Gale Warning 186, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
421 PM AKST Fri Feb 22 2019

.ANALYSIS AND UPPER LEVELS...
A ridge continues to build over mainland Alaska, with a trough
digging in over the Bering, which will be the dominant synoptic
pattern through early next week. Southcentral is experiencing dry
conditions and offshore flow as this ridge starts to amplify over
the area. A front stretching from the North Pacific to the
northern Bering along the western part of the ridge is bringing
gusty conditions and rain showers across the western and central
Aleutians and Bering.

&&

.MODEL DISCUSSION...
The numerical models are generally in very good agreement through
Sunday afternoon. Out west, there are some differences in how the
models handle a low moving across the central/western Aleutians
Friday afternoon and then northward across the central Bering
Friday night and Saturday. With a ridge and subsidence over
Southcentral, some fog is expected and the models generally do not
do very well with low level moisture. The forecast confidence
for the afternoon package is above normal, except below normal
for the fog.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist into this
evening. LIFR conditions in fog are expected beginning around
midnight, and continuing through the morning. The timing and
density of the fog around there area is uncertain, but judging
from last night conditions, it should move in. Then conditions
should improve Saturday afternoon as the fog lifts/clears.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...
(Tonight through Monday)

As a former colleague of mine would say, "big bubble...no
trouble". That is the best way to some up at least the next few
days, as high pressure will continue to build across Southcentral
through the period. This will allow fog to develop during the
nocturnal hours and persist through the morning. For tonight, it
looks like portions of Copper Basin and Western Kenai up through
the western Anchorage Bowl will see the greatest impact from this.
Then, as the ridge will continue to strengthen over the next few
days, look for the fog to increase in coverage and density, with
visibilities 1/2 mile or less a real possibility for many
locations.

With a higher sun angle compared to the past few months, most
locations should experience enough heating to help mix out some or
all of the fog during the afternoon hours, except for the Copper
Basin. Here, much colder air being trapped in the valleys will
make it more difficult for this to occur.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Saturday
and Sunday)...

As upper level ridging builds over the southern mainland, and
moisture from a low and its associated front over the Central
Aleutians flows over the top of this ridge, the resulting moist
southwest onshore flow over the Kuskokwim Delta will keep a chance
of precipitation across the region through tonight. As this ridge
is pushed east by the low and front moving into the Bering Sea
Saturday, increased moisture moves to the Kuskokwim Delta coast
once again increasing the chance for precipitation Saturday
afternoon and night. Along with this increased moisture will be
very warm low level air, and the 850 mb level temperatures are
expected to warm from -10C to 0C Saturday. This will result in
precipitation, if any, in the form of rain. By Sunday the upper
ridge is shoved over the Southcentral region, thus increased
cloud cover and chances of precipitation can be expected over the
southwest mainland Sunday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Saturday
and Sunday)...

A low now over the central Aleutians will continue to trek north
with its associated frontal boundary pushing east to south of the
Aleutians Sunday producing a strong gradient in advance of the
front and around the low resulting in strong wind. Temperatures
will support snow at first then a mix of rain and snow. This
elevates the concern for a brief period of possible blizzard
conditions for the central and western Aleutians Sunday.

&&

.MARINE (Days 3 through 5)...

There is high confidence in a deep storm-force low crossing the
Western Aleutians late Sunday into Monday. A widespread area of
southeasterly gales, with the potential for smaller core of
storm- force winds, are expected across the central and western
Bering ahead of this system through late Monday. Westerly storm-
force winds will wrap underneath the low, impacting the coastal
water surrounding the western and central Aleutians. Waves with
this system will rise to 30 to 40 feet on the Pacific side of the
western and central Aleutians and to 20 to 30 feet across the
western and central Bering. Winds and waves will slowly decrease
in intensity and height as the low lifts north Tuesday. Confidence
is also above average with a second deepening storm-force low
moving across the central Aleutians and into the central and
eastern Bering late Tuesday into Wednesday with widespread
southerly gales, areas of storms, and wave heights building to
over 20 feet.

High pressure anchored over the Gulf of Alaska will allow
generally westerly, sub small-craft winds along the coast from
Kodiak Island to Cordova through midweek.

&&

.LONG TERM FORECAST (Days 3 through 7...Mon through Fri)...

An established omega block stretching from the western Aleutians
east to the Canadian Interior will anchor a ridge directly over
Southcentral through much of next week. The vertically stacked
ridge will result in a rather tranquil weather pattern, with the
biggest forecast challenge through the period being fog, due to
the weak flow in the lower levels and warmer air advecting
northward in the mid-levels. Drier conditions and light winds will
also allow for larger diurnal temperature swings, with daytime
highs across Southcentral running slightly above average and
nighttime lows below average. A few areas of scattered cloud cover
may develop late Monday into Tuesday from Kodiak north through the
Cook inlet and again late Wednesday into Thursday across western
areas of Southcentral as two shortwaves rotate around the ridge.
Both of these systems, however, are expected to weaken and shear
apart as they lift northward.

For the southwest mainland, conditions will remain more unsettled
with the region sitting in the transition zone between the main
trough to the west and the ridge to the east. Here, a series of
fronts will track toward the coast, move inland and fall apart as
they run into the ridge. The first will arrive late Sunday and
linger through Monday before a second reinforces the southerly
flow and precipitation (especially along the coast) late Monday
through Tuesday. These systems then fall apart as the ridge to
the east shifts a bit to the west. A third front is then expected
to impact the southwest late Thursday into Friday. The challenge
here will be precipitation type, especially as a persistent
southerly flow continues to bring warmer air into the region. The
first two systems look to be mostly a rain event along the coast
(with snow aloft and over the interior), but slightly colder air
in place over the region prior to the third frontal system may
bring a wintry mix to all locations during the onset of
precipitation before the warmer air arrives.

Farther west, the weather pattern is expected to remain quite
active with a deep trough anchored over the Bering. we have a
very active pattern on tap for next week. The week starts with a
strong storm-force low crossing the western Aleutians into the
western Bering. This low lifts north, a second gale-force low
follows behind the first and tracks north into the western Bering
beginning early Wednesday. Broad, cyclonic flow and areas
of showers continue across the Bering as this second storm lifts
north. A newly developing North Pacific low is then expected to
advance toward the western Aleutians by the end of next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning: 150-170, 172-177, 179-185, 411-414.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...AH/BL
SOUTHCENTRAL ALASKA...PD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...SA
MARINE/LONG TERM...TM