Area Forecast Discussion
Issued by NWS Honolulu, HI

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000
FXHW60 PHFO 271331
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
331 AM HST Thu Jul 27 2017

.SYNOPSIS...
Trade winds will continue to fan the islands for at least the next
week, bringing a few brief showers to windward slopes and coasts
today. From late tonight through Saturday, moisture associated with
the remnants of Greg is expected to bring increased humidity, while
also fueling an increase in showers, 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. A trend toward drier
trade wind weather is expected from Sunday into early next week.

&&

.DISCUSSION...
Currently, moderate to locally breezy ENE trade winds are being
supported by surface high pressure centered about 1400 miles NE of
the islands. The center of the remnant low that was once Greg is
estimated to be about 650 miles ESE of the Big Island, and MIMIC TPW
imagery shows that an associated area of increased PWAT (2-2.3
inches) extends from 17N to 22N between 150W and 137W, with
additonal moisture trailing SE from Greg. Water vapor imagery
highlights a closed low aloft centered about 500 miles NW of Kauai,
and a ridge aloft centered about 750 miles SSE of the Big Island.
The resultant SW flow aloft is advecting some high level cloudiness
toward the islands from the S/SW. Early morning 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 ft keeping a cap on the incoming low clouds.

A relatively quiet trade wind weather pattern is expected into
tonight, with a few showers gracing windward slopes and coasts, with
showery low clouds currently in greater concentration upstream of
Maui and the Big Island. An isolated heavy shower developed over the
leeward slopes of Mauna Kea yesterday afternoon, and a repeat
performance is possible Friday and Saturday afternoons, but the
current sounding doesn`t support their development today. High
clouds are expected to pepper skies over the Big Island and portions
of Maui county today before the bulk of this moisture aloft slides E
of the area tonight and Friday.

Low- to mid-level moisture associated with the remnants of Greg will
begin streaming over the islands from the E 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 (mainly Friday and Friday night), as the low NW 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, and they have been added to the
forecast with the latest forecast package. 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.

06Z GFS continues to keep this moisture over windward Big Island and
Maui into Sunday while the ECMWF indicates a state-wide drying trend
on Sunday. With the trough aloft expected to have moved E of the
islands by Sunday, the ECMWF solution is favored at this time. A
ridge aloft building over the islands early next week should support
a drier than normal trade wind pattern, but windward areas will
likely still receive a few brief showers. With respect to trade wind
speeds, some slight reduction to the locally breezy conditions is
expected over the next day or so, and a slight increase is possible
late in the weekend as the trough marking the remnants of Greg
passes S of the islands.

&&

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

AIRMET Tango remains in effect for moderate low level turbulence
over and south through west of all mountains below 8000 feet.

&&

.MARINE...
High pressure north of the islands will maintain moderate to locally
breezy trade winds through the upcoming weekend. A Small  Craft
Advisory (SCA) is posted for the typically windier waters around the
Big Island and Maui through this afternoon. Winds are expected to
drop just below SCA levels tonight, but could pick up to SCA levels
again this weekend.

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. However, Niihau and Kauai will help effectively block most
of 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.

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

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST today for Maalaea Bay-Pailolo
Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island
Southeast Waters.

&&

$$

DISCUSSION...Birchard
AVIATION...JAF
MARINE...M Ballard


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