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Hawaiian Islands Synoptic Discussion and Guidance

212
FXHW60 PHFO 281332
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
332 AM HST Fri Feb 28 2020

.SYNOPSIS...
Gusty trades will continue to focus modest rainfall along
windward slopes today. High pressure will strengthen north of the
state, boosting trades further through the weekend. An upper low
will drift over the islands late today through the weekend,
leading to an increase in showers, some possibly heavy with a
slight chance of thunderstorms. Wet and unsettled conditions may
persist into early next week.

&&

.DISCUSSION...
Trade winds will increase through the weekend as a surface high
far to our north strengthens to around 1040 mb. However, an upper
low currently east of the state will approach today and drift
overhead during the weekend, likely weakening the local surface
pressure gradient and keeping the strongest trade winds to the
northeast of the islands. Overnight soundings show the inversion
has lifted to about 9400 feet, high enough to allow mixing through
a deep layer and decreasing the likelihood of advisory level winds
across lower elevations of the Big Island. The The Wind Advisory
for that area has therefore been cancelled.

Showers will increase late today through the weekend. As the
upper low moves overhead through early Saturday, exceptionally
cold temperatures aloft will produce unstable conditions that will
likely generate spotty showers, some briefly heavy, and perhaps a
few thunderstorms. Rainfall will continue to focus along windward
slopes, but gusty trades will easily push fast-moving, briefly
heavy showers across leeward areas. Models hint at deep moisture
wrapping around the mid level low on Sunday. At this time, there
does not appear to be a significant flash flood threat.

Winter weather is possible this weekend at high elevations,
including Haleakala. By early Saturday, freezing levels will drop
unusually low to 9,000, possibly lower, bringing chances for snow
and/or freezing rain as showers pass over the high summits of the
Big Island and Haleakala. The highest probability for winter
weather on the summits appears to be on Saturday night into
Sunday. Wet conditions may linger into early next week. Models
show the upper low dissipating and lifting northward on Monday,
while an associated surface trough is pulled over the islands from
the east. Trades should drop and may shift out of the southeast,
depending on the strength of the trough. Moisture convergence on
the east side of the trough could maintain wet weather for some
windward areas.

&&

.AVIATION...
A strong surface high located far northeast of the main Hawaiian
Islands will maintain gusty northeast to east trade winds through
Saturday. As a result, AIRMET TANGO remains in effect for
moderate low-level turbulence below 9 thousand feet immediately
south through west of the higher terrain on all islands.

The strong trades will also continue to transport low clouds with
embedded showers into northeast facing sections of the islands
today, producing brief periods of MVFR conditions due to lower
ceilings and reduced visibilities. Otherwise, expect VFR
conditions to prevail across most of the state. Low cloud and
shower coverage may increase from later tonight through early
Saturday morning. This would result in more widespread MVFR
conditions developing over the higher terrain and northeast
facing slopes. This may require the issuance of AIRMET Sierra for
mountain obscuration over some islands.

&&

.MARINE...
A high far northeast of the area is producing strong to near gale
force easterly trade winds over Hawaiian coastal waters. A Small
Craft Advisory (SCA) is in effect for all coastal waters through
the weekend. Winds are expected to be strongest from tonight into
Sunday as the high builds to near 1040 mb. A low aloft is
expected to induce troughing at the surface near the islands,
which will keep the winds from becoming even stronger. The low
will bring a slight chance of thunderstorms through the weekend.

A new northwest swell has been building through the night. Because
the swell is sufficiently large to produce high surf along north
and west facing shores of the smaller islands, a High Surf
Advisory (HSA) is currently in effect through this afternoon.
Also, surf will slowly build along east facing shores as the
trade winds strengthen, and will remain elevated into the new work
week. An HSA for east facing shores will likely be needed later
this weekend, and it is possible the surf could reach warning
levels along east facing shores. No other significant swells are
expected through the middle of next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST this evening for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Olomana-Molokai-Maui Windward West-Maui Central Valley-
Windward Haleakala.

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

&&

$$

DISCUSSION...Powell
AVIATION...Houston
MARINE...Kinel

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