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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 191340
Area Forecast Discussion
National Weather Service Honolulu HI
340 AM HST Wed Jun 19 2019
Moderate to locally breezy trade winds will deliver just a few brief
windward showers through Thursday, but then winds will weaken and
shift to the southeast from Friday through the weekend. The lighter
winds will lead to daytime sea breezes, spotty showers and muggy
The current synoptic features affecting island weather include a
broad ridge aloft, a persistent surface trough to the NW, and the
trade wind supporting high centered far to the NE. A ridge from this
high extends SW to about 450 miles N of the islands, and the
pressure gradient S of these features is supporting locally breezy
trade winds which are currently delivering a field of primarily
stable broken to overcast low clouds, with these clouds dropping a
few windward showers. Overnight soundings indicate a stable island
atmosphere capped near 7000 feet, and little windward rainfall is
expected in the short term, while leeward areas will be mostly dry.
Relatively thin high clouds are possible into Friday, but they may
briefly thicken on Thursday due to the passage of a weak trough
While little significant change is expected with the high to the NE
into next week, the trough to the NW will become reinvigorated by a
developing closed low aloft beginning Thursday. This cutoff low and
associated trough aloft is expected to linger NW of the islands well
into next week, supporting the development of several weak surface
lows, while also causing the persistent trough axis to drift closer
to the islands. The ridge to the N will move over the islands in
response, causing winds to weaken and veer to the S and SE along the
W end of the island chain, with light to moderate E to SE winds near
the Big Island.
Initially, this flow looks to support only limited showers, although
increasing surface dew points will combine with the diminished winds
to bring increasing mugginess. As the deep-layer trough axis nears
Kauai Friday night and Saturday, models indicate increasing moisture
advecting northward over waters W of Kauai, which could lead to a
period of showery weather over Kauai county as well. Meanwhile, a
mostly dry weather pattern is expected over the remainder of the
state through Saturday.
Later in the weekend into early next week, there is increased
uncertainty as to where clouds and showers will occur (and when),
due to the veered wind flow and gradually increasing low-level
moisture. While mostly dry weather (outside of a few brief showers),
will continue over the Big Island, the other islands could see
spotty showers from Sunday into Wednesday. Some showers could form
in the afternoons over interior areas due to daytime heating and sea
breeze convergence, with others forming within eddies and cloud
plumes peeling off other islands, potentially bringing much needed
showers to leeward areas, with a chance of briefly heavy downpour.
Additionally, conditions will be warm and muggy.
Long range outlook presented by latest GFS/ECMWF guidance has
delayed the return of trade winds until the end of next week, as low
pressure lurking NW of the islands continues to veer and weaken the
normally-expected trade winds.
Most of the shower activity early this this morning is off to the
southwest of the island chain although there are a few showers on
the windward sides of the islands. Relatively strong high pressure
far to the northeast of Hawaii will remain stationary through the
period. This will keep light to moderate trade winds from the
east/northeast through at least tonight. There will be fairly stable
conditions with drier air over most parts the islands. Most of the
shower activity will be confined to the windward slopes, especially
on Maui and the Big Island. Winds will be moderate again this
afternoon in the same areas as Tuesday but they should not be strong
enough to produce strong low level turbulence.
There are currently no AIRMETs in effect.
A surface high far northeast of the area will maintain locally
strong trade winds over the coastal waters through tonight. The
Small Craft Advisory has been extended through tonight for the
Pailolo and Alenuihaha Channels and waters south of the Big Island.
Winds will weaken and shift out of the southeast beginning Thursday
as a trough deepens west of the offshore waters. The southeast flow
will be partly blocked by the Big Island, resulting in light winds
around the smaller islands Friday through the weekend.
Existing small surf will fade even more today and remain well below
the advisory threshold along all shores through the coming weekend.
A small boost in east shore surf is possible as a short-period,
northeast swell builds over the weekend.
A Red Flag Warning (RFW) for extreme fire behavior requires that the
Keetch-Byram Drought Index (KBDI) be greater than 600 (it's
currently near 670, while normal for this time of year is near 510),
sustained wind speeds be greater than 19 mph, and RH be less than 46
percent - for two hours or more - based on observations at PHNL.
While sustained winds are not expected to reach criteria for the
issuance of a RFW, dry fuels combined with RH values near 50% around
midday will combine with the locally gusty trade winds to bring
increased fire weather concerns today and Thursday. Decreasing winds
and increasing RH vales from Friday onward will diminish those
Small Craft Advisory until 6 AM HST Thursday for Pailolo Channel-
Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office