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Hawaiian Islands Synoptic Discussion and Guidance

384
FXHW60 PHFO 282007 CCA
AFDHFO

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
1007 AM HST WED SEP 28 2016

.SYNOPSIS...
Locally breezy trade winds will maintain a somewhat stable
rainfall pattern focusing showers across windward slopes into
Friday. Showers are expected to increase during the weekend, and
possibly into early next week, as deeper low level moisture moves
over the islands and a mid to upper level trough strengthens near
the state.

&&

.DISCUSSION...
Water vapor imagery shows a broad upper level low east of the
state, with a ridge just to the west. At the surface, high
pressure is located far north of the state, with a trough to our
northwest. 12z soundings show low inversions of 5-6kft.
Precipitable waters range from 0.94 at Lihue to 1.44 at Hilo.
Early morning MIMIC total precipitable water imagery shows a wedge
of drier air around Kauai, with higher values from Oahu to the Big
Island. This area of moisture extends 250 miles upstream to the
northeast of the state.

The surface high north of the state has kept most clouds and
showers focused across windward sections, though rainfall has
been limited overnight. Most rain gages only measured up to a few
hundredths of an inch over the past 12 hours (greatest over the
eastern half of the state). Satellite and radar show showery
clouds over the windward offshore waters, which matches up with
the pocket of higher PW air mentioned above. This moisture will
move across the islands tonight, enhancing the trade showers.
Forecast soundings/cross sections show elevated inversions/
moisture depth this evening into Thursday morning, followed by
more typical trade wind showers through Friday.

Rainfall chances will increase heading into the weekend. The 12z
GFS/ECMWF both show a nearly east to west oriented upper level
trough setting up just north of the state on Friday and gradually
strengthening through the weekend. This trough will erode the mid
level ridge over the islands and weaken the inversion. Meanwhile,
the models are showing deeper low level moisture moving in from
the east. This should result in an increase in rainfall,
especially across the eastern end of the island chain.

The GFS and ECMWF are convergence toward a similar solution early
next week, showing a low developing along the western end of the
upper trough. The GFS is still farther west than the ECMWF with
the low in relation to the state, but has trended eastward
compared to the 06z run. During this time, moisture from the
remnants of tropical cyclone Ulika will be carried near or over
the islands by the trade wind flow. This could create the
potential for heavy rainfall given the degree of instability that
would accompany the upper low. We maintained wet conditions early
next week due to the amount of uncertainty with the placement of
the upper low, but if the 12z model trends pan out we may need to
add a mention of heavy showers as well.

&&

.AVIATION...
AIRMET Tango remains posted for low level turbulence south through
west of mountain ranges as high pressure continues to provide for
moderate to fresh trade winds across the area.

VFR conditions prevail at the TAF sites with some brief MVFR
conditions expected along windward slopes as moisture embedded in
the trades interacts with terrain.

&&

.MARINE...
High pressure far north of the state will maintain locally strong
trade winds across the area. An ASCAT pass from last evening
showed 25 knots of wind through the Alenuihaha Channel and near
South Point, which matches well with the existing small craft
advisory. This advisory is now in effect through Thursday, though
it would need to extended if conditions remain steady as currently
expected.

&&

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

&&

$$

Discussion/Marine...Bravender
Aviation...Burke

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



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