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Hawaiian Islands Synoptic Discussion and Guidance

122
FXHW60 PHFO 070153
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
353 PM HST Sat Mar 6 2021

.SYNOPSIS...
A high to the north will maintain breezy to locally strong trade
winds for the next several days. Continued heavy showers are
expected to move ashore over Kauai through tonight, with passing
windward showers elsewhere. Starting early Monday, deep moisture
will move in from the southeast, first affecting the the Big Island,
then spreading to all islands later Monday and Tuesday. This
increase in moisture will bring the potential for heavy rainfall
that will likely remain for most of next week.

&&

.DISCUSSION...
In summary, breezy to locally strong trade winds will prevail for
the next several days. Wet trade wind weather is expected over Oahu
and Kauai tonight, while windward portions of Maui County and the
Big Island will see a few showers as well. The focus for increased
showers, and potentially heavy rainfall, is expected to shift to the
Big Island starting early Monday as a trough develops near the
islands. The threat of widespread rainfall will spread to all
islands later Monday and Tuesday, with the potential for wet weather
persisting for most of next week. Most showers will focus over
windward areas, but there is a chance for increased rainfall over
leeward areas if the low-level trough sharpens near the islands.

Surface high pressure consolidating far N of the islands will
support breezy to locally strong trade winds for the next couple of
days, with the wind forecast becoming increasingly uncertain
thereafter as a surface trough may or may not sharpen near the
islands. Moisture associated with a dissipated front will continue
to move into Kauai and Oahu from the E overnight, while an E-W
oriented trough aloft near Kauai acts to enhance this moisture.
Although moving rather quickly, heavy showers repeatedly moving in
from the E have led to saturated grounds on Kauai. A Flash Flood
Watch (FFA) is in effect for Kauai through tonight as high-
resolution model guidance indicates that showers will persist, if
not intensify. This moisture is expected to gradually clear Oahu
this evening and Kauai by Sunday morning, leading to a fairly
typical distribution of trade showers statewide on Sunday.

On Sunday night, a low aloft is expected to develop within the
trough aloft just W of the islands. This will lead to a realignment
of the flow aloft, further destabilizing the island atmosphere while
drawing moisture NW over the area. As this moisture moves over the
Big Island, guidance depicts a surface trough developing near the
islands, with the associated low-level moisture convergence bringing
an increased potential for heavy rain (and winter weather to the Big
Island Summits). Since the threat on the Big Island appears to start
early Monday, it was decided to hold off on a Winter Weather
Watch/Flash Flood Watch for the Big Island at this time, but these
may need to be posted on Sunday.

As the trough moves W (and potentially sharpens) later Monday and
Tuesday, the potential for heavy showers will spread to all islands
while the low aloft persists. Trade winds will likely focus most of
the rainfall over windward areas, but if winds turn toward the S or
SE (per latest GFS), potential for significant leeward rainfall will
increase. Depending on when/where/if the trough sharpens, winds
could become locally strong. The lingering low aloft will prevent a
significant drying trend until next weekend.

&&

.AVIATION...
VFR conditions prevail at all TAF sites with some brief periods of
MVFR conditions conditions occurring at PHLI and PHTO in low clouds
and shra. Expect these conditions to continue into the overnight
hours.

AIRMET Sierra is currently in effect for Kauai due to mountain
obscuration. Conditions are expected to improve overnight. They will
likely be brief periods of MVFR conditions elsewhere along windward
slopes through the overnight hours as low clouds interact with
island terrain.

AIRMET Tango is currently in effect over and immediately south thru
west of mountain ranges on Kauai due to low level mechanical
turbulence. Winds may diminish a bit during the overnight hours.

&&

.MARINE...
Large, strong high pressure located about 1,000 miles north of the
islands will slowly drift southeastward the next few days. Trade
winds will remain fresh with locally strong magnitudes through the
windier channels and bays tonight. The high sliding southeastward
will tighten the pressure gradient back across the islands enough to
support sustained strong trades with gale gusts through the eastern
island channels Sunday and Monday. Strong to near gale force trades
will likely occur most of next week around Maui County, through the
Alenuihaha Channel and over those waters south of the Big Island.

There is a Small Craft Advisory (SCA) in effect through 6 AM HST
Sunday for all of the Hawaiian coastal waters, with the exception of
the Oahu and Maui leeward waters, due to strong trades and elevated
seas. Strengthening trades will have this SCA expanding into the
Oahu and Maui leeward waters from 6 AM HST onward Sunday. This all
water SCA should remain in effect for the foreseeable future.

North swells that have recently boosted north and west shore surf
will slowly diminish tonight and level out Sunday. Northern and
western exposures will experience well below advisory level surf the
next couple of days as incoming overlapping north and northwest
swells produce medium size surf. Surf along northern shores will
become small from Tuesday onward as lingering north and northwest
swells fade. The next moderate, low period northwest to north swells
will arrive next weekend. Strengthening trades the first half of
next week will build in an easterly swell and ultimately increase
eastern-facing shoreline surf heights. There is a good chance that
surf along Oahu and Kauai's eastern exposures will reach 8 foot surf
advisory thresholds Sunday night or Monday morning and then build
further statewide into Tuesday. Low, long period background
southerly swells will continue to create small near seasonal surf
along south-facing shores the next several days.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through late tonight for Kauai.

Small Craft Advisory until 6 AM Sunday for all Hawaiian waters
except Maui County Leeward waters and Oahu Leeward waters.

Small Craft Advisory from 6 AM Sunday through 6 AM Monday for all
Hawaiian waters.

&&

$$

DISCUSSION...Birchard
AVIATION...Burke
MARINE...Blood

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

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