Wildfire in Hawaii, July 27 to 28, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
957 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 fresh trade winds are being supported by
surface high pressure centered about 1400 miles northeast of the
islands. The center of the surface trough from ex-Tropical
Cyclone Greg is estimated to be about 600 miles east of the Big
Island, and MIMIC total precipitable water (PW) imagery shows that
an associated area of increased PW (2-2.3 inches) extends from
17N to 22N between 153W and 138W, with additonal moisture trailing
SE from Greg. Water vapor imagery highlights a closed low aloft
centered about 500 miles northwest of Kauai, and a ridge aloft
centered about 700 miles south southeast of the Big Island. The
resultant southwest flow aloft is advecting some high level
cloudiness toward the islands. 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 feet 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
east 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 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, 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.

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 east
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 south 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 over the next several days. 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 and Friday, 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
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 locally breezy 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 this evening for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

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