Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 240128
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
328 PM HST Tue Apr 23 2019

.SYNOPSIS...
A stable, moderate to locally breezy trade wind flow will persist
through Thursday as surface high pressure dominates north of the
state. A typical rainfall pattern will continue, featuring modest
showers across windward slopes and mainly dry conditions over
most leeward areas. Trade winds will weaken on Friday and be
disrupted during the weekend, leading to a land and sea breeze
regime that will produce daytime clouds and showers over interior
sections.

&&

.DISCUSSION...
A stable trade wind flow remains in place. Moderate to locally
breezy trades are being driven by surface high pressure far
northeast of the state. A strong ridge aloft is maintaining
stable conditions with a well-defined inversion between 4,000 and
6,000 ft, and precipitable water (PW) values in the trade wind
flow are holding near to slightly below April normal. Under these
conditions, modest rainfall has been focused on windward terrain,
and with the exception of a few afternoon showers on the South
Kona slopes of the Big Island, leeward areas have been dry.

Little change is due over the next couple of days. Another surface
high will pass far north of the islands, leading to steady trade
winds. Stable conditions will hold as a ridge aloft remains
overhead. No significant areas of moisture are seen in the upwind
trade wind flow, pointing toward a continuation of the current
shower pattern.

The trade wind flow will weaken on Friday and will be disrupted
during the weekend and Monday as a weak cold front approaches the
state. The approaching upper level trough supporting the front
will erode the ridge aloft on Friday. A generally stable trade
wind flow should hang on, though afternoon clouds and a few
showers will be possible over leeward terrain. Trades will veer
out of the southeast on Saturday and will likely be completely
disrupted on Sunday and 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. While there does not appear to be a
significant increase in moisture, an isolated heavy shower will be
possible, mainly in the afternoon.

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, minimizing
chances for heavy rainfall.

&&

.AVIATION...
Surface high pressure to the northeast of the state will continue
to push further off to the east. This pattern should allow the
moderate easterly trades to persist this evening and into
tonight. Slightly weaker flow is expected on Wednesday. Expect a
slight chance of showers over the windward and mauka locations,
especially over Maui and the Big Island, with mainly dry and VFR
conditions elsewhere.

AIRMET Tango remains in effect for areas south thru west of
mountains below 7000 feet for tempo moderate turbulence.

&&

.MARINE...
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.

&&

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

&&

$$

DISCUSSION...Wroe
AVIATION...JT
MARINE...TS



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