Small Craft Advisory 890, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
352 PM AKST Mon Nov 12 2018

.ANALYSIS AND UPPER LEVELS...
The fairly active pattern continues. On this afternoons weather
map a weakening low was located south of the eastern Aleutians
with another developing center heading rapidly north across the
southwestern Gulf toward Kodiak Island. A frontal zone extended
north from the low south of the AK Pen to across the western
interior. A broader frontal boundary extended across Southcentral,
through the northern Gulf to the developing center over the
southwest Gulf, then southeast to the northeast Pacific. Snow and
rain were evident along the front extending through the Pribilofs
to over the western mainland with areas of light freezing rain
reported over the Delta. This band was exiting to the north across
delta. Widespread rain, moderate to locally heavy at times was
observed across the Gulf Coasts with lighter amounts west of the
Chugach mountains across the Anchorage Bowl and Matanuska Valley.
Freezing levels were fairly high across Southcentral with values
in excess of 4000 ft. This produced rain or a mix even in the
higher elevations along the road system.

Fairly strong easterly winds were observed over the northern Gulf
in advance of the north moving low and frontal boundary. Gale force
northerly winds continued across Central and eastern Bering
Sea/Aleutians along and west of the frontal zone in the region.
Cold air pooling aloft over the north Pacific destabilized the
atmosphere and was by the presence of a large area of open cell
cumulus clouds and showers/thunderstorms well south of the AK Pen.

&&

.MODEL DISCUSSION...
The numerical models are generally in good agreement with the
synoptic features. Namely with the main player which is heading
toward Kodiak Island and its associated frontal bounder heading
toward the north Gulf Coast.

&&

.AVIATION...
PANC...Generally VFR conditions with periods of rain. Some IFR
conditions are possible this evening, and then again Tuesday
morning as some low stratus and fog may linger near the airport.
LLWS will be a concern through tonight as surface winds will
remain from the north with brisk southeast winds aloft.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A strong low in the southwestern Gulf of Alaska is moving
westward and just clipping the southernmost part of Kodiak Island
late this afternoon. Heavy rain and strong winds are accompanying
this low over Kodiak and along the north Gulf coast with
downsloping developing on the lee sides of the mountains.

This also means that the warm advection over Southcentral Alaska
has now reached its peak and there will be a slow cooling trend
the next few days as colder air aloft will begin to move into the
area overnight. Emphasis on 'slow.' With colder air aloft the
precipitation will become more convective in nature and there
will be the chance for some thunderstorms over the Gulf of Alaska
offshore waters overnight tonight and maybe into tomorrow morning.

With the Gulf low moving westward and then southwestward, it will
increase the offshore flow over Southcentral tomorrow into
Wednesday; therefore lowered the POPs around Southcentral for
those days. While the temperatures will be slowly dropping the
next two days, there will still be widespread high temperatures
above freezing Tuesday. Wednesday will be a little cooler and
while the airmass still is warm enough to support above freezing
temperatures, if the cloud cover diminishes a sooner on Tuesday
night, it could bring in a strong inversion that will be difficult
to break and lead to widespread stratus cloud development and
cooler daytime temperatures. Will have to wait and see if this
tips its hand over the next day or so.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Another frontal wave rotating from the western Gulf through
Southwest Alaska tonight through Tuesday morning will bring a
variety of precipitation types to the area. In the greater
Bristol Bay where temperatures are predominately above freezing,
precipitation will fall mainly as rain with a mix of freezing rain
and snow further inland and to the north of Dillingham.
Additionally, the breezy northerly winds will turn easterly and
increase again in speed as the surface low crosses the Alaska
Peninsula late tonight and tracks through Bristol early Tuesday
morning. Further to the north, in lower Kuskokwim Valley and
across the Kuskokwim Delta, with the entrenched cold low level
airmass and persisting northeasterly winds, precipitation with the
next front late tonight through midday Tuesday will fall as a mix
of freezing rain and snow. Weak upper level ridging will start
building over Southwest Alaska Tuesday night and then strengthen
Wednesday through Wednesday night for a brief midweek break in the
weather.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
North to northeasterly Gales across the Bering will gradually
diminish tonight through Tuesday as the mean center of the complex
upper level low rotating over and to the south of the Alaska
Peninsula shifts to the further to the east. The last of the
stronger frontal waves associated with the low will cross the
eastern Alaska Peninsula late tonight, rotate through Bristol Bay
Tuesday morning and then track back to the west down and south
across the Alaska Peninsula through Tuesday afternoon and evening.
A north Pacific frontal system will lift north over the western
Aleutians on Thursday.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The long-term forecast begins Wednesday night with a continuation
of an active pattern across Alaska. The main features are a broad
upper-level trough axis positioned from the Bering to well south
of the AKPEN with a pair of flattened mid-level ridges bookending
the trough. There is also a (sub) tropical connection to the
trough as it draws up plenty of warmer air and moisture
originating north of Hawaii. This is reflected at the surface by a
low moving north through the Pacific on Thursday and centering
near Kodiak Island for Friday with its warm front stretching along
the southern coast. This feature and the upper-level trough then
both become negatively tilted as the system occludes and the ridge
downstream re-amplifies across the Alaska Panhandle. By Saturday,
the low begins to weaken as it and its corresponding upper-level
shortwave eject northeast over the northern Alaska Panhandle in
response to a second, deeper trough sliding east across the
Aleutians and reinforcing the longwave trough centered over the
eastern Bering/AKPEN.

As is the usually the case in the long term, there are plenty of
subtle differences concerning the exact timing of the shortwaves
moving through the Gulf and across the Aleutians. The GFS has been
the most bullish (most progressive) of the suite of extended
range guidance, but it has also been performing the best of late
(although the EC and Canadian also agree on the long wave setup).
Therefore, the extended forecast is weighted more toward the
GFS/GEFS solution. This keeps a weak trough over the eastern
Bering through the end of the week as the northward moving Pacific
low spreads clouds and precipitation over the AKPEN by Thursday
and over southcentral by Friday. Farther west, a weakening
occluded front over the western Aleutians on Thursday will spin up
a new triple point low over the central Bering by Friday. This is
the feature that will intensify quickly as it moves into the
eastern Bering. Precipitation type may be a concern again later in
the week as the weather pattern remains similar to that (areas of
colder air at the surface with much warmer air over-running
aloft) which brought the mixed bag of precipitation to much of
southern Alaska this past weekend.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory (Mixed Precip) 152 155.
MARINE...Storm Warning 119.
 Gale Warning 120 125 130>132 136>140 160 175 180 181 185.

&&

SYNOPSIS AND MODEL DISCUSSION...RC
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JR
LONG TERM...TM