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Hawaiian Islands Synoptic Discussion and Guidance

492
FXHW60 PHFO 220649
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
849 PM HST Sat Oct 21 2017

.SYNOPSIS...
Trade winds will diminish tonight with clouds and showers remaining
focused over windward and mauka areas. Sunday, light and variable
winds with afternoon clouds and sea breezes are expected. Some
heavier showers over the eastern half of the Big Island are possible
Sunday afternoon and night. Monday through Wednesday, southerly
winds ahead of a front will bring deep tropical moisture with the
potential for flooding for mainly the eastern half of the state.
Cool and dry air will fill in behind the front.

&&

.DISCUSSION...
After a long stretch of breezy to windy conditions across the
islands, trades are forecast to quickly trend down over the upcoming
24 hrs as a cold front approaches and moves into the area from the
northwest Sunday through midweek. The latest surface analysis showed
this frontal boundary north of Midway extending westward from a
developing storm-force low. A weak surface trough was analyzed east-
northeast of the islands, marking the leading edge of a broad area
of higher moisture. The latest satellite-derived precipitable water
(PW) imagery reflected this and showed a large area of 1.75 to 2"
PWs a few hundred miles east of the Big Island tracking toward the
islands. Drier than normal conditions, however, currently remain in
place across the state (PWs < 1.25"), which should hold through the
overnight hours into Sunday with quiet conditions continuing.

The latest short-term (Sunday-Tuesday) guidance remains in good
agreement and indicates increasing moisture advancing westward
across the islands as the area of higher moisture to the east
moves into the area and a cold front approaches from the
northwest. Model PWs are forecast to rise into 1.75 to 2" range
late Sunday through Monday, with the highest moisture focused over
the eastern half of the state as a pre-frontal convergence
boundary sets up through the day Monday. The low-level flow is
forecast to shift toward the south through the day Sunday, then
toward the south-southwest and strengthen Monday and Monday night
(850 mb SW winds 25-35 mph) ahead of the approaching front. The
front is forecast to reach Kauai Monday night, then progress
eastward over the rest of the island chain Tuesday through Tuesday
night.

A combination of falling upper heights associated with an upper
trough digging southward over the region, increasing instability
and isentropic lift, shear (effective 0-6 km bulk shear up to 35
kt), and plenty of deep tropical moisture pulling northward over
the islands will support heavy rain and thunderstorms developing.
At this time, guidance supports the best chance for heavy rain and
thunderstorm development late Sunday night through Monday night
from Oahu to the Big Island, then shifting east of Oahu late
Monday night through Tuesday and focusing over Maui County and the
Big Island.

Impacts associated with this front and upper trough expected
across the state will be most likely Monday through Tuesday and
include: the threat for flash flooding, strong thunderstorms (an
isolated severe storm can't be ruled), gusty downslope winds
mixing down over windward areas, and a wintry mix over the summits
Monday night into Tuesday. As a result, a flash flood watch will
be possible in the late night package tonight for much of the
state for this period.

North winds behind the front will bring cool and dry conditions to
Kauai Tuesday, and possibly Oahu Wednesday; while the convergence
band keeps things moist and humid over Maui County and the Big
Island.

Model guidance is a bit cloudy for the second half of the week
with the GFS maintaining a weak trough east of the state, while
the ECMWF places a stronger surface low southeast of the Big
Island. The forecast maintains light to moderate northeast trade
winds with windward showers in the extended forecast.

&&

.AVIATION...
Moderate trade winds are trending down, as the high pressure north
of the islands moves closer to the islands. Scattered clouds and
showers continue to be carried in on the trade wind flow, and will
primarily impact the windward sides of the islands overnight. VFR
conditions are expected to prevail, with some isolated MVFR
conditions in passing showers. With lighter winds expected Sunday,
expect localized afternoon sea breezes that will allow clouds and
shower to form over the interior of the islands. Isolated MVFR
conditions are possible with these showers as well.

No AIRMETs anticipated at this time.

&&

.MARINE...
A ridge north of the area will produce gentle to fresh easterly
trade winds overnight. The ridge will shift south over the weekend
ahead of a front approaching from the northwest. Winds will
weaken further and shift out of the southeast tomorrow as the
ridge moves over the area. Winds may shift out of the south Monday
and could become locally fresh again as the front approaches. The
front is expected to reach the Kauai waters Monday evening and
push down to the Big Island waters by Tuesday evening. Heavy
showers and thunderstorms are possible ahead of and along the
front. Fresh to locally strong north winds will spread over the
islands behind the front. The winds will slowly veer to northeast
by the end of next week.

A northwest swell expected tomorrow and Monday, will likely produce
surf near the advisory threshold along north facing shores. That
swell will diminish Monday night into Tuesday, but a bigger
northwest swell Tuesday and Wednesday will likely require a High
Surf Advisory for north and west facing shores. The swell will
also boost combined seas above the 10 foot level, so a Small Craft
Advisory will likely be needed. An even larger northwest swell is
forecast to arrive next Friday. That swell could produce warning
level along north and west facing shores.

A low east of New Zealand generated a swell which will produce surf
near the advisory level along south facing shores Sunday and Monday.
See the Collaborative Surf Discussion for details.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...GIBBS
AVIATION...BALLARD
MARINE...DONALDSON

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