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fxhw60 phfo 200636 

Area forecast discussion
National Weather Service Honolulu HI
836 PM HST Thu Sep 19 2019

island weather will trend wetter through the weekend, with
diminishing trade winds and increased moisture also making it feel
quite muggy. The chances for a few locally heavy showers will
increase as well, as low pressure in the upper atmosphere develops
over the islands. A trend toward more settled trade wind weather is
in the forecast for the middle of next week as the low moves away,
but trade wind speeds will likely remain on the lighter side.


in the short term, a somewhat dynamic situation is in place, as a
sharp trough in the mid- to upper-atmosphere is quickly digging
toward the island chain from the north. Water vapor nicely
highlights this feature, currently about 400 miles north of Kauai,
and moving south near 30 mph. It will bring increasing layered high
clouds to all islands overnight, while also helping to carve out a
cut-off closed low aloft Friday night into Saturday. This low is
expected to develop just northeast of the islands, then drift slowly
west and northwest into early next week before dissipating and
moving north. Unclear how much instability this trough and
developing low will bring in the short term, but a few locally heavy
downpours can't be ruled out overnight into Friday as divergence
aloft associated with the trough moves from north to south across
the islands. With moderate to locally breezy trade winds in the low-
levels supplied by a surface high to the distant northeast, the most
likely area for a downpour will be windward areas, while the trades
will also keep showers from sitting over any one area for too long.

A brief period of convergence aloft/summat increased stability
(and decreased high clouds over the western islands) is highlighted
by latest model guidance as the upper trough stalls over the islands
during the day Friday, and before the low aloft consolidates. From
Saturday into Tuesday, the low will move slowly west, and will
induce the development of a low-level trough over the area that will
weaken the low-level gradient, leading to light winds that will
favor an east to southeast direction. Layered high clouds south and
east of the low's center will spread from The Big Island to the
other islands by Sunday and Monday, and periods of considerable
cloudiness are possible.

500 mb temperatures are forecast to drop 4-5 degrees c through the
weekend (bottoming out near -10c), as the low aloft moves directly
over the island chain. Although only about one Standard deviation
colder than normal, given the unusually warm water temperatures near
the islands and the expected increase in low-level moisture, locally
heavy showers and thunderstorms will be possible, and may need be
added to the forecast overnight. Difficult to pinpoint exactly where
and when these would occur, but the diminished winds may allow sea
breezes to drive heavy shower development over leeward and interior
areas during the afternoons and evenings, while the light east to
southeast winds deliver windward showers that would favor nights and
mornings. The peak of the instability and deep moisture is expected
Sunday and Monday, so those with weekend outdoor plans should plan
accordingly. A Flash Flood Watch may be required at some point, but
timing and confidence do not support its issuance at this time.

The low aloft will move west of the islands Tuesday before being
drawn northward into the circulation of a new low aloft that is
forecast to approach from the distant northwest by the middle of
next week. This appears to allow a mid-level ridge to build over the
islands from the northeast, bringing a more settled weather pattern,
although trade wind speeds look to remain light as a surface ridge
will be just north of the area. Another low aloft may move toward
the islands from the northeast by the end of next week as light to
moderate trade winds prevail.


trade wind flow continues, but a low northwest of the state will
bring an uptick in clouds and showers to the island chain for
Friday. Mid and high level clouds associated with this low are
beginning to fill in over the state from west to east. Airmet Zulu
for icing may be needed later on tonight.

Brief periods of MVFR conditions are possible through tonight for
windward areas, especially The Big Island, but overall, VFR should


fresh to locally strong, easterly trade winds will continue through
Friday. A Small Craft Advisory remains in effect for most of the
typical windy areas around Maui and The Big Island. Winds will
gradually weaken into the weekend as a surface trough approaches
from the east. The trough will increase showers starting Saturday,
as well as shift winds out of the southeast by Sunday, allowing sea
and land breezes to develop over leeward areas.

A series of small swells from the southwest and south are expected
through the rest of this week. A small northwest swell is possible
late in the weekend into early next week. A small east swell from
tropical cyclone Kiko will persist into the weekend.


Hfo watches/warnings/advisories...
Small Craft Advisory until 6 PM HST Friday for Pailolo Channel-
Alenuihaha Channel-Big Island leeward waters-Big Island
southeast waters.



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