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Hawaiian Islands Synoptic Discussion and Guidance

899
FXHW60 PHFO 230204
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
404 PM HST Mon Apr 22 2019

.SYNOPSIS...
Moderate to fresh trade winds will continue through the work week,
bringing periods of clouds and showers to windward areas, while
leeward areas remain mostly dry. The trades will begin to decrease
over the weekend allowing for the development of sea and land
breezes. This will favor showers developing over leeward and
interior sections of the islands during the daytime.

&&

.DISCUSSION...
A 1030 mb surface high is centered about 1450 miles northeast of
Hawaii is driving moderate to fresh trade winds across the state
this afternoon. The atmosphere is currently dry and stable with
weak upper level ridging over the area. 00Z soundings show strong
inversions between 5 to 6 kft and precipitable water values
between 0.9 to 1.1 inches. Visible satellite shows scattered to
broken low clouds focused over windward sections of the islands.
A few showers are associated with the low clouds. Leeward areas
are mostly sunny this afternoon, and aside from the typical cloud
builds ups along the Kona coast on the Big Island.

Trade winds will maintain speed through Wednesday, then weaken
slightly towards the end of the work week. Low clouds and
scattered showers will continue to funnel over the islands
focusing over windward and mauka areas. These showers will
generally be light and occur mostly during the overnight and early
morning hours. Warm and mostly sunny conditions will continue for
leeward areas.

A weak surface trough will develop over the islands in response
to an approaching upper level trough approaching. This will result
in the trade winds weakening, veering east southeastly and allowing
daytime sea breezes and nighttime land breezes to set up. A weak
front will also approach the islands from the northwest, but will
likely fade before reaching Kauai. This weekend, expect an overall
increase in clouds and showers, particularly across leeward and
interior sections of the islands during the daytime. Conditions
are also expected to be warm and muggy.

The influence of the upper trough is still up in the air given the
weaker depiction of this feature in the GFS and versus the
stronger ECMWF. There is the potential that some of the shower
activity could be locally heavy or produce some thunderstorms on
Sunday and Monday, but given the differences in the models and
being so far out in the forecast period, will hold off on adding
any mention of thunder to the forecast until details become more
clear.


&&

.AVIATION...
Surface high pressure to the far northeast of the islands will
allow for moderate easterly trades to persist this evening into
tonight. As typical with this type of pattern, expect a slight
chance of showers along the windward and mauka slopes with mainly
dry conditions for leeward locations. Highest chance for
precipitation will be late tonight over the windward coastal
slopes and waters.

AIRMET Tango remains in effect below 6000 ft for south through
west of all mountains due to tempo moderate turbulence.


&&

.MARINE...
Expect fresh to locally strong trades to continue into Thursday,
with a decrease Friday on into the weekend. The ridge currently north
of the islands will be disrupted by a front that is expected to
stall north of the islands Friday. The current Small Craft
Advisory (SCA) for the windier areas near Maui and the Big Island
continues through Wednesday afternoon, but may need to be extended
into Wednesday night or Thursday.

Surf is expected to remain below advisory levels through the week
for all shores. A few small west-northwest and north-northwest
swells will provide small surf along north and west facing shores
throughout the week. A series of small south swells will provide
small surf starting today and will likely continue through most of
the week and into the weekend. Continue to expect the swell along
south shores to peak tonight and slowly dropping over the next
couple of days. Several reinforcing south swells are expected
towards the latter half of the week. Surf along east facing shores
will hold steady in the small to moderate range through Thursday
due to the breezy to locally strong trades.


&&

.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...Foster
AVIATION...Thomas
MARINE...M Ballard

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



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