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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 240610

Area Forecast Discussion
National Weather Service Honolulu HI
810 PM HST Tue Apr 23 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
Thursday through Friday, which will lead to light and variable winds
for the upcoming weekend as a cold front approaches. Clouds and
showers will then become focused over interior locations through the
afternoon periods into early next week.


The latest surface analysis depicted a weak front passing north of
the islands and a 1034 mb area of high pressure centered to its
north. Water vapor imagery showed a ridge extending northward
several hundred miles to the west and mostly dry mid- to upper-level
air over the islands. Satellite and the afternoon upper air
soundings showed moisture levels below normal for this time of year
and a strong subsidence inversion between five and six thousand
feet. Rain gages support the dry conditions with minimal
accumulations measured over the past 12-hrs (through 7 PM HST).

Guidance supports the mostly dry and stable trade wind pattern to
continue 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 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.
Additionally, 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 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.


A surface high pressure northeast of the state will continue to
advance further eastward. This will result in slow and slight
easing of the easterly trades through Wednesday. Low clouds and
embedded showers are still favored over the windward and mauka
locations under the easterly winds and may bring brief MVFR
ceiling to those areas. Otherwise VFR conditions are expected to
prevail for all sites.

AIRMET Tango remains in effect for areas south thru west of
mountains below 7000 feet for tempo moderate turbulence this
evening. Winds may ease enough overnight leading to the possible


Expect moderate to locally strong 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 for the typical windy waters around
Maui and south of the Big Island goes through at least Wednesday at
this time. Will keep an eye on winds to see if the SCA needs to be
extended a bit further.

Surf is expected to remain below advisory levels through the week
for all shores. There will be a series of small west-northwest to
north-northwest swells over the rest of this week, with a moderate
size northwest swell arriving late Sunday afternoon, peaking Sunday
night, then lowering through Monday night. A series of small swells
will maintain relatively small surf along south facing shores through
the period with the largest bumps today and Saturday. Surf along east
facing shores will hold steady in the small to moderate range through
Thursday due to the breezy trades, then lower over the weekend as the
trades lessen.


Small Craft Advisory until 6 PM HST Wednesday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.




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
Send comments to: Ryan Lyman