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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 210150

Area Forecast Discussion
National Weather Service Honolulu HI
350 PM HST Fri Sep 20 2019

Island weather will trend wetter through the weekend, with
diminishing trade winds and increased moisture also making it feel
quite muggy by Sunday. The chances for a few locally heavy showers
will increase as well, as a low pressure system in the upper
atmosphere passes near the islands. Light winds may continue for
most of next week.


Have made some tweaks to the forecast primarily with winds in the
longer range and PoPs and associated fields in the short term. The
back end of the high clouds is over Maui County this afternoon, so
have cleared out some of the PoPs and thus the clouds over the
leeward waters for this evening. Still seeing some showers
upstream of the showers, and expecting more tonight, with the land
areas seeing more showers than the waters. Anticipating the high
clouds to move back to the west this weekend as the upper level
trough moves west. The latest ECMWF model backed off on wind
speeds for the Monday to Wednesday time period, more in line
with the GFS. As such knocked the winds down a bit during that
time period to better align with the global models.

General forecast philosophy hasn't changed. The upper level trough
over the central islands continues to bring high clouds to the
east islands. Afternoon soundings from Lihue and Hilo had 1.52 and
1.62 inches of precipitable water (PW), and 500 mb temperatures
of -7 and -6 celsius, respectively. Normal values for September
would have PW around 1.4/1.5 and 500 mb temperatures of -6
celsius. Models continue to show 500 mb temperatures will cool
further over the next 24 hours or so, before beginning to rebound.
PW will hold relatively stable for the next 24 hours or so, before
increasing Saturday night and Sunday.

Thunderstorms remain in the forecast for the Big Island Saturday
afternoon and evening mainly as a result of the colder 500 mb
temperatures. Will be monitoring the need to extend or expand the
area of thunderstorms.

The upper level trough is expected to move to the west through
Sunday and then start to move to the north late Monday and
Tuesday. Showers will continue to be possible statewide through
at least Monday with the presence of the upper trough, and the
surface trough that is expected to develop. The unstable airmass
will help to enhance showers so expect some heavy showers mainly
Sunday and Monday. As the trough moves to the north, it will merge
with a new system to the northwest of the islands that looks to
maintain some convergence over and near the islands for the first
half of the week. This would continue a wetter pattern over the

As mentioned above, the winds Tuesday and Wednesday have been
trended towards a lighter wind pattern in line with the latest
ECMWF and GFS model runs.


A surface high far NE of the area will maintain locally strong
low level east winds over the area through Saturday. Winds are
expected to subside early next week. The low level winds will
push some clouds and showers over windward (east) facing sections
of the islands and produce ISOL MVFR ceilings and visibilities.
VFR conditions will prevail over most leeward areas, but afternoon
sea breezes may produce ISOL MVFR ceilings over the southwest
(leeward) slopes of the Big Island of Hawaii and Maui. There will
be a slight chance of thunderstorms and heavy showers tomorrow on
the Big Island.

A northeast to southeast trough aloft over the Kauai channel is
moving southeast around 15 mph. A low within the trough about 300
nm northeast of Maui is moving southwest. A band of layered
middle and high clouds associated with the trough blankets Maui
County and the Big Island. The trough is producing light to moderate
turbulence aloft over the area and light icing has been reported
in the layered clouds.

AIRMET TANGO is in effect for the moderate turbulence. Light
icing (below AIRMET intensity) is noted in the AIRMET bulletin.


High pressure far north-northeast of the state will maintain fresh
to locally strong northeast trades through Saturday. Starting
Saturday night, a trough passing just south of the state will
bring increasing showers, and a decrease in wind speeds. Winds are
expected to decrease to light to moderate speeds by Sunday and
veer to the east-southeast. Sea and land breezes will be possible
on Sunday especially along the leeward coast. Models are currently
indicating fresh east to east-southeast winds returning on Monday.
Beyond Monday, models are indicating a front developing northwest
of the state. There are still uncertainties on how strong and how
close the front will develop. Stay tuned for more updates.

The Small Craft Advisory for most of the typical windy areas
around Maui and the Big Island remains in-effect through Saturday
and is expected to drop below advisory levels by Saturday night.

A series of small swells from the southwest, west-southwest and
south are expected through the rest of this week. A small
northwest swell is expected late Saturday into early next week.

Looking out into next week, a strong storm tracked west to east
south of New Zealand centered around 55S over the last several
days. This storm is expected to track north with a captured fetch
along the great circle route to Hawaii today through Saturday.
This should bring a prolonged south swell starting mid next week
with surf potentially reaching advisory levels towards the end of
next week.


Small Craft Advisory until 6 PM HST Saturday for Pailolo Channel-
Alenuihaha Channel-Big Island Leeward Waters-Big Island
Southeast Waters.




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

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It was last modified on: Wed, Dec 17 2014 - 1841 UTC
Send comments to: Ryan Lyman