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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 280209

Area Forecast Discussion
National Weather Service Honolulu HI
409 PM HST FRI MAY 27 2016

A trough northwest of the state keeps winds light and from the
east-southeast through the weekend. Onshore sea breezes during
the day will lead to afternoon clouds and showers across interior
and leeward areas. Offshore land breezes at night will clear out
much of the area, but weak east to southeast winds will keep some
clouds across windward sections. A shower band will move south
over the islands from late Sunday into Tuesday, with a brief
return to trades behind it.


Water vapor imagery shows a broad upper level trough west of the
state, with a west-southwesterly jet over the islands. At the
surface, high pressure is located far to the northeast, with a
trough a few hundred miles northwest of Kauai. 00z soundings show
the difference in airmass across the state, with inversions
ranging from around 8kft at Hilo to a strengthening one near
14kft at Lihue. Precipitable waters are also 1.3 and 1.5 inches
respectively. Early afternoon MIMIC total precipitable water
imagery shows drier air across most windward waters from the Kauai
channel eastward, with greater moisture in a band from the lee of
the Big Island west-northwest across Kauai.

Heavy showers flared up again this afternoon mainly over
Kauai/Oahu (greatest coverage over Kauai), with additional showers
over the interior/leeward sections of the remaining islands as
well. These showers are diurnally driven and will weaken with
sunset, but we have seen the past few days that they may linger
longer especially within the main moisture axis. This is also
reflected in the high-resolution WRF NMM and ARW runs, and we have
held off on the bulk of the land-breeze induced clearing until
after midnight.

A similar pattern

showers flared up late last night within the moisture axis over
Kauai, with a few gages on Kauai reporting over 1 inch in the past
12 hours and the Mount Waialeale gage measuring over 3 inches.
Shower activity has died down this morning, but will flare up
again during the late morning/afternoon. Given the moisture and
instability, the heaviest showers are expected across Kauai.
Because of the light winds, sea breezes will again develop across
leeward sections. High-resolution guidance from the WRF ARW and
NMM runs shows activity greatest across Kauai and Oahu, but with
afternoon showers across all islands. The main focus will also be
across leeward coastal sections, where convergence between the sea
breezes and the weak background flow will be greatest. Have
updated the forecast to reflect the higher rainfall chances across
these areas for today.

Gradual stabilization is expected through early next week as
heights slowly rebuild over the state. The wind and weather
pattern we saw today will continue at least for Saturday and
Sunday, with afternoon showers interior and leeward and some
lingering showers windward overnight. (Although the pattern won't
be exactly the same; it will be improving on Kauai, similar to
what the other islands saw today). Monday may also fall into the
same pattern as the rest of the weekend, but there is the
potential for greater shower coverage. Differences emerge among
the extended models with respect to an old frontal boundary/cloud
band this is expected to reach the area early next week. The GFS
is fastest bringing this band to Kauai/Oahu by early Monday, then
continue on to the Big Island on Tuesday. Other extended models
including the ECMWF and NAM have been slower, not bringing the
boundary this far south until late Monday/Monday night. The trend
has been faster though (which was also seen between the 00z and
12z ECMWF runs), so with this in mind have increased rainfall
chances a bit Sunday night/Monday across the northwest portion of
the area. If the faster solution does pan out, we would see higher
rainfall chances for Memorial Day.

The other accompanying impact from this old front/cloud band will
be an increase in northeasterly winds with it. A new surface high
will setup along 30N, providing at least a few days of trade winds
during the course of the new work week. At some point another
system passing to our north may weaken and displace the high,
cutting off the trades and returning us to another light wind
pattern. However, both the 12z GFS and ECMWF has backed off on
this next system, keeping us with a trade pattern for a couple
days longer. Given the time of year, would expect this trend
toward a more entrenched high to continue.


Light winds will continue through Saturday for most of the state
with the prevailing east to southeast wind flow being blocked by
the Big Island. Expect many areas to feel land breezes at night
and sea breezes during the day. The Big Island should see most of
the clouds from convection late in the morning to early evening with
the east sides seeing some cloudiness overnight as well due to
moisture within the easterly winds. The smaller islands will see
cloudiness building mainly over interior sections during the
afternoon. So some MVFR conditions are expected here and there.
Lanai, as usual, will have the best chance for IFR ceilings during
the daytime.

Daytime cloudiness may prompt the issuance of an AIRMET for
mountain obscuration at times.


Winds and combined seas will remain below small craft advisory
criteria through the forecast period.

The current southwest swell will gradually diminish through the
holiday weekend. A small northwest swell arriving Sunday will
peak Monday well below the advisory threshold. See the the
Collaborative Near-shore Swell and Wind Forecast (SRDHFO) for
details on these swells. Otherwise rather small surf is expected.






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
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