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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 261311

Area Forecast Discussion
National Weather Service Honolulu HI
311 AM HST Tue Sep 26 2017

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 tonight 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.


Currently at the surface, a 1024 mb high is centered around 1900
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
200 miles east of Hilo. Infrared satellite imagery shows clear to
partly cloudy skies in most areas, with some pockets of enhanced
cloud cover in windward areas as well as southern sections of the
Big Island near South Point. Radar imagery shows a few light
showers drifting into windward areas, with generally rain free
conditions elsewhere. Main short term concerns revolve around
rain chances and the strengthening trades over the next few days.

Today 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 tonight. 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 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. Additionally, some
volcanic haze will likely work northward into the smaller islands
Saturday through early next week.


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 at night. 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.


The trades are expected to strengthen Wednesday night and reach
Small Craft Advisory criteria Thursday for some of the waters
around Maui County and waters south of the Big Island. This boost
in the trades is brief as they will lower to below advisory level
on Friday. 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. At this time, the front is expected to stall some 350
miles northwest of Kauai, or just outside of our offshore waters
northwest of Kauai.

A weakening 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. This trough is currently 160 miles
east of the Big Island. The strengthening of the trades Wednesday
night coincide with the passage of this trough.

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. A small bump from the NW is expected tonight.
It will gradually fade after Wednesday. A 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 relatively small through the period, even with
the brief boost in the trades between Wednesday night and Friday.






Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office