Area Forecast Discussion
Issued by NWS Honolulu, HI

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000
FXHW60 PHFO 260629
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
829 PM HST Tue Feb 25 2020

.SYNOPSIS...
Breezy trade winds will continue through Wednesday with isolated
to scattered showers mainly over windward and mountain areas
favoring the overnight and early morning hours. Significant
weather changes expected by Thursday as we transition to a five
day wet weather pattern, and breezy trade winds increase into the
windy range by Friday likely exceeding wind advisory and high wind
warning thresholds through Sunday. Wet weather will continue
through next week Tuesday as an upper level low lingers over the
Hawaiian Islands.

&&

.DISCUSSION...
Breezy trade winds continue through Wednesday with limited shower
activity under an upper level ridge and low subsidence inversion
heights around 7000 feet. Isolated to scattered showers remain in
the forecast along windward slopes and mountain areas. The
highest coverage for showers will occur in the overnight to early
morning hours. On Thursday, the upper level ridge breaks down as
an unstable trough moves in from the north. This trough will lift
the trade wind subsidence inversion heights, increase low level
cloud heights, and begin to transition the Hawaiian Islands into a
five day period of wet weather across the state.

On Friday, a strong 1040+ high pressure center drifts into the
Central Pacific basin north of Hawaii producing three days of windy
weather conditions across the region. Meanwhile, an upper level
cold core low will form at the base of the upper trough with the
low center setting up just east of the islands. This unstable low
will raise the subsidence inversion cap even higher producing
numerous showers across the islands. The combination of strong
gusty winds and upper level forcing will result in Blasting Wet
Trades from Friday through Sunday. Wind speeds will likely exceed
Wind Advisory levels with some wind favored areas reaching High
Wind Warning thresholds through the weekend.

Global weather models remain in good agreement with the
development of these strong winds and the unstable upper low on
Friday. Both the American (GFS) and European (ECMWF) models show
this cold core low center drifting westward over the Big Island on
Sunday. Although the strong surface winds tend to be generally
unfavorable for flooding, the significant increase and duration
of this upcoming wet weather event means that localized flooding
cannot be ruled out. Windy wet conditions over three days will
also mean trees may be uprooted with added potential for some
property damage and power outages through the weekend. A High Wind
Watch and Gale Watch will likely be issued later this week as the
forecast time period grows closer.

The longer range forecasts from both the GFS and ECMWF show the
upper low slowly weakening and drifting northward on Monday and
Tuesday. Enough instability will remain over the islands for
enhanced showers through the first half of next week. Trade winds
appear to decrease into the more moderate range on Tuesday and
Wednesday as the high center moves into the Eastern Pacific and a
cold front passes by far north of the island chain.

&&

.AVIATION...
Tonight, a 1033 mb high to the north of the island chain will
allow for moderate east northeast trade flow to persist. Thus,
expect scattered showers to persist along the windward coasts and
slopes of the islands with isolated activity elsewhere. Periods
of MVFR conditions may develop due to lower ceilings from the
passing shower activity.

AIRMET TANGO remains in effect for areas below 8000 feet south
thru west of mountains for tempo moderate turbulence.

&&

.MARINE...
High pressure will strengthen far northeast of the islands over
the next couple days, resulting in the strengthening of the trade
winds across area waters, to near gale force in some of the
channels. Currently, a broad 1035 mb high sits some 1300 nm NE of
the islands. This high will be reinforced by a new high of 1036
mb 1030 nm N of Kauai early Thursday evening. The high even
strengthens to 1040-ish mb over the weekend. Should the models
projection is right, it is almost certain of gale warning with
some of the channels such as the Alenuihaha and Pailolo over the
weekend.

A Small Craft Advisory (SCA) is currently in effect for all
Hawaiian waters through 6 pm HST Friday. But the SCA is due to
not only winds but also above normal seas due to the large
northwest swell that is now peaking. The SCA due to strong trades
cover waters around Maui County and waters south of the Big
Island. The rest of the SCA is from the large northwest swell
that is now peaking at the Pauwela buoy, with a height of 11 and a
half feet. This swell will be traversing eastward through the
rest of the island waters the rest of tonight. A gradual down
trend is expected through Wednesday night, but a new northwest
swell of about 8 feet max is due to arrive Thursday morning. And
given with the new high moving in, we will likely see the SCA for
all Hawaiian waters continuing through the coming days. On top of
that, at some point, gale warning for some of the near shore
waters. There is currently a High Surf Warning for the north and
west facing shores of some of the smaller islands, to until 6 am
HST Wednesday. A High Surf Advisory is in effect for the west
facing shores of Niihau, Oahu, Molokai, and north facing shores of
the Big Island.

The reinforcing northwest swell is slated to peak Thursday night,
then gradually lower Friday through Sunday. Advisory level surf
is expected with this swell along most north and west facing
shores.

Short period choppy surf along east facing shores will gradually
rise through the week and become rather large by the weekend due
to strengthening trade winds. Advisory level surf is likely by the
weekend, with possible warning level surf Sunday through early
next week. There will also be a series of small background south
swells during the next 7 days.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning until 6 AM HST Wednesday for Niihau-Kauai
Windward-Kauai Leeward-Oahu North Shore-Oahu Koolau-Molokai-Maui
Windward West-Maui Central Valley-Windward Haleakala.

High Surf Advisory until 6 PM HST Wednesday for Waianae Coast-
Big Island North and East.

Small Craft Advisory until 6 PM HST Friday for all Hawaiian
waters.

&&

$$

DISCUSSION...Bohlin
AVIATION...Thomas
MARINE...Lau


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