Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 302039
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1039 AM HST Sun Apr 30 2017

.SYNOPSIS...
A strong upper level low just southwest of the islands will
interact with a weakening stationary frontal boundary near the Big
Island today. This will result in cool, rainy weather for the
smaller islands and a chance of thunderstorms mainly near the Big
Island. The upper low will weaken and slowly move away through
midweek, and the stationary front will weaken and push westward as
well. This will result in a gradual return to trade wind weather
by midweek.

&&

.DISCUSSION...
Lost the first discussion I crafted so will keep this one
relatively brief in comparison.

Threat for heavy rain and flooding will continue for the islands
from Oahu to the Big Island as deep moisture on the periphery of
an upper low brings band of heavy rain and embedded
thunderstorms. The heavy rain may be enhanced along and to the
east of a stationary frontal boundary near the Big Island where
instability is the greatest. This stationary front will gradually
push westward and weaken to a trough tonight and Monday and
continue moving away from the islands. In the meantime, the upper
low just to our SW will gradually weaken and drift away. Wind
fields aloft are relatively strong, so some of the thunderstorms
near the Big Island could be strong or locally severe, along with
strong winds at summit level. Also the cold air aloft combined
with deep layered moisture there could result in periods of heavy,
squally snow at the summits.

Tonight into Monday will feature a diminishing threat for heavy
rainfall, but deeper moisture undercutting the lingering cold air
aloft could still result in hefty downpours in bands and anchored
on terrain. Will likely need to extend the Flash Flood Watch for
some areas into tonight, but confidence beyond that drops quickly.
A warm, moist, humid airmass near and SE of the Big Island will
spread up the chain, in stark contrast to the very cool
temperatures over some of the smaller islands so far this morning.

Gradually warming temperatures aloft Tuesday and beyond will be
accompanied by a slow reestablishment of the subtropical ridge to
the N of the islands. Moist trades at gentle speeds should return
by Wednesday, becoming breezy and drier late in the work week.

&&

.AVIATION...
A compact but rather strong mid to upper level dropping southward
over Kauai will produce widespread MVFR conditions across the
eastern half of the island chain through much of the day. The
greatest amount of moisture is focused over the Big Island and
Maui, where isolated thunderstorms are are moving up from the
south. AIRMET Sierra for mountain obscuration will remain in
effect for the Big Island and Maui County through the day and
likely into tonight for some areas. A shield of mid level level
clouds with bases around 080 will continue to generate light to
moderate rain over Oahu through much of the day. AIRMET Sierra of
mountain obscuration does not extend to Oahu, though we will
closely monitor through the day. On Kauai, only isolated MVFR
conditions are expected.

The low is also producing areas of turbulence. Moderate to
isolated severe turbulence above FL240 is expected along the
western and northern flank of the upper low today, including over
and around Kauai. Moderate low to mid level turbulence will occur
across the rest of the island chain, in addition to areas of
icing.

&&

.MARINE...
The forecast has been updated this morning to include a Small
Craft Advisory (SCA) for the Alenuihaha Channel and over the
waters to the east and west of the Big Island. The surface
boundary is near Maui County, with some strong and gusty southerly
winds over portions of these areas. As the day wears on, the
winds are expected to turn to the north and weaken. Will be
continuing to evaluate the need for an SCA beyond today with this
uncertain weather situation.

Thunderstorms remain a possibility through tomorrow night over
the waters of Maui County and the Big Island. The most unstable
airmass, combined with the necessary moisture, is found over the
waters near the Big Island, where thunderstorms have the greatest
chance for gusty winds, small hail and frequent lightning today.
Will be continuing to monitor the situation to see if that threat
expands either time wise or area wise.

Another front is forecast to pass north of the waters Tuesday
through Wednesday, with strong high pressure expected to build
north of the waters Wednesday through Thursday. This should return
a more typical trade wind pattern to the region for the middle to
latter part of the week.

Higher than normal high tides are expected to continue over the
next couple of days. The most recent south swell is on the
decline, but another south swell will help to increase the total
water levels over the next several days. This incoming bump on the
south shore is expected to arrive tonight, hold into the middle
of the week near advisory levels, and then trend downwards. The
greatest chance for beach inundation is during the evening hours
when the high tides will be their highest. The higher than normal
high tides are expected to return to more normal levels during the
second half of the week.

Rough surf along the north facing shores, particularly over Kauai
and Oahu, are the result of the fresh northerly winds and a short
period northerly swell. The winds and surf will trend downwards
this week. A couple of small, long-period, northwest swell will
impact north facing shores this week. The first one is forecast
to fill in late Monday and hold through Tuesday night. The second
source is expected to arrive Wednesday and hold into the second
half of the week.

As high pressure builds north of the regions during the second
half of the week, expect a trade wind surf to build along east
facing shores.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through this afternoon for Oahu-Molokai-Lanai-
Kahoolawe-Maui-Big Island.

High Wind Warning until 6 AM HST Monday for Big Island Summits.

Winter Storm Warning until 6 PM HST this evening for Big Island
Summits.

&&

$$

DISCUSSION...R Ballard
MARINE...M Ballard
AVIATION...Wroe



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