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Hawaiian Islands Synoptic Discussion and Guidance

117
FXHW60 PHFO 170202
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
402 PM HST Mon Sep 16 2019

.SYNOPSIS...
A return of drier air and more stable conditions is anticipated
from east to west Tuesday through Thursday. Increasing moisture
and rain chances will become a possibility once again over the
upcoming weekend.

&&

.DISCUSSION...
At the surface, a 1025 mb high is centered about 1400 miles north
of Hawaii. A passing trough and front to the northwest between
the islands and the high is allowing for the otherwise moderate to
fresh trade winds to veer east southeasterly across the smaller
islands this afternoon.

Water vapor imagery shows an mid to upper low centered about 600
miles north northwest of Kauai and moving away from the area.
Southwest winds along the southeastern flank of the low is
currently drawing scattered to broken mid and high level clouds
across the state as shown on visible satellite. A few low clouds
are anchoring along the windward slopes as well. MIMIC Total
Precipitable Water (PW) shows a swath of PW near 2 inches across
the central portions of the state, but 00Z soundings indicate
values at 1.5 to 1.7 inches at Hilo and Lihue.

The mid and high level clouds kept showers from popping up like
yesterday, but there's still a small chance for leeward/interior
showers into early this evening. Drier and more stable air should
begin to funnel in with the upstream trades tonight.

Over the next couple of days, we will be watching for development
of a tropical disturbance southwest of Hawaii. Both GFS and ECMWF
show a low forming and immediately moving north as an upper low
far to our northwest digs southward. While neither model shows an
impact to the main Hawaiian Islands, the synoptic setup may keep
us in a similar east southeast flow pattern through the next
several days, but will depend on how this system develops.
Upstream of Hawaii, drier and more stable air will funnel in so
clouds and showers should be kept to a minimum.

Despite some differences between the various model solutions, a
return of light winds and increasing moisture (two inch PWs) will be
possible Friday night into next weekend as a trough associated
with a tropical disturbance to the south moves through. Added
instability associated with an upper low setting up over the state
combined with this moisture will support increasing rainfall
chances.


&&

.AVIATION...
High clouds continue to stream up over the state from the
southwest, especially over Oahu this afternoon. Despite the
elevated moisture, shower activity has been more scarce than
anticipated much of the day. Some scattered showers can still be
expected for the rest of the day and into tonight. Winds continue
to have more of a east-southeasterly component to them, but should
turn back towards a more typical trade direction through
tomorrow. A drying trend with increasing trade wind speeds is on
tap as we head into Tuesday thanks to the upper low north of Kauai
moving away from the state.

AIRMET Zulu remains in effect for light icing 160-FL280.


&&

.MARINE...
High pressure far north of the state is expected to drive
moderate trade winds across the area as it slowly moves east
through the week. An ASCAT pass from late this morning shows winds
have dropped below advisory threshold and we have allowed the
Small Craft Advisory (SCA) to expire this afternoon. However,
winds near advisory threshold are expected into Tuesday.

A series of small swells from the southwest, south, and southeast
is expected this week. The largest of these swells looks to be
towards the end of the week as a combination of a long-period
south-southwest swell and potential short-period southwest swell
moves in across the area. Surf heights are expected to be below
advisory levels with these swells.

Small surf is expected along north, east and west facing shores
through the new week. Expect a small east swell from Tropical
Cyclone Kiko as early as Wednesday along exposed east facing
shores and likely continuing through the weekend. Exposed west
shores may see some of the aforementioned southwest swell on
Friday. A small northwest swell will be possible late in the
weekend into early next week.


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$


DISCUSSION...Foster
AVIATION...Dye
MARINE...Powell

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