Heavy Freezing Spray Warning 114, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
423 PM AKST Fri Mar 12 2021

.ANALYSIS AND UPPER LEVELS...

A highly amplified pattern exists in the upper levels. The
previous "snow-bringer" low pressure system is weakening in the
Gulf of Alaska as it slides eastward. The other deep system is
well out west near Kamchatka, though the front associated with it
is extending from the Bering Strait region southward through the
North Pacific. There is a decent moisture tap down into the
tropics as precipitable water value range 100-200 percent of
normal. Between these two features is a ridging that extends onto
the west coast and some stronger northwest flow, bringing some
colder air from the mainland across the Alaska Peninsula.

From a sensible weather perspective, the departing system has left
a very cold air mass and mostly clear skies, save for some
lingering high clouds over the Copper River Basin. Cold air
flowing off the mainland has fueled gap winds in favored locations
along the north Gulf Coast. Out west, a gale-force front keeps
marching from west to east across the Bering Sea.

&&

.MODEL DISCUSSION...

No major issues with the departing system in the Gulf and frontal
timing is well agreed upon. Differences begin to arise as a low
center forms along the front when it clears the Alaska Range.
Currently guidance is clustered in the Barren Islands region but
the exact track has yet to be nailed down.

&&

.AVIATION...

PANC...VFR conditions and light winds will persist.

&&

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

The pattern across Southcentral remains quiet overall. A
shortwave ridge passes over the area tonight, allowing offshore
flow to continue through Saturday. Gusty winds are expected
through Whittier, Seward, Valdez, Thompson Pass, and the Copper
River Delta. Additionally, a High Wind Warning has been issued
for Thompson Pass late tonight through tomorrow morning as winds
continue to increase. Weather becomes active again with the
approach of a low pushing across the Barren Islands towards Prince
William Sound (PWS) on Sunday. Most of the precipitation will be
along the coastal areas, especially along the North Gulf Coast and
PWS. Western Kenai and the Anchorage area will likely be
downsloped out initially, with light snowfall on the backside of
the low. There is a lot of model inconsistency as to whether or
not Anchorage will see some snow or if it will stay downsloping
the entire time. This will need to be fined tuned in the coming
days. The highest accumulations will be along the upslope areas
(eastern side of mountain ranges).

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...

Weak, transient high pressure will continue to move eastward
while an occluded front moves in from the west this evening and
tonight. Snow will begin to fall across Nunivak Island late
tonight and reach the Kuskokwim Delta coast early Saturday
morning. Temperatures will remain cold due to a significant
offshore flow which will result in the snow being highly
transportable. Strong winds in conjunction with snow will result
in blowing snow and reduced visibility. Visibility will be reduced
to a quarter of a mile across Nunivak Island and the Kuskokwim
Delta coast. The strongest winds will occur in and around Etolin
Strait due to the localized funneling of winds. The front will
significantly weaken as it moves over the Kuskokwim Delta and thus
the winds will ease Saturday evening and Saturday night. A
Blizzard Warning has been issued for Nunivak Island and the
coastal Kuskokwim Delta for areas north of Kipnuk from 6am
through 9pm Saturday. With respect to snow accumulations, 1-3" is
expected for the Kuskokwim Delta and 3-5" across Bristol Bay and
2-4" with localized 5" across the Lower Kuskokwim Valley. The
front will exit to the east on Sunday. Temperatures will trend
warmer behind the front as westerly onshore flow moderates the
cold airmass.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...

A front currently over the central Bering/Aleutians will continue
to move eastward. This front will continue to bring widespread
gale force winds and storm force gusts across the area. Large
20-30ft long period swell in the western Bering will continue to
subside as the front moves farther to the east. The front will
move over Southwest Alaska Saturday afternoon and evening. In the
wake of the front, rain/snow showers and strong west-northwesterly
winds are expected and will reach gale force at times. These
strong west-northwesterly winds and rain/snow showers will
continue through Sunday.

&&

.MARINE (Days 3 through 5)...

The low pressure/frontal system developing in the Gulf of Alaska
on Sunday will feature gale-force winds, getting close to storms.
Backside gales will also be possible with this system before it
weakens late Monday. The Bering Sea will be relatively quiet with
steady westerly winds and an Arctic air mass bringing continued
freezing spray concerns.

&&

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

Another northern Gulf system will should bring some snow to the
the Gulf Coast, less inland. Out further in time, ensemble
guidance is consistent with building high pressure at the surface,
first over the Bering Sea, then transitioning onto the mainland.
The second part of the equation is a complex low pressure system
in the Northeast Pacific which seems very plausible given the
extent of the Arctic air over the Pacific. This pattern will favor
shifting the storm track further eastward. What this means for
sensible weather over Southern Alaska is continued colder and
drier into next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard Warning: 155.
High Wind Warning: 131.
MARINE...Gale: 121 127 128 131 132 155 165 170 172-414.
Heavy Freezing Spray: 119 121 126-131 138 139 141 160 180.
&&

SYNOPSIS AND MODEL DISCUSSION...MTL
SOUTHCENTRAL ALASKA...KM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...ED
MARINE/LONG TERM...MTL