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Hawaiian Islands Synoptic Discussion and Guidance

739
FXHW60 PHFO 250652
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
852 PM HST SUN JUL 24 2016

.SYNOPSIS...
Tropical Storm Darby will continue to bring unsettled weather to
much of the state overnight as it tracks northwestward near or
over Kauai. Improving conditions are then expected to spread
across the island chain from east to west on Monday as Darby
departs. More typical trade wind weather will then return for the
remainder of the work week as high pressure builds well northeast
of the Aloha state.

&&

.DISCUSSION...
The 8 pm HST advisory from the Central Pacific Hurricane Center
(CPHC), indicated that Tropical Storm Darby was located 20 miles
east-northeast of Lihue. Infrared satellite imagery shows cloudy
or mostly cloudy skies in place across the state, with radar
imagery showing some intense bands of showers and thunderstorms
primarily impacting Oahu. Main short term concern for tonight
revolves around the potential for heavy rainfall and flash
flooding.

Rest of tonight...
The latest track from CPHC takes Darby northwestward near or over
Kauai to a location around 125 miles northwest of Kauai by 8 am
Monday. Deep southerly flow on the eastern side of Darby will
continue to transport tropical moisture with precipitable water
values in excess of 2.0 inches across the island chain through the
overnight hours. The combination of moisture along with large
scale synoptic forcing provided by an upper level trough to the
northwest of the state and the right rear quad of an upper level
jet will promote numerous to widespread showers and thunderstorms
overnight. These features will best align over central and
western portions of the state, particularly Oahu and Kauai, where
the potential for locally heavy rainfall and flash flooding will
continue. Deep layer ridging will begin to build westward over the
Big Island overnight, so conditions are expected to improve here.

Monday through Friday night...
Darby will continue to track northwestward and away from the Aloha
state, with conditions steadily improving from east to west during
the day on Monday. As high pressure builds well northeast of the
island chain, a more typical trade wind pattern is expected
through the remainder of the week, with clouds and showers
focused primarily over windward and mauka locales.

Next Weekend...
Model guidance is in good agreement showing the trade wind flow
moistening up in response to a lowering of upper level heights
over the state. There is also some indication that moisture
associated with the remnants of what is now Hurricane Georgette in
the eastern Pacific, will get caught up in the trade wind flow. As
a result, there should be an increase in trade wind showers
across the state, with the majority of the activity impacting
windward and mauka locales.

&&

.AVIATION...
Tropical Storm Darby continues to bring clouds and rain to most
areas of the state, with the heaviest rainfall with thunderstorms
over Oahu this evening. Tropical SIGMET Whiskey remains in effect
for Darby which covers most of the state.

AIRMET Sierra is in effect for IFR conditions across Kauai and
Oahu, and mountain obscuration for the rest of the state. AIRMET
Sierra is likely to remain up most of the night.

AIRMET Tango continues for both mechanical turbulence downwind of
the mountains on all islands, and for strong surface winds over
Kauai and Oahu.

Darby is expected to continue a northwest track, so expect
clearing from east to west over the next 24 hours.

&&

.MARINE...
There will continue to be rainbands and pockets of deep
convection associated with Darby, and any of these could result
in strong gusty winds, intense rainfall, low visibilities, and
frequent cloud-to-surface lightning. Although the wind field will
continue to gradually diminish, seas will continue to be elevated
for awhile longer. Considering all of this, a Tropical Storm
Warning remains in effect for some of the coastal waters, in
deference to the CPHC warnings up for the adjacent land areas.

Surf along east facing shores will remain elevated but should
continue to decline as Darby pushes through the leeward waters.
The shorter fetch downwind from the islands will result in a bit
of a bump on some south facing shores, but likely not enough for
surf headlines.

The returning trade winds will probably be strong enough for Small
Craft Advisories (SCA) for the usual windier marine areas and
channels (starting near the Big Island), possibly as soon as
Monday night, and even more likely on Tuesday. Will keep an eye on
this and will probably need to fire out SCA later tonight or
Monday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through Monday morning for Oahu.

Tropical Storm Warning for Niihau-Kauai-Oahu.

Flash Flood Watch through Monday afternoon for Niihau-Kauai.

High Surf Advisory until 6 AM HST Monday for Kauai Windward-Oahu
Koolau-Olomana-Windward Haleakala.

Flash Flood Watch until 6 AM HST Monday for Molokai-Lanai-
Kahoolawe-Maui.

Tropical Storm Warning for Kauai Northwest Waters-Kauai Windward
Waters-Kauai Leeward Waters-Kauai Channel-Oahu Windward Waters-
Oahu Leeward Waters-Kaiwi Channel.

&&

$$

DISCUSSION/MARINE...Jelsema
AVIATION...Foster

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