Gale Warning 647, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
659 PM AKST Mon Nov 25 2019

Updated for Long Term and Marine Discussions

.ANALYSIS AND UPPER LEVELS...
An upper ridge built over the mainland in the wake of an upper
trough which moved into Canada. Cold air spread across the
mainland as morning sounding from Southwest Alaska showed 850mb
temps in the minus teens. Cold dense air over the mainland
generally increased thermal gradients between the Southcentral
Mainland and warmer maritime air over the Gulf. As a result
pressure gradients were fairly strong with lower pressure over the
Gulf and higher inland. This enhanced outflow winds along the
Gulf Coasts and also northerly gap winds through the Alaska
Range. A band of snow showers, associated with a leading wave,
moved across the delta bringing locally low visibilities. A
northeast moving low over the western Bering Sea brought westerly
storms and warm air with rain to the Bering Sea and Aleutians.
Temperatures were in the 40s from the Aleutians to the Pribilof
Islands and rose to the 30s along the Delta coast.

&&

.MODEL DISCUSSION...
Models are reasonably handling the storm force front moving east
across the Bering Sea. The biggest challenge over Southwest
Alaska will be precipitation types. The next challenge will be
the handling of the frontal trough over the northeast Pacific and
associated movement of the frontal wave. Models also differ in
temperature advection and wind fields. The ECMWF seems to be the
most consistent and will be used in the afternoon Package.

&&

.AVIATION...
PANC...Ceilings should remain VFR through the period, but could
flirt with MVFR as stratus builds along the mountains. Otherwise,
light northerly winds will persist.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2/Tonight
through Wednesday)...
An amplifying upstream ridge pushing onshore of Southwest Alaska
this afternoon will build over Southcentral tonight through
Tuesday. A strengthening jet on the leading edge of the ridge
along with a short-wave dropping south through the Copper River
Basin will enhance offshore gap winds in the Thompson Pass/Valdez
area as well as through the Copper River Delta. There isn't much
cold advection to speak of, since the arctic air mass has already
moved out into the Gulf. This is the big limiting factor for
strength of winds and this would be characterized as a "weak"
offshore wind event. Gap winds along the north Gulf coast and
Prince William Sound will die down during the day Tuesday as the
ridge moves overhead. For the remainder of the region, expect
mostly clear skies tonight with seasonably cold temperatures.

On Tuesday a weak warm front lifting northeastward across
Southwest Alaska will just clip Southcentral. Low levels will
be dry, but weak overrunning could produce some very light snow
from the Kenai Peninsula to the Susitna Valley. With a dry airmass
in place, the mountains are most likely to see a dusting or light
accumulation while the valleys likely won't see more than
flurries.

As the upper ridge continues to slide eastward Tuesday night,
a digging trough over the north Pacific (just south of the
Aleutians) will pick up energy from a trough and low sitting
farther south over the Pacific. Warm air and moisture will stream
northward up the west side of the ridge Tuesday night and continue
northward across the western Gulf and into Southcentral on
Wednesday. A couple weak leading upper waves could produce some
snow Tuesday night into Wednesday morning across the Susitna
Valley, but notably heavier precipitation will arrive in
Southcentral Wednesday afternoon and beyond.

There are some key differences in model guidance on the
progression of the frontal system during the day Wednesday,
along with weak lows moving up the cold front. This introduces
large uncertainty into the details of the wind, temperature, and
precipitation-type forecast - particularly for the western Kenai
and Anchorage. The overall set-up is favorable for widespread
winter weather impacts across Southcentral. However, there are
key questions about strength of downslope flow and whether
precipitation for the population centers will fall as snow or
freezing rain. The current forecast attempts to reflect a forecast
in the middle of the two extremes. The one high confidence aspect
of the forecast is that the Susitna Valley will see significant
snow accumulation and the eastern Kenai Peninsula will see some
snow and blowing snow when precipitation first arrives on
Wednesday. Outside of that, will have to wait to refine details
and weather impacts until have more confidence in the synoptic
scale solution.

&&

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

A storm force front will move across the Bering tonight and bring
a mix of rain and snow into the Kusko Delta and southwest Alaska.
Expecting areas of blowing snow with strong winds inland. While
coastal areas see a mix of rain and snow where strong warm air
advection ahead of the front will mix with colder air in place
this evening. Reinforcements to precipitation will arrive
Wednesday morning in the Bristol Bay area as the Bering front
slows down and another north Pacific low pushes a front into the
area. The main driver at this point will be the ridging to the
east over Southcentral Alaska that will have a blocking effect on
these two systems and their progress east.

&&

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

A storm force front will move from the western Aleutians to the
Alaska Peninsula tonight through Wednesday. Strong warm air
advection will bring rain ahead of the front, while the strongest
areas of cold air advection behind the front will be responsible
for storm force winds in the Central Bering tonight. Wednesday
morning a frontal boundary from a north Pacific low will merge
with the Bering front and bring more rain to the Alaska Peninsula.
Interaction of these two fronts will depend heavily on the ridging
over Southcentral Alaska.

&&

..MARINE (Days 3 through 5)...
Thursday through Friday several storm systems are expected across
area waters. The first storm will impact the Eastern Aleutians,
Alaska Peninsula and Western Gulf with gale to storm force winds
around the low core as it deepens to 970 mb on its track toward
Sand Point Thursday afternoon. Winds associated with this storm
taper down Friday night with the low filling on a southward track.
To the west...a low rapidly develops and swiftly tracks north
along the western Bering Thursday, with a second low developing
south of the western Aleutian Chain taking a northward track. This
second feature will bring gale to storm force winds as it lifts
north over the Western and Central Aleutians Thursday and Friday.
Saturday, a weaker low may develop over the easter Bering to
western Gulf of Alaska, with the potential for gale winds. There
is high uncertainty in the mid range forecast on timing and
strength of the next several lows...so monitor the going forecast
for changes.

&&

.LONG TERM FORECAST (Days 3 through 7)...The long range forecast
beginning on Wednesday is an active pattern across the southern
mainland and surrounding waters. Overall, a wetter pattern can be
expected through next Monday as low pressure over the Bering on
Wednesday, is shoved eastward on Thursday, then followed by
several other systems moving from west to east. The upper level
jet stream will go through several phases of amplifying which will
enhance precipitation Wednesday into Thursday and Sunday into
Monday. Models are not in very good agreement on timing and
strength of the systems rapidly moving through the flow;
therefore, expect many updates throughout the next week. A blend
of models (with a heavy amount of WPC) were used for afternoon
updates...with quite a few changes for wind directions/speed and a
general increase for precipitation.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Coastal Flood Advisory 155 161.
MARINE...Storm Warning 412-414 179 181 185.
         Gale Warning 352 132 411 150 165 170 173 175-178 180.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...RMC
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...LTB
MARINE/LONG TERM...BL