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Hawaiian Islands Synoptic Discussion and Guidance

362
FXHW60 PHFO 161328
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
328 AM HST Thu Jul 16 2020

.SYNOPSIS...
Trade winds will gradually trend down today into Saturday as a
trough passes north of the islands. Lighter winds combined with
increasing moisture will translate to warm and humid conditions
through this time. Although most of the shower coverage will focus
over windward and mountain locations, some leeward showers will
be expected, especially where afternoon sea breezes develop. A
return of breezy trade winds and drier conditions is anticipated
Sunday into next week.

&&

.DISCUSSION...
Latest surface analysis showed the subtropical ridge anchored north
of the area and a remnant trough associated with former Tropical
Storm Cristina far northeast of the Big Island. Satellite-estimated
precipitable water (PW) imagery reflected this and depicted a large
area of higher moisture over and around this feature (2+ inch
PWs).

The short-term guidance remains in decent agreement and shows this
weak surface trough continuing westward and passing north of the
islands Friday through Saturday. The pressure gradient will respond
and weaken through this time beginning today, which will allow
the trades to trend down into the light to moderate range. Warm
and humid conditions will result due to a combination of the
lighter winds and increasing moisture/dewpoints (lower 70s).
Although most of the shower coverage will focus over windward and
mountain locations with the background trades in place, some
leeward showers will remain possible, especially where afternoon
sea breezes develop. Some showers may be briefly heavy in some
locations.

Although breezy trade winds are expected to return by Sunday,
another area of moisture moving up the island chain from east to
west will be enough to keep the rainfall chances in place, mostly
for windward and mountain locations. Drier trade wind conditions are
anticipated early next week as the area of moisture shifts to the
west.

&&

.AVIATION...
High pressure to the north-northeast of Hawaii is maintaining the
southeast to east tradewind pattern we have been experiencing.
Winds should be relatively light over the next few days. Not a lot
of moisture and the showers that we do see should be confined to
windward mauka areas. The MVFR conditions related to these showers
should only be passing in nature.

There are no AIRMETs in effect and none anticipated through this
forecast period.

&&

.MARINE...
Surface high pressure centered far north-northeast of the area is
maintaining moderate trade winds across most of the island chain
early this morning. A weak surface trough, which is the remnant
of former East Pacific Tropical Cyclone Cristina, is slightly
more than 600 nm east-northeast of Hilo. As this feature
continues to move westward at about 15 kt, it will disrupt the
pressure gradient across the islands from later today through
Friday. This will cause the trade winds to weaken during the next
couple of days. As a result, local sea breeze circulations may
develop during the afternoon over some leeward nearshore waters
today and Friday.

By this weekend, the forecast models indicate the surface trough
will eventually dissipate after it moves northwest of the
islands. At the same time, surface high pressure will build far
north-northeast of the state. This will cause the trade winds to
gradually strengthen from Saturday through Sunday. The latest
forecast indicates Small Craft Advisory conditions may develop
over the typically windier waters adjacent to the islands of Maui
County and the Big Island starting Saturday night, and persist
into early next week.

No significant swells are expected into the middle of next week.
Small south and southeast swell energy will continue to produce
small background surf along south facing shores through Friday. A
small, long-period south-southwest swell may cause a slight bump
in surf heights along south facing shores this weekend. The
weakening trade winds will cause surf to decline along east
facing shores through Friday. A slight increase in surf along
east facing shores is expected later this weekend as the trade
winds strengthen. Surf along north facing shores will remain
nearly flat for the foreseeable future.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Gibbs
AVIATION...Chevalier
MARINE...Houston

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



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