Small Craft Advisory 264, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
556 AM AKDT Sat Mar 21 2020

.ANALYSIS AND UPPER LEVELS...

The upper level weather pattern consists of ridging over the
western Bering, a trough over the eastern Bering from the
Pribilofs north and east, so the Pribilofs are at the base of the
trough. A northwest to southeast oriented 115 kt jet streak
extends north and west of the Pribilofs. A ridge is over
Southcentral. The ridge axis is along the Alaska/Yukon border. A
second southwest to northeast oriented 130 kt jet streak lies
from Unimak Island to Denali.

Snow is ongoing across nearly all of Southwest Alaska this
morning, with rain falling from King Salmon south. With full cloud
cover in place across all of Southern Alaska, temperatures are
largely homogeneous as well, nearly everywhere away from the coast
in the 25 to 32 degree range. From King Salmon south, temperatures
are between 35 and 40, with any precipitation remaining all rain.

&&

.MODEL DISCUSSION...

The models are in very good agreement on the synoptic scale
through this weekend's forecast, including timing and
precipitation amounts. The main forecast challenge will be
temperatures and precipitation type. With temperatures hovering
right around freezing, even a degree or two will be the difference
between rain and snow and accumulating snow. Thus, while forecast
confidence is high regarding that there will be widespread
precipitation today, there is low confidence on the precipitation
type, snowfall accumulations, and temperatures.

&&

.AVIATION...
PANC...VFR conditions will become MVFR early this morning as snow
begins, then IFR for the late morning and early afternoon hours.
By late afternoon the snow will diminish from west to east,
resulting in VFR conditions returning. VFR conditions will persist
through Sunday and into the early workweek. Light winds are
expected during the snow, which will become northerly with
occasional gusts will develop tonight.

&&

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

A front is moving through Southcentral Alaska today bringing a
mix of precipitation types to the region. Figuring out where the
rain-snow line will be is the biggest challenge to the forecast
package. Snow should be the rule from Kenai northward but high
temperatures in the mid to upper 30s will help keep accumulations
lower due to a much lower snow ratios and compaction of the heavy
wet snow. In the city of Homer it should get warm enough to mix
with rain or turn to all rain. However the Homer Bluff and East
End Road are more likely to see all snow. Considered a Winter
Weather Advisory for snow for those areas, but decided to keep
amounts less than the 6 inches necessary for the advisory due to
the amount of compaction of the snow.

The Copper River Basin is also interesting as southeast winds
should downslope out the precipitation for most of the morning
near Glennallen. However, these winds should diminish in the
afternoon which will allow temperatures to drop again (they could
approach 40 degrees in the southeast winds). That will bring a
period of snow to the area, but not nearly as much as there would
have been without the winds.

This front will move through rather quickly, allowing northerly
outflow winds to develop again tomorrow allowing skies to begin to
clear again over the region.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
(Today through Sunday evening)

The main focus in the short term is the snow currently moving
through the region. It has already ended near the Kuskokwim coast.
Further south, the snow extends further back into Bristol Bay, as
this region has better jet support aloft leading to increased
ascent and higher precipitation intensities. While both areas
will gradually see the snow end from west to east as we head
through the day, earlier thinking of 1 to 2 inches (maybe 3) for
the Kuskokwim Valley (lower and upper) and a more general 2 to 4
inches for Bristol Bay eastwards towards Iliamna seems on track. A
brief burst of cold advection will follow the precipitation
shield, with clearing skies in its wake. This will lead to
substantially colder temperatures observed by this time Sunday
morning. Given the clearing skies and fresh snowfall, some patchy
fog may also develop. Temperatures will begin their rebound on
Sunday as warm advection aloft begins to move back into the
region.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
(Today through Sunday evening)

High pressure currently over the western Bering and Aleutians will
rapidly build eastward through the day, reaching the Alaska
Peninsula by this evening. This will allow for only scattered
cloud cover in most locations, with perhaps some patchy fog
developing, especially across the western half of the region as
warm advection aloft commences. Otherwise, look for dry conditions
to prevail until Sunday afternoon/evening, when an approaching
front brings a return to wet weather for the western and west
central Aleutians.

&&

.MARINE (Days 3 through 5)...

No gales are expected in the Gulf in days 3 through 5. Gales are
possible on Thursday across the Western Aleutians with the
approach of a low. Elsewhere across the Bering, no gales or high
seas are expected.

&&

.LONG TERM FORECAST (Days 3 through 7)...
There remains strong confidence that a strong upper level ridge
will be in place over the southern half of the state early next
week. An upper level low is expected to move north towards the
central Aleutians from the North Pacific weakening as it does so.
This upper level low will eventually force the ridge to weaken
and move southward by midweek, opening up a zonal flow pattern
aloft. By the end of the week, models continue to suggest a
strong low will approach the Bering from the west with a track
just west of the Aleutian Chain. There still remains uncertainty
with the exact placement of the low but models continue to show
better agreement on this feature leading to moderate confidence in
the forecast.

Further east across Southcentral and Southwest Alaska, models are
in disagreement on what the upper level pattern may look like
across the mainland. Some guidance shows an upper level trough
diving southward while other guidance shows an upper level ridge
leading to a tough temperature forecast. Also of note, each model
differs on the strength of this upper level trough leading to low
confidence in the long term.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning: 127, 130-136, 150, 351, 352.

&&

SYNOPSIS AND MODEL DISCUSSION...JPW
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
MARINE/LONG TERM...JPW/MV