Drought in Hawaii, February 1 to 25, 2020

Area Forecast Discussion
National Weather Service Honolulu HI
839 PM HST Fri Jan 31 2020

.SYNOPSIS...
A modest increase in moisture associated with the tail-end of a
dissipating frontal boundary moving into the area will bring the
rain chances up going into the weekend across the state. A few
thunderstorms and heavy showers can't be ruled out, especially
Saturday due to a strong disturbance aloft moving into the area.
Trades will briefly return for the weekend as high pressure passes
to the north behind the front, then shift out of the southeast early
next week as a stronger front approaches. This front is forecast
to quickly advance down the island chain Wednesday through
Thursday bringing another round of showers and potentially a few
thunderstorms. Drier air along with gusty northerly winds
following its passage will be possible through the latter half of
the week.

&&

.DISCUSSION...
Water vapor imagery showed a potent shortwave trough digging
southward into the area this evening with plenty of middle- to upper-
level moisture pooling northward over the islands to its southeast.
The tail-end of what is left of the attendant front at the surface
extends southwestward from an area north of Oahu through Kauai. This
moisture combined with these synoptic features and a sufficient
amount of instability have led to increasing shower coverage this
evening, mostly over Kauai and the adjacent waters. Expect this
trend to continue overnight into Saturday, especially over the
western end of the state as this boundary inches toward Oahu. Trades
have begun to fill in along and behind this boundary, which should
support the best shower coverage over windward and mountain
locations.

Upper heights will quickly lower late tonight through Saturday as
the aforementioned shortwave drops southeastward over the
central/eastern islands and closes off an upper low. Cold pool (-15
C) aloft accompanying this feature will destabilize the atmosphere,
potentially enough to trigger a few rumbles of thunder.
Heavy showers will become a concern through this time, that could
translate to localized flooding. The tail-end of the front will
diminish into a surface trough and stall somewhere around Maui
County through the day Saturday. Although confidence remains low,
some wintry mix can't be ruled out for the Big Island summits
late Saturday through early Sunday.

A drying trend from west to east appears likely as the upper low
begins to shift east of the area and the trough over the central
islands diminishes. Guidance reflects this and shows precipitable
water (PW) values dipping back down to around and just below average
for this time of year. Any showers that develop will favor windward
and mountain locations with the moderate trades holding over the
islands.

Guidance shows the low-level flow shifting out of the southeast
early next week as another front approaches. Mostly dry and stable
conditions along with light winds will likely give way to a land and
sea breeze regime, especially over Kauai and Oahu Monday through
Tuesday. The best chance for clouds and showers will favor windward
locations, except across the interior areas through the afternoon
hours where sea breezes setup.

The extended guidance remains in decent agreement and shows an
amplifying upper pattern evolving over the central Pacific due a
strong shortwave trough approaching. If this materializes, this
shortwave will drive a much stronger cold front through the islands
quickly Wednesday through Thursday. A line of showers and
potentially a few thunderstorms will accompany this feature. Strong
northerly winds and some drier air will follow its passage through
the second half of the week, especially for the western end of the
state.

&&

.AVIATION...
Through tomorrow evening, VFR conditions are expected to prevail.
However, tempo MVFR conditions are possible as scattered showers
and storms develop from the remnants of frontal system. Moderate
trade winds will continue into the evening.

As of 06Z, surface analysis shows the remnants of a frontal
system being pushed over Kauai tonight. This is due, in part, to a
shortwave moving along the lower extension of the surface trough
assoicated with the previous front. This shortwave will help to
further push this area of moisture and instability over the rest
of Hawaii tomorrow. This will bring in MVFR conditions to the
island chain from the NW to the SE tonight through tomorrow
evening, with the best chances of deteriorating visibility
conditions due to weather over Kauai and Oahu.

Near the end of the 06Z TAFS tomorrow, moderate to strong trade
winds will begin to set up and will further help to generate
showers and storms due to upslope flow over the windward portions
of the island chain aiding in convective development along Mauka
areas of the islands.

&&

.MARINE...
High pressure building NNW of the islands tonight will move
steadily E through the weekend, while also strengthening to a
potent 1042 mb by Sunday. The high is pushing a rapidly weakening
frontal boundary over the area from the NW, leading to increasing
showers in a strengthening trade wind flow. As the front devolves
to a N-S oriented low-level trough near and N of the islands on
Saturday, the strongest winds associated with the building high
look to remain N of island waters. Instead, moderate to briefly
strong NE trade winds are expected near Kauai/Oahu, with slightly
lighter ESE winds expected near Maui and the Big Island. A Small
Craft Advisory (SCA) is posted for most marine zones, due to a
combination of seas and winds, with a downward trend in both later
in the weekend likely leading to its cancellation.

As the trough gradually diminishes and moves W of the area
Saturday night and Sunday, moderate to locally strong ESE trade
winds are expected over all waters. A weakening trend to this flow
is expected on Tuesday as a more organized front approaches from
the NW and weakens the ridge. The front is forecast to move over
the islands Wednesday, bringing a shift to strong N winds as it
passes. In the meantime, despite the low-level front weakening to
a trough, an approaching strong trough aloft is expected to carve
out a low that grazes the island chain on Saturday, bringing the
potential for a few thunderstorms and locally heavy downpours that
lead to reduced visibility.

A High Surf Advisory (HSA) remains posted for shores exposed to a
medium-period WNW swell (see CFWHFO for details), which is
expected to gradually diminish through Saturday, just as a new N
swell arrives. Peak surf heights with the new swell will approach
advisory levels along N facing shores, and buoy observations will
be monitored to see if the swell arrives larger than expected. If
not, the HSA will likely be dropped Saturday morning. Diminishing
surf is expected Sunday into Tuesday, when a new WNW swell will
begin to arrive. This swell may peak near advisory levels at its
peak Wednesday, potentially getting even bigger on Thursday.
Otherwise, no other significant swells are expected. See the
recently updated Oahu Surf Discussion (SRDHFO), as it contains
detailed information on the source of the swells for the islands.

&&

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

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

&&

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