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Hawaiian Islands Synoptic Discussion and Guidance

148
FXHW60 PHFO 011353
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
353 AM HST Wed Apr 1 2020

.SYNOPSIS...
An approaching front will cause the light to moderate trade winds
to become light south and southeast on Friday, then continue into
the weekend. Expect the weather to become more unstable over the
weekend as the approaching front gets support from an upper level
low, forecast to settle northwest of the islands.

&&

.DISCUSSION...

The main Hawaiian Islands will be undergoing a change in weather
regime in the next 24 hours, from trades to convective. The
lighter trade winds in store for later this morning will promote
an onshore sea breeze flow over the smaller islands. Meanwhile,
the upper level short wave trough, now over Maui, continues to
push east, taking most of the dense mid and high clouds with it.
The end result is most islands will have a sunny morning. The
daytime heating will help promote an onshore sea breeze flow.
Coupled with ample moisture and instability, the end result is the
likelihood of some scattered showers this afternoon, including a
slight chance of a thunderstorm. For tonight, the land breeze
takes over, clearing or pushing most of the daytime clouds out
into the adjacent waters, resulting in a mostly fair evening. The
only exception is for the windward shoreline, where a weak
easterly flow may carry a stray showers or two onshore. The
sea/land breeze, convection regime continues into Thursday,
although not as showery, and no thunderstorms.

An approaching cold front from the west is the reason for the
trades becoming southerly. The front will have some upper level
support from a upper level low, forecast to set up shop some 850
miles northwest of Kauai on Friday. In doing so, we expect the
weather to start deteriorating on Friday. The low will be drawing
moisture from the tropics over the weekend. Further more, a pre-
frontal trough is forecast to appear just west of Kauai on Friday
then drift over the western islands over the weekend and act as a
focusing mechanism for some enhanced showers. All said, at this
time, there is potentially another round of unsettled weather
heading our way for especially the weather weary islands of Kauai
and Oahu this coming weekend.

The models disagree at the moment as to when the front arrives.
The latest ECMWF is faster with it, having it over Kauai Monday
morning. The GFS is slower by about 24 hours. Officially, we are
following the GFS solution.

&&

.AVIATION...
An upper level trough mainly affecting the Big Island will keep a
few showers there into the early morning hours with heavy showers
in the southeast waters of the Big Island.

Otherwise, lighter trades are in store for later this morning,
leading to daytime sea breezes. Along with daytime heating, ample
moisture and instability, this will likely trigger a few inland
showers, including a slight chance of a thunderstorm. Will issue
AIRMET for mountain obscuration as needed. As night approaches,
the islands looses the daytime heating, an offshore land breeze
develops, pushing any daytime borne clouds into the adjacent
waters.

&&

.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
tomorrow.

A low south of the Aleutians is forecast to move south through Friday.
As the low approaches, winds will veer out of the southeast starting
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 and could
produce strengthening south winds over the western waters and
bring a chance of more 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
surf is expected to remain well below the advisory threshold.
Swells from the southern hemisphere will maintain small surf along
south facing shores through tomorrow. Swells from the distant
southeast Pacific will be partly blocked by the islands of French
Polynesia but will 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.
Surf will subside by the end of the week as the upstream fetch
weakens.

&&

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

&&

$$


H Lau/Chevalier/Donaldson

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