Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 06/28/19

Area Forecast Discussion

National Weather Service Honolulu HI

 336 PM HST Thu Jun 27 2019

.SYNOPSIS...
Low pressure west of the state will maintain wet and unsettled
weather over Kauai into Friday. Across the rest of the island
chain, east to southeast winds will produce showers across
windward slopes, as well as a few afternoon showers over leeward
terrain. As the area of low pressure weakens late Friday, drier
conditions will gradually move in from the east, and a less humid
and more seasonable trade wind weather pattern will prevail from
Saturday into early next week.

&&

.DISCUSSION...
A moist and unstable southeast to south flow will remain in place
over Kauai into Friday. A broad and persistent area of low
pressure at the surface and aloft is holding over and just west of
the islands, and an embedded surface low within this area is
currently located about 375 miles northwest of Kauai. An area of
deep moisture convergence associated with this low has been
focused just west of Kauai since last night, but intermittent
heavy showers with rain rates of mainly 1 to 2 inches per hour
have been moving up over the Garden Isle during the much of the
day. Very high precipitable Water (PW) values of around 2.25
inches will hold in this area into mid day Friday, and the GFS and
ECMWF models are showing a shortwave aloft passing overhead
around daybreak Friday. Additional heavy showers and thunderstorms
can be expected across already saturated ground, with the main
time of concern being late tonight and early Friday morning as the
shortwave moves through. The Flash Flood Watch has been extended
into Friday afternoon.

A slightly drier, though still humid, and more stable east to
southeast flow will persist from Oahu to the Big Island into
Friday. The low level flow remains moist, as PW drop only to
about 1.5 inches at Hilo, and recent aircraft data and the
afternoon Hilo sounding show a weak inversion between 4,000 and
7,000 ft. Modest showers have been focused along east and
southeast facing slopes of the Big Island and east Maui, and a
weak plume off the Big Island produced a few showers on Oahu
today. Expect showers to continue to favor east and southeast
slopes through Friday. A few showers will flare over interior and
leeward sections during the afternoon hours, especially on the Big
Island. It will remain humid, since dew points will hover in the
low to mid 70s.

A more stable easterly trade wind flow is expected for the weekend
and into early next week. The area of low pressure to the
northwest of the state will weaken and shift to the west. This
will allow surface high pressure northeast of the islands to build
westward, leading to easterly trade winds over the state. A ridge
aloft will nose in from the east, bringing a drier and more stable
air mass. As a result, expect a typical rainfall pattern focusing
showers over windward slopes. Dew points will drop closer to
seasonal levels (upper 60s), bringing relief to the humidity.

A decrease in trades is looking increasingly likely by the middle
of next week. Broad low pressure will once again develop north of
the islands around Wednesday. This will weaken the high driving
the trades, causing winds across the state to decline.

&&

.AVIATION...
A low pressure trough will remain west of Kauai through the
period. This feature will generate instability and maintain thick
cloud cover and showers across the western end of the state. In
addition, a slight chance of thunderstorms will exist for areas
west of Oahu. These storms may bring occasional IFR conditions...
ceilings below 1500 ft and/or vis below 3sm to Kauai. Elsewhere,
expect isolated to scattered showers with a light southeasterly
background flow.

AIRMET SIERRA has been issued for Kauai above 1500 ft due to
mountain obscuration from clouds and showers.

&&

.MARINE...
Latest satellite and radar imagery is showing a line of showers
developing over the waters around Kauai. Over the next 24 hours,
showers will continue to develop across the same area with
a chance of thunderstorms possible around the waters of Kauai
tonight and Friday. The unstable weather near Kauai is due to a
weak surface low centered about 330 nm WNW of Kauai. This low is
slowly moving west and away from the state. Meanwhile, a surface
ridge near Molokai is providing stable weather across Big Island
and Maui waters.

As the low to the WNW of Kauai gradually moves away through
Saturday, the ridge will move N of the islands, and the stable
trade wind weather prevailing near the Big Island will eventually
spread to all waters. Winds accelerating around the Big Island
will be strong enough to warrant a Small Craft Advisory, which
will go into effect tonight, and continue through the weekend.
The returning trade wind flow will be veered to the E or ESE
(especially on the Kauai end of the island chain) as a trough will
linger to the distant W of the islands, with moderate to locally
fresh wind speeds prevailing. Wind speeds may begin to diminish
Monday/Tuesday as the ridge weakens.

Surf along all shores will remain below High Surf Advisory criteria
through Friday. A long-period SSW swell arriving over the weekend
is expected to bring high surf to S facing shores of all islands,
with the peak of the swell expected around Sunday. Latest readings
from the Pago Pago buoy is running higher than guidance, which
increases the confidence for advisory level surf late this
weekend into Monday. Several smaller reinforcements arriving from
the S Pacific will continue to support small to moderate surf
along S facing shores into next week. Otherwise, the only other
surf source will be a modest increase in short-period wind waves
on E facing shores the next couple of days.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through Friday afternoon for Niihau-Kauai.

Small Craft Advisory from 6 PM this evening to 6 PM HST Sunday
for Alenuihaha Channel-Big Island Leeward Waters-Big Island
Southeast Waters.

&&

$$

DISCUSSION...Wroe
AVIATION...JT
MARINE...Kino