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Temp0.7 C
RH13 %
WindE 12 mph
RoadOpen (4x4)
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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 301330

Area Forecast Discussion
National Weather Service Honolulu HI
330 AM HST FRI SEP 30 2016

High pressure north of the state will keep moderate to breezy trade
winds in place through the weekend, with showers focused primarily
over windward and mauka areas. Showers are expected to increase
in coverage and intensity over the weekend through much of next
week, as a moist airmass associated with the remnants of Ulika
moves over the state.


Currently at the surface, a 1037 mb high is centered well north
of the state, while Tropical Depression Ulika is located around
850 miles east-southeast of Hilo. Aloft, a broad upper level low
resides between Ulika and the islands, with upper troughing
extending westward over the state. Infrared satellite imagery
shows variably cloudy skies across the state, with some high
level jet stream cirrus enhancing cloud cover over Maui County
and the Big Island. Meanwhile, radar imagery shows scattered to
numerous showers over windward areas, with isolated showers
spilling over into leeward locales. Main short term concern
for today revolves around rain chances.

High pressure well north of the state will keep a moderate to
breezy trade wind flow in place, with clouds and showers focused
mainly over windward and mauka areas. MIMIC total precipitable
water (PW) values show an area of slightly higher moisture with
PW values in the 1.2 to 1.4 inch range currently over and upstream
of Maui County, Oahu and Kauai. Meanwhile, a drier airmass with
PW values around 1 inch resides immediately upstream of the Big
Island. With slightly higher moisture along with steep low level
lapse rates around 9C/KM in the 0-1 KM layer, we should see fairly
good coverage of showers over windward/mauka areas from Maui
County to Kauai this morning. This afternoon, we should see a
decrease in shower coverage as a slightly drier airmass moves in.
Leeward areas will see isolated showers, mainly during the
morning hours.

Across the Big Island, shower coverage should be reduced this
morning in windward areas, then increase by mid-late this
afternoon as a band of deeper moisture now approaching 152W begins
to move in. Leeward areas will see some scattered showers develop
on the mountain slopes this afternoon as a result of daytime

Tonight through Saturday night,
Moderate to breezy trade winds will continue as the gradient
remains tight due to a large surface high northeast of the
islands and the approach of Ulika from the east. Aloft, there are
some differences between the 00Z GFS and 00Z ECMWF, with the GFS
indicating more pronounced upper level troughing over the islands,
while the ECMWF is not as aggressive. Deep layer moisture will
increase from east to west across the islands tonight through
Saturday morning, with PW values will rising into the 1.5 to 1.8
inch range for Saturday afternoon and Saturday night. Overall,
with troughing aloft and an increase in deep tropical moisture,
we should see shower coverage and frequency increase across the
state, with showers favoring windward and mauka areas.

Sunday through next Thursday,
Both the 00z GFS and 00z ECMWF are in fairly good agreement,
showing high pressure remaining to the northeast of the state,
keeping a moderate trade wind flow in place through the period.
Aloft however, there are some significant differences and model
run to run continuity has been poor. The GFS and ECMWF both close
off a pair of upper level lows to the east and west of the state
Sunday through early next week, but the GFS solution shows these
features weaker with more residual troughing remaining in place
across the island chain. The ECMWF on the other hand shows
stronger upper level lows displaced further apart, with upper
level ridging over the state. Obviously, this has significant
impacts on the weather pattern across the islands. The GFS
solution suggests a wetter and more unstable pattern, while the
ECMWF suggests more of a wet trade wind set up.

Given that both models indicate plenty of deep tropical moisture
associated with the remnants of Ulika, along with at least some
weak troughing aloft, we should see a wetter than normal pattern
across the state through the period. Showers will continue to
favor windward and mauka areas, with the potential for locally
heavy rainfall at times.


Fairly typical, and breezy, trade wind weather expected. Most
areas will see VFR most of the time. However, isolated to TEMPO
MVFR conditions will likely affect windward and mauka areas as
showery low clouds move along on the low-level flow. Some of this
cloudiness may make its way over leeward sections of the smaller
isles and West Maui from time to time as well.

An AIRMET for moderate, intermittent low level turbulence to the
lee of the mountains, all islands, remains in effect.


High pressure far to the north of the state will keep moderate to
breezy trade winds in place through the weekend. A Small Craft
Advisory is in effect for the typically windier waters around
Maui and the Big Island through Saturday, and will likely need to
be extended through the rest of the weekend.

Short period easterly swell resulting from the trade winds will
continue through the remainder of the week. A bit of a bump in
surf heights is expected for east facing shores over the weekend
as the remnants of Ulika approach from the east. Elsewhere, surf
heights will remain small through the middle of next week.


Small Craft Advisory until 6 PM HST Saturday 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