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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 280702

Area Forecast Discussion
National Weather Service Honolulu HI
902 PM HST Mon Feb 27 2017

A period of unsettled weather is upon the islands this Monday
evening. Spotty heavy showers and thunderstorms will focus on the
windward and mountain areas tonight, but will become widespread
Tuesday and Tuesday night as an upper level low forms over Kauai.
This upper low will begin lifting northeastward Wednesday and
Wednesday night that will lead to fewer showers across the area
espousal on Friday. However, a southerly wind flow will keep the
islands under a humid and voggy air mass until a front arrives
over the weekend that will clear out these elements.


Maui has been the hardest hit so far this evening, warranting a
flash flood warning for the eastern side of the island, east of
Kahului to Makena line. Radar is estimating up to 5 inches of
rain has fallen over the upper slopes above Hana up until 7 pm.
The heavy showers remains active over other parts of Maui as
this time. Oahu has seen a couple of pulses of heavy showers
especially across the Koolau range. A heavy shower did popped up
in the Makakilo and Nanakuli area likely as a result of day time
heating and light winds. A thunderstorm is in progress over the
interior area of Kauai. We are closely monitoring the rain and
river gages on the Garden Island given the Kauai radar is down
for extended repairs and maintenance.

We have a Winter Storm Watch from Tuesday morning through Wednesday
afternoon for the Big Island summits. The GFS points to at least
advisory conditions as of now. We will have a bulletin out with the
4 am HST forecast package.

The low level flow this evening is from the ESE. It is expected
to remain out of this direction through tonight and into Tuesday
morning. The wind speeds ranged from moderate to strong over the
exposed waters around the Big Island to light to moderate around
Kauai. Expect the winds to become SE over the area west of Maui
Tuesday afternoon while a light to moderate easterly wind flow
covers the Big Island. The winds will continue veering to southerly
area wide Tuesday night and early Wednesday morning at light to
moderate speeds.

As the upper low pulls northeastward further away from the islands
on Thursday, both the ECMWF and GFS models are suggesting some
lingering moisture to affect Maui and Big Island counties.
Therefore, the Flash Flood Watch (FFA), which is good to until
6 pm HST Wednesday, could be extended through Thursday.

The prevailing winds after the heavy rains have abated, that is
on Friday, will continue to be out of the south. This is due to
a broad surface low setting up shop just NW of Midway Atoll.
A front associated with the low will swing eastward and reach Kauai
late Saturday afternoon or evening. The front may take its time
eastward which may raise some concern for flooding potential.
This is quite a ways out, however, and the models may have a
different picture. So, I will leave it at that.


Unsettled conditions will continue across the island chain over
the next 24 hours as an upper level trough approaches from the
west. Widespread MVFR conditions and isolated IFR conditions in
SHRA will continue to impact mainly east facing portions of the
islands through the TAF forecast period. Reduced cigs/vsbys may
occasionally impact leeward areas through Tuesday morning, with
deteriorating conditions then expected in these areas Tuesday
afternoon and especially Tuesday night. Additionally, due to the
unstable airmass in place, thunderstorms will be possible through
the next 24 hours as well.

AIRMET Sierra is in effect for east facing slopes of Kauai, Oahu,
Molokai, Maui and the Big Island. These conditions are expected to
continue through the night.

AIRMET Tango is in effect for moderate upper level turbulence from
Molokai to the Big Island between FL250 and FL400. The AIRMET may
need to be expanded to include the rest of the island chain later

AIRMET Zulu is in effect for moderate icing from Molokai to the
Big Island between 150 and FL200. The AIRMET may need to be
expanded to include the rest of the island chain later tonight


The current WNW (300-320 deg) swell will ease for the next couple
of days with surf remaining below high surf advisory levels. A
small reinforcement out of the WNW (300 deg) associated with a
recent gale over the far NW Pacific is forecast to fill in Tue
through mid week.

A moderate NE swell (030 deg) associated with a recent gale
centered a thousand NM northeast of the islands will remain in
place through Tue night before easing. This swell will bring near
advisory-level surf to select E facing shores.

Moderate to strong winds out of the ESE direction will continue
through tonight for exposed waters around the Big Island and the
Pailolo Channel. A Small Craft Advisory (SCA) is in place for these
waters to until 6 AM HST Tuesday. The models have these winds
subsiding to below SCA by noon Tuesday, so there is a possibility
the SCA will be extended to noon Tuesday for the Alenuihaha

For the extended, models depict a gale- to storm-force low
developing a few hundred miles WNW of Midway Atoll Wed through Thu,
then tracking toward the NNE across the Date Line Thursday.
Along the way, the low will spit out a large WNW swell toward the
islands that may result in advisory-level surf over the weekend.


Flash Flood Watch through Wednesday afternoon for all Hawaii

Winter Storm Watch from 6 AM HST Tuesday through Wednesday
afternoon for Big Island Summits.

Small Craft Advisory until 6 AM HST Tuesday for Pailolo Channel-
Alenuihaha Channel-Big Island Windward Waters-Big Island Leeward
Waters-Big Island Southeast Waters.



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

This page is maintained voluntarily by the MKWC and the UHMET faculty, staff, and students.
It was last modified on: Wed, Dec 17 2014 - 1841 UTC
Send comments to: Ryan Lyman