Area Forecast Discussion
Issued by NWS Honolulu, HI
FXHW60 PHFO 270319
Area Forecast Discussion...UPDATED
National Weather Service Honolulu HI
520 PM HST Sun Feb 26 2017
Latest satellite and radar trends indicate that a shortwave moving
toward the islands from the W is destabilizing the atmosphere to the
point that isolated thunderstorms have been observed over Hawaiian
Offshore waters to the S and SW of the islands, and over Kauai
leeward coastal waters. As the trough passes quickly overhead
tonight, the potential for thunderstorms cannot be ruled out, and a
slight chance of thunderstorms has been added to the overnight
Light to moderate easterly trade winds will strengthen slightly on
Monday before weakening again on Tuesday. High clouds, currently
blanketing Big Island skies, will thicken over the remainder of the
island chain on Tuesday, remaining in place through Wednesday. A
nearby developing low will bring a trend toward unsettled weather
the next couple of days, with the potential for heavy showers, and a
chance of thunderstorms, especially Tuesday through Wednesday. The
low will move north of the area on Thursday, and island weather will
become more settled, but light winds are expected to continue
through next weekend.
Latest surface analysis features lows centered to the distant NE and
NNW of the islands, with a surface ridge about 150 miles N of Kauai
extending from a high far N of the area. Water vapor imagery shows a
broad trough aloft over the area, with a swath of broken to overcast
mid-and high-level clouds blanketing Big Island skies. With the
ridge nearby, E to ESE winds were sufficiently light to allow sea
breezes to drive cloud formation over the island slopes from Maui to
Kauai, while it appears that high clouds precluded significant low
cloud cover from developing over the Big Island slopes. Some small
heavy showers are currently falling from these convective clouds
over portions of leeward and interior Oahu.
Little overall change to the current weather pattern is expected
tonight, with high clouds persisting over the E half of the state
while a mostly dry trade wind flow persists. Land breezes over
leeward areas should preclude showers after sunset while trade winds
may deliver a few windward showers. However, there is a chance that
showers start to increase over windward portions of the Big Island
and Maui later tonight, and forecast PoPs are higher there. Trade
winds, while remaining veered to an E to ESE direction, are expected
to gradually increase overnight into Monday as the low NNW of the
islands moves NW and the ridge to our N strengthens. This locally
breezy flow will persist through Tuesday before lighter winds return
for Wednesday through the end of the week.
Latest guidance continues to indicate that a trough aloft will
deepen NW of Kauai through Tuesday before moving over the islands
Tuesday night and Wednesday. This development will support
thickening high clouds over the whole state on Monday, while also
bringing increased instability and moisture. Additionally, as the
sharp trough aloft passes over the islands, guidance indicates the
development of a weak surface low near the islands on Wednesday. We
therefore anticipate a period of unsettled weather, but the
specifics of where remain uncertain, with some uncertainty as to
when as well.
There is enough certainty that a deep layer of mid- and upper-level
moisture will affect the state beginning as early as Monday, and
with temperatures aloft cold enough to support wintry weather over
the summits of the Big Island, a Winter Storm Watch has been issued
for Monday night through Tuesday night. Otherwise, the best chance
for wet weather appears to be Tuesday through Wednesday, with
potential for heavy showers and thunderstorms, but still little
confidence as to what areas may or may not be impacted. Confidence
and timing do not warrant issuing a Flash Flood Watch with this
package. As a note to customers, this type of watch can be issued
when forecaster confidence exceeds 50% that conditions will be met
within 48 hours.
A trend toward drier weather is expected Wednesday night/Thursday as
the trough aloft and associated surface low move N, and a surface
ridge develops over the islands. Light winds and mostly dry weather
are then expected to persist through next weekend, although the flow
around the ridge may advect vog from the Big Island over the smaller
islands, bringing hazy conditions Friday through Sunday.
Moderate east southeast flow expected at the low levels through
tonight, which is expected to strengthen on Monday. Aloft, a 110
knot subtropical jet will bring moderate turbulence over most of the
state into the evening, but the turbulence area is expected to move
northeast of the state tonight. AIRMET Tango is posted for moderate
turbulence for Oahu to the Big Island between FL200 and FL350.
AIRMET Zulu is in effect for light icing associated with the jet
stream clouds over just the Big Island.
No AIRMETs are in effect for mountain obscurations at this time, but
the potential threat for this will be across the Big Island summits
as the mid and high clouds build lower tonight. There is also a
chance some lower clouds and showers may lift up over the islands
from the south overnight and Monday.
The current west-northwest swell (300-310 deg) will continue to
slowly lower into the early portion of the week. A small
reinforcement out of the west-northwest is forecast to fill in
Tuesday through mid week. Surf along exposed north and west facing
shores will remain below high surf advisory levels through this
time. A small to moderate north-northeast swell (030 deg) due to a
pair of compact lows to the north of the islands will fill in across
the local area through Tuesday night, peaking early Monday. This
swell could produce surf near high surf advisory threshold (8 ft
faces) for exposed east facing shores around the peak of the swell.
For the extended, the latest model guidance depicts a gale to storm-
force low developing and tracking a few hundred miles north of
Midway Wednesday through Thursday, sending a large west-northwest
swell toward the local waters for the upcoming weekend.
Light to fresh breezes out of the east-southeast will continue
through Tuesday morning, becoming locally strong tonight. The
strongest winds will occur across the channels and around the Big
Island early Monday morning through early Tuesday morning, prompting
the issuance of a Small Craft Advisory for those areas and time
period. Models continue to depict a developing area of low pressure
near the state by mid-week, lifting northward across the state
through Thursday. Plenty of moisture and instability associated with
this feature will lead to increasing shower/thunderstorm chances
through this period.
Winter Storm Watch from Monday evening through late Tuesday night
for Big Island Summits.
Small Craft Advisory from 4 AM Monday to 6 AM HST Tuesday for
Pailolo Channel-Alenuihaha Channel-Big Island Windward Waters-Big
Island Leeward Waters-Big Island Southeast Waters.