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Hawaiian Islands Synoptic Discussion and Guidance

410
FXHW60 PHFO 160631
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
831 PM HST Wed Jul 15 2020

.SYNOPSIS...
Trade winds will gradually trend down Thursday 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 Thursday, 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 evening. A weak surface trough, which is the remnant
of former East Pacific Tropical Cyclone Cristina, is evident
about 700 nm east-northeast of Hilo. As this feature continues to
move westward at 10 to 15 kt, it will disrupt the pressure
gradient across the islands from Thursday 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 over
some leeward nearshore waters Thursday and Friday afternoon.

By this weekend, the forecast models indicate the surface trough
will eventually dissipate 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 suggests that
Small Craft Advisory conditions may develop over the typically
windier waters adjacent to the islands of Maui County and the Big
Island by Sunday.

No significant swells are expected through early next week. Small
south 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 during the
next few days. 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 through
early next week.

&&

.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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