Drought in Hawaii, February 1 to 28, 2021

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
912 PM HST Sun Jan 31 2021

.SYNOPSIS...
The high pressure system north of the islands will slowly drift
eastward as a cold front moves into the Central Pacific from the
west. Easterly trade winds will diminish on Monday, and veer to
the southeastly and southerly direction by Tuesday ahead of an
approaching cold front. A cold front will sweep from west to east
down the island chain from Tuesday night through Thursday morning.
Clouds and showers along the frontal band will produce widespread
rain, heavy at times, with a few thunderstorms. Colder air behind
the front will bring drying trends with below normal temperatures
spreading across the islands from Wednesday into the weekend.

&&

.DISCUSSION...
The satellite picture this evening shows a high pressure system
sandwiched between a cold front moving into the Pacific Northwest
and a cold front moving into the Central Pacific basin from the
west. Low level cloud bands, likely unstable remnants of an old
frontal boundary, drifting in on the trade winds from east to
west. These cloud bands will be enhanced in the overnight hours
with developing shower bands favoring windward and mountain areas
through early Monday morning. A cold front, roughly 1000 miles
northwest of Kauai, continues to move into the Central Pacific
region from the west. This cold front will pass through the
islands later this week spreading widespread rainfall across the
state. A weak jet stream north of Hawaii will produce some ice
crystal cirrus clouds on Monday enhancing sunrise and sunset
colors.

Easterly trade winds will diminish on Monday as the high pressure
system north of the islands drifts eastward away from the state.
A cold frontal system will move into the region from the west
causing wind directions to veer southeasterly and southerly from
Monday night through Tuesday. Numerous shower activity favoring
windward and mountain areas will also diminish by Tuesday morning,
as the wind directions shift to a more southerly direction.
Prefrontal shower bands will develop on Tuesday producing brief
periods of showers moving up over the islands from a southerly
direction.

On Wednesday and Thursday, a fast moving cold front sweeps through
the Hawaiian Islands from west to east. Southwest winds will
develop ahead of the front on Wednesday, quickly shifting to a
cool and breezy northwesterly winds after the front passes each
island. Widespread rainfall will develop along the frontal band
with showers, heavy at times, spreading across all islands in the
state. Enough instability will develop with this system for a few
thunderstorms. The latest models are showing a prefrontal
convergence band of showers developing just ahead of the cold
frontal boundary, producing two rounds of showers adding to storm
total rainfall amounts for each island. The initial timing on the
cold frontal passage starts with Kauai and Niihau from Tuesday
night to Wednesday morning, reaching Oahu by Wednesday afternoon,
moving into Maui County from Wednesday afternoon to evening, and
finally drifting into the Big Island from Wednesday night to
Thursday morning. Just remember that the prefrontal rain band will
arrive roughly 12 hours ahead of the cold front, so expect two
rounds of widespread rain over most islands. Get your jackets
ready, as much colder air will move into the islands behind the
front with high and low temperatures falling below normal into the
weekend.

The cold front will move fairly rapidly from west to east through
the state this week decreasing statewide flooding threats.
However, small scale heavy rain bands within this system may line
up over individual islands keeping a possibility for more
localized flooding in the forecast. For now, our confidence is not
high enough to issue Flash Flood Watches for any islands at this
time. We will continue to monitor this rainfall event closely with
regular forecast updates as the weather situation evolves.

Thursday and Friday, more stable northerly flow will develop over
the islands on Thursday as the front moves eastward away from the
state. Much colder temperatures are forecast with highs for most
islands in the mid to upper 70's and the overnight temperatures
falling into the low 60's. Moderate northeasterly trade winds
return on Friday with isolated to scattered showers.

Saturday and Sunday, moderate northeasterly winds continue on
Saturday. Trade winds may weaken on Sunday as a low pressure
system drifts into the region from the east. Long range forecast
guidance remains poor between the American and European models on
the movement and intensity of this low. However, we continue to
have a high confidence in below normal temperatures lasting
through Sunday.

&&

.AVIATION...
A surface high far N of the islands will support moderate to
fresh easterly trade winds. Winds will slowly weaken and veer
out of the southeast over the next 24 hours. VFR conditions will
prevail this evening, with isolated MVFR ceilings and visibilities
over east sections. Some increase in coverage is anticipated later
tonight as a band of showery low clouds arrives on the trade wind
flow. Widespread-enough mountain obscuration may require issuance
of AIRMET Sierra by Monday morning.

&&

.MARINE...
Winds are expected to remain below Small Craft Advisory levels
through much of the week. High pressure to the north of the region
will maintain winds out of the east through Monday, but expect
winds to shift to the southeast beginning Monday night ahead of a
cold front that will approach the region from the northwest. The
front is expected to reach the northwestern coastal waters by
Tuesday night, and push down the island change through Wednesday
night, spreading some heavy rain and thunderstorms as it does so.
Breezy northwest winds are expected behind the front, followed by
northerly winds by Thursday.

The current northwest swell will continue to lower through Monday,
followed by a series of small, northerly swells early in the week.
A much larger northwest swell will likely produce very large to
extra large surf along north and west-facing shores of the
smaller islands toward the end of this week, potentially affecting
west facing shores of the Big Island as well. Early energy from
this swell will likely arrive on Kauai Tuesday afternoon, and then
steadily build, peaking late Thursday into Friday. Surf along east
facing shores will continue to trend down as the trade winds ease
and turn to the southeast. South shore surf will remain small
through the forecast period.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

DISCUSSION...Bohlin
AVIATION...Donaldson
MARINE...M Ballard