Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 280315 AAA
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
515 PM HST Thu Jul 27 2017

.SYNOPSIS...
Moisture associated with the remnants of Greg is expected to
bring increased humidity and an increase in showers beginning
later tonight, some of which may be heavy on Friday. Most of the
showers will affect windward areas, but a few will stray leeward
on the smaller islands, while leeward Big Island slopes could see
a few afternoon downpours over the next couple of days. A trend
toward drier trade wind weather is expected from Sunday into early
next week. High pressure north of the state will drive primarily
moderate trade winds.

&&

.DISCUSSION...
Moderate trade winds are being supported by a surface high
centered about 1400 miles northeast of the islands, which will
continue through the forecast period. Moisture from ex-Tropical
Cyclone Greg is estimated to be about 150 miles east of the Big
Island, and MIMIC total precipitable water (PW) imagery shows that
an associated area of increased PW (around 2 inches) extends from
17N to 22N between 153W and 140W, with additonal moisture
trailing SE from Greg. Water vapor imagery highlights a closed low
aloft centered about 400 miles northwest of Kauai, and a ridge
aloft centered about 600 miles south southeast of the Big Island.
The resultant southwest flow aloft is advecting some high level
cloudiness towards Maui County and the Big Island, which is
expected to slide east of the state tonight as the low advances
eastward towards us.

Afternoon soundings indicate modest instability/CAPE in the mid
and upper levels of the island atmosphere, with a strong low level
subsidence inversion based around 6000-8000 feet keeping a cap on
the incoming low clouds.

A relatively quiet trade wind weather pattern is expected into
this evening, with a few showers gracing windward slopes and
coasts. Low to mid-level moisture associated with the remnants of
Greg will begin streaming over the islands from the east later
tonight, bringing an increase in humidity as dew points climb into
the lower 70`s, and fueling an increase in trade wind showers.
Some of the showers may be briefly heavy on Friday and Friday
night as the upper low northwest of Kauai opens up to a trough,
and passes over the island chain. A significant reduction of mid-
level temperatures is not expected at this time, but the abundance
of low-level moisture may be sufficient to help fuel a few heavy
showers Friday and Friday night. With trade winds continuing,
showers will focus over windward slopes and coasts, and these are
the areas most likely to see a briefly heavy downpour, aside from
leeward Big Island slopes. With trade winds keeping most showers
on the move, and with relatively warm mid-levels, the idea of a
Flash Flood Watch was rejected.

Late Saturday into Saturday night look to be a slow transition
back a drier and more stable trade wind weather pattern as models
push the moisture west of the area. Some showers may still hang
onto windward slopes and Kona slopes of the Big Island during this
period.

Sunday into the first half of next week look to be drier with
moderate trades continuing. Windward areas may still see a few
stray showers tracking in with the trades, but rainfall amounts at
this point should be light.


&&

.AVIATION...
VFR conditions will prevail at all TAF sites tonight, though
passing low clouds embedded in the trade winds will bring brief
MVFR conditions to windward and mountain areas.

No AIRMETS are in effect and none are expected.


&&

.MARINE...
High pressure north of the islands will maintain moderate trade
winds over the next several days. The forecast calls for winds to
stay just below or near the Small Craft Advisory threshold during
this time.

A distant typhoon in the west Pacific is expected to send a small
mid to long period west swell towards the islands Friday into the
weekend. The exposed western to southwestern shores could see a
a bump in surf above nil. However, Niihau and Kauai will help
effectively block several areas on the remaining smaller islands
from this swell.

Another round of distant east Pacific tropical cyclones could send a
series of mid to long period east swells to the islands late
Saturday into midweek. This will need to be monitored over the next
several days for more specific details on timing and height, and
if a High Surf Advisory will be needed.

Otherwise, the moderate trade winds will keep choppy breakers
along along the east facing shores. A series of small longer
period south, southeast, and southwest swells will produce small
background surf through the period.


&&

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

&&

$$

DISCUSSION...Foster
AVIATION...Powell
MARINE...Eaton


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