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Temp-4.4 C
RH96 %
WindSW 57 mph
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Hawaiian Islands Synoptic Discussion and Guidance

590
FXHW60 PHFO 200720 AAA
AFDHFO

Area Forecast Discussion...UPDATED
National Weather Service Honolulu HI
920 PM HST Mon Feb 19 2018

.UPDATE...
The winter weather advisory for the Big Island summits has been
cancelled as drier air has filled in across the area. Rainfall
chances were also lowered across the state overight and Tuesday
based on the latest trends and short-term guidance.

&&

.SYNOPSIS...
Dry and pleasant conditions will likely hold into midweek as the
upper low that brought the heavy rains across the state on Sunday
lifts north and away from the area. A return of unsettled weather is
possible through the second half of the week and the upcoming
weekend as another upper low moves drops south into the area.

&&

.DISCUSSION...
A mostly dry pattern with hold through Tuesday with dewpoints
remaining in the low 60s. The upper low that brought the flooding
rains and thunderstorms across the state on Sunday continues to pull
away to the north this evening as plenty of middle- to upper-level
dry air fills in across the area. Upper air data and satellite-
derived precipitable water (PW) values have lowered as a result and
should remain around and potentially just under an inch across much
of the state through Tuesday. The evening update will reflect this
with lower rainfall chances through Tuesday.

Increasing moisture and rain chances are anticipated by late
Wednesday with another period of unsettled weather setting up for
the second half of the week and into the upcoming weekend. Despite
some subtle differences between solutions, guidance remains in
decent agreement and depicts deep tropical moisture being drawn
northward over the state as another upper low drops south into the
area Wednesday night into Thursday and holds through Saturday. The
upper low is forecast to stall over an area located a few hundred
miles northwest of Kauai. Strong southwest winds aloft will setup
over the western end of the state with 0-6 km bulk shear magnitudes
increasing into 35 to 45 knot range by Thursday afternoon. This
combined with increasing lift and instability will support heavy
showers and thunderstorms developing once again, especially over the
western end of the state where the better upper support will be.
Model PWs will climb into the 1.5-1.8" range, which is well above
normal for this time of year. Winds are expected to increase and
could become gusty for some locations out of the east-southeast
direction through this time as the surface trough shifts west of the
state and high pressure builds to the northeast.

No significant changes are shown Sunday through early next week.
Guidance continues to advertise a wet pattern with a deep trough
positioned west of the state and moist east-southeast flow
continuing (PWs holding above average in the 1.4-1.7" range).

&&

.AVIATION...
VFR conditions will prevail across the state overnight and
Tuesday. The upper trough axis has now moved northeast of the
islands, taking the chance of thunderstorms with it beyond the
windward coastal waters. Dry and more stable conditions are
expected over the state through Tuesday, with just some afternoon
clouds and showers along with isolated MVFR conditions possible
over the island interiors as seabreezes develop.

Moderate turbulence aloft remains possible over the state overnight
due to the nearby upper trough, and AIRMET Tango remains posted.
This turbulence should subside on Tuesday. No other AIRMETs are
anticipated during the overnight period.

SIGMET Romeo was recently issued for thunderstorms developing
along the primary West Coast to Hawaii air routes near the upper
trough.

&&

.MARINE...
Small to moderate northwest swells will continue through Tuesday,
with surf remaining below advisory level along north and west
facing shores. However, a longer period northwest swell is
expected to spread down the island chain beginning Tuesday night
and peaking Wednesday. This may result in advisory level surf
along some north and west facing shores, especially Kauai and
Niihau, during the peak of the event. This swell will gradually
subside Thursday, but another west-northwest swell is expected
Thursday night into Friday, before fading into the weekend.

A long fetch due to the flow around a surface high over the
northeastern Pacific will send a short-period east swell into the
local waters during the next several days. This swell is expected to
increase later tonight and Tuesday, with resulting surf likely
reaching the advisory threshold of 8 feet along most east facing
shores Tuesday. A High Surf Advisory has been issued for these
areas and will go into effect Tuesday morning. A further increase
in this swell is expected Thursday, remaining elevated into the
weekend, with high surf likely continuing along east facing shores
through that period.

Combined seas may increase to around 10 feet over most windward
waters starting late Wednesday due to the combination of west-
northwest and east swells, and local wind waves. A Small Craft
Advisory may be needed for the latter portion of the work week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory from 6 AM Tuesday to 6 AM HST Wednesday for
Kauai Windward-Oahu Koolau-Olomana-Molokai Windward-Maui
Windward West-Windward Haleakala-Big Island North and East.

Wind Advisory until 6 AM HST Tuesday for Big Island Summits.

&&

$$

AVIATION...Jacobson
MARINE...Kinel
REST OF DISCUSSION/UPDATE...Gibbs

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