Gale Warning 156, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
429 PM AKST Wed Feb 13 2019

.ANALYSIS AND UPPER LEVELS...
Synoptically, there are three main features aloft. The first is a
trough that is digging southeastward over the Gulf of Alaska. At
the base of the trough is a vertically stacked low in the
southeastern Gulf. The flow around the trough is advecting cold
air over the western Gulf, western mainland and AK-Pen. The second
feature is an amplified ridge that is building over the central
Bering and Aleutians. The third aspect of the pattern is a
deepening vertically stacked low sitting just to the west of the
Aleutian chain. Cold air is wrapping around this Kamchatka low,
and a warm tongue is pushing over the central Aleutians (ahead of
the system) from the North Pacific. This low sits in the left exit
region of the jet where winds are approaching 120 kts at 30,000
ft. The jet is meridional with the highest wind speeds
northwesterly oriented over the AK-Pen and Bristol Bay.

At the surface, offshore flow in Southcentral is increasing winds
through the northern Gulf coast and western Gulf. Northerly flow
is advecting cold, dry air at the surface into Southcentral,
causing temperatures to drop and keeping precipitation limited.
Most of the activity is present in the southwest as strengthening
front moves over the western Aleutians. This front is on the
leading edge of the 956 mb surface Kamchatka low that is skirting
the western edge of Shemya.

&&

.MODEL DISCUSSION...
Models are in relatively good agreement with handling the active
weather pattern through the end of the week, so only minor
changes were made using NAM data for Southcentral and the Gulf and
GFS for the Southwest and Bering. Good run to run consistency
through Friday has led to greater confidence on the timing of the
front moving across the Bering and Southwest. There is still
uncertainty on the track and organization of this front as it
moves to Southcentral on Saturday. All 12Z operational models are
in sync on developing a surface low along the western gulf early
Saturday morning, however they appear a bit aggressive on breaking
down the amplifying ridge. Kept the inherited forecast mainly
intact for Saturday; however, precipitation may need to be trended
down if the ridge holds.

&&

.AVIATION...
PANC...VFR conditions prevail through Thursday afternoon. Northerly
flow could be gusty early this evening, then again beginning late
Thursday morning.

&&
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2/Wednesday
night through Friday)...
High pressure and fair weather will continue for the next couple
of days in southcentral AK. Temperatures will continue to cool
gradually with northerly flow bringing in cold air from the
Arctic. Cold air and high winds will continue to push off shore as
well; this phenomena is being induced by the strong low moving
through the Gulf of Alaska and the high pressure ridge to our
west. Winds will gradually decrease from west to east but overall
not much change in the weather for all of southcentral AK.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A ridge of high pressure that has brought fair conditions to much
of Southwest today will continue moving inland through early
Thursday. A front associated with a Bering low lifts over
Southwest Thursday morning, bringing gusty south to east winds and
accumulating snow. Along the northern Kuskokwim Delta coast,
winds combined with snow will create limited visibility at times.
We have issued a Winter Weather Advisory for that area for
Thursday.

By Friday morning, a second front associated with a strong 950 mb
Bering low lifts inland over Southwest, bringing another round of
southerly flow and a chance for mixed rain and snow across the
Southwest coast. Warm air advection will allow for precipitation
to fall mostly as rain over the AKPEN through Fri afternoon. We
will continue to monitor these systems for potential coastal
hazards through the weekend.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The battle will rage on over the Bering Sea. The area is ripe for
cyclogenesis due to a strong baroclinic zone in the low levels
and an impressive 150 kt upper level (30,000') jet over the North
Pacific. So while the low from last night is just about (or has
just) reached peak intensity, another near storm-force low will
move in later this period to take its place. First, tonight's low
will track over the Western Bering as it reaches full maturity.
Its associated front will elongate across the entire Bering Sea
before reaching the West Coast early Thu morning. As the front
slides through, expect storm-force southeast winds ahead of it and
then another round of storm-force southwest winds behind it.
There is a healthy amount of precipitation right along the front,
but most of it will fall as rain with some decent warm air
advection. Behind the front, cold air will plow through leaving
all locations open to numerous snow showers.

The next system is due to take shape near Atka and Adak by Thu
afternoon. Originally it was thought that this would be a weaker
secondary system wrapping around the parent low near Kamchatka.
But models came in today and showed this low actually potentially
becoming the dominant low in the area. With that, they all trended
stronger on both winds and waves. We trended the forecast in that
direction to an extent, but did not go as bullish as the GFS
(strongest). Either way, this system intensifies rapidly Fri
morning over the Central Bering. South to southeast winds could
bring some hurricane force gusts back to the marine waters. It
will sling another warm front right at the West Coast by late Fri
as it follows a similar track to the system earlier this week
(Monday).

&&

.MARINE (Days 3 through 5/Saturday through Monday)...
Winds will gradually diminish and seas subside through the day
Saturday over the western Bering as a low departs northward over
Siberia. Generally expect sub-gale force winds for the Bering as a
weak ridge builds in from the south. A second low will move into
the western Bering Sunday. There is some uncertainty to the exact
strength, with model solutions varying between gale and storm
force winds, though there seems to be a general consensus of gale
force winds over the Bering with this system. There is confidence
that the active pattern will continue into early next week with
gales and building seas likely as a series of lows continue to
move over the Bering.

A weakening front will move into the region Saturday, however
winds are expected to remain well below gales. Another low will
track in from the southwest on Sunday and brings the potential for
small craft winds and slightly building seas for the western and
southern Gulf.

&&

.LONG TERM FORECAST (Days 3 through 7)...
A low departing northward over Siberia will cause winds to
gradually diminish over the Aleutians by Saturday. An active
pattern will continue Sunday into early next week. A low is
expected to move into the western Bering on Sunday morning. The
timing of the associated front is generally well agreed upon,
bringing moderate precipitation to all the Aleutians and western
mainland through Monday. The low intensity remains somewhat
uncertain with the ECMWF and Canadian models similar in strength
and position. The GFS has a weaker low and is centered further
south. We are slightly leaning towards the Canadian/ECMWF
solutions however cautious with going towards stronger solutions
given the recent model strength inconsistencies. Models are in
poor agreement regarding position and strength of another low
tracking into the region Tuesday, but as previously mentioned the
active pattern is likely to continue through next week.

A weakening cold front will track into the southwest mainland
Saturday morning. It remains uncertain how much a ridge out ahead
of the front will impede its progress, however a quick shot of
snow is likely over the Anchorage area during the day, though
accumulations are uncertain at this time. Sunday looks to be more
promising as all models are indicating a weak low approaching
from the south. The GFS solution keeps it as a single closed low
moving roughly over Valdez whereas the ECMWF and Canadian models
split the low south of Anchorage. What is more certain is that the
southwesterly flow will advect a significant amount of moisture
into the area and whatever track verifies, accumulating snow is
looking more likely. The active pattern continues into early next
week, however current model agreement is poor, though snow showers
appear to remain possible.

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning 178.
 Gale Warning 119>121 127>132 136>138 165 170 173>177 179>181 185.
 Heavy Frz Spray 127 139 140 180 181.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...RA/KH
SOUTHCENTRAL ALASKA...BB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KO/MSO
MARINE/LONG TERM...AP