Drought in Hawaii, September 26 to 30, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
857 PM HST Mon Sep 25 2017

.SYNOPSIS...
A slow increase in the trades is expected over the next couple
days as a surface ridge to our north strengthens. The trades will
peak on Thursday at locally breezy levels. Some increase in trade
wind showers is expected Tuesday night through Wednesday night as
a band of moisture pushes across the islands. Our winds will shift
around to the southeast by the weekend as a cold front approaches
from the northwest. This will allow for daytime sea breezes and
overnight land breezes in most areas, along with bringing some vog
northward and into the smaller islands.

&&

.DISCUSSION...
Currently at the surface, a 1024 mb high is centered around 1700
miles northeast of Honolulu, with a ridge axis extending
southwestward to a location around 250 miles north of Kauai.
Meanwhile, a weakening trough of low pressure is located around
250 miles east of Hilo. Infrared satellite imagery shows some
lingering clouds over leeward sections of the Big Island, with
mostly clear to partly cloudy skies across the remainder of the
State. Radar imagery shows a few light showers moving into
windward areas, with some lingering showers over leeward sections
of the Big Island as well. Main short term concerns revolve around
rain chances and the strengthening trades over the next few days.

Tonight through Friday night,
The weak surface trough to the east of the State will weaken as
it shifts westward, reaching windward sections of the Big Island
late Tuesday night. The trough will dissipate by the middle of the
week, but will send a band of deeper moisture westward across the
island chain Wednesday and Wednesday night, bringing an increase
in trade wind showers to the area. At the same time, the surface
ridge axis will lift northward over the next couple of days,
resulting in a gradual strengthening of the trade winds. Light to
moderate trade winds are expected through Wednesday, with the
trades then increasing to moderate and locally breezy levels
Wednesday night through Thursday night. The trades will then
begin to ease again by the end of the work week and heading into
the weekend as a cold front begins to approach from the northwest.
Clouds and showers will favor windward and mauka areas through
the period, with a stray shower reaching leeward areas from time
to time. Rainfall amounts should generally remain on the light
side.

Saturday through next Monday,
There remains a significant amount of uncertainty with respect to
the overall synoptic pattern evolution over the weekend into
early next week, with quite a bit of wavering in the model
solutions from run to run as well as significant differences
between the GFS and ECMWF. The big question is how far east the
cold front manages to move, with the GFS showing it possibly
reaching the western islands, while the ECMWF keeps it to the west
of the State. For now will follow the more optimistic and
consistent ECMWF solution, keeping the cold front and deeper
moisture associated with it, to the west of the island chain.
Overall, with the cold front approaching the islands from the
northwest, we should see a weakening of the trades, with the
winds also shifting around to a more southeasterly direction over
the weekend into early next week. This will likely allow for
overnight land breezes and daytime sea breezes across the island
chain, with showers most common over leeward and interior sections
of the islands during the afternoon and evening hours and near
the coast during the overnight and early morning hours.

&&

.AVIATION...
A cold front far to the north of Hawaii will continue to weaken
the high pressure ridge producing light to moderate trade winds
over the islands through Wednesday. Localized sea breezes will
develop in some areas during the day with clouds ceilings
building over interior sections of each island. Localized land
breezes will clear out cloud cover in most areas. Large scale
stability across the region will limit any shower potential to
mostly isolated coverage over island interior and mountain
regions during the day and windward areas at night.

No AIRMETs are currently in effect or anticipated.

&&

.MARINE...
Trades will remain below Small Craft Advisory criteria through the
rest of the work week. There is a slight bump between Wednesday
night and Friday to moderate speeds, but not any higher as noted
in the previous discussion. This warrants closer attention,
however. Over the weekend, the trades are replaced by a light SE
to S wind flow, as a front nears the NW limit of our offshore
waters. According to the latest GFS run, the front may advance
further into the NW coastal waters of Kauai on Monday with light
southerly winds as the primary wind flow ahead of it. Light
northerly winds follow in behind the front if this solution pans
out.

A low level trough embedded in the trade wind flow, will pass
through the main Hawaiian islands on Wednesday, adding a boost to
the trade showers. Also, the passage of the trough will bring a
slight bump in the trades to the area.

The spotty thunderstorm activity just outside the NE offshore
waters has ceased as the upper low has weakened.

Surf is expected to remain below advisory levels through the
forecast period. Surf along south facing shores will remain small
through the week with mainly a mix of background southeast and
southwest swells. The next long-period pulse out of the southwest
should fill in Tuesday night through midweek, leading to a slight
rise along the south facing shores. A small bump from the NW is
expected Tuesday night. It will gradually fade after Wednesday. A
more moderate size NW swell is slated to arrive next week Monday.
Forerunners from this swell should be showing up Sunday afternoon.
Surf along east facing shores will remain small through
Wednesday, A slight rise is expected on Thursday as the trades
strengthen slightly.

&&

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

&&

DISCUSSION...Jelsema
AVIATION...Bohlin
MARINE...Lau