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Hawaiian Islands Synoptic Discussion and Guidance

919
FXHW60 PHFO 191400
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
400 AM HST Thu Apr 19 2018

.SYNOPSIS...
High pressure north of the state will keep a moderate to breezy
trade wind flow in place through the weekend, before
strengthening to locally windy levels for the first half of next
week. A band of an enhanced clouds and showers associated with an
old front combined with an upper level trough moving through the
state, will keep showery weather and abundant cloud cover in
place through Friday, particularly over windward areas. Some of
the showers may be heavy at times and an isolated thunderstorm
can't be entirely ruled out. A more typical trade wind pattern
will return Friday night through the middle of next week, with
showers favoring windward and mauka areas.

&&

.DISCUSSION...
Currently at the surface, an east to west oriented ridge of high
pressure centered around 600 miles north of Kauai, is driving
moderate trade winds across the island chain early this morning.
Aloft, an upper level trough is located around 250 miles west-
northwest of Kauai, with an upper level ridge a few hundred miles
to the east of the Big Island. Infrared satellite imagery shows
mostly cloudy to overcast conditions across the state, with an
extensive shield of mid and high cloud extending well to the
southwest of the island chain. Meanwhile, radar imagery shows
scattered to numerous showers moving into windward areas in
association with an old frontal band, with the greatest coverage
over Maui County and the Big Island. Main short term concern
continues to revolve around the potential for heavy rainfall and
flash flooding.

Today through Friday,
A ridge of high pressure will remain to the north of the state
through the period, maintaining moderate to locally breezy trade
winds across the island chain. The old frontal remnants will
linger across the state over the next couple days, while the
upper level trough shifts slowly southeastward, moving through
the islands tonight and Friday. As a result, the airmass will
destabilize, and deep tropical moisture will increase.
Precipitable Water (PW) values will climb into the 1.6 to 1.9
inch range today and hold at these levels through tonight. The
airmass will begin to stabilize and deep moisture will decrease
from northwest to southeast down the island chain on Friday as
the upper trough shifts southeastward, with PW values ranging
from 1.2 inches near Kauai to around 1.6 inches over the Big
Island by early Friday evening.

As for sensible weather details, shower coverage and intensity is
expected to increase statewide today as the old frontal remnants
and upper level trough begin to interact, with showery weather
then continuing tonight. As deep moisture decreases and the upper
level trough shifts southeastward on Friday, shower coverage and
intensity will decrease from northwest to southeast. The highest
potential for heavy rainfall is expected from Oahu east to the
Big Island, particularly for windward areas. Given the high
levels of saturation from recent rainfall over Kauai however, any
additional moderate to heavy rain that occurs could quickly
result in flooding problems. As a result, the Flash Flood Watch
remains in effect for the entire state from 6 AM this morning
through 6 PM Friday. Thunderstorm probabilities don't appear all
that great, but given the unstable airmass and deep tropical
moisture in place, we can't rule out a rumble or two of thunder.
As a result, we will keep a slight chance of a thunderstorm in
the forecast, but we wouldn't be surprised if no thunder occurs.

Additionally, with temperatures near and below freezing and deep
moisture increasing over the Big Island summits, snow is expected
to develop for elevations above 12000 feet today. Periods of
snow, heavy at times, are then expected to continue through
Friday. As a result, the Winter Storm Watch has been upgraded to
a Winter Storm Warning for the highest elevations of Mauna Kea
and Mauna Loa from 6 AM this morning through 6 PM Friday.

Friday night through Wednesday,
A ridge of high pressure north of the state will maintain
moderate to locally breezy trade winds Friday night through the
weekend. High pressure will then strengthen as it shifts to the
northeast of the state Monday through the middle of next week,
allowing the trade winds to strengthen to breezy and locally
windy levels. Overall, a fairly typical trade wind pattern is
expected through the period, with bands of enhanced clouds and
showers drifting through the islands from time to time. Windward
and mauka areas will remain the primary targets for showers, with
an occasional shower spreading leeward due to the strength of the
trades. Model time/height relative humidity cross sections show
the mid and high cloud cover decreasing as the upper level trough
exits eastward, so more sunny conditions are expected to return
to the island chain. That said, some high cloud cover will likely
linger through the weekend, particularly over the eastern
islands, as a weakening upper level jet remains overhead.

&&

.AVIATION...
Moderate trade winds will continue across the Hawaiian Islands
producing moderate low level mechanical turbulence over and
downstream of mountains for the next 24 hours. Low cloud ceilings
and showers will linger over mountain areas of all islands.

A band of unstable clouds and showers will enhance shower
activity across all islands with MVFR ceilings along windward
slopes producing widespread mountain obscurations.

AIRMET Sierra in effect for Tempo Mountain Obscurations over
north through east facing slopes of all islands for elevations
above 2000 feet.

AIRMET Tango in effect for low level mechanical turbulence for
all islands over and south through west of mountains below 7000
feet. Another AIRMET Tango remains in effect for upper level
moderate turbulence due to the jet stream between FL280 and
FL400.

&&

.MARINE...
Mariners should be aware that an upper-level trough moving over
the area will likely produce unstable weather across the state
during the next couple of days. This will bring the threat of
locally heavy downpours and a chance of thunderstorms through
Friday.

A surface ridge far north of the area will maintain locally
strong trade winds over the area today. Therefore, a Small Craft
Advisory (SCA) remains in effect for most of the waters
surrounding the islands of Maui County and the Big Island through
this afternoon. The trade winds are expected to weaken slightly
by this evening as the high moves farther east and a front
approaches from the north-northwest. Starting this evening, the
SCA has been scaled back to only include the typically windy
waters adjacent to the islands of Maui County and the Big Island.
This SCA is in effect through Friday afternoon for now. However,
the latest forecast suggests that it will likely be extended from
Friday night into early next week for these same waters.

Surf will likely remain below the High Surf Advisory (HSA)
thresholds along all shorelines through this weekend. Rough,
moderate surf is expected to persist along most east facing
shores through Sunday night. Surf heights will increase again
along east facing shores early next week due to strengthening
trade winds. Surf may reach the HSA threshold along east facing
shores starting late Monday, and persist into the middle of next
week.

A moderate northwest swell arriving later today is expected to
peak Friday, and then gradually subside this weekend. A slightly
larger northwest swell is expected to spread down the island
chain late Sunday and peak on Monday. This swell will gradually
subside from Tuesday through Wednesday.

A series of small swells from the southern hemisphere will
maintain modest surf along most south facing shores across the
island chain through this weekend.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch from 6 AM HST this morning through Friday
afternoon for the entire state of Hawaii.

Winter Storm Warning from 6 AM this morning to 6 PM HST Friday
for Big Island Summits.

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

Small Craft Advisory until 6 PM HST this evening for Kaiwi
Channel-Maui County Windward Waters-Big Island Windward Waters.

&&

$$

DISCUSSION...Jelsema
AVIATION...Bohlin
MARINE...Houston

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

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