Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 200713 CCA

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
913 PM HST Wed Jun 19 2019

Moderate trade winds will focus showers mainly across windward
areas through Thursday. Winds will weaken and shift to the
southeast from Friday through the weekend, leading to daytime sea
breezes, spotty interior showers, and more muggy conditions. Trade
winds may try and return early next week, but will likely remain
lighter than normal for early summer.


Currently at the surface, a 1033 mb high is centered around 1800
miles northeast of Honolulu, while a weak surface trough remains
nearly stationary several hundred miles northwest of Kauai. Aloft,
upper level ridging remains over the islands, with a broad upper
level trough northwest of the state. Infrared satellite imagery
shows clear to partly cloudy skies in most areas, with a few
pockets of enhanced cloud cover over windward slopes and leeward
sections of the Big Island. Radar imagery shows very little shower
activity, with nothing more than isolated coverage. Main short
term concerns revolve around the diminishing trades and rainfall
chances over the next couple days.

Model solutions are in good agreement showing the upper level
trough northwest of the islands, amplifying and cutting off into
a closed upper low over the next couple days. This is expected to
result in the surface trough northwest of the islands sharpening
up with a surface low developing along the trough. Locally, this
will lead to the winds shifting around to the southeast and
decreasing through the rest of the work week and over the
upcoming weekend. The trades appear to hold on through Thursday at
moderate strength, then a sea and land breeze pattern will likely
develop in the shadow of the islands Friday through the weekend.
There is some disagreement regarding the potential return of trade
winds next week, with the ECMWF suggesting the trades may
redevelop by Monday, while the GFS keeps a lighter wind regime in
place through the middle of next week. Regardless of which model
is followed, it appears that winds will remain lighter than normal
through the middle of next week as the semi-permanent surface
trough continues to meander several hundred miles northwest of
the state.

As for weather details, expect a fairly dry trade wind pattern to
hold in place tonight and Thursday, with a few showers mainly
affecting windward and mauka areas. As the trades diminish and
the low level boundary layer flow shifts around to the southeast,
speed convergence could result in a few more showers Thursday
night and Friday. Showers are expected to favor windward and mauka
areas and locations near the coast Thursday night and Friday
morning, and interior and leeward areas during the afternoon
hours Friday. A fairly dry pattern should then hold in place
Friday night through the weekend, with a few showers possible near
the coast at night and a few showers developing over the island
interiors each afternoon. Details become a bit more unclear next
week, but with the lighter wind regime and potential for returning
trades, showers should favor windward and mauka areas during the
day and interior and leeward areas during the afternoon and
evening hours. No significant rainfall is expected through the
forecast period.


A 1033 MB surface high will continue to sit stagnant far to the
northeast of the state. This will allow for moderate easterly
trade winds to persist across the islands. The atmosphere will
remain mostly dry and mostly stable. Limited shower activity will
be focused along windward slopes and coasts. Partial clearing
expected along leeward portions of the Big Island overnight.

No AIRMETs currently in effect or anticipated at this time.


Fresh to strong trades will hold tonight, then begin to weaken
Thursday through Friday as the ridge shifts southward into the
area in response to low pressure developing along an old frontal
boundary several hundred nautical miles northwest of Kauai. This
transition will allow the low-level flow to shift out of the
southeast direction, likely giving way to localized land and sea
breeze conditions near the coasts Friday through the weekend.
Forecast confidence begins to lower early next week due to
notable model differences. The ECMWF depicts a return of moderate
to fresh trades, while the GFS continues to show the light wind

Surf along south facing shores will remain small through early
next week with mainly a combination of short-period southeast
trade wind energy and overlapping background southerly swells
moving through.

For the long range, guidance is advertising a gale- to storm-
force low passing south to southeast of New Zealand over the
upcoming weekend with seas forecast to reach the 35 to 45 ft
range. If this scenario materializes, a decent south swell will
become a possibility locally at the end of the month through the
first couple days in July.

Small to moderate short-period surf will hold along east facing
shores through Thursday, then trend down Friday as the trades
ease. An upward trend, however, is anticipated over the weekend as
a moderate period (10-12 seconds) northeast swell from a pocket
of strong breezes that has setup within the 040-060 degree band
relative to the islands moves through. Heights should remain
below the advisory level for east facing shores through the peak
this weekend.


Fuel conditions are drier than normal, with the Keetch-Byram
Drought Index (KBDI) already over the red flag warning threshold
of 600. Conditions at HNL reached Red Flag Criteria briefly today,
sustained winds of 20+ mph and relative humidity values less than
or equal to 45 percent, but did not last for the required 2
consecutive hours. While we`re still not expected to reach all
the red flag thresholds on Thursday (wind will likely be the
limiting factor), conditions will be dry and somewhat gusty, which
bring increased fire danger. Decreasing winds and increasing RH
vales from Friday onward will diminish those concerns a bit.


Small Craft Advisory until 6 AM HST Thursday for Pailolo Channel-
Alenuihaha Channel-Big Island Leeward Waters-Big Island
Southeast Waters.



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