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RH44 %
WindNNE 0 mph
RoadOpen (4x4)
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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 290159

Area Forecast Discussion
National Weather Service Honolulu HI
359 PM HST WED SEP 28 2016

Locally breezy trade winds will focus showers across windward and
mauka areas through 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.


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. 00z soundings from Lihue and Hilo show inversions
around 6kft and precipitable waters of 1.1-1.2 inches. Early
afternoon MIMIC total precipitable water imagery shows similar or
higher moisture values within 200 miles upstream to the

The surface high north of the state has kept most clouds and
showers focused across windward sections, though rainfall has
remained limited through the day. Most rain gages only measured
up to a few hundredths of an inch over the past 12 hours (with one
recent shower bringing 2 tenths of an inch to the Niu Valley gage
on Oahu). Satellite and radar show showery clouds associated with
the upstream moisture starting to move ashore over Maui/Big
Island. These showers will spread to the rest of the state early
this evening. Forecast soundings/cross sections show inversions/
moisture depth increasing from this evening through Thursday
morning, then lowering during the afternoon. More typical trade
wind showers are expected through Friday.

Rainfall chances will increase heading into the weekend. The
GFS/ECMWF 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 had started to converge toward a similar
solution early next week, but the 18z GFS seems to develop a
problem with gridscale convective feedback. It now spins up a
compact low over/north of the state Monday/Tuesday, while the
ECMWF develops at most an inverted surface trough. 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
combination of tropical moisture and increasing instability that
would accompany the upper low. While we will continue to keep the
forecast wet through the extended, have held off on mentioning any
heavy rain due to uncertainty with the pattern and timing.


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


High pressure far north of the state will maintain locally strong
trade winds across the area, and a small craft advisory is in
effect for the typically windier areas near Maui and the Big
Island. The morning ASCAT pass showed only 20 knots of wind
through the Alenuihaha Channel, in an area that had 25 knots from
the pass last night. High-resolution models picked up on this
brief downward trend, and also show wind speeds increasing again
this evening and continuing near small craft levels through much
of the week. This advisory is in effect through Thursday, though
it would need to extended if conditions remain steady as currently


Small Craft Advisory until 6 PM HST Thursday 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