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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 250200

Area Forecast Discussion
National Weather Service Honolulu HI
400 PM HST Wed Apr 24 2019

Seasonal trade wind conditions will prevail through Thursday as
high pressure remains in place to the north. Clouds and showers
will favor the typical windward locations through this time,
leaving leeward areas mostly dry. Trade winds are forecast to
weaken Friday, which will lead to light and variable winds for the
upcoming weekend through early next week as a cold front
approaches. Clouds and showers will then become focused over
interior locations through the afternoon periods each day.


The latest surface analysis depicted a weak front passing north
of the islands and a 1032 mb area of high pressure centered to its
north. Water vapor imagery showed an upper ridge extending
northward several hundred miles to the west and mostly dry air
aloft over the islands. Satellite derived precipitable waters and
00Z soundings showed moisture levels below normal for this time
of year and a strong subsidence inversion positioned between 4500
feet at Lihue to 7500 feet at Hilo. Rain gages support the dry
conditions with light rainfall accumulations measured over the
past 12 hours.

Guidance supports the mostly dry and stable trade wind pattern
continuing through Thursday. Upper heights will rise as the
previously mentioned upper ridge shifts eastward over the state.
This upper ridge and dry air aloft combined with a strong
subsidence inversion will limit the rainfall potential/coverage,
even for windward locations. The exception will be due to pockets
of higher moisture moving through periodically within the trades,
which will translate to better windward rainfall coverage.

A transition period is expected Friday into the weekend as upper
heights lower and the surface ridge to the north weakens in response
to a shortwave trough and surface front moving into the region.
Trades will weaken and veer out of the southeast on or by
Saturday and likely be completely disrupted Sunday through Monday
as the upper level trough moves overhead. An unstable land and sea
breeze regime will result. Expect afternoon clouds and showers
over the islands, followed by partial clearing at night.
Increasing instability associated with the upper trough combined
with increasing moisture and daytime heating will support locally
heavy showers. Although confidence this far out remains low, a
thunderstorm or two can't be ruled Sunday through Monday,
especially through the afternoon hours.

Though there remains some run-to-run inconsistency in ECMWF and GFS
models, the latest runs suggest that the upper level trough will
move east of the state on Tuesday or Wednesday as the remnants of
the above mentioned front stall near Kauai. Though there remains
considerable uncertainty regarding the winds and rainfall pattern,
the atmosphere will likely be increasingly stable, with the
likelihood for heavy rainfall trending down.


High pressure to the northeast will keep easterly trade winds in
place for Hawaii through the period. The trades which were a
little weaker today will probably be even a little weaker
Thursday as a cold front to the north of the state gets into the
mix slightly reducing the pressure gradient. Therefore we should
not see the need to issue AIRMET Tango for low level turbulence.
We also have had no PIREPs for turbulence in the region today.

There will be clouds on the windward sides of the islands and the
chance for showers, especially for the Big Island and Maui.
Northeast sections of the Big Island and Maui have a good chance
to have isolated passing low ceilings and rain showers. Mountain
obscurations are possible but they should remain intermittent.

There are currently no AIRMETs in effect. We will monitor the
possibility for low level turbulence and mountain obscuration and
issue AIRMETs if needed.


Expect moderate to fresh trades to continue into Thursday, then
ease through the weekend as the surface ridge, currently north of
the islands, gets nudged eastward by a front stalling northwest of
Kauai. The Small Craft Advisory (SCA) for the typical windy
waters around Maui and south of the Big Island has been expired
for all zones except the Alenuihaha Channel. The SCA for the
Alenuihaha Channel has been extended through the day Thursday.
Winds are expected to drop below SCA levels by Friday.

Surf is expected to remain below advisory levels through the week
for all shores. There will be a series of small northwest to
north-northeast swells over the rest of this week, with a
moderate size northwest swell arriving late Sunday afternoon,
peaking Sunday night, then lowering through the first half of next
week. A series of south swells will maintain small surf along
south facing shores with reinforcing swells expected every few
days through early next week. Surf along east facing shores will
hold steady in the small range through Friday due to the breezy
trades, then lower over the weekend as the trades ease.


Small Craft Advisory until 6 PM HST Thursday for Alenuihaha




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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