Gale Warning 562, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
503 AM AKDT Sat Nov 4 2017

.ANALYSIS AND UPPER LEVELS...
A weak shortwave trough is pushing out of the Copper River Basin,
while ridging begins to build back into southern Alaska. The ridge
is not as strong as it once was, it is a dirty ridge as several
weak disturbances are beginning to erode it from the west. The
most significant feature is a shortwave trough in the northern
Bering Sea. A front extends from south of the Aleutians toward the
southwestern Gulf of Alaska.

The cold advection and northerly flow was not able to break the
inversion and mix out the fog yesterday, however the dry air
associated with it has made the fog much more patchy this morning.
The stratus deck still remains and stretches from the Kenai
northward into the southern Susitna Valley.

&&

.MODEL DISCUSSION...
Guidance is in great agreement with the approaching shortwave
trough and the resulting isentropic lift in the Cook Inlet region.
The trend has been to cut the system off sooner in the western
Gulf and dig the trough further south. The outlier with our
snowfall/precipitation amounts would be the NAM which has too
much downslope drying for the synoptic scenario. Otherwise,
snowfall timing and amounts are very well agreed upon.

&&

.AVIATION...
PANC...Fog and stratus continue to drive forecast uncertainty.
While fog appears to be lifting across the area, the atmosphere
remains conducive for patchy fog to remain through the morning
hours. The higher confidence portion is the stratus deck. There is
no mechanism to mix out the low clouds until the snow starts, and
will likely stick around through the snow event.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Areas of fog and stratus will persist this morning from the
western Kenai Peninsula to Anchorage and the Mat-Su with the
upper ridge overhead and a steep surface-based inversion.
However, with a fairly thick cirrus shield moving overhead fog
will be more localized than it was during the overnight hours.
Increasing flow ahead of an approaching trough this afternoon,
along with an overall increase in mid level clouds should bring
an end to the fog and stratus. Meanwhile, gusty winds along the
north Gulf coast will rapidly die off during the morning hours
as high pressure builds in.

The forecast for a quick shot of snow or rain across Southcentral
tonight through Sunday is generally on track. One notable trend in
the model guidance is to dig the trough further south and develop
a cut-off low sooner. There is good vertical motion out ahead of
the approaching trough this evening into the overnight hours, but
the best lift will then quickly shift down into the Gulf late
tonight through Sunday. Still expect a widespread 1 to 4 inches
from the Kenai Peninsula to Anchorage and the Mat-Su. Areas
further east, Valdez, Cordova, and the Copper River Basin will
never really see the strong upward motion, so precipitation will
generally be lighter. There will likely be a band of heavier snow
along a deformation zone which develops north of the developing
upper low, but it's hard to say exactly where that will be. It
could bring marginally higher snow totals to the western Kenai,
southern Susitna Valley or further east across portions of
Anchorage and the Mat Valley. Lastly, the dendritic growth zone
looks like it will be fairly shallow and fairly high up in the
atmosphere. Thus, snow ratios look like they will hover in the
10:1 to 12:1 range with generally small snowflakes. This supports
the idea of a light accumulation of snow. Lastly, temperatures
along the Gulf coast might still be above freezing when
precipitation starts, but wet bulb temperatures will be below
freezing, so expect most places to change over to snow.

The digging trough and strong cold air advection behind it will
lead to intensification of a surface low over the southern Gulf
on Sunday. With another high amplitude ridge upstream and a strong
upper level jet, a strong offshore wind event is taking shape
Sunday through Monday. Kodiak and the western Gulf will see the
initial brunt of winds Sunday. Winds will then rapidly kick up
along the north Gulf coast and Prince William Sound Sunday night
into Monday, with snow quickly ending as winds arrive.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA...
Ridging that has been over southwest Alaska is pushing off to the
east as a shortwave over the northern Bering is moving in. As the
shortwave pushes through today it will bring a mix of rain and
snow to the area. Areas along the coast should see mostly rain
while interior locations will see primarily light snow. On Sunday
high pressure will move in over southwest Alaska from the Bering
helping to clear out the precip until a front makes it over the
Kuskokwim Delta Monday night.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS...
High pressure over the western Bering will track east through the
weekend making it to the eastern Bering on Sunday and then slowly
track further to the east over mainland Alaska. This high pressure
is preventing a weak system in the North Pacific from pushing into
the Bering keeping the rain from this system confined to the
Eastern Aleutians and Alaska Peninsula before it moves off
Saturday night. Behind the high a front will move over the Western
Aleutians tonight and the Central Aleutians Monday. It will then
continue its easterly track through Monday night.

&&

.LONG TERM FORECAST (Days 3 through 7: Monday through Friday)...

As the low in the southern Gulf shifts south to the North Pacific
Monday, deep northerly flow will establish itself over the
Mainland. Expect gusty outflow winds along favorable gaps over the
northern Gulf Coast to peak Sunday night through Monday evening,
then generally subside through Tuesday. Amid the strong offshore
flow, skies will quickly clear out Monday morning from west to
east over Southcentral and a general cooling trend will set up
through the middle of the week. By Tuesday night, the ridge will
erode as a series of shortwaves approach from the west. These
shortwaves will bring some more clouds to Southcentral and a
return of the chance for some isolated showers through the end of
the week.

Over Southwest Alaska and the Bering Sea, the dominant ridge will
break down early next week. A front will push through the Bering
Sea from west to east Sunday night through Tuesday morning,
spreading rain and gale force winds eastward before weakening as
it moves onshore Tuesday. Ahead of the front, southerly flow will
bring warm air and a marine stratus layer northward. Once the
front has passed, colder air will begin to filter out of Russia
and into the Bering Sea. This will destablalize the atmosphere and
re- introduce convective showers to the Bering Sea as a series of
upper-level disturbances push east Tuesday through Thursday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 150 177 178.

&&

SYNOPSIS AND MODEL DISCUSSION...MTL
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DK
LONG TERM...RD