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Temp1.3 C
RH57 %
WindSE 10 mph
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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 220141

Area Forecast Discussion
National Weather Service Honolulu HI
341 PM HST Sat Sep 21 2019

Increasing moisture and instability will translate to a wet
pattern through the weekend. Shower coverage will continue to
focus over windward areas tonight, then become more widespread
Sunday through Monday as the trades diminish. Although some drier
air is expected to fill in over the Big Island and Maui County by
Tuesday, the wet pattern is forecast to linger over Kauai and Oahu
through midweek.


Currently at the surface, a trough of low pressure is located
around 200 miles east-southeast of the Big Island, while a 1026
mb high is centered around 1150 miles north-northeast of
Honolulu. The resulting gradient is producing moderate to breezy
trade winds across the island chain this afternoon. Meanwhile
aloft, a closed low is located in the vicinity of Oahu. Visible
satellite imagery shows partly cloudy skies in most areas, with
some pockets of more extensive cloud cover, particularly over the
Big Island. Radar imagery shows scattered showers moving into
windward areas. A few showers continue to spill leeward from time
to time, will scattered sea breeze and daytime heating driven
showers notes over leeward sections of the Big Island. Main short
term concerns over the next few days revolve around the potential
for heavy rainfall and thunderstorms.

Tonight through Tuesday,
Model solutions are in good agreement the next several days,
showing the upper level low retrograding westward, dragging a
trailing surface trough and plume of deep tropical moisture with
2.0 to 2.5 inch precipitable water values westward and through the
island chain. This is expected to result in an unsettled period
of weather across the state, with the potential for producing
locally heavy rainfall and thunderstorms at times. Showers are
expected to favor windward areas, although leeward areas will see
a fair amount of afternoon and early evening shower activity due
to the unstable airmass in place. The plume of deep tropical
moisture is expected to arrive over the eastern end of the state
tonight, then spread westward over Maui County and Oahu on Sunday
and into Kauai Sunday night. Most of the state will be in the
soupy airmass on Monday, with some drier air beginning to work in
from the east, particularly across the Big Island and Maui
County, Monday night and Tuesday. For now the most unstable
airmass associated with the upper low and deeper tropical moisture
associated with the trailing surface trough don't appear to align
in a manner consistent with a widespread flash flood threat. That
said, given the copious amount of deep tropical moisture moving
through, we will continue to closely monitor trends for potential
flash flooding as the pattern evolves.

Tuesday night through Friday,
There are some conflicting signals in the guidance in the longer
range periods, with the ECMWF suggesting the surface trough and
trailing plume of deep tropical moisture will shift just west of
the state Tuesday night, although it will remain in close
proximity to Kauai. The GFS on the otherhand, lingers the surface
trough and deep tropical moisture over Kauai through Thursday,
before bringing in a drier trade wind pattern for the end of the
work week. For now will keep the forecast more wet across the
western islands through Thursday until details become more clear.


A moist and somewhat unstable trade wind flow will veer out of
the east-southeast on Sunday. High pressure far northeast of the
state will maintain moderate trades tonight while an upper level
low centered over the western end of the island chain drifts
westward. The unstable trade wind flow will produce a rather
active shower pattern tonight, leading to periods of MVFR ceilings
and visibility (and perhaps an isolated thunderstorm) along
windward slopes and waters in passing moderate to heavy showers.
As deeper moisture approaches from the southeast late tonight,
AIRMET Sierra for mountain obscuration may be needed, mostly
likely for the Big Island and Maui. Afternoon convection across
leeward and interior Big Island will diminish during the evening,
while leeward areas on the other islands will experience brief
MVFR conditions in passing showers. A weak surface trough and its
associated moisture will spread up the rest of the island chain on
Sunday, causing winds to decrease and veer and producing higher
chances for MVFR conditions in showers and thunderstorms.

A band of layered middle and high clouds associated with the
upper low is expected to spread northwestward over the state late
tonight and Sunday. AIRMET Zulu for light icing may be needed.

After reports of low level turbulence earlier in the day, AIRMET
Tango was issued below 8,000 ft. Conditions for turbulence are
marginal this afternoon, and the AIRMET will likely be dropped by
sundown as winds slowly decrease.


A trough approaching the state tonight will cause the winds to
veer to the east-southeast and decrease to fresh speeds tonight.
The Small Craft Advisory (SCA) has been cancelled due to the
expected decrease in wind speeds. As the trough passes over the
state on Sunday, winds will drop to light to moderate speeds over
most areas. The channels and windward zones near the Big Island
and Maui County will likely see fresh speeds on Sunday due to
terrain accelerations. Fresh to locally strong speeds are expected
to return late Monday as the trough moves west of the state and
the persistent high, far north-northeast of the state builds back
across the area.

For the upcoming week, a front developing northwest of the state
Monday into Tuesday, should allow winds to decrease to light to
moderate east-southeasterlies by the middle of the week. Winds
are expected to remain east to southeast and below SCA levels
through most of the week as the front remains northwest of the
state and a high persists north-northeast of the state.

A series of small swells from the southwest, and south are
expected through the upcoming week. A new moderate period south-
southwest swell is due on Monday and will be followed by a larger
long-period swell later in the week. The swell later in the week
may reach advisory levels along south facing shores late Thursday
into Friday. Forerunners from a new northwest swell are starting
to trickle in at the Hanalei Buoy this afternoon. We should see
the swell continue to fill in overnight across the state, peak on
Sunday, then lower through the day Monday. A few other small
northwest swells will be possible towards the end of the upcoming






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