Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 290212

Area Forecast Discussion
National Weather Service Honolulu HI
412 PM HST SUN AUG 28 2016

Moderate trade winds will continue for the next few days as high
pressure persists north of the state. Showery conditions this
evening will diminish overnight, with much drier conditions on
Monday. 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 weekend.


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. 00z soundings show the
changing airmass across the state, with an inversion around 7kft
and precipitable water of 1.31 inches at Lihue and an inversion
over 10kft and PW of 1.64 inches at Hilo. Early afternoon MIMIC
total precipitable water imagery shows the band of highest
moisture centered near Maui County and the Big Island, which
extends over 100 miles upstream to the northeast and east. Drier
air is located near and to the northeast of Kauai. 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 maintain a moderate
pressure gradient across the islands. Rainfall that picked up
last night continued through the day. Rain gage measurements over
the past 12 hours showing widespread quarter to half inch amounts
with a handful of one to two inch totals. Drier air has already
reached Kauai, and will spread across the rest of the state
through the night. Forecast soundings/cross sections show
moisture depth decreasing tonight/Monday, with inversion heights
and precipitable water bottoming out Monday afternoon. Have
trimmed back rainfall chances a bit, with just isolated to
scattered showers across windward sections by afternoon.

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 less than
1000 miles east of the state. Madeline is expected to move
northwest for the next day or so 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, and 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.


The band of clouds and showers continues to fall apart slowly
across the main Hawaiian Islands this afternoon. AIRMET Sierra,
for mountain obscuration, was canceled for Kauai and Oahu a short
while ago, and should remain this way through the evening. However,
AIRMET Sierra remains in effect for the windward and mountain
areas of Molokai and Maui. Scattered showers developed over the
windward Big Island slopes east of Laupahoehoe and down the Kau
slopes warranting AIRMET Sierra. Conditions should be improving
during the first half of the evening where Sierra can be lowered
by the 10z package. Otherwise, VFR conditions will prevail during
overnight hours.


The morning ASCAT pass showed at most 20 knots of wind through
the Alenuihaha Channel and south of the Big Island. High
resolution model guidance showed stronger winds today, with a
weakening trend overnight. With no strengthening expected, we have
dropped the small craft advisory. 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 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. However, surf up to warning level surf
along east facing shores is possible. We may also start to see a
swell from Hurricane Lester as early as Wednesday or Thursday,
with heights building into the weekend.





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