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Hawaiian Islands Synoptic Discussion and Guidance

453
FXHW60 PHFO 212007
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1007 AM HST Sat Oct 21 2017

.SYNOPSIS...
Moderate trade winds will continue today with clouds and showers
remaining focused over windward and mauka areas. Some brief
showers will reach leeward sections of the smaller islands. By
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...
Moderate trades will remain today 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. Current model statistics are showing 850 mb
southwest winds below the threshold for damaging winds, but this
forecast will be monitored closely as the event grows near.

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...
Strong and gusty trade winds will continue to support AIRMET
Tango through the day for low level turbulence downwind of the
mountains on all islands.

Clouds and showers will favor windward slopes and coasts.
Isolated MVFR CIG/VIS are to be expected in passing showers today and
may become more frequent across windward Big Island late tonight.
Otherwise, VFR conditions will prevail all areas.

&&

.MARINE...
Trade winds are continuing to blow in the moderate to fresh
range over the coastal waters, with locally Small Craft Advisory
(SCA) level winds today in the usually windiest spots between the
islands and near South Point. Winds will drop off quickly
overnight and turn 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 some
storms could be strong or severe along and ahead of the front.

Although short-period wind waves will be easing, 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 in the S Pacific. 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 long-period NW swell
is possible late in the week as Lan becomes extratropical in the
NW Pacific. 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...
Small Craft Advisory until 6 PM HST this evening for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

$$

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

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

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