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Hawaiian Islands Synoptic Discussion and Guidance

483
FXHW60 PHFO 081340
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
340 AM HST Mon Mar 8 2021

.SYNOPSIS...
Cloudy and showery weather is expected for the first half of the
week as an upper level disturbance interacts with tropical moisture
moving in from the east and south. Breezy trade winds will give
way to lighter winds by mid-week as a surface trough moves over the
islands. Strong and gusty northeast winds are in the forecast for
next weekend.

&&

.DISCUSSION...
The combination of cold temperatures aloft and a deep tropical
air mass will result in an extended period of widespread heavy
showers and possible thunderstorms through mid week. A Flash Flood
Watch is currently in effect for the Big Island and Maui County
through Tuesday night. This watch could be extended to the other
islands if the conditions warrant. This moisture will also bring
a potential for winter weather to the Big Island summits above 11k
feet through mid week. A Winter Weather Advisory is effect through
early Tuesday morning with the likelihood of additional extensions.

The latest ECMWF model run has nudged toward the GFS solution. Not
completely but closer by placing the surface trough over the central
islands on Thursday. The trough presented by the EC, although not
as strong as the GFS, will still cut off the trades. This surface
trough not expected to clear the Big Island until Saturday afternoon.

There is no disagreement with the models in addressing the upper
level low/trough. Both models have the low southwest of Kauai opening
up to a trough Wednesday. The upper trough then pushes eastward across
the island chain, finally moving past the Big Island by Friday morning.
With the upper level trough moving east of the surface trough, there
will only be limited support for heavy rains. So, expect an improving
trend Thursday through Friday, from west to east.

Heavy showers ended across the Hilo and Puna districts just before
midnight. This dry period will help the ground recover from the
drenching it took earlier. Now, the leading edge of a new area of
clouds and showers has just moved ashore into the Puna and Kau
districts. Hopefully, the land breeze will slow down its progress
inland, and/or weaken the showers. Passing showers have been
observed with the remaining islands. Satellite imagery shows mid
and high clouds streaming across the eastern half of the island
chain that will get thicker through the rest of today.

Following the trough, strong northeast winds are expected to
spread down the island chain from east to west, that is between
Friday and Saturday. It won't be until Sunday night and Monday
when the northeast winds weakens.

&&

.AVIATION...
VFR conditions prevail at all TAF sites except PHTO where MVFR to
IFR conditions are expected through the night due to low clouds
and showers. As increasing moisture moves in from the southeast
tonight, conditions will deteriorate mainly over windward slopes
of the Big Island and Maui County. AIRMET Sierra for mountain
obscuration is currently in effect for the NE slopes of the Big
Island and will likely be expanded to Maui County.

AIRMET Tango is currently in effect over and immediately south thru
west of mountain ranges on all islands due to low level mechanical
turbulence.

&&

.MARINE...
A high far north of the main Hawaiian Islands will slowly drift
southeast over the next few days as it drives moderate to locally
strong trades across Hawaiian waters through 6 am HST Tuesday
morning. Trade wind waves, combined with northerly swell, will
maintain elevated seas over exposed waters. A Small Craft Advisory
is in effect through early Tuesday morning for all Hawaiian
waters. Models develop a surface trough east the area in the
coming days and slowly drag it across the state through the week.
This would disrupt mid to late week trade flow.

Strong trade winds will cause surf to remain elevated along east-
facing shores today and tonight. A High Surf Advisory for eastern
exposures remains in effect through tonight. North swell will
continue to decline through today. However, reinforcing north and
northwest swells, in tandem with a trade wind wave/east swell
wrap, will generate medium surf heights along most north and west-
facing shores through today. Diminishing northerly swells will
cause surf heights along north and west shore to decrease from
Tuesday into next weekend. Small, long period south and southwest
swells, along with some trade wrap, will provide enough background
to support near seasonable surf along most south-facing shores.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Tuesday for Kauai Windward-
Oahu Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-South Big Island-Big Island North and East.

Flash Flood Watch through late Tuesday night for Big Island.

Flash Flood Watch through late Tuesday night for Molokai-Lanai-
Kahoolawe-Maui.

Winter Weather Advisory until 6 AM HST Tuesday for Big Island
Summits.

Small Craft Advisory until 6 AM HST Tuesday for all Hawaiian
waters-


&&

$$

H Lau/Morrison/Powell

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



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It was last modified on: Wed, Dec 17 2014 - 1841 UTC
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