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Hawaiian Islands Synoptic Discussion and Guidance

964
FXHW60 PHFO 260113
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
313 PM HST Tue Jun 25 2019

.SYNOPSIS...
A wet and unsettled weather pattern will continue through
Wednesday across Kauai and Oahu, with locally heavy rainfall and
thunderstorms possible. The heavier rainfall may try to sneak
into Maui County tonight into early Wednesday, but should shift
back westward Wednesday afternoon. Conditions will begin to
improve from east to west across the island chain Wednesday night
through Thursday night, with a more typical trade wind pattern
featuring windward and mauka showers expected Friday through
early next week.

&&

.DISCUSSION...
Currently at the surface, a broad area of low pressure is located
several hundred miles northwest of state, with a surface trough
extending southward from the low, and just to the west of Kauai.
Meanwhile, a weak ridge of high pressure is located around 1350
miles northeast of Hilo. The resulting gradient is producing a
light south to southeasterly flow across the island chain this
afternoon. Aloft, an unusually strong upper level trough is
situated to the west of the state with shortwave energy rotating
around the base of the upper trough and across the western
islands. Quite a difference in airmass remains in place across the
island chain this afternoon. The Hilo sounding continued to show
a low level inversion around 6,000 feet, with a precipitable water
(PW) value of 1.37 inches, while the Lihue sounding had no
inversion and a PW value of 2.14 inches. The latest MIMIC total PW
imagery correlates well with the afternoon soundings, and shows
PW values of 2.0 to 2.25 inches across Kauai and Oahu, with PW
values much lower, between 1.3 and 1.5 inches, across the far
eastern end of the state. Main short term concerns continue to
revolve around the potential for thunderstorms and flash flooding.

Tonight through Thursday,
Model guidance is in good agreement showing the upper level
trough remaining to the west of the state, with spokes of
shortwave energy moving through the base of the trough and across
the western islands. At the surface, the trough of low pressure
will remain nearly stationary to the west of Kauai, while surface
high pressure holds in place well to the northeast of the island
chain. This will keep a light south to southeast flow in place
through Wednesday, with the trades trying to return from east to
west across the state Wednesday night and Thursday as high
pressure strengthens to the northeast. Deep tropical moisture will
remain over Kauai and Oahu through the period, but the most
unstable conditions are expected tonight and Wednesday as
shortwave energy rotates over the western end of the state. Heavy
rainfall and thunderstorms will remain possible, and as a result,
the ongoing Flash Flood Watch which runs through Wednesday
afternoon appears on track. There is the potential the watch may
need to be extended through Thursday across Kauai, where the
deeper moisture will hold in place longer. The probability of
needed to extend the watch on Oahu into the overnight hours
appears to be lowering.

Across Maui County and the Big Island, the airmass won't be
nearly as wet or unstable, so expect showers over interior and
mountain areas during the afternoon and evening, and locations
near the coast at night. There is the potential for some heavier
showers and storms to sneak into Maui County tonight, but the
probability of heavy flooding rainfall is not high enough to
warrant a Flash Flood Watch. If the deeper moisture does indeed
push into Maui County, it appears to be short lived with drier
air moving back into the region on Wednesday.

Thursday night through Monday,
The GFS and ECWMF are in good agreement with the large scale
synoptic features, although they differ on some of the details,
particularly for the end of the work week. Overall, high pressure
will strengthen to the northeast of the state, and this will push
the nearly stationary trough of low pressure slowly westward and
further away from the island chain. The ECMWF suggests some heavy
rain could linger over Kauai through the day on Friday while the
GFS suggests any lingering heavy rain should have shifted west of
the state by Thursday evening. Will follow a blended solution for
forecast details, and keep conditions unsettled across Kauai
through Thursday night, with improving conditions then expected
for Friday. A more typical trade wind pattern should overspread
the island chain Friday through early next week, with showers
favoring windward and mauka areas. The trades will likely be
lighter than the summertime average however, as the trough of low
pressure continues to linger a few hundred miles northwest of the
state.

&&

.AVIATION...
A trough of low pressure west of Kauai will continue to help
usher in deep tropical moisture/cloud cover from the south.
Instability from this feature will generate scattered showers with
isolated thunderstorms for the remainder of the day and into the
overnight hours. In general, Oahu and Kauai will see the greatest
shower and storm coverage, though some activity may drift as far
east as Molokai and Maui. Ceilings below 1500 ft and vis less than
3 SM will be possible with the most intense activity. Winds will
generally remain light and variable at most of the island airport
locations through tonight.

AIRMET SIERRA remains in effect for Kauai, Oahu, Lanai and
Molokai for tempo mountain obscuration above 2000 ft due to clouds
and showers.

&&

.MARINE...
A trough west of Kauai will cause unsettled weather over waters
around Kauai and Oahu through Thursday and potentially spread
east to include Maui county waters this evening into Wednesday.
Currently, the showers and thunderstorms are mainly over the Kauai
waters and is tracking towards the waters around Oahu. We are
expecting an increase in showers and thunderstorms across the Oahu
waters and possibly Maui County waters this evening into tonight.
Threats to mariners include locally erratic wind and reduced
visibility in areas of heavy showers and thunderstorms, as well as
the potential for cloud-to- surface lightning strikes. This
system is not expected to bring impactful weather to Big Island
waters.

Outside areas of convection, expect moderate to locally fresh
east to southeast winds over Big Island waters, while the
remainder of the waters will see light to moderate southeast to
south winds that may occasionally shift southwest. South winds may
briefly become fresh to strong near Kauai late Wednesday into
Thursday. Beyond Thursday, there is still some uncertainty in the
wind forecast due to the disagreement on the placement of the
trough to our west. We could possibly see east trade winds fill in
on Friday or we may see southeast winds continue through the
weekend.

Surf along all shores is expected to be below High Surf Advisory
levels through the week, but a long-period south swell arriving
this weekend is expected to bring advisory-level surf to south
facing shores. Long-period forerunners are expected to arrive
Friday night and fill in through the day Saturday. Surf is
expected to reach advisory levels along south facing shores of all
islands Saturday night through Monday. A small short- period
northeast swell will linger for another day or so before
diminishing.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through Wednesday afternoon for Niihau-Kauai-
Oahu.

&&

$$

DISCUSSION...Jelsema
AVIATION...Thomas
MARINE...Kino

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