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Hawaiian Islands Synoptic Discussion and Guidance

656
FXHW60 PHFO 220153
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
353 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...
Trade will diminish tonight with showers relegated to mainly
windward areas. By Sunday, a front approaching from the northwest
will push a surface ridge over the islands allowing for light and
variable winds over the smaller islands with widespread afternoon
sea breezes and interior clouds and showers. Near and over the Big
Island, southeasterly winds will bring increasing humidity and
moisture from the tropics allowing for heavier showers starting
Sunday and continuing Sunday night, mainly over the Kau, Puna, and
Hilo districts.

As the front approaches Kauai Sunday night and Monday, both the
GFS and ECMWF are depicting a moisture convergence band forming
over Oahu, Maui County, and the Big Island. The near surface
convergence, tropical moisture, and island terrain will likely produce
heavy rain, isolated thunderstorms, and the potential of flash
flooding for areas of the eastern islands through Tuesday.
Sufficient deep layer shear, as well as turning of the winds with
height in the lower levels, suggest that some of those isolated
thunderstorms could be strong or severe along and ahead of the
front.

Kona winds (southerly) will increase Monday with some potential
for gusty winds accelerating down the terrain into windward areas
of Kauai and Oahu. The front will push over Kauai Monday night
bringing moderate rain with the heaviest falling over the north
and west facing slopes of the Garden Isle. The front will push the
aforementioned convergence band slowly eastward, likely pushing
east of Oahu Tuesday, and then east of the Big Island sometime
Wednesday. 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 to breezy trade winds will trend down this evening as a
ridge of high pressure approaches the islands from the north.
AIRMET Tango for low level turbulence downwind of the mountains on
all islands will likely be canceled later this evening.

Ragged clouds and scattered showers will persist along windward
slopes and coasts overnight. Partial clearing is expected across
leeward slopes and coasts after sunset once weak sea breezes
abate. Isolated MVFR CIG/VIS are to be expected in passing
showers this afternoon may become more frequent across windward
Big Island late tonight. Otherwise, VFR conditions will prevail
all areas.

&&

.MARINE...
Trade winds continue to gradually diminish but are still blowing
in the moderate to fresh breeze range across the coastal waters
this afternoon. Will be discontinuing the Small Craft Advisory as
we should see winds will drop off overnight and start turning SE
as a subtropical ridge near 30N pushes rapidly S over the islands
on Sunday. Kona winds from southerly directions will then take
over and increase by Monday as a developing gale low gets cranking
600 nm or so to our N. Some of the guidance suggests that
prefrontal konas might approach SCA levels on Monday or Monday
night, something we will need to keep an eye on. There will also
be fresh northerlies immediately behind the cold front as it
pushes down the coastal waters from NW to SE Monday night and Tue,
and these could also approach SCA criteria in some spots before
the low starts to move away in the midweek time frame.

Deep tropical moisture drawn northward ahead of the front is
expected to lead to some thunderstorms over the coastal waters
starting early Monday, mainly from around Oahu eastward.
Sufficient deep layer shear, as well as significant turning of
the winds with height in the lower levels, suggest that a couple
of the storms could be strong or severe along and ahead of the
front.

Although short-period wind waves will continue to ease, building
NNW and S swells Sunday and Monday are expected to keep combined
seas elevated, but below the SCA criteria of 10 feet. A larger,
shorter-period NNW swell building on Tuesday will likely lead to
SCA in all zones as seas exceed 10 feet once again. The first NNW
swell will produce surf near the advisory threshold along exposed
N facing shores, and the second swell is expected to produce high
surf along N facing shores. Advisory-level surf along S facing
shores is possible Sunday and Monday, with the swell generated by
a distant low near New Zealand last weekend. A small long-period W
swell is possible next week, generated by large and slow-moving
Super Typhoon Lan in the W Pacific. An even larger, possibly
warning level NW swell is possible late in the week as Lan becomes
extratropical in the NW Pacific. The GFS and ECMWF are in good
agreement on the evolution of this feature and the fetch
generation area. SE swell is also possible over the next several
days. The recently updated Oahu Surf Discussion (SRDHFO) has
details on the swell sources affecting the islands.


&&

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

&&

$$

Morrison...Discussion
RBallard...Marine
Bedal...Aviation

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