Area Forecast Discussion
Issued by NWS Honolulu, HI

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701
FXHW60 PHFO 041400
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
400 AM HST Mon Nov 4 2024

.SYNOPSIS...
A plume of tropical moisture will bring the potential for locally
heavy rain and a few thunderstorms through Tuesday, particularly
over Maui and the Big Island. A cold front will approach the
islands on Wednesday then stall and dissipate as a new high builds
to its north. This will bring a period of wet trades and breezy to
locally windy conditions to the islands later this week.

&&

.DISCUSSION...
Early morning satellite imagery shows a weak surface trough
approximately 200 miles southeast of the Big Island. As this
trough moves west over the next couple of days it will be the
primary forecast concern due to the potential for heavy rain and
localized flooding, particularly over windward Big Island and
Maui. CIMSS MIMIC precipitable water imagery continues to show a
broad area of 2+ inches of precipitable water around this trough.
The morning sounding from Hilo measured around 1.77 inches of
precipitable water which is close to the 90th percentile for this
time of the year.

Guidance continues to show the trough and its plume of moisture
moving west-northwestward across the islands through Tuesday. This
surface trough looks to be supported by a passing upper level
trough from the west which will help to increase instability
across the state. The combination of the troughs at the surface
and aloft along with enhanced tropical moisture will bring the
potential for heavy rain and a few thunderstorms over mainly
windward Big Island and Maui. A Flood Watch remains in effect for
these islands through Tuesday. Otherwise, expect trade winds to
strengthen a bit today as the surface trough increases the local
pressure gradient, with clouds and passing showers affecting
mainly windward and mauka areas of the smaller islands west of
Maui.

Trade winds look to weaken slightly on Tuesday as the surface
trough dissipates and a cold front approaches from the northwest.
Oahu and Kauai may see a brief period of heavy rain on Tuesday
before the trough dissipates, but confidence is not high enough to
expand the Flood Watch to these islands.

Guidance continues to diverge beyond Wednesday and generally falls
into two camps. Some model guidance (mainly the ECMWF) shows the
cold front moving across the state sometime Wednesday through
Thursday. However, other guidance suggests that the front will
briefly stall over or just north of the islands during this time
period before dissipating. Will continue to favor the non-ECMWF
solution of the front reaching the state by Wednesday then
stalling and dissipating over or just north of the islands by
Thursday as a new high builds in behind the front. Based on the
current forecast expect breezy to locally windy conditions to
return later this week and into the weekend, but will likely
need to make some adjustments as model guidance hopefully starts
to converge on a single solution.

&&

.AVIATION...
Moderate to locally breezy trades for today. Low cigs, SHRA and
some isol TSTMs are possible across the eastern side of the state
through tomorrow. With an increase in SHRA coverage and intensity
expect MVFR and isol IFR conds. On the western side of the state
expect the typical windward and mauka SHRA with brief MVFR conds
possible with those SHRA.

AIRMET Sierra is in effect for mtn obsc for windward Big Island.
This may be expanded to include other islands when/if conds
warrant.

&&

.MARINE...
A weak surface trough passing across the southern offshore waters
today will tighten the regional pressure gradient enough to
subtly strengthen trades for those waters surrounding Big Island
and Maui County. A Small Craft Advisory is in effect for these
waters through early Tuesday. The passage of this trough this
afternoon through early Tuesday will introduce an eastern island
nearshore and southeast-south offshore waters isolated thunderstorm
threat. Fresh to locally strong trades will transition west-to-
east across the local waters Wednesday as a Central Pacific cold
front and its associated backing surface high north of the state
move east. The pressure gradient over and upstream of the state
may be tight enough in the front`s wake or along the southern
periphery of the surface high to support fresh trades into the
weekend.

The ongoing medium period, moderate size northwest (330 degree) swell
that peaked yesterday afternoon will gradually subside through the
day. Near double head high surf will persist along better focused
reefs (in relation to this 330 degree swell) to maintain advisory surf
heights past noon. The swell and subsequent surf is expected to make a
significant fall from this evening through Tuesday. To account for
this slower decline, the High Surf Advisory for the north and west-
facing shores of the smaller islands, except for only the north on
Maui, will remain in effect through this afternoon. A mid to late week
frontal passage north of the islands will be the source of the next
medium to long period, moderate size northwest swell scheduled to
arrive sometime Wednesday night into Thursday morning. Surf along
east-facing shores will remain relatively small and choppy with select
exposures still experiencing some of this northwest swell wrap. As
trades strengthen during the second half of the week, east-facing
shore chop will likely be trending upward.

&&

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

Flood Watch through Tuesday afternoon for Maui Windward West-
Maui Leeward West-Haleakala Summit-Kona-Kohala-Big Island
Interior-Maui Central Valley North-Maui Central Valley South-
Windward Haleakala-Kipahulu-South Maui/Upcountry-South Haleakala-
Big Island South-Big Island Southeast-Big Island East-Big Island
North.

Small Craft Advisory until 6 AM HST Tuesday for Maui County
Windward Waters-Maalaea Bay-Pailolo Channel-Alenuihaha Channel-
Big Island Windward Waters-Big Island Leeward Waters-Big Island
Southeast Waters.

&&

$$

DISCUSSION...Ahue
AVIATION...Walsh
MARINE...Blood