid to
upper level low moves overhead, precipitable water does not
change substantially, but very cold temperatures aloft will
produce unstable conditions that will likely generate spotty heavy
showers and perhaps a few thunderstorms. Rainfall will continue to
be focused along windward slopes, but the gusty trades will easily
push brief showers across leeward areas. Winter weather will be
possible on the high summits, and freezing levels could drop low
enough to affect Haleakala.

&&

.AVIATION...
A strong surface high northeast of the Hawaiian Islands will
maintain locally strong northeast to east trade winds around the
main islands. AIRMET TANGO remains in effect for TEMPO MOD TURB
below 8000 feet south thru west of the mountains of all the
islands.

VFR conditions will prevail, but the trade winds will push broken
clouds and isolated showers over windward areas of the islands
producing isolated MVFR ceilings and showers.

&&

.MARINE...
Strong to near gale force trade winds are expected to prevail
through Saturday as a surface high strengthens to near 1040 mb N
and NE of the islands. Winds may reach gale force in the windier
areas around Maui and the Big Island as winds reach their peak
Friday into the weekend, but there is some uncertainty surrounding
the weekend wind forecast. Combined seas will respond to the
nearby and upstream fetch of trade winds, with increasing short-
period wind waves dominating the wave spectra, but a couple of
long-period NW swells arriving from now through the weekend will
add to the seas. A Small Craft Advisory (SCA) is in effect for all
zones through the weekend due to the combination of winds and
seas.

A High Surf Advisory for N and W facing shores remains posted for
areas exposed to NW swell, which is expected to gradually diminish
through tonight. A new long-period WNW swell is expected to arrive
Thursday, peak Friday near advisory-levels, then gradually diminish
through the weekend, with no other significant NW swells in the
forecast through the middle of next week.

Surf along E facing shores will increase over the next several
days, with surf rising to advisory-levels by Saturday. Latest
guidance places a belt of enhanced trade winds immediately
upstream of the islands late in the weekend as a weak surface
trough develops in response to a nearby developing mid-level low.
This increase in winds is expected to bring a surge in mid-period
ENE swell/wind wave, with warning level surf along E facing shores
a distinct possibility Sunday and Monday.

Exact placement and movement of the surface trough will have
profound effects on wind speeds in island waters, and confidence
in the Sunday/Monday forecast (and beyond) is reduced because of
uncertainty in its evolution. For the record, GFS/ECMWF are
currently indicating that the trough will move over the islands
Sunday/Monday, but this may change. This would lead to reduced SE
winds over island waters, with near gale force winds over Hawaiian
Offshore waters just to the E and NE of the islands. There is
increased confidence however that the associated mid-level low
will bring increasing shower coverage and intensity starting
around Friday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST this evening for north and west
facing showers of Niihau, Kauai, Oahu, and Molokai, and for north
facing shores of Maui.

Small Craft Advisory until 6 PM HST Sunday for all Hawaiian
waters.

&&

$$

DISCUSSION...Wroe
AVIATION...Donaldson
MARINE...Birchard

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