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Eastern Pacific

NHC

Outlook | Discussion

Central Pacific

NHC

Outlook | Discussion

Central Pacific Discussion


000
FXHW60 PHFO 251302
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
302 AM HST Sun Jun 25 2017

.SYNOPSIS...
A mid level trough passing over the western islands today, will
keep the trade winds in the light to moderate range and result in
an increase in leeward shower activity during the afternoon and
evening hours. This mid-level trough will shift west of the State
tonight, with high pressure then building north and northeast of
the islands through the upcoming work week. This will result in a
drier and more stable trade wind shower regime, with the trades
increasing into the breezy to locally windy range Tuesday through
the end of the work week. A more showery trade wind pattern will
return late in the work week and next weekend, as an upper level
low approaches the islands from the east.

&&

.DISCUSSION...
Currently at the surface, a 1033 mb high centered around 1650
miles north of Honolulu, is driving light to moderate trade winds
across the island chain early this morning. Meanwhile aloft, a
mid-level 700 mb trough is enhancing some of shower activity over
the western islands. Infrared satellite imagery shows partly cloudy
skies in most areas, with some localized areas of mostly cloudy
conditions. Radar imagery shows scattered showers moving into
windward areas, with rain free conditions in leeward areas at
this time. Main short term concern revolves around rain chances.

Today,
High pressure will track slowly eastward well to the north of the
islands, maintaining a light to moderate trade wind flow across
the State. Localized sea breezes will likely develop again this
afternoon in the more sheltered leeward areas due to the weakened
trade wind flow. Meanwhile aloft, the weak mid-level trough will
shift slowly westward, moving over Kauai this afternoon. As for
sensible weather details, showers will continue to favor windward
and mauka areas through the period as the trades continue to blow
across the island chain. In leeward areas, can't rule out an
isolated shower this morning, but better chances will come this
afternoon with the assistance of sea breeze convergence in the
weakened trade wind regime. There could be a few enhanced showers
this afternoon as well, particularly over Kauai in association
with the mid-level trough moving overhead.

Tonight through Thursday,
The mid-level trough will exit to the west of the state, while
surface high pressure builds to the northeast of the island chain.
This will increase the pressure gradient across the islands,
resulting in strengthening trade winds tonight and Monday. The
trades are expected to strengthen to breezy and locally windy
levels Tuesday through Thursday. In addition to the strengthening
trades, with the exit of the mid-level trough to the west of the
islands, a drier more stable airmass will move into the area.
There will likely be a bit more windward shower activity tonight
due to the convergence associated with the returning trades. By
Monday and continuing through Thursday, precipitable water values
will drop into the 1.0 to 1.2 inch range, below normal for this
time of year, and this will result in a drier than normal trade
wind shower pattern. Showers will continue to favor windward and
mauka areas, with a stray shower reaching leeward areas from time
to time due to the strengthening trades. Rainfall amounts are
expected to remain light through the period.

Thursday night through next Saturday,
Both the GFS and ECMWF are in good agreement showing an upper
level low tracking eastward toward the islands Thursday night and
Friday, then lifting northward and away from the islands for the
first half of next weekend. High pressure will continue to hold in
place north of the islands through the period, with breezy trade
winds expected to continue through at least Thursday night. The
trades are then expected to weaken Friday through Saturday as the
gradient slackens due to an upper level low approaching from the
east. As for sensible weather details, both models show deep
layer moisture increasing beginning Thursday night, with this
enhanced moisture remaining over the islands through next
Saturday. As a result, we should see a more showery trade wind
pattern through the period, with showers continuing to favor
windward and mauka locales.

&&

.AVIATION...
Light to breezy northeast to east trade winds will continue
today. Expect onshore sea breezes to develop mainly along western
slopes of all islands with cloud ceilings building over mountain
areas after 20Z. TEMPO showers and isolated MVFR conditions are
forecast.

No AIRMET's in effect or anticipated over the next 24 hours.

&&

.MARINE...
Light to moderate trade winds associated with 1033 mb high
pressure far north of the state will continue into tonight. Fresh
to strong trades are expected to return through the day Monday and
continue through the week as the gradient tightens over the
region. Small craft conditions will be likely through this period
over the typically windier channel waters, Maalaea Bay and south
of the Big Island.

Surf along east facing shores will build and become rough through
the week due to a combination of increasing trades and a moderate
northeast swell associated with a recent batch of gales off the
west coast. Surf will approach advisory levels along east facing
shores through this time, especially Monday through Tuesday as the
northeast swell peaks.

Although surf along south facing shores will trend down toward
average going into the upcoming week, small long-period reinforcing
pulses from the southern Pacific will keep things from going flat. A
blocking pattern has become established east of New Zealand, which
is leading to fetch regions associated with passing storms
setting up farther east than normal or more toward French
Polynesia. Another pulse from this region out of the south-
southeast is forecast to fill in Wednesday across the local waters
and bring the surf back up a notch going into the second half of
the week. For the long range, nothing significant is anticipated
at this time as model guidance continues to support this blocking
pattern holding down south.

Up north, mainly wrap into the typically exposed locations from the
northeast swell Monday through Tuesday and the increasing trades
will be expected for surf. Otherwise, nothing significant is
anticipated through the week.

Extreme tides that have been observed over the past several days
will trend back toward normal into the upcoming week. As a result,
flooding impacts along the coast will diminish.

&&

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

&&

$$

DISCUSSION...Jelsema
AVIATION...Bohlin
MARINE...Gibbs

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