Current Conditions
Temp3.7 C
RH42 %
WindNE 0 mph
RoadOpen (4x4)
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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 180602

Area Forecast Discussion
National Weather Service Honolulu HI
802 PM HST Thu Sep 17 2020

The dry and stable trade wind pattern will continue into next week,
with the best chance for showers developing overnight through the
early morning periods across windward and mountain locations.
Moderate trade winds may become locally breezy over the weekend,
then relax by midweek as the ridge weakens to the north.


Latest surface analysis and satellite imagery showed the
subtropical ridge anchored north of the islands and patchy low-
level clouds moving through from east to west. Water vapor imagery
depicted mostly dry middle- to upper-level air over the state
between an upper trough to the northwest and an upper ridge to the
east and southeast. The afternoon upper-air soundings at Lihue
and Hilo reflected this and exhibited mostly dry and stable
profiles with PWs measured around 1.2 inches. Satellite-derived
precipitable water (PW) imagery, however, does reveal a decent
sized area of moisture skirting the islands to the south due to
low pressure far south of the area in the deep tropics. Some
upper-level debris clouds from the convection associated with this
feature have made it as far north as the Big Island. Showers have
been sparse with little accumulations over the past several days
across the state.

Guidance has initialized well with the current pattern and depicts
the stable and dry trade wind conditions continuing through early
next week as the upper heights rise over the weekend. Dewpoints
reflect this dry pattern and dipped several degrees below
predicted this afternoon, which drove relative humidities into the
upper 30s to lower 40s for some leeward areas. Although a slight
increase in moisture is possible as the aforementioned moisture
skirts the islands to the south, showers will be sparse with the
best chance occurring overnight through the early morning periods
each day. Moderate trade winds may become locally breezy over the
weekend, then relax by midweek in response to a weakness
developing in the subtropical ridge as a front passes to the


A surface ridge north of the islands will maintain moderate trade
winds through early Friday morning. Expect mainly dry conditions
also, with some low clouds and brief showers possible tonight
over windward facing sections. This may produce brief periods of
MVFR conditions along windward facing slopes, especially later
tonight and early Friday morning. Otherwise, expect VFR
conditions to prevail overnight.

The trades are forecast to increase slightly on Friday, but
mainly dry trade wind weather is expected to persist. Some
windward sections may see some clouds and light showers from mid-
morning through the afternoon on Friday. In addition, clouds and a
chance of showers are possible along the upslope sections of the
Leeward Big Island Friday afternoon.

No AIRMETS are currently in effect, and none are expected through
Friday afternoon.


Strengthening high pressure to the northeast of the islands has
created a tight enough pressure gradient across the chain to
support a Small Craft Advisory (SCA). The latest afternoon
scatterometer pass showed 25 knot winds in the Alenuihaha Channel
and over those waters south of the Big Island. The SCA is in
effect for these waters, as well as Maalaea Bay and Pailolo
Channel, as winds are expected reach SCA criteria and hold at
those magnitudes through the weekend.

Surf will remain below advisory levels along all shores. Longer
period small south-southwest swells are expected to arrive over
the next couple of days. North shore surf will remain nearly flat
through tonight, then increase a bit as small north-northeast and
northwest swells arrive Friday and last through early next week.
East shore surf will increase a touch due to strengthened trade
flow but will still stay below average through the middle of next


Small Craft Advisory until 6 PM HST Sunday 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