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Hawaiian Islands Synoptic Discussion and Guidance

717
FXHW60 PHFO 020147
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
347 PM HST Wed Apr 1 2020

.SYNOPSIS...
Deep low pressure developing far northwest of the state on
Thursday will cause trade winds to veer out of the southeast,
leading to a somewhat unstable land and sea breeze regime that
will produce spotty showers, some briefly heavy, over the islands
during the afternoon hours. Winds will turn more southerly on
Friday and the weekend, likely drawing up deeper moisture over the
islands and fueling more shower activity. A front associated with
the low is expected to maintain wet conditions over the state on
Monday and Tuesday.

&&

.DISCUSSION...
Weak trades have been overpowered by sea breezes this afternoon,
while an upper level trough overhead is maintaining unstable
conditions. As a result, heavy showers and isolated thunderstorms
have flared over interior sections, mainly on the Big Island and
also on Oahu, where a weak surface trough is passing by. Expect
heavy shower activity to taper off late this afternoon, with
showers lingering on the Big Island a bit longer. The weak trough
will move westward, bringing a few showers to windward Kauai.

For tonight, weak trades will hang on with modest showers
favoring windward slopes. Deep troughing at the surface and aloft
will continue to dominate across the Pacific north of the state,
splitting a pair of strong surface highs and suppressing the local
trades. leeward shower activity will clear out quickly by
evening. With available moisture limited, modest showers will
favor windward slopes, even though conditions will remain
unstable.

Trade winds will gradually shift out of the southeast on Thursday
as an area of deep low pressure develops in the trough far
northwest of the state. While showers will favor east and
southeast slopes, a land and sea breeze regime will dominate over
leeward areas, leading to clouds and spotty afternoon showers,
some briefly heavy. Aside from the interior slopes of the Big
Island, thunderstorms are unlikely.

As the deep low settles about 850 miles northwest of the state on
Friday, shower chances will increase as winds shift more
southerly and draw deeper moisture over the islands. The state
will be under the outer portion of the upper level trough
associated with the low, likely leading to enough instability to
trigger isolated heavy showers. Thunderstorms cannot be ruled out
but confidence is not high enough to include them at this time.
As a front associated with the low edges closer to the state on
Sunday, shower chances should increase, especially across the
western end of the state. In addition, the southerly flow will
push dew points up to around 70, bringing an increase humidity
that we have not experienced for a while.

On Monday and Tuesday, the front, or what is left of it, is
expected to push across portions of the island chain, stalling
perhaps as far east as Maui during the middle of next week. The
latest runs of the GFS and ECMWF have come into better agreement
on this scenario, but somewhat high uncertainty remains.

&&

.AVIATION...
Weak flow will persist this afternoon as a trough passes over the
islands. Sea breeze formations along the coast have allowed for
shower development over the interiors of the islands. Enough
instability exists for isolated areas of heavier showers or
thunderstorms.

Most activity is expected to dwindle by sun down. Tonight, light
winds will ensue state wide with a few showers along the windward
coasts and slopes.

No AIRMETS are in effect at this time.

&&

.MARINE...
A surface high far northeast of the area is maintaining easterly
trade winds across Hawaiian waters, but a trough over the eastern
offshore waters will keep the winds gentle to locally fresh though
Thursday.

A low south of the Aleutians is forecast to move south and approach
the area, causing winds to veer out of the southeast beginning
Thursday. Winds will weaken over the western part of the coastal
and offshore waters but winds may strengthen over eastern waters,
especially where the southeast flow accelerates around the Big
Island and Maui. A convergence band associated with the low will
likely move over the northwest offshore waters Saturday,
strengthening south winds over the western waters and bringing
another chance of thunderstorms.

The current small northwest swell will slowly subside through
Friday night. A new swell building Saturday will produce moderate
surf along north and west facing shores over the weekend, but is
expected to remain well below the advisory threshold. Swells from
the southern hemisphere will maintain elevated surf along south
facing shores through tomorrow, but should remain just below
advisory levels. Swells from the distant southeast Pacific will
be partly blocked by the islands of French Polynesia, but should
be enough to produce small surf through the rest of the week. Surf
along east facing shores will remain up through midweek due to an
upstream fetch, despite the trades relaxing locally, then subside
by the end of the week.

&&

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

&&

$$

DISCUSSION...Wroe
AVIATION...Thomas
MARINE...TS

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