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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 251324
Area Forecast Discussion
National Weather Service Honolulu HI
324 AM HST Tue Apr 25 2017
A ridge north of the islands will maintain moderate east
southeast winds into midweek. Limited shower activity is expected
through Tuesday. From late Tuesday onward, more windward and
mauka showers are expected, with the potential for a period of
unsettled weather this weekend.
The 1027 mb high is about 1400 miles northeast of Oahu overnight
while the associated ridge remains about 400 miles north of Kauai.
Satellite derived precipitable water shows values right around 1
inch near the islands, which is a touch on the dry side for this
time of year. This dry air extends about 200 miles to the
northeast of the islands. Have dried out the PoPs further for this
morning taking into account the drier airmass, and the lack of
clouds and showers in the upstream area.
There is an increase in the precipitable water about 200 miles to
the northeast. Initial timing indicates that some of that moisture
will reach the islands this afternoon, particularly over the
central islands. Have left the PoPs pretty much as is for the
afternoon hours. Made some minor tweaks to the PoPs for
Tuesday night, primarily to indicate a slightly better chance for
showers during the second half of the night.
Otherwise the forecast remains largely intact. Look for passing
showers mainly over the windward sides tonight through Thursday
night. As we head into the end of the week through the beginning
of next week, a cold front approaching from the northwest brings
a level of uncertainty to the forecast. Both the GFS and ECMWF
continue to agree with the front reaching the Kauai on Friday.
Moisture lingering near the islands has the potential to form
pre-frontal bands over the central islands Thursday night or
Although both models indicate unsettled weather through the
weekend and into early next week, there are still several details
to work out. Behind the front, there could be some stronger north
winds ushering in a drier airmass. However a mid to upper level
low dropping into the islands behind the front would bring much
colder temperatures to the upper levels. Any break in shower
activity would likely be short lived due to additional pools of
moisture and the colder temperatures aloft. Normal 500 mb
temperatures for late April/early May would be around -8 celsius.
For Friday and Saturday, the models are in good agreement with -12
celsius at 500 mb, significantly below normal. Combined with
precipitable water values around 1.3, at or slightly above normal,
we could see some heavy showers. The ECMWF proceeds to bring much
colder temperatures aloft over Kauai and then to the west of the
islands Monday and Tuesday with a closed upper low dropping over
the islands. The GFS is not as aggressive with the upper level
feature with the latest run, but that has been changing from run
to run. The forecast remains wet from Friday through the middle of
next week. There will likely be periods of time that are wetter
than others, but will await further runs to nail down the
VFR conditions will dominate through most of today under a rather
dry and stable trade wind flow. With limited low level moisture,
expect isolated MVFR ceilings to be confined to windward terrain,
as well as leeward slopes of the Big Island during the afternoon
hours. Increasing clouds and showers will be possible, mainly over
windward areas, beginning later this afternoon or early this
evening as an area of enhanced moisture embedded in the trades
moves into the state from the east. Areas of MVFR conditions will
become more likely at that time over windward and mauka locations.
No AIRMETs are anticipated through early this afternoon. AIRMET
SIERRA for mountain obscuration may become necessary for some
windward areas during the later afternoon or evening hours.
Guidance remains persistent and depicts a ridge of high pressure
north of the waters weakening Wednesday through Friday as another,
late season, cold front approaches and moves over the waters. Local
winds will respond and become light between Wednesday night and
Friday. Fresh to strong northerly winds will become a possibility
over the northwest waters Friday into the weekend as strong high
pressure builds eastward in the wake of the front.
A small northwest (310-330 deg) swell currently impacting the
northern shores should hold today before dropping off through
midweek. A reinforcement out of the same direction is forecast to
fill in later today and hold through midweek before easing into
the second half of the week. Strong northerly winds associated
with high pressure building east in the wake of the previously
discussed front moving into the local area could translate to
rising, mainly short period, surf along exposed northern shores
into the weekend.
Small surf along eastern facing shores will hold steady into
Wednesday before dropping off Thursday and Friday as the local winds
Small surf along southern shores will hold through midweek, before
rising Thursday through Saturday as a new, long period, south-
southwest (190 deg) swell from the southern Pacific fills in. Surf
will near, if not reach, the high surf criteria (8 ft faces) at some
point from Friday through Saturday. This swell is currently
impacting the American Samoa buoy and will continue to be monitored
through tonight as the swell moves through. If it ends up peaking
slightly higher than predicted, the chances for advisory-level surf
will be greater locally once it fills in.
For more detailed information on the local surf and outlook for
Oahu, see the latest Collaborative Nearshore Swell and Wind Forecast
for Oahu that was issued Monday afternoon.
Small Craft Advisory until 6 AM HST Wednesday for Alenuihaha
Channel-Big Island Windward Waters-Big Island Leeward Waters-Big
Island Southeast Waters.
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office