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Hawaiian Islands Synoptic Discussion and Guidance

644
FXHW60 PHFO 270657
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
857 PM HST Wed Jun 26 2019

.SYNOPSIS...
Low pressure northwest of the islands will maintain a unsettled
weather pattern over Kauai through at least Thursday, while a
mainly southeast wind flow will bring a few showers into the
windward and mountain areas of the remaining islands. As the low
moves away late Thursday and Friday, drier conditions will move in
from the east, and a somewhat less humid, more seasonable trade
wind weather pattern should prevail by the weekend.

&&

.DISCUSSION...
Although the models are suggesting a somewhat quiet evening
ahead, we will remain vigilant, as the air mass remains moist and
unstable across the western part of the island chain. The Kauai
radar has started picking up newly developed showers in the past
hour, with the southerly winds carrying them on to island's
southern coast. Oahu is rather quiet so far, compared to 24
hours, where the island got drench, along with a couple of
injuries from lightning strikes. The same goes for the other
islands, with a couple of showers moving across Maui.

The focus of the instability will be or has shifted to Kauai from
Oahu. This is due in part of a surface low, located 255 mies west
northwest of Kauai as of 7 pm HST. The low is moving northward,
but is dragging a trough with it that will remain just west of
Kauai through at least Thursday. This trough will help maintain
the unstable southerly winds across Kauai County through Thursday.
The upper level low northwest of Kauai will be shifting northward
as well, and in due course, the supporting cyclonic flow aloft
flattens out about Friday night and Saturday. With this said, the
latest GFS guidance still has Kauai under some lingering
instability as of Friday afternoon. The newly arrived ECMWF is
also suggesting the same. At this point, the Flash Flood Watch for
Kauai, that ends late Thursday afternoon, could be extended. But
we will how things transpire in the hours to come, for Kauai has
been pretty quiet thus far today.

The current surface wind, area wide, is south across the smaller
islands and east-southeast across the Big Island, thus maintaining
a humid airmass across the area. The winds will be trending to
trades starting Thursday evening, and through Friday afternoon,
with Kauai being the last to feel the trades. Surely by Saturday
afternoon, all islands will be under a more comfortable and stable
trade wind pattern, mainly in the moderate range. This trade wind
pattern should last through the rest of the weekend and into the
first half of next week. The trades may be breaking down once more
during the second half of next week as a upper level trough forms
over the islands.


&&

.AVIATION...
A north to south trough will remain west of Kauai for at least the
next 24 hours. Layered clouds, frequent showers and isolated
thunderstorms associated with the trough will remain over Kauai,
Niihau and surrounding waters. There will be isolated to scattered
showers from Oahu eastward.

AIRMET Sierra is currently in effect for Kauai for mountain
obscuration from clouds and showers.


&&

.MARINE...
A persistent complex low NW of the islands has spawned a surface
low, which is currently centered about 250 nm WNW of Kauai, moving
NE at 10 kt. Meanwhile, a surface high centered to the distant NE
has a ridge that extends SW over waters near the Big Island. Deep
southerly flow to the E of the low continues to bring abundant
moisture northward over waters around Kauai and Oahu, while
mostly stable E to SE winds prevail near the ridge axis. The deep
moisture will combine with instability provided by a low aloft to
bring bouts of heavy showers, and a slight chance of thunderstorms,
mainly to waters around Kauai and Niihau. While weather has been
somewhat benign over most of the area over the last 12 hours or
so, guidance continues to indicate one or more convergence bands
developing near Kauai later tonight into Thursday, possibly
lingering into Friday.

As the low to the NW gradually moves away Friday into Saturday,
the ridge will move N, and the stable trade wind weather prevailing
near the Big Island will spread westward. Over the weekend, the
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 wind speeds prevailing.
Wind speeds may begin to diminish on Monday as the ridge N of the
area weakens.

Surf along all shores is expected to remain well below High Surf
Advisory (HSA) criteria through Friday. However, a new long-
period SSW swell arriving over the weekend is expected to reach
HSA criteria along exposed S facing shores, with the peak of the
swell expected around Sunday. Swell energy arriving from the S
Pacific will continue to support some 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 Thursday afternoon for Niihau-Kauai.

&&

$$


H Lau/Donaldson/Birchard

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