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Hawaiian Islands Synoptic Discussion and Guidance

235
FXHW60 PHFO 231326
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
326 AM HST Mon Sep 23 2019

.SYNOPSIS...
A band of deep tropical moisture will move across the islands
today, bringing a chance for some locally heavy showers and even
the possibility of a thundershower or two. This moisture will
push west of the state on Tuesday, before stalling and pushing
back to the east on Wednesday and Thursday. An upper level
disturbance may put this moisture to work, leading to a few
downpours over the western end of the chain. For the most part,
winds will be rather light this week. A gradual shift toward
wetter than normal trade winds is expected for the weekend.


&&

.DISCUSSION...
The upper level low responsible for the uptick in showers (and
even a couple of thundershowers) Sat night and Sunday is moving
away and weakening to the W of the islands. In the wake of this
feature, a relatively narrow swath of deep tropical moisture is
moving across the islands with precipitable water values at or
above 2 inches from Oahu to the Big Island. The moisture hasn't
been particularly active, however, owing to the rising mid-level
heights and temperatures over the main Hawaiian Islands as the
upper low departs. There are a couple of locally intense showers
out there early this morning over the coastal waters, though, as
the moisture continues to interact with the flow around the island
terrain. With high pressure far to our NE, the local low level
background flow is gentle SE, with land breezes prevailing in most
spots early this morning.

Models show some hint that the flow may try to turn a little more
easterly later today, which could allow for locally breezy trades
in the typically windier spots this afternoon. Otherwise, gentle
sea breezes will dominate. Although upper support is relatively
weak, we should be able to pop a few brief downpours over interior
and leeward sections this afternoon, as well as a thundershower or
two once again. Best chance for any thunder would seem to be over
portions of interior and leeward Big Island, but hard to
completely rule out over the interior or lee sections of the
smaller islands.

Any trade wind relief we are able to manage today will be short-
lived. The surface high to the NE will move even farther away
tonight, and the background flow turns SE again by Tue. A weak NE-
to-SW oriented surface ridge builds over the islands for the
latter half of the work week, keeping light background winds from
the SE or even S. Models take the weak ridge to the NW of the
islands. If that holds true, it would allow gentle trades to
gradually return over the weekend.

Models take the axis of deepest moisture to Kauai by tonight,
with drier air east of the Big Island trying to push in from the
E through Tue. As the low level flow slows down once again, the
deeper moisture is expected to stall just W of Kauai Tue night,
and then begin sloshing back E over Kauai and possibly Oahu on Wed
and Wed night. Guidance suggests that a weak mid level shortwave
may activate this moisture band bringing a chance for heavier
downpours and even a couple of thundershowers once again. Highest
chance would be near and around Kauai, with a lesser risk around
Oahu. The band should then weaken and become less active during
the latter part of the week as the mid-level energy weakens.
Nevertheless, as the flow becomes more trade-like again next
weekend, moisture levels are expected to remain above normal so
somewhat wet trades are expected.

&&

.MARINE...
No significant changes to the marine elements of the forecast this
morning. Moderate to fresh east-southeast winds are expected to
make a brief return later today and last into tomorrow. Winds
are expected to near Small Craft Advisory (SCA) levels in the
typical windier areas near Maui County and the Big Island. An
overnight ASCAT pass showed 15 to 20 kt winds in these areas. Will
continue to monitor for the need for an SCA should winds exceed
advisory levels.

Winds will once again be disrupted by midweek in part due to a
front developing to the northwest of the islands. This will
maintain a east-southeast flow with winds weakening to light to
moderate levels.

A series of small swells from the southwest and south are
expected through the week. A new moderate period south-southwest
swell is due today, and will be followed by a larger long-period
swell later in the week. This latter swell may reach advisory
levels along south facing shores late Thursday into Friday. Surf
along north facing shores is expected to steadily trend down today
as that swell subsides. A few other small to moderate northwest
swells will be possible towards the end of the week from the low
pressure system developing northwest of the state over the next
few days.

&&

.AVIATION...
An area of moisture moving in from the east on a renewed trade
flow is producing areas of MVFR conditions in lower ceilings and
showers, mainly over windward and mauka sections. Lingering
instability has also been enhancing some of the showers over the
coastal waters this early morning. With the atmosphere remaining
somewhat unstable with a nearby upper trough west of the islands,
pockets of heavy downpours are possible during the day, with a
slight chance of thunderstorms in the afternoon over the Big
Island.

Trade winds will be in the light to moderate range, with some
locations seeing wind speeds at breezy levels by late morning and
into the afternoon. Trades may veer east-southeast at times as
well.

An AIRMET for mountain obscuration has been posted for north thru
east sections of Maui and the Big Island. More isles may need to
be added later.

&&

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

&&

$$

DISCUSSION...R Ballard
MARINE...M Ballard
AVIATION...Kinel

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