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Hawaiian Islands Synoptic Discussion and Guidance

309
FXHW60 PHFO 231941
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
941 AM 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 persists. Moderate to locally breezy
trades are being driven by surface high pressure far northeast of
the state. A strong ridge aloft is producing stable conditions
with a well-defined inversion between 4,500 and 6,500 ft, and
precipitable water (PW) values in the trade wind flow are running
near to slightly below April normal. Two small patches of moisture
are producing some enhancement to cloud cover and showers on
Kauai and windward Big Island this morning. Otherwise, modest
rainfall will be focused on windward terrain. With the exception
of a few afternoon showers on the South Kona slopes of the Big
Island, leeward areas will be 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 as a weak cold front approaches the state. The
approaching upper level trough driving 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 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. Forecast models have yet to come into agreement in the
longer range, with the ECMWF hinting at lingering instability on
Monday.

&&

.AVIATION...
Surface high pressure to the northeast of the state will continue
to push further off to the east. This pattern should allow the
light to moderate easterly trades to persist for the remainder of
this afternoon and into tonight. Slightly weaker flow is expected
on Wednesday. Similar to last couple of days, there will be a
slight chance of showers over the windward and mauka locations
with mainly dry conditions elsewhere.

AIRMET Tango remains in effect for area 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 the 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

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office



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