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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 250630

Area Forecast Discussion
National Weather Service Honolulu HI
830 PM HST Mon Oct 24 2016

High pressure north of the state will maintain moderate trade
winds through Wednesday. The trades will strengthen Wednesday
night and Thursday, then remain strong through the weekend.
Showers will favor windward and mountain areas, with an increase
in showers expected late this week into the weekend as the trade
winds strengthen and a weak upper level trough develops over the


Dry conditions prevail across much of the state on this Monday
evening. The exceptions are parts of windward Maui and windward
sections of the Big Island, where rainfall amounts between one
quarter and one inch have been measured at some gauges during the
past six hours. Early evening infrared satellite images show a
band of low clouds associated with remnants of an old frontal
boundary sinking slowly southward over the eastern end of the
state. These clouds have cleared south of Kauai, Oahu and Molokai,
allowing for drier conditions on those islands. The leading edge
of another frontal remnant cloud band is currently located about
60 miles north of Kauai and about 100 miles north of Oahu, and
these clouds are also moving slowly southward. Surface analysis
depicts a 1028 mb high far north of the state, with moderate to
locally breezy trade winds prevailing over the islands. Aloft,
a strong ridge is centered several hundred miles to our northwest
and is maintaining stable conditions over the islands. The 00Z
Hilo sounding measured PW of 1.32 inches with the inversion based
near 8500 feet, while the 00Z Lihue sounding found a below normal
PW value of 0.97 inches and the inversion based near 5500 feet.
MIMIC-TPW satellite imagery shows narrow stripes of slightly
enhanced moisture with estimated PW values up to 1.5 inches along
the frontal remnants over the Big Island and just to the north of
the state.

Overnight, showers may decrease late over the windward sections
of the Big Island and Maui, as the frontal remnant cloud band
shifts south of those islands. At the same time, showers will
probably increase late tonight over windward parts of Kauai and
Oahu as the next remnant frontal cloud band moves south into those
islands. Leeward areas will remain drier, although a few showers
may spill over into those sections of the smaller islands. Have
made minor updates to the gridded/text forecasts to reflect the
above thinking.

High pressure far to our north will maintain moderate trade winds
for Tuesday into Wednesday. A couple of additional cloud bands
associated with frontal remnants will move south through the state
during this time, causing periodic increases of shower activity
mainly over windward areas. The leeward Big Island will continue
to see the typical diurnal pattern of clouds and widely scattered
showers forming over the slopes during the afternoons, followed by
clearing at night.

Trade winds will increase beginning Wednesday afternoon or evening
through the later part of the week, as a new high pressure area
builds north of the state. This high will sink slowly southward
into the weekend, maintaining strong trade winds with windy
conditions over the islands. Persistent upper level ridging will
continue to keep a cap on the trade wind showers through Thursday
night, with only light to moderate rainfall amounts expected
mainly over windward areas. A weak mid/upper level trough is
expected to develop over the state beginning Friday and continuing
into the weekend. The resultant cooler temps aloft will increase
instability and allow for increased shower coverage and intensity
beginning Friday or Friday night and into the weekend. With the
strong trades and increased vertical cloud depth, expect a wet
trade wind regime over the weekend and into early next week, with
scattered showers readily spilling over into leeward areas.


Isolated to TEMPO MVFR conditions are likely to persist over
windward and mauka areas, mainly from Oahu to the Big Island,
through the evening hours and overnight as moisture from an old
front continues to affect the isles. However, VFR will prevail

An AIRMET for mountain obscuration remains in effect for the
northeast section of the Big Island. The one for low level
turbulence has been cancelled since the trade winds have eased.


A small craft advisory remains posted for the typically windier
locations around Maui County and the Big Island through Tuesday.
Winds in these areas may temporarily drop below advisory levels
Tuesday night and Wednesday. Another high pressure area will
build north of the state once again beginning late Wednesday
through the rest of the week, bringing strong trade winds back to
the islands. Another round of near gales in the windier marine
zones and advisory-level surf along east facing shores will be
possible from Thursday night or Friday into the weekend.

Surf along east-facing shores will ease through Wednesday with
values remaining below advisory levels. A new long-period
northwest swell will fill in later tonight, peak late Tuesday
through Tuesday night, then gradually ease later Wednesday through
Friday. Surf heights will respond, but should remain well below
advisory levels through the peak. A small subsiding south swell
will remain into Tuesday, translating to a slight bump in wave
heights along south facing shores.


Small Craft Advisory until 6 PM HST Tuesday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.




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