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Hawaiian Islands Synoptic Discussion and Guidance

354
FXHW60 PHFO 260701
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
901 PM HST Sat Jan 25 2020

.SYNOPSIS...
Dry weather and moderate trade winds will continue through Monday
morning, with a few light showers favoring windward areas. The
winds will then shift southeasterly Monday afternoon through
Thursday as a series of cold fronts approach from the northwest,
with daytime sea breezes and overnight land breezes becoming
common across most of the smaller islands. Rather dry conditions
will prevail through much of the work week, although a few light
showers will continue to trickle into windward sections of the
eastern islands, and a few cloud build ups and interior showers
will be possible across the island interiors each afternoon. A
strong cold front will move into the islands Friday into next
weekend, potentially bringing a wet and unsettled weather pattern
to portions of the state.

&&

.DISCUSSION...
Currently at the surface, a 1026 mb high is centered well east-
northeast of the state, with the ridge axis extending westward to
a location around 275 miles north of Kauai. The resulting
gradient is producing light to moderate trade winds across the
island chain this evening. Infrared satellite imagery shows clear
to partly cloudy skies in place, with a few pockets of enhanced
cloud cover over windward slopes. Radar imagery shows very little
shower activity, with just a few light showers moving into
windward sections of the smaller islands. Main short term concern
revolves around the trade wind trends.

The surface ridge will remain north of the island chain tonight
through Tuesday, before sliding southward and over the state
Wednesday and Thursday. The first in a series of cold fronts will
approach from the northwest through the first half of the work
week, with the second and stronger cold front moving into the
western islands Friday, then stalling somewhere across the western
or central islands next weekend. Moderate trade winds are
expected to prevail through Monday morning, with the strongest
trades across the eastern islands which will be further away from
the ridge axis. Boundary layer winds begin to shift around to the
southeast Monday afternoon, and will hold in this general
direction through much of the work week. This will keep moderate
east-southeast to southeast winds in place across the eastern end
of the state, while most of the smaller islands see light and
variable winds featuring daytime sea breezes and overnight land
breezes. Strong trade winds should return from northwest to
southeast down the island chain late Friday into next weekend as a
strong high builds in behind the cold front progressing down the
island chain.

Rather dry weather will prevail across the islands through late
in the work week, as precipitable water values hold generally at
or below 1 inch and low/mid level ridging persists over the
region. A few light showers will be confined primarily to windward
areas tonight through Monday morning. By Monday afternoon, we
should begin to see a few cloud buildups and interior showers
develop as sea breezes become more common. This pattern featuring
a few cloud build ups and showers over the island interiors will
then hold in place through Thursday, although some windward
showers will continue across the eastern islands in areas favored
under east-southeasterly to southeasterly flow. We will likely
begin to see some jet cirrus begin to spread across the islands on
Wednesday as well, it's difficult to know how thick this will be
at the moment, but it does appear to thicken up a bit Wednesday
night and Thursday. More unsettled weather appears to be heading
this way beginning Thursday night or Friday, and holding into next
weekend as the stronger of the two fronts enters the islands.
Although the models differ on the specifics, it appears that some
wet weather is in store for at least portions of the state,
particularly windward locales Friday and next Saturday.

&&

.AVIATION...
A ridge of high pressure to the north of the islands will persist
through twenty-four hours and beyond. Light to locally moderate
trade winds will continue to carry bands of low topped cumulus and
stratocumulus towards north and east facing slopes and coasts.
Isolated MVFR CIG and VIS are to be expected in passing showers.
Otherwise VFR conditions will prevail all areas.

&&

.MARINE...
A ridge of high pressure north of the area will sustain moderate
to locally breezy trade winds across most coastal waters through
Monday. Beyond that time frame, a couple of fronts approaching the
area from the northwest through next week will weaken the winds
and turn them toward the east and southeast.

The current northwest swell continues to remain elevated this
evening, and a High Surf Warning remains in effect for exposed
north and west facing shores of the smaller islands through Sunday
and a High Surf Advisory for west facing shores of the Big Island
through Sunday night. Latest buoy readings from NOAA buoy 51101
and 51001 continue to show swell heights of between 13 feet to 16
feet with a period of about 16 seconds as of 8 pm this evening.
With an average travel time from these buoy to Oahu of around 11
hours, this would suggest extra large breakers continuing for Oahu
and Maui County through at least the morning hours on Sunday. The
swell is expected to gradually drop during the day Sunday with
surf dropping to advisory levels by Sunday night or Monday.

A Small Craft Advisory is in effect for all Hawaiian waters
except Maalaea Bay through Sunday due to a combination of winds
and large seas from the northwest swell. Some marine zones will
likely hold on to SCA level seas into early next week.

Surf will remain elevated along north and west facing shores as a
progressive weather pattern across the northern Pacific keeps a
series of large northwest swells moving through the islands, at
least through the middle to latter part of next week. The next
northwest swell is expected to arrive Tuesday night from a large
fetch of gales currently blowing in the northwest Pacific. Latest
model guidance is suggesting the swell to peak at high end
advisory levels sometime on Wednesday.

Surf along exposed east facing shores will remain rather small
through the period due to the lack of persistent trades locally
and upstream across the eastern Pacific.

Surf along south facing shores will remain small through next week.
A couple background south swells expected over the weekend and
again on Tuesday should be enough to keep things from going flat.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning until 6 PM HST Sunday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala.

High Surf Advisory until 6 AM HST Monday for Kona-Kohala.

Small Craft Advisory until 6 PM HST Sunday for all Hawaiian
waters except Maalaea Bay-

&&

$$

DISCUSSION...Jelsema
AVIATION...Bedal
MARINE...Kino

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



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