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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 240153

Area Forecast Discussion
National Weather Service Honolulu HI
353 PM HST Fri Mar 23 2018

Unsettled weather conditions are expected through the Saturday
night as a series of low pressure troughs move through the state.
Heavy rainfall and thunderstorms will be possible at times as
these disturbances move through. Conditions are expected to
improve across the area on Sunday. A light wind regime will allow
for daytime sea breezes and overnight land breezes Sunday through
the middle of next week. This will allow for daytime shower
development over interior and leeward sections of the islands,
with showers hugging the coast at night. Light trade winds may
return late next week.


Currently at the surface, a trough of low pressure is evident in
satellite imagery around 125 miles southwest of Honolulu, and was
captured well by an ASCAT pass earlier today. Another trough of
low pressure which has lingered for several days now to the west
of the state, is located around 400 miles west of Kauai this
afternoon. Meanwhile aloft, a strong shortwave trough can be seen
in water vapor imagery around 750 miles west-northwest of Kauai,
and this feature is tracking southeastward at around 35 mph.
Satellite imagery shows mostly cloudy to overcast conditions
across most of the state, with a few breaks evident in the cloud
cover over the Big Island. Radar imagery shows mainly light
showers over the islands of Kauai and Oahu, with a few heavier
showers now beginning to develop over the waters to the south and
east of Kauai. Meanwhile across Maui County and the Big Island, a
few light windward and daytime heating driven showers are
occurring. Main short term concerns revolve around the potential
for heavy rainfall and severe weather through Saturday night.

Tonight through Saturday night,
The GFS and ECMWF are in good agreement showing a pair of low
pressure troughs shifting across the state. The first trough south
of the state is expected to sharpen up this evening with a weak surface
low developing just south of Oahu and Maui County by daybreak
Saturday. The models then show this weak low tracking eastward
near or over the Big Island Saturday afternoon into Saturday
evening, before exiting east of the state after midnight Saturday.
The other trough currently west of the state is expected to
approach from the west tonight, then shift eastward across the
smaller islands Saturday into Saturday evening.

Overall, we expect a fairly wet night on Kauai and Oahu, with
showers increasing across Maui County, particularly after
midnight. The most persistent shower activity is then expected to
shift from west to east across the island chain Saturday and
Saturday night, exiting to the east of the state after midnight
Saturday. Although the deepest moisture will shift east of Kauai
and Oahu by Saturday afternoon, there will likely remain some
lingering shower activity through Saturday night here in
association with the secondary trough of low pressure.

Across the Big Island, scattered showers are expected across
windward areas tonight, with isolated showers possible in leeward
locales. Rain is then expected to increase in coverage and
intensity on Saturday with showers continuing into the evening
before decreasing after midnight.

As far as flooding potential goes, the combination of deep
tropical moisture surging northward in advance of the pair of low
pressure troughs along with strong forcing for ascent, could
result in some heavy rainfall and thunderstorms across the
islands. The most probable time frame for flash flooding across
the smaller islands is expected tonight through Saturday, so will
adjust the timing of the Flash Flood Watch to run through 6 PM
Saturday. Heavy rainfall is expected to hold off tonight across
the Big Island before increasing on Saturday. As a result, have
adjusted the Flash Flood Watch timing to run from 6 AM Saturday
through 6 AM Sunday for the Big Island.

In addition to the potential for heavy rainfall, there is a threat
for severe weather mainly across the Big Island as the lead
trough of low pressure moves through. Model solutions show plenty
of deep layer shear, with 45-55 knots in the 0-6 km layer to
support organized convection. The limiting factor will be surface
based instability, with Most Unstable (MU) CAPE values in the 500
to 1000 J/KG range. The other factor that lowers confidence about
severe weather occurring is that the model solutions have shifted
quite a bit from the previous run regarding location where the
severe weather potential would be highest. That said, based on the
latest guidance, the highest probability of severe weather
appears to be across the Big Island, with damaging straight line
winds of 60+ mph and isolated tornadoes not out of the question.
As a result, have included a mention of possible severe
thunderstorms across the Big Island Saturday afternoon and
Saturday evening.

Finally, the summit weather on the Big Island will likely go down
hill tonight. H6 winds are expected to increase significantly,
and a High Wind Warning has been issued through 6 AM Sunday.
Additionally, with the deep moisture moving through, there is the
potential for heavy snow at the summits. This remains a little
uncertain given summit level temperatures around or a little above
freezing. Nevertheless the potential appears to be there for a
heavy snow event, so a Winter Storm Watch has been issued running
from 6 AM Saturday through 6 AM Sunday.

Sunday through next Friday,
Conditions should improve across the islands on Sunday as a
surface low intensifies north of the state. A weak wind regime
will remain in place Sunday through the first half of the week,
with light trades returning by late next week. As a result, we
should see a more convective weather pattern featuring daytime
shower development over the interior and leeward sections of the
islands, with shower activity remaining offshore or near the coast
at night.


A surface trough moving in from the west is already impacting
western portions of the state with widespread cloud cover
stretching over the smaller islands. The trough is expected to
continue moving eastward similarly impacting Lanai Maui and the
Big Island. AIRMET Sierra is in effect for mountain obscurations
for the Big Island Kauai Oahu and Molokai due to the clouds. Will
probably be updating AIRMET Sierra to include Maui as the surface
trough is expected to be over Maui County overnight tonight into
the morning Saturday. Isolated periods of IFR conditions may

Temporary light in-cloud icing expected for Oahu Kauai Molokai and
surrounding waters through tonight.


Surf reports and the nearshore buoys indicate the northeast swell
continues to decline, with surf along east facing shores below
High Surf Advisory levels. Thus the advisory is being cancelled
with the afternoon package. The current northeast swell will
continue to subside this weekend, and no other significant swells
are expected through early next week. A modest north-northeast
swell is expected from Saturday night into Monday. A small
northwest swell is also expected from Sunday night through
Tuesday, followed by another small northwest swell during the
middle of next week. Small south-southwest and southwest swells
are also expected from today into early next week.

Winds have been on the decline, and have fallen below Small Craft
Advisory (SCA) criteria, so the SCA has been cancelled with the
afternoon package. Even though winds and seas are expected to be
below SCA levels starting tonight, mariners should be aware of the
threat of inclement weather this weekend.

An area of low pressure is expected to move across the state this
weekend, producing an extended period of rain, with a chance of
thunderstorms and locally heavy rainfall. The current forecast
continues to show a belt of localized south to southwest winds
approaching 25 knots developing along the eastern flank of this
low. Should this pan out, we may need to re-post an SCA Saturday
for portions of the waters adjacent to the western islands. Once
this low begins to depart the state, expect westerly winds to
spread across the islands from Saturday night through Sunday,
followed by light and variable winds early next week.


Flash Flood Watch through Saturday afternoon for Niihau-Kauai-

Flash Flood Watch from Saturday morning through late Saturday
night for the Big Island.

High Wind Warning until 6 AM HST Sunday for Big Island Summits.

Winter Storm Watch from Saturday morning through late Saturday
night for Big Island Summits.



MARINE...M Ballard

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