690
FXHW60 PHFO 111929
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
929 AM HST Sat May 11 2024

.SYNOPSIS...
Heavy rain and isolated thunderstorms are possible over all
islands again today. The threat will diminish by tonight and a
land and sea breeze pattern favoring clouds and showers over
island interiors will continue into the middle of next week.
Another low will bring the threat of heavy rain to the islands
during the second half of next week.

&&

.DISCUSSION...

Local radar composite loop hints at the present state of the middle
and upper atmosphere as heavy showers and thunderstorms rotate
cyclonically around the area. Heavy rain up to 3"/hour lifting
northeastward over northern Big Island has recently dissipated while
a fresh round of heavy showers developed and moved over Kauai
from the north this morning. In between, steering flow remains
rather nebulous and slow-moving showers and thunderstorms have
developed off the windward coasts of Oahu and Molokai. The onset
of daytime heating will allow these showers to redevelop inland.
Worth noting that all guidance has quite a poor handle on the
current distribution of precipitation across the state this
morning. The Flood Watch remains in effect for the islands through
today.

Focus for today will shift toward another west-east progression of
heavy rain potential as a shortwave embedded within the parent low
aloft pivots through the area during the next 12-18 hours. The
thermal trough associated with this second wave is of similar
magnitude to yesterday`s, if not slightly colder (-14 to -16C at
500mb). 700-500mb lapse rates over Kauai have almost certainly
steepened from this morning`s 12z sounding and are probably solidly
in excess of 7.0 C/km. This favorable environment for deep
convective growth leading to heavy showers and isolated
thunderstorms will sweep eastward as the day progresses. Heavy rain
leading to flash flooding will be a possibility for all islands once
again today.

The resident upper low will very gradually reabsorb into the
baroclinic westerlies during the next few days leaving the islands
firmly within the trailing envelope of cyclonic flow aloft, though
forcing and instability will diminish. In the near term (Sunday into
Monday), the established airmass will remain in place with continued
weak troughing over the islands supporting a light easterly gradient
and the consequent land and sea breeze pattern. This will favor
afternoon cloud build ups and interior showers of less intensity
than those of yesterday and today.

Strong consensus exists among the global models that high amplitude
mid-latitude troughing will beget another closed low in the vicinity
of the islands during the middle of next week. The local gradient
responds to the associated height falls as early as Monday-Tuesday
when surface winds veer to southeasterly and advect higher dewpoints
(near 70F) into the region. In addition to the lack of meaningful
wind and increasingly uncomfortable humidity, this will supply
plenty of boundary layer moisture for afternoon showers as the sea
breeze pattern continues.

The second half of next week bears watching as the aforementioned
closed low reaches its southernmost extent. Current consensus
indicates PWATs rising toward 2" as convergent southwesterly flow
establishes over the islands leading to an elevated heavy rain
threat. The inherited forecast handles this potential well,
including the risk of thunderstorms.

&&

.AVIATION...

A strong upper level disturbance will continue to gradually move
eastward across the state today. As a result, enhanced shower
activity and isolated thunderstorms will produce periods of MVFR
and local IFR. Light winds should allow for sea breeze development
this afternoon, with scattered showers possible over the interior
and sheltered leeward areas.

AIRMET Sierra for tempo mountain obscuration is in effect for
all of Kauai and windward portions of Oahu this morning. This
AIRMET will likely be needed on and off throughout the day for
portions of island chain as showers develop and dissipate.

AIRMET Tango for moderate to isolated severe mid to upper level
turbulence remains in effect as the upper level disturbance moves
across the state. Conditions will likely improve heading into the
evening hours.

AMD NOT SKED for PMDY continues due to equipment issues, with no
timeline for restoration.

&&

.MARINE...

Thunderstorms, some of which could be strong to severe, will
remain possible across Hawaiian coastal waters today as an upper
disturbance moves across the area. Any thunderstorms that develop
will have the potential to produce gusty winds, small hail, and
frequent lightning. Marine interests should continue to monitor
for any Special Marine Warnings that may be issued. Light and
variable winds will prevail today, except within and near
thunderstorms. Light to moderate easterly trade winds will begin
building back into the region on Sunday. Winds will weaken again
and become more southerly by Monday night as high pressure becomes
established both far northeast and far northwest of the islands
and a trough in between moves into the region.

Surf along south facing shores remains elevated, but will slowly
decline through Sunday and then hold into early next week as a
long-lived, small south swell continues to affect the islands.
Although the swell size itself will remain largely unchanged
through Sunday night, the period will gradually shorten from 15
seconds today to near 13 seconds by Sunday night, allowing surf
heights to slowly trend down. Another series of overlapping,
long-period south swells will begin moving in on Tuesday, with
reinforcements giving another noticeable boost to south shore
surf from Wednesday through the rest of next week.

A small, short-period north-northwest swell has increased north
shore surf this morning, but this will decline later today into
Sunday. A new small, long-period northwest swell will build Sunday
night, giving north shore surf a boost early next week. Surf
along east facing shores will be well below normal levels through
the middle of next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flood Watch through this evening for all Hawaiian Islands.

Wind Advisory until 6 PM HST this evening for Big Island Summits.

Winter Weather Advisory until 6 PM HST this evening for Big
Island Summits.


&&

$$


DISCUSSION...JVC
AVIATION...Farris
MARINE...Vaughan