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Hawaiian Islands Synoptic Discussion and Guidance

346
FXHW60 PHFO 200635
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
835 PM HST Mon Apr 19 2021

.SYNOPSIS...
Light and variable winds will prevail the next couple of days, with
trade winds gradually strengthening Wednesday and Thursday before
becoming breezy next weekend. The light winds will allow low clouds
and showers to focus over interior locations during the day, but an
overall increase in showers is expected through Tuesday over
Kauai and Oahu as an upper-level disturbance moves through, with a
few heavy showers possible. The upper-level disturbance will also
bring a thick layer of mid and high clouds on Tuesday that will
gradually diminish on Wednesday. The returning trade winds will
bring just a few windward showers.

&&

.DISCUSSION...

Weakly convergent southeast flow is supporting scattered showers
around Kauai while mostly clear skies and dry weather prevail for
all other islands. Aside from a gradual increase in mostly
transparent high cloud cover, this should generally remain the trend
through most of the night.

The afternoon sounding out of Lihue was very dry in the mid-levels
and featured a PWAT of 0.87", which is on the low side of normal for
April. Likewise, dewpoints at Barking Sands were in the upper 50s as
recently as this morning. However, the dewpoint there has jumped up
to 70 degrees over the last couple of hours which corresponds to a
satellite analysis of PWATs up to 1.4" spreading over Kauai at press
time. As this airmass filters into the area during the next 12 to
18 hours a similar increase in surface humidity can be expected
for the other islands. This will represent a noticeable increase
in mugginess compared to the last couple of weeks.

Attention for the evening forecast package revolves around shower
coverage and heavy rain potential on Tuesday. Model consensus has
held firm over the last 24 hours or so with no forecast changes
needed and no substantial revisions to overall forecast reasoning.
Advection of warm and moist air within a deep layer of confluent
southwesterly flow will contribute to modest forcing for ascent
through the forecast period while a fairly dynamic 60kt jet provides
favorable support by Tuesday afternoon. With that said, the
potential for deep convection and thunderstorms is expected to be
limited due to paltry mid level lapse rates and increasing high
clouds limiting instability over the islands. The greatest potential
for deep inland convection will probably be over Maui and Big
Island where high clouds are not expected to arrive until later
Tuesday evening. Overall flavor of the event is still expected to
be one of increasing shower coverage and locally heavy rainfall
for portions of Kauai and Oahu.

A building ridge over the central Pacific will cause a return to
trades by the end of the week. The returning trade winds will be on
the drier side as a building mid-level ridge brings stability,
although there may be a weak low aloft bringing more trade showers
over the weekend. Long range outlook shows a trend toward light
winds in about a week or so.


&&

.AVIATION...
The cold front far northwest of the Hawaiian Islands continues
its slow march eastward. Light mostly offshore winds will
predominate across the islands tonight. Winds will shift to the
south on Tuesday as the front edges closer to Kauai. Current radar
imagery reveals minimal shower activity statewise this evening.
Shower coverage and intensity is expected to increase over Kauai
late tonight or early Tuesday, but the remainder of the state
should experience mostly clear skies overnight.

Prefrontal showers will spread from Kauai to Oahu by Tuesday
afternoon and to Maui County by Tuesday evening. On the Big Island,
inland cumulus are expected to dissipate overnight, but redevelop
Tuesday afternoon.

The air mass over the state is expected to become increasingly
unstable on Tueday. Slight chance of Thunderstorms over Kauai,
Oahu, and Interior Big Island on Tuesday.

No AIRMETs in effect at this time. Mountain obscuration could
become an issue for Kauai on Tuesday.

&&

.MARINE...

A front approaching from the northwest will keep moderate east-
southeast winds in place over the eastern islands and light and
variable winds over the western end of the state through
Wednesday. A few heavy showers and even a thunderstorm will be
possible through Tuesday evening over the Kauai and Oahu waters
as an upper level disturbance moves through. Moderate trades are
expected to return from east to west across the state Wednesday
night and Thursday as high pressure builds north of the islands.
The trades are forecast to strengthen into the fresh to strong
range Friday through the weekend, and Small Craft Advisories will
likely be needed for some marine zones at that time.

Surf along north facing shores will remain small through
Tuesday. A new short period north-northwest swell will then give
north shore surf a boost during the middle of the week, with a
slow decline expected Friday through the weekend. Head high to
slightly over head sets can be expected along north facing shores
as this swell peaks, but surf will remain well below advisory
thresholds.

East shore surf will remain small through the next 7 days,
although we should see a slow trend upward Thursday into the
weekend as the trades over and upstream of the islands strengthen.


Surf along south facing shores will remain small during the next
7 days, with mainly background south and south- southwest swell
energy moving through.


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$


DISCUSSION...JVC
AVIATION...Bedal
MARINE...Jelsema

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



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