Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 281338
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
338 AM HST Fri Jul 28 2017

.SYNOPSIS...
Moisture associated with the remnants of former Tropical Cyclone
Greg will bring increased humidity and more showery weather to the
area today and tonight, some of which may be locally heavy. Most
of the showers will affect windward areas, but showers will also
reach leeward sections of the smaller islands, while the leeward
Big Island slopes will see a few afternoon downpours over the next
couple of days. A trend toward drier trade wind weather is
expected Saturday and Saturday night, with more typical trade wind
weather then continuing through the middle of next week. High
pressure north of the State will drive primarily moderate trade
winds for the next week.

&&

.DISCUSSION...
Currently at the surface, the remnant trough of former tropical
cyclone Greg is located around 400 miles southeast of Hilo.
Meanwhile, a 1026 mb high centered around 1500 miles northeast of
Honolulu, continues to drive light to moderate trade winds across
the island chain early this morning. Infrared satellite imagery
shows partly to mostly cloudy skies in place, with some scattered
to broken jet stream cirrus seen moving across Maui County and the
Big Island. Radar imagery shows numerous showers moving into
windward sections of the smaller islands, with windward sections
of the Big Island remaining mostly dry for the time being due to
the offshore land breeze. Meanwhile a few showers are spreading
int leeward areas of the smaller islands, while leeward sections
of the Big Island remain rain free. Main short term concern
revolves around the potential for heavy rain today and tonight,
as the remnants of Greg pass just to the south of the island
chain.


Today and tonight,
The 12Z soundings indicate that the airmass is slowly moistening
up across the island chain, with precipitable water (PW) values
climbing to 1.57 inches at PHLI and 1.41 inches at PHTO. MIMIC
total precipitable water imagery appears to be handling the
increasing moisture moving into the islands well, with a plume of
PW values between 1.7 and 2.0 inches just upstream of the island
chain. This higher PW airmass will overspread the islands this
morning and remain in place through tonight. As a result, showery
weather is expected across much of the State. Aloft, an upper
level disturbance moving overhead will act to further erode the
inversion over the area, allowing the depth of deep moisture to
increase. As a result, some of the shower activity could be heavy,
particularly over windward areas of all islands, as well as
leeward sections of the Big Island. With the trade winds expected
to continue at moderate levels, the threat for flash flooding does
not appear particularly high. The 12Z soundings from PHTO and PHLI
showed some mid-level dry air in place along with weak mid-level
inversions. Some moistening of the mid-levels and erosion of the
inversions is expected, but lapse rates are forecast to remain
unfavorable for thunderstorm development. Given all these factors,
the threat for flash flooding although not zero, appears low.
Conditions will be quite humid across the area, with dewpoints
rising into the lower 70s in most areas this morning. Moderate
trade winds will prevail, with showers most frequent over windward
areas. Showers will reach leeward areas more regularly however,
given the higher moisture depth and prevailing trade wind flow.

Saturday and Saturday night,
The plume of higher PW air will continue to affect the Big Island
through the day on Saturday, but a drying trend is expected across
the smaller islands by Saturday afternoon, possibly earlier.
Conditions are then expected to begin drying out and stabilizing
over the Big Island Saturday night. As a result, we will likely
continue to see showery trade wind weather affecting windward
areas of all islands Saturday morning, with the shower coverage
dropping off across the smaller islands Saturday afternoon and
Saturday night. Across the Big Island, wet conditions are expected
to continue for windward areas through the day on Saturday, with
localized downpours also affecting leeward slopes. Shower coverage
should begin to decrease across the Big Island Saturday night as
the drier more stable airmass works its way in. Showers will
continue to favor windward areas through the period, with a stray
shower reaching leeward areas of the smaller islands from time to
time. Moderate trade winds are expected to prevail.

Sunday through Thursday,
A dry and stable trade wind pattern is expected Sunday through
Monday, with lower than normal shower coverage. A more typical
trade wind pattern will then return Monday through late next week
as the airmass moistens up. Showers will continue to favor
windward and mauka areas through the period, with a stray shower
reaching leeward locales mainly during the overnight and early
morning hours.

&&

.AVIATION...
A high pressure center far north of the Hawaiian Islands will keep
moderate strength northeasterly trade winds in the forecast.
Remnant moisture from former tropical cyclone Greg will continue
to move across the islands this morning with increasing showers
and periods of MVFR conditions expected. The best shower coverage
along with Tempo MVFR conditions will occur over northern and
eastern slopes of all islands. The Big Island eastern slopes will
become predominate MVFR ceilings in showers after 14Z. Scattered
showers with more isolated MVFR conditions are forecast elsewhere.

No AIRMETS are in effect. AIRMET Sierra for mountain obscurations
are likely later this morning.

&&

.MARINE...
Surface high pressure far northeast of the islands will maintain
trade winds across the region into early next week. The latest
forecast indicates trade wind speeds will strengthen to the Small
Craft Advisory (SCA) threshold over Maalaea Bay, the Pailolo
Alenuihaha Channels from this evening into early next week. The
trade winds will also eventually strengthen south of the Big
Island, so the waters south of the Big island will likely be
added to the SCA starting late Saturday.

Note that a large area of enhanced tropical moisture associated
with former Tropical Cyclone Greg is already moving into the
islands from the east this morning. Although we do not anticipate
any major changes to our typical trade wind weather pattern,
mariners should expect to see an increase in low clouds and
showers over some parts of the coastal waters from later today
into this weekend.

The trade winds will keep moderate choppy surf along east facing
shores of most islands into early next week. In addition, the wave
model guidance continues to indicate mid- to long period east
swells from distant east Pacific tropical cyclones Irwin and
Hillary may reach the islands starting this weekend. Once these
swells arrive, they will need to be monitored for more specific
details on the timing and resultant surf heights along east facing
shores. In addition, we will also need to determine if this swell
energy combined with the trade wind waves might produce surf
approaching the High Surf Advisory criteria along some windward
shorelines.

A small mid- to long period west swell produced by a distant
typhoon in the west Pacific is expected to spread across the area
from later today into this weekend. Therefore, some exposed west
facing shores could see a small bump in surf. However, Niihau and
Kauai will likely block much of this swell energy from the west
facing shores of the remaining islands.

Elsewhere, a series of small mid- to long period southeast, south
and southwest swells will produce small background surf along
most south facing shores through the middle of next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory from 6 PM this evening to 6 AM HST Monday
for Maalaea Bay-Pailolo Channel-Alenuihaha Channel.

&&

$$

DISCUSSION...Jelsema
AVIATION...Bohlin
MARINE...Houston


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