High Surf Advisory 41, NWS Honolulu, HI

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
1025 PM HST Tue Oct 24 2017

.SYNOPSIS...
Locally heavy showers and a slight chance of thunderstorms
associated with a slow moving front may lead to flash flooding
over the Big Island tonight and possibly into Wednesday. The
front will remain stationary just to the west of Big Island
tonight before shifting eastward across the Big Island late
Wednesday and Wednesday night. Deep moisture will linger over or
near the Big Island through Thursday, keeping shower chances
highest over the eastern end of the State. Cooler and drier
conditions with north winds will prevail across Kauai and Oahu
through the rest of the week, and will slowly track onto Maui
County by late in the day on Wednesday or Wednesday night. Weaker
winds, more humid weather, and increasing shower coverage may move
back across the island chain this weekend and early next week.

&&

.DISCUSSION...
Currently at the surface, a strong 999 mb low is centered around
675 miles north of Kauai, with a trailing cold front extending
southward through the Alenuihaha Channel between Maui and the Big
Island, where it has become nearly stationary. Additionally, a
weak low is evident in satellite imagery around 350 miles due
south of the Big Island. Meanwhile, a 1021 mb high is centered
around 1400 miles west-northwest of Kauai, with ridging nosing
into the western end of the island chain. Light northerly winds
have overspread the smaller islands, while light southerly winds
continue across the Big Island. Infrared satellite imagery shows
mostly cloudy to overcast conditions to the east of Oahu, with
generally partly cloudy conditions across Oahu and Kauai. Radar
imagery continues to show scattered light showers streaming
northward across Maui County, with shower coverage increasing over
south and southeast sections of the Big Island. Deeper convection
is also evident in satellite imagery developing along the
stationary front to the southwest of the Big Island. Main short
term concern revolves around the potential for heavy rainfall and
flooding overnight and possibly lingering into the day on
Wednesday across the Big Island.

Tonight through Thursday night,
Model solutions are in fairly good agreement showing the front
remaining nearly stationary between Maui and the Big Island
tonight. The front should then get kicked slowly eastward across
the Big Island late Wednesday and Wednesday night as the upper
level trough axis currently just west of the State, shifts
eastward. At the same time, the weak low due south of the Big
Island will lift steadily northward tonight and Wednesday, with
the low merging with the weak front or frontal remnants just east
of the Big Island late Wednesday night through Thursday night.

Light to moderate northerly winds are expected across the smaller
islands through the period, while light southerlies over the Big
Island shift around to the north late Wednesday and Wednesday
night as the front gets kicked eastward.

Scattered showers will continue to stream northward across Maui
County and the Big Island tonight and Wednesday, with the showers
driven by isentropic ascent over Maui County and the combination
of large scale ascent and deep tropical moisture over the Big
Island. Given the convection developing along the stalled front
to the southwest of the Big Island, some heavier showers and
isolated thunderstorms can't be ruled out here overnight and
Wednesday. Will keep the Flash Flood Watch in place for the Big
Island tonight, and re-evaluate if this will need to be extended
through the day on Wednesday with the 4 AM forecast package. The
deeper moisture and best forcing for ascent will shift east of the
Big Island Wednesday night through Thursday night, leading to a
decrease in shower coverage across Maui County and the Big Island,
with the shower focus shifting into mainly windward areas. Across
Oahu and Kauai, a drier northerly flow should keep showers light
and confined to mainly windward (north facing slope) areas through
the period.

Friday through next Tuesday,
Details become much less clear in the extended portion of the
forecast with notable differences between the reliable model
guidance along with poor run to run continuity. Generally, the
models suggest that a lingering trough of low pressure and
associated deep moisture east of the Big Island, will begin to
shift westward and back across the island chain over the weekend
into early next week. For now will take a blended forecast
approach, which would suggest we would see an increase in shower
coverage moving from east to west across the islands over the
weekend into early next week. Additionally, we should see an
increase in humidity as winds once again shift around to the
southeast and south as another cold front approaches from the
northwest.

&&

.AVIATION...
A band of clouds and showers continues to produce MVFR conditions
over Maui, Molokai and Lanai this evening. Cloud ceilings will
remain low enough overnight to produce Tempo Mountain Obscurations
and AIRMET Sierra remains in effect.

Light icing will continue to linger within this deep moisture
layer between 150/250 over Maui County and the Big Island
overnight.

This band of clouds and showers will slowly lift northward over
the next 24 hours with drier conditions returning to the island
chain.

&&

.MARINE...
A front in the Alenuihaha Channel this evening will be moving
slowly eastward through Wednesday, finally clearing the Big Island
Wednesday night. The front is expected to stall and weaken to a
trough over the eastern offshore waters during the second half of
the week.

A large north-northwest swell, generated by a strong low pressure
system 600 nm north of Kauai, is spreading across the island
waters this evening. This swell is large enough to warrant a high
surf advisory for the north and west facing shores of most
islands. This advisory is good through 6 pm HST Wednesday. This
swell has also prompted the issuance of a Small Craft Advisory
(SCA) for above normal seas. The SCA is good through Wednesday
night. The swell will be peaking tonight and Wednesday morning
before trending down.

A much larger, northwest swell (320 degrees) is set to arrive as
early as Thursday. This swell originated from once Typhoon Lan in
the northwest Pacific several days ago. Wave model guidance
projects a warning-level surf event for the north and west facing
shores with long-period forerunners peaking on Friday. A moderate
size northwest swell is expected to come in Tuesday of next week.

The south facing shores will be getting some small pulses mainly
from the southeast through Friday. After that, the surf will trend
smaller.

For more detailed information on the surf for Oahu, see the latest
collaborative forecast at: www.prh.noaa.gov/hnl/pages/SRF.php.

&&

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

Flash Flood Watch until 6 AM HST Wednesday for Big Island.

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

Small Craft Advisory until 6 AM HST Thursday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Kaiwi Channel-Maui County Windward Waters-
Big Island Windward Waters.

&&

DISCUSSION...Jelsema
AVIATION...Bohlin
MARINE...Lau