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Hawaiian Islands Synoptic Discussion and Guidance

910
FXHW60 PHFO 012010
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1010 AM HST SAT OCT 1 2016

.SYNOPSIS...
High pressure northeast of the state will drive fresh trade winds
through the weekend. Showers are expected to increase in coverage
during the weekend, with locally heavy downpours becoming more
likely early next week, as a moist airmass associated with the
remnants of Ulika moves over the state.

&&

.DISCUSSION...
High pressure to the northeast of the state will maintain fresh
trade winds across the region through Sunday. The high is expected
to move to the east over the next few days, and the associated
ridge will sink a bit closer to the islands. This would weaken
winds near the islands, and turn them more southeasterly for
Monday and Tuesday.

Ulika remnants are about 400 miles southeast of the Big Island
this morning with the bulk of the associated moisture to the
northeast of the remnants near 18N 149W. As this system moves
closer to the islands, precipitable water (PW) values will
increase. The overnight soundings from Lihue showed near normal
PW for October of 1.19 inches, while Hilo was above normal with
1.86 inches. The models are in good agreement with the influx of
moisture through the start of the work week, with the highest PW
values expected over and near the Big Island.

Looking at the upper level temperatures, both the GFS and ECMWF
indicate 500 and 700 temperatures will cool a little Monday
through Tuesday, but remain near normal for October. The coolest
temperatures remaining to the northwest of Kauai. This is in part
due to an upper level low that the models form to the northwest of
Kauai. The disconnect between the bulk of moisture to the southeast
and colder upper level temperatures to the northwest introduces an
uncertainty with the organization of this event.

What is certain and agreed upon with the models is that the
increase in moisture will bring an increase in rain chances, and
the possibility of locally heavy rainfall. Some heavy rain could
make it into the islands as early as tomorrow afternoon and
overnight, but the greater chance begins Monday. The grids have
the possibility starting Monday and at this time do not have
overwhelming evidence to change that. At this time, the threat
appears the highest Monday night. Will be monitoring future model
runs to evaluate the situation.

During the second half of next week, look for trades and trade
wind showers to gradually return.

&&

.AVIATION...
Pockets of showery clouds over and upstream of the state will
lead to occasional MVFR conditions, mainly across windward
sections. Brief IFR visibilities are also possible in heavier
showers. After an improving trend this afternoon, an additional
area of moisture over the windward offshore waters will enhance
showers and lead to an increase in MVFR conditions again tonight.
Have expanded the AIRMET for mountain obscuration to include all
windward sections, though if conditions continue to improve we
may be able to drop Maui County and Oahu.

AMDAR soundings from Honolulu and Kahului show up to 30 knots of
wind below a strengthening inversion. We have not received any
pilot reports with more than light turbulence near the islands.
However, with winds as strong as they are it remains prudent to
keep the AIRMET for moderate low-level turbulence in the lee of
the mountains.

&&

.MARINE...
High pressure to the northeast will maintain fresh to locally
strong trade winds through the weekend. As a result, a small craft
advisory remains posted for the typical windier areas near Maui
and the Big Island through Sunday. Winds and seas are expected to trend
down early next week as the high moves further away and winds
become more southeasterly.

The trades will continue to bring rough surf to east facing
shores into Monday...before trending down as the winds weaken. Elsewhere,
small surf is expected through the upcoming week. Surf along north
facing shores will likely build next weekend due to a gale that is
projected to develop over the northwest Pacific by Wednesday.


&&

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

&&

$$

DISCUSSION/MARINE...M Ballard
AVIATION...Bravender

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