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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 282001

Area Forecast Discussion
National Weather Service Honolulu HI
1001 AM HST SUN AUG 28 2016

Locally breezy trade winds will continue for the next few days as
high pressure persists north of the state. Passing showers will
favor windward and mauka areas, with showers most active through
this evening. By the middle of the week, we may begin to feel
impacts from Tropical Storm Madeline, with Hurricane Lester
possibly affecting the islands over the upcoming Labor Day


Water vapor imagery shows an upper level ridge to the northwest of
the state, with a large low far to the northeast. At the surface,
high pressure is located far to the north, with a ridge passing
several hundred miles northwest. The 12z sounding from Lihue shows
an inversion over 8kft and a precipitable of 1.41 inches. The
sounding from Hilo appears contaminated by a shower, but shows a
greater moisture depth and higher PW. Early morning MIMIC total
precipitable water imagery shows pockets of higher values up to
2.0 inches near the Big Island and Maui, with elevated values
across much of the coastal waters. Drier air with PW values less
than 1.3 inches is located within 100-200 miles northeast of the
state. Farther upstream the moisture associated with Madeline
exceeds 2 inches, and these high values extend almost as far north
as 30N.

Trade winds will be the dominant feature for the next couple
days. Even though the high is far north of the state, a nearly
stationary low to our south will help to enhance the pressure
gradient across the islands. This will lead to locally breezy
conditions, especially through Monday. Rainfall picked up last
night, with rain gage measurements over the past 12 hours showing
widespread quarter to half inch amounts with a handful of one to
two inch totals. Little change is expected today, with abundant
moisture in place and elevated inversion heights.

The pocket of high moisture over the state will continue to move
southwest, with drier air filtering in tonight from the northeast.
Forecast soundings/cross sections show moisture depth decreasing
tonight/Monday, with inversion heights and precipitable water
bottoming out Monday afternoon. The current forecast is
particularly dry, with low-end scattered showers across windward
sections by Monday, and this may even be on the high side given
the amount low-level drying.

On Tuesday/Tuesday night, both the GFS and ECMWF show a surge of
deep moisture moving across the state from the east. This can be
traced back to the northern extent of the tropical moisture north
of Madeline. This gets pulled ahead in the trades and will reach
the area ahead of the main system. Because this moisture spans a
larger area, there is better confidence in it affecting the state
for a period Tuesday/Tuesday night. Beyond that, we head into the
particularly uncertain period of the forecast.

The Central Pacific Hurricane Center in Honolulu issuing
bulletins on Tropical Storm Madeline, which is located just west
of 140W. Madeline is expected to move northwest for the next day
or two before turning west, and may impact the state during the
late Tuesday through Thursday time frame. There is a large
degree of uncertainty with the speed, track, and intensity of
tropical systems like this, which can result in significant
changes to the types of impacts we could see. Be sure to stay up
to date with the latest forecasts, which are issued every six
hours. In addition, Hurricane Lester is located far to the east,
but may affect the state over the upcoming holiday weekend. There
has also been a lack of consistency in model guidance with Lester
this far out in time, which leads to even greater uncertainty as
to possible impacts to the state.


A band of low clouds and showers is slow to clear the smaller
islands. Thus, AIRMET Sierra for widespread mtn obscuration for
the smaller islands will continue into the afternoon hours.
We anticipate VFR conditions to set in with most of the islands
by 04z. Scattered showers may develop across leeward Big Island
this afternoon after 00z, and this may prompt an AIRMET Sierra for
the Kona slopes. VFR conditions will prevail elsewhere. No other
AIRMETS are expected.


A Small Craft Advisory remains in effect for the typically windier
locations around Maui county and the Big Island, due to locally
breezy trade winds accelerating around the island terrain. These
conditions are expected to continue for the next few days.
Depending on the eventual track of Tropical Storm Madeline,
a Tropical Storm watch may be needed for portions of the marine
area within the next day or so.

No significant swells are expected through Monday, though surf
along east facing shores will be slightly elevated due to the
stronger trades. Winds and seas, as well as surf along east facing
shores, will likely increase from east to west late Tuesday/
Tuesday night with the approach of Madeline. The east swell may
peak Wednesday, and heights will depend on the eventual track and
intensity of the system. We may also start to see a swell from
Hurricane Lester as early as Wednesday or Thursday, with heights
building into the weekend.


Small Craft Advisory until 6 PM HST Monday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.




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