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Hawaiian Islands Synoptic Discussion and Guidance

188
FXHW60 PHFO 202005 CCA
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1003 AM HST Tue Apr 20 2021

.SYNOPSIS...
Scattered showers capable of producing locally heavy rain and
possibly a thunderstorm are expected today as a front approaches and
a disturbance aloft moves over the islands. Fewer showers are
expected thereafter as the front dissipates and the disturbance
moves east of the islands. Light and variable winds will continue
into mid-week, followed by a return to moderate to breezy trades
by the weekend.

&&

.DISCUSSION...
Weakly convergent S to SW flow ahead of an approaching surface front
is supporting the development of heavy showers near Kauai, but so
far this morning, they have remained offshore just W of Niihau.
Meanwhile, a surface ridge extends SW over Maui and the Big Island
from a high centered to the distant NE, where winds are light and
variable, primarily blowing offshore. However, that is in the
process of changing as this morning's abundant sunshine is already
fueling the onset of sea breezes, and the associated increase in low
clouds over the slopes. Relatively thick high clouds, associated
with a sharp trough aloft a couple hundred miles NW of Kauai, are
slowing the heating process over Kauai and Oahu.

The expectation is that the trough aloft (and associated jet max)
will move over the islands today and tonight, then move E of the
islands on Wednesday. There are some conflicting forces at work
acting to complicate the short term forecast, with the trough aloft
expected to promote large-scale rising motion as associated
divergence aloft passes over the islands. However, with the front
expected to stall and wash out just NW of the islands over the next
24 hours, there lacks a clear significant surface focusing mechanism
- other than the surface convergence over the islands provided by
sea breezes. So the forecast anticipates some locally heavy showers
over land this afternoon, and possibly a thunderstorm. As the trough
aloft progresses eastward overnight, divergence aloft will be
greatest over Maui and the Big Island, but offshore-flowing land
breezes may limit any convection over land.

A return to stable conditions can be expected by late tonight as a
building mid-level ridge and associated subsidence acts to support a
low-level capping inversion. Although the front will wash out to the
NW of the islands, a resultant surface trough will linger N of the
islands, keeping the trade winds from significantly increasing on
Wednesday/Thursday. As the trough diminishes and high pressure
builds to the distant N, trade winds will increase Friday/Saturday.
While a mid-level ridge will be over the area for most of the
period, limiting shower coverage and intensity, a mid-level trough
moving S along the periphery of the building ridge will move over or
just E of the area Friday and Saturday. This could bring a reduction
in stability, potentially leading to a wetter trade wind weather
pattern.

&&

.AVIATION...
A trough approaching the islands from the west will bring increased
shower activity and a slight chance of thunderstorms for mainly
areas west of Molokai through tonight. MVFR conditions ceilings and
visibility will likely occur with the heavier showers. Isolated
showers will exist elsewhere.

Light and variable prevail over the next 24 hours.

AIRMET SIERRA remains in effect for Kauai above 2000 feet due to
tempo mountain obscuration from clouds and showers.

&&

.MARINE...
Winds will remain light through Wednesday as a weakening front
approaches from the northwest. Instability associated with the front
may allow for a few thunderstorms to form around Kauai and Oahu
today. A new high will build in north of the area beginning Thursday
with trade winds gradually strengthening toward the weekend. A Small
Craft Advisory will likely be posted as the winds increase
especially for the typically windy zones around Maui County and the
Big Island.

Several small short period northwest swells can be expected through
the remainder of this week and on into early next week keeping surf
above flat levels. Small, mainly background long period southerly
swells can also be expected throughout the forecast period. Surf
will remain small along east facing shores for the next couple of
days with a slight uptick later in the week and on into the weekend
as the trade winds strengthen.

&&

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

&&

$$

DISCUSSION...Birchard
AVIATION...Thomas
MARINE...Burke

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



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