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Hawaiian Islands Synoptic Discussion and Guidance

772
FXHW60 PHFO 021349
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
349 AM HST Tue Mar 2 2021


.SYNOPSIS...
Windy trade winds will persist through today before trending down
to locally strong by Wednesday night. A new high pressure system
will usher moderate to strong trades into the area during the
second half of the week, and continue into the weekend. Strong and
gusty trade winds will be returning by Monday and continue through
the first half of next week. A wet trade wind pattern is expected
Thursday night into the weekend with the aid of another upper
level disturbance.

&&

.DISCUSSION...
A series of surface highs will be replacing one another through
the rest of the week. The surface high north of Kauai has been
taken over by a new 1044 mb surface high 1700 miles northwest of
Kauai. This high will be moving east-southeast through Wednesday
night to a position 1200 miles northwest of Kauai with a central
pressure of 1035 mb. Into Friday this high will be taken over by a
1040 mb high located further north. This new high will move
southeast to 1200 nm northeast of Kauai on Sunday. Translated,
the trade winds will ease down to locally strong over the common
waters of Maui County and waters south of the Big Island by
Wednesday night and hold through Thursday. Trade winds will start
strengthening over the waters around Kauai Thursday night and
expand eastward to the rest of the area Friday, and persist
through Saturday. Another boost in the trades will come Sunday
into Monday. This boost may warrant a Wind Advisory for the common
windy areas.

The current Wind Advisory (WA) for most the islands is set to
expire late this afternoon at 6 pm HST today. We will see how the
winds are trending, and extend the advisory if needed for tonight.
The advisory is not expected to last into Wednesday. The winds
atop Mauna Kea have level off below advisory criteria, so no
advisory is needed. There were some clouds moving through the
summits of Mauna Kea earlier, causing a slight rise in relative
humidity, but have since cleared out.

Satellite imagery shows plenty of low level moisture about to cut
through most of the islands. So there will be some passing showers
during the morning rush. With the aid of the strong trades, many
of these showers are making their way into the lee areas of the
smaller islands with ease. The 2 am HST weather balloon sounding
from Lihue and Hilo shows a inversion at 10 to 11k feet. This is
expected to hold through Thursday. Models are indication that this
afternoon to be rather dry, ahead of a band of moisture that will
be moving down on the islands this evening. The clouds and showers
then dissipate Wednesday morning, followed by another band of
moisture. On Thursday, we will likely see the increase in trade
showers, especially in the afternoon and into the night as
remnants of a front moves down on the islands. These showers will
have the support of an upper level trough that stall over the
islands on Friday. The trough will drift down to the Big Island on
Sunday. In essence, wet trades are expected between Thursday
afternoon and at least Sunday. We will be taking a closer look in
the days ahead to see what are the impacts of this wet and windy
outlook.


&&

.AVIATION...
Strong trade winds will continue to be supported by a surface
high to the N, delivering stable and showery low clouds that will
focus over windward slopes and coasts, resulting in periods of
MVFR VIS/CIG. AIRMET Tango remains in effect for moderate to
severe low-level turbulence near and downwind of the terrain, for
locally strong surface winds, and for moderate mid-level
turbulence between FL220 and FL300 due to a trough aloft. Some
decrease in low-level wind speed is expected tonight into
Wednesday while the turbulence aloft should also diminish as the
trough aloft moves E of the area.

&&

.MARINE...
Strong to locally gale force trade winds continue through today
as strong high pressure holds far north of the state. A Gale
Warning is in effect for the typical windier waters around the Big
Island and Maui County through this afternoon. A Small Craft
Advisory (SCA) remains posted for the remaining waters due to the
combination of both strong trade winds and resultant rough seas.
The surface high is being replaced by another surface high far
northwest of the islands this morning, and commence a short-
lived downward trend in the trades. The Gale Warning will be
replaced by an SCA this evening. The weakening trades along with
the subsiding seas result in the reduction of the SCA to
waters around Maui County and waters south of the Big Island by
Wednesday night. Building high pressure to the northwest of the
state will bring a boost in the trades to the waters west of
Kauai, thus warranting an SCA there. The SCA will likely expand
eastward and join up with the SCA in effect for the Maui County
waters and waters south of the Big Island. Over the weekend, the
SCA will cover most, if not all of the nearshore waters.

Large trade wind-driven seas will maintain high surf along east
facing shores into Wednesday. Nearshore buoys have been holding at
9 to 11 feet with near 11 second periods. Fully-developed seas will
continue to produce rough east shoreline surf above the 8 foot High
Surf Advisory (HSA) threshold through Wednesday afternoon. As trade
winds over and upstream of the islands weaken through the middle of
the week, surf along eastern exposures will lower to borderline HSA
levels by Wednesday evening. Expect east shore surf to be just below
HSA levels late this week and during the weekend.

Surf will remain below advisory levels on all other shores through
Thursday. Exposed sections of both north and south-facing shores will
pull in some trade wind swell wrap and keep south shore surf near
winter averages today to be followed by a decline from today through
Wednesday. Moderate height, long period northwest swells will begin
arriving Wednesday afternoon and evening, peaking surf along north and
west-facing shores late Thursday into Friday. Fortifying secondary
north swells will arrive on Friday and clip the islands. While the
bulk of this northern swell energy will pass to the east of the chain,
there will be enough energy to keep surf at, or slightly above, HSA
heights both Friday and Saturday. These swells will diminish through
the weekend and lower surf below HSA thresholds by Saturday evening.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Wind Advisory until 6 PM HST this evening for Kauai-Oahu-Molokai-
Lanai-Kahoolawe-Maui Windward West-Maui Leeward West-Maui
Central Valley-Windward Haleakala-Leeward Haleakala-South Big
Island-Big Island North and East-Kohala.

High Surf Advisory until 6 PM HST Wednesday for Kauai Windward-
Oahu Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-South Big Island-Big Island North and East.

Small Craft Advisory from 6 PM this evening to 6 PM HST
Wednesday for Maalaea Bay-Pailolo Channel-Alenuihaha Channel-Big
Island Leeward Waters-Big Island Southeast Waters.

Gale Warning until 6 PM HST this evening for Maalaea Bay-Pailolo
Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island
Southeast Waters.

Small Craft Advisory until 6 PM HST Wednesday for Kauai
Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-
Kauai Channel-Oahu Windward Waters-Oahu Leeward Waters-Kaiwi
Channel-Maui County Windward Waters-Maui County Leeward Waters-
Big Island Windward Waters.


&&

$$

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



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