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Hawaiian Islands Synoptic Discussion and Guidance

852
FXHW60 PHFO 301953
AFDHFO

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

.SYNOPSIS...
High pressure north of the state will drive fresh trade winds in
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.

&&

.DISCUSSION...
A 1036 mb surface high is centered far north of Hawaii and is
driving fresh trade winds across the area. Meanwhile, former
tropical cyclone Ulika is located around 800 miles east of the Big
Island and slowly moving to the west. Aloft, a broad trough
resides between former Ulika and the hawaii with second, narrow
trough northwest of the area. A weak upper ridge is wedged between
these two features and is over the islands this morning.

Visible satellite imagery and radar over Hawaii show windward
clouds and showers moving in with the trade wind flow. A few
clouds and showers are blowing over to leeward sides of the
smaller islands. 12Z soundings show a typical 6 to 7 kft inversion
over the state with precipitable water values of 1.18 to 1.24
inches. MIMIC total PW satellite shows an area of higher PW about
70 miles upstream of the Big Island. This higher moisture will
move towards the state today and will likely result in an increase
in showers for the eastern end of the state by late this
afternoon, then spread westward across the rest of the islands
beginning tonight. Until then, anticipating somewhat of a dropoff
in shower coverage today.

Fresh trade winds will continue into the weekend as the pressure gradient
remains tight due to a the surface high northeast of the islands
and the approach of former Ulika from the east. Aloft, there are
some differences between the GFS and ECMWF, with the GFS
indicating more pronounced upper level troughing shifting closer
to 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.

The GFS and ECMWF both close off a pair of upper level lows to
the east and west of the state Sunday while moisture from Ulika
filters in. With 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 on Sunday, with the potential for locally heavy
rainfall at times. Next week, the trades will weaken slightly and
shift slightly east southeast as the upper low to the northeast
induces a surface trough just west of the state.

&&

.AVIATION...
A strong surface high pressure area sitting north-northeast of the
state is generating moderate to strong and gusty trade winds
throughout the islands. Random bouts of low level moisture and
clouds riding in with the trades will produce MVFR conditions
mainly along the windward sections of the islands today through
the weekend. Currently most of the clouds and moisture are
affecting Maui and Molokai and could affect Oahu later today. A
large cluster of low clouds will also move into the Big Island
this afternoon. Cloud tops are mostly around 8000 to 9000 feet.

AIRMET TURB for moderate, intermittent, low level turbulence lee
of the mountains continues and is likely to remain in effect
through Saturday.


&&

.MARINE...
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.


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Saturday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.


&&

$$

Discussion/Marine...Foster
Aviation...Fujii

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



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