Heavy Rain in Hawaii, March 8 to 9, 2022

Area Forecast Discussion
National Weather Service Honolulu HI
325 PM HST Tue Mar 8 2022

.SYNOPSIS...
High pressure north of the islands will allow trade winds to fill
in across the islands through Wednesday. A nearby upper level
trough will help to enhance showers, and could trigger a
thunderstorm or two on the Big Island during the afternoon hours
tomorrow. A more stable trade wind pattern is expected for the
second half of the week.

&&

.DISCUSSION...
As anticipated, no significant changes to the forecast with the
afternoon package. Have boosted some PoPs on the Big Island
through tomorrow morning mainly over the windward side of the
islands based on the FV3 model and latest radar imagery. That
being said, the general flavor of the forecast remains the same.

The surface analysis shows the weakening front to the northeast of
the islands with surface high pressure north of the central
islands. The upper level trough remains near the islands, with
some thunderstorms to the east through southeast of the Big Island
over the offshore waters.

The high pressure will allow the trade winds to fill in across the
island chain through tomorrow, and then persist through the end
of the week. The upper level trough over the region will continue
to provide some instability, which will allow for some higher
rainfall totals in some showers. Most of the instability remains
to the east of the islands, thus the thunderstorms noted on
satellite far to the east, and not so far to the southeast of the
Big Island. The afternoon soundings shows the 500 mb temperature
at Lihue as warmed to -11 celsius, while Hilo remains near -13
celsius. Precipitable water (PW) near the Big Island has dropped
to around 1 inch, while Lihue had 1.20 inches. The showers over
the Big Island this afternoon show that even with the lower PW,
the colder temperatures aloft are enhancing the showers. Still
can't rule out a thunderstorm or two during the remainder of the
afternoon, or again tomorrow afternoon.

As the upper level trough moves away on Thursday, upper level
ridging is expected to move over the islands. This will help to
stabilize the airmass, which should reduce some of the rainfall
totals. High pressure at the surface will allow for a more
typical trade wind pattern for the second half of the week.

&&

.AVIATION...
Showers will remain limited across the central and western
portions of the state through this evening, as a cold front to
the northeast continues to dissipate. However, satellite and
radar trends continue to support better chances for showers,
along with a few thunderstorms, remains around the Big Island
this evening. Otherwise, easterly trades will continue to
strengthen across the state, which will focus a majority of clouds
and showers over windward and mountain areas through Wednesday
night. Sea breeze clouds which formed in sheltered leeward zones
on the Big Island during the afternoon, will diminish tonight as
the diurnal trend toward land breeze begins. Mainly VFR conditions
are expected through the TAF period with possible MVFR cigs in
developing showers.

No AIRMETs are currently in effect, however a period of tempo
light ice in cloud may be possible through this evening over
central and eastern portions of the state above FL180, as a band
of upper lvl moisture pushes eastward across the state. Additionally,
a period of potential moderate upper level turb is possible after
10z as upper jet dynamics increase in the region.

&&

.MARINE...
High pressure building in north of the state is currently
producing light to locally moderate northeast trade winds across
the area. The high will move east over the next several days
causing winds to become more easterly. Trade winds may strengthen
slightly as we head into the weekend as the pressure gradient
tightens up a bit across the area.

The current northwest swell will lower gradually tonight through
Wednesday. Another northwest swell is expected to arrive late
Thursday, peak on Friday near advisory levels, the lower gradually
over the weekend. A third, northwest swell is expected to arrive
on Sunday, peak Sunday night and early Monday near advisory levels,
then lower gradually into Tuesday.

Small background south swells are expected through Wednesday with
a slightly larger long period south swell expected Thursday and
Friday. Then a return to small background surf is expected
through the weekend and on into early next week. Short period choppy surf
will generally remain on the small side through Sunday as trade
winds remain in the light to moderate range over and upwind of the
state. Some increase in east shore surf is likely early next
week with the expected increase in trade winds.

&&

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

&&

DISCUSSION...M Ballard
AVIATION...SWR
MARINE...Burke