Winter Weather Advisory 21, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
455 AM AKDT Tue Mar 19 2019

.ANALYSIS AND UPPER LEVELS...
There is a train of lows from Kamchatka to the Oregon Coast that
are moving across the north Pacific. The latest ASCAT/Advanced
Scatterometer pass has detected gale force winds south of the
chain and near Kamchatka associated with the aforementioned
systems. There is also a weak low in the West Bering with a
frontal boundary attached to it. Looking at the upper levels,
there is jet core pushing over Prince William Sound and another
jet core south of the chain.

&&

.MODEL DISCUSSION...
Used the ASCAT/advanced scatterometer data for initialization
purposes and the GFS and ECMWF did very well pinging into the
location of the surface low south of the chain between Adak and
Unalaska and they also did well with the placement of the low near
Southern Kamchatka. The global models remain in good agreement
with the synoptic features through Thursday. Even more spectacular
is the tight grouping or clustering of the GFS ensemble members
in both the Bering and the North Pacific. On Friday, the solutions
begin to diverge introducing some minor differences in timing and
the exact low track. Very confident about the forecast for most
of the area of responsibility for the the ensuing 48 hour period.

&&

.AVIATION...
PANC...Some low cigs were observed this morning along with both rain
and snow being observed after midnight. As of 3 AM ADT, the Kenai
radar actually has another band of precipitation near Fire Island
which may also clip PANC. By 18z VFR conditions are likely which is
also when the winds should lay down.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: today
through Wednesday night)...

A surface low over western Cook Inlet and associated upper trough
will move rapidly north across the Cook Inlet to MAT-SU Valleys
corridor this morning. This system will bring light snow with it
which should end across the Anchorage Bowl this morning and linger
on a little longer over the MAT-SU Valleys. A weak frontal band over
the eastern Gulf will head west today as a developing surface low
along its southern boundary heads northwest toward Kodiak. Latest
model runs suggest a slower westward progression of the frontal
boundary across the Gulf. That said, the brunt of the precipitation
will remain mostly over the Gulf through Wednesday night and will
not likely arrive over the western Gulf/Kenai Peninsula until this
evening. Warm air and rising freezing levels will accompany the
front so most locations below a 2000 ft will generally see rain.
There could be a little spill over precipitation over the lee of the
Chugach/Kenai Mountains tonight before the easterly cross-barrier
flow sets up, but it would be fairly short lived and should not
amount to much. Strong winds will once again develop from Portage
Valley to along Turnagain Arm and the Matanuska Valley tonight and
will slowly diminish Wednesday night. Gusty south winds will develop
along the Copper River and out of the Knik River Valley Wednesday
afternoon/evening.

&&

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

Scattered showers lingering from Bristol Bay north along the
western edge of the Alaska Range will diminish today as a weak
upper-level trough lifts north over central Alaska. Behind it, an
upper-level ridge will build in across the southwest mainland,
providing a brief respite from the wintry conditions through early
Wednesday. By Wednesday, a frontal band lifting northwest from a
low tracking toward the AKPEN will move over Bristol Bay bringing
another round of snow. This system will continue to push north,
with snow reaching the Kuskokwim Delta by Wednesday afternoon as
the low and associated front continues to track to the northwest.
As this system enters the eastern Bering, the pressure gradient
between it and a 1026 mb high anchored over eastern Russia will
increase over Southwest Alaska. The resulting gusty northeast winds
from coastal Bristol Bay north to the Y-K Delta will produce areas
of blowing snow, reducing visibility at times. The low will then
weaken as the occluded front pivots over the eastern Bering.

A second low on a similar track to the first will then absorb the
remnants of the first as it lifts over Bristol Bay and swings a
front across Southwest Alaska Thursday. Warmer air with this second
system will mean a changeover from snow to rain as the low moves
into the Bering. Precipitation type and amount of snow is still a
challenge as there is some discrepancy between the models with the
extent of warm air advection. At this time, however, it does appear
that there will be enough warmer air working in aloft to facilitate
a quick changeover to rain from the AKPEN north to Dillingham and
east to Sleetmute and Stony River. For the Kuskokwim Delta, enough
cold air may hold on to keep most, if not all, the precipitation as
snow, where a few inches could fall from Bethel on east through
early Friday.

&&

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

The western Bering continues to be under the influence of an
upper-level trough with a series of weak surface lows positioned
from the Kamchatka Peninsula north to the Chukchi Sea. Farther
east, weak upper-level ridging over the eastern Bering and a
southwest-northeast oriented surface ridge over the central Bering
are keeping things fairly quiet over the open water. Low pressure
south of the eastern Aleutians will stall and remain in this
general location through Wednesday as it becomes vertically
stacked and slowly fills. This system will then get absorbed by a
north Pacific low as it lifts towards the Southwestern Gulf. Snow
and rain will develop over the eastern Bering as this second
system lifts toward the AKPEN for Thursday. As this system lifts
north, northerly gales will develop over the central Bering.
Farther west, a surface low tracking off the Kamchatka Peninsula
will turn northeast, lifting a front over the western Aleutians
Wednesday.

&&

.MARINE (Days 3 through 5, Thursday through Saturday)...

...Bering Sea/Aleutians...

Confidence is still rather high that north/northeasterly gales
will develop over the eastern half of the Bering Thursday through
Friday. Gales will become more widespread on Friday and extend
south across the AKPEN and eastern Aleutians before diminishing on
Saturday. Wave heights will peak Friday afternoon over the
southeastern portion of the Bering (north side of the
Aleutians/AKPEN) approaching 20 ft. Further west, confidence
increases that a strong low will push east across the western
Bering by Friday afternoon, with winds ranging between upper end
gales to low end storms with wave heights increasing to 20-25 ft
through Saturday.

...Gulf of Alaska...

Decent model agreement surrounding a 960 mb low moving into the
Gulf south of Kodiak on Thursday leads to increasing confidence
that gale force winds will spread across most of the Gulf through
Friday afternoon. Waves will also increase to 25 ft over the open
Gulf by Friday as well. Winds should diminish below gale force by
Saturday though a secondary low may move into the eastern Gulf
over the weekend but likely to remain mostly sub-gale force.

&&

.LONG TERM FORECAST (Days 3 through 7, Thursday through
Monday)...

The warm and wet pattern looks to continue into the weekend as
upper ridging over western Canada/US will help to direct the storm
path north into the southern mainland. On Thursday morning, the
Gulf coast will see a short break before a broad 960 mb low tracks
north, with the center moving to a few hundred miles south of
Kodiak. A front moving well ahead of the low will slam into the
coast by Thursday afternoon bringing another round of heavy precip
and mountain snow to the Prince William Sound/eastern Kenai
coastline, with snow levels rising to above 2000 ft. Strong cross
barrier flow will promote downsloping on the west side of the
mountains keeping inland areas dry, while also ramping up the gap
winds through Friday, including through Turnagain Arm. The low
looks to stall out then shift south while weakening as it runs
into the upper ridge nosing into eastern interior AK.

By the weekend, a strong low pressure system tracks east into the
western Bering. This system looks to be strong enough to push the
persistent upper ridge to the east allowing the overall longwave
pattern to become more zonal (west to east) instead of
meridional (south to north). The pattern still looks to remain
rather unsettled as a parade of lows push east across the Bering
into the mainland, though specific timing and strength of these
systems is still to be determined

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Heavy Freezing Spray 181 185. Gale 181 185 172 179 180
119 120 125 130 131 138 139 352 351.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...PJS
SOUTHCENTRAL ALASKA...BC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TM
MARINE/LONG TERM...KVP