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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 110137
Area Forecast Discussion
National Weather Service Honolulu HI
337 PM HST Mon Aug 10 2020
Trade winds will continue to provide cooling breezes to the islands
for the foreseeable future, but they will ease slightly over the
next day or two. The trade winds will periodically deliver low
clouds and showers to windward areas, with showers most active
during the overnight and early morning hours.
A surface high far N of the area will change little this week,
thereby keeping the islands embedded within the trade wind belt.
However, forecast guidance indicates some loosening of the low-level
pressure gradient the next day or two, with some reduction in trade
wind speeds expected. Moderate (bordering on light at times) trade
winds are then expected to prevail through next weekend, but winds
may back to the NE over the weekend as the high moves W in response
to a front passing far NNE of the area.
The atmosphere is expected to remain stable, but randomly
distributed low clouds and showers will occasionally dampen windward
areas. Nocturnal cloud-top cooling means shower formation is most
favored during nights and mornings. On the other hand, daytime
heating of the leeward Big Island slopes will drive the daily
formation of afternoon and evening shower-bearing clouds. To a
lesser extent, this could occur on the other islands with the
diminished trade wind speeds later in the week.
Forecast models have been bullish on tropical cyclone development
near and within the central Pacific, and a trough far SE of the
islands is being monitored for potential development later this
week. This trough is now expected to spawn a westward-moving
tropical depression by the weekend (70% chance), with any potential
impacts to Hawaii being unknown at this time. Details on formation
probabilities are contained in regularly-issued Tropical Weather
Outlooks, which are available at hurricanes.gov, as are forecast
advisories for active tropical cyclones.
Moderate to locally breezy trade wind flow will persist through
twenty-four hours. Clouds and scattered showers will continue to
favor northeast coasts and island interiors. Partial clearing
expected across Leeward slopes after midnight. Isolated MVFR CIG and
VIS are to be expected in passing showers, otherwise VFR conditions
are expected to prevail all areas.
AIRMET TANGO remains in effect below 8000 feet over and south thru
west of mountains for occasional moderate turbulence.
Subtle weakening of the high north of the islands will take
currently fresh to strong trades down into the moderate to fresh
range Wednesday. A Small Craft Advisory is in effect through
tomorrow afternoon for Maalaea Bay, the Pailolo and Alenuihaha
Channels along with the waters south of the Big Island. Seas will
remain in the 4 to 6 foot range, 7 to 8 feet in the Alenuihaha
Channel and off of South Point tomorrow.
Fresh trades will maintain rough surf along eastern facing shores
through tomorrow. Slightly weaker trades will lower mid week surf
but an upward trend may arrive Thursday. A 25 to 30 knot
northeasterly fetch area forming off the Oregon and Northern
California coastlines today could correspond to a medium period
north northeasterly swell arriving Thursday. This will pick up late
week north facing shore surf. Not much southern energy translates to
continued small surf along south facing shores.
Small Craft Advisory until 6 PM HST Tuesday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office