Current Conditions
Temp10.4 C
RH62 %
WindE 11 mph
RoadOpen
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Hawaiian Islands Synoptic Discussion and Guidance

831
FXHW60 PHFO 240139
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
339 PM HST Sun Sep 23 2018

.SYNOPSIS...
Low pressure northwest of Kauai will lift to the northeast over
the next couple of days, bringing a weakening trough and
increasing moisture towards the western islands. Showers will
increase mainly over the western islands Monday night through
Tuesday. Weak, south winds and higher than normal dewpoints will
continue muggy conditions into the middle of the week.

&&

.DISCUSSION...
No major changes since this morning. Have made some tweaks to the
PoP/Sky/Wx grids through Tuesday night, but no significant changes
to the flavor of the forecast.

The morning analysis puts the surface low of 1007 mb about 675
miles northwest of Kauai, and shifts the surface ridge to the
northeast of the smaller islands. Wind speeds around the islands
have been light, with a south to southeast flow that can also be
noted in the low cloud movements on satellite. Satellite also
shows an abundance of clouds to the northwest of the Kauai coastal
waters in association with the low and its associated trough. Some
isolated thunderstorms have been noted about 250 miles northwest
of Kauai.

The afternoon soundings from Lihue and Hilo show lower
precipitable water (PW) values than the overnight soundings. Lihue
came in at 1.49 which is near normal for September, while Hilo had
1.2 inches which is about 1 standard deviation on the dry side.
The big question over the next 36 to 48 hours is how much moisture
will reach Kauai and the surrounding area.

The GFS remains the most aggressive with the latest run have more
than 2 inches of PW reaching Kauai by Tuesday morning. The ECMWF
is still on the lower side, but does have some 2 inch values
reaching parts of the Garden Isle. Both models show 500 and 700
temperatures remaining near normal throughout, so will lack the
instability that would favor heavy rain or thunderstorms. That
being said, having noted thunderstorms in the area closer to the
low, will have to keep an eye on future runs.

The longer time range continues to show discrepancies in the
model runs. The ECMWF continues to build high pressure in to the
northwest behind the trough, returning a more typical trade wind
flow by the end of the work week. The GFS holds onto the trough
for a little longer, which would delay the onset of trades a day
or two.

Dewpoints have reached 70+ degrees at several reporting sites
across the state, and with the lighter winds, its increasingly
likely to feel a bit on the muggy side. Expect dewpoints to
increase a couple degrees Monday night and Tuesday, and linger in
the low to mid 70s through much of the work week. With winds
remaining on the light side, muggy conditions will continue. As
winds possibly pick up at the end of the week, we could see some
relief.

&&

.AVIATION...
Winds will continue to veer to the southeast across the state
tonight as a low pressure trough slowly approaches from the west.
Land and sea breezes will develop across many areas over the next
few nights/afternoons.

Clouds and passing showers will remain mostly confined to the
windward and mauka areas tonight, with isolated/brief MVFR
conditions possible. Some clouds and a few showers will linger
over interior/leeward areas with the sea breeze into this
evening. Otherwise, VFR conditions will prevail. As the trough
edges closer to the state tonight, clouds and showers will begin
to increase over and around Kauai with ISOL-SCT MVFR conditions
at times.

No AIRMETs are currently in effect, or expected through tonight.

&&

.MARINE...
An area of low pressure far northwest of Kauai will continue to
move slowly northeast and east through the middle of the week.
In response, surface winds have been weakening and shifting
southeasterly. They will then become more southerly by Monday
night through Wednesday. This will also bring an increase in
showers, especially across the western coastal waters throughout
the week.

Surf will remain below advisory levels along all shores through the
middle of the week. Rough, short-period surf along east facing
shores will gradually decline throughout the work week as the
winds ease and shift. A north-northwest swell is expected late
this week, which may increase surf to near the advisory level
along north facing shores. Small, long-period southwest and south
swells will give surf along south facing shores a small boost
throughout the week, but remain well below advisory level.

&&

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

&&

$$


DISCUSSION...M Ballard
AVIATION...TS
MARINE...Kinel

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