Small Craft Advisory 216, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
432 PM AKDT Tue Mar 13 2018

.ANALYSIS AND UPPER LEVELS...
A weakening weather front which moved west across the Cook Inlet
to MAT-SU corridor this morning has hung up along the Alaska
Range and is dissipating. A broad area of overrunning associated
with a baroclinic zone is producing snow across the Copper River
Basin. This boundary is also producing rain and snow along the
North Gulf Coast. A broad area of low pressure over the eastern
Bering Sea/Southwest Mainland is producing widespread cloud cover
along with light snow, which is mixed with rain over the Alaska
Peninsula. A weak ridge over the central Bering Sea/Aleutians and
weak troughing over the western Bering Sea brought relatively
benign conditions to that region.

&&

.MODEL DISCUSSION...
The models are in general agreement in the broad upper level
pattern but struggle with the positioning of surface features.
This is especially noticeable with the next low heading toward
the Alaska Peninsula Wednesday night/Thursday morning. While
models differ on positioning of surface low center, the actual
trough axis position and sensible weather is pretty similar
between the GFS/ECMWF. Will be leaning that way for this package
out west and a NAM/GFS solution in the east.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist through much
of the night. There is potential for MVFR conditions with snow
Wednesday morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A weak disturbance moving northward through the Gulf will serve
to reinforce precipitation over eastern portions of Prince William
Sound beginning this evening, with snow then spreading into the
Copper River Basin overnight into Wednesday morning. Temperatures
should be warm enough to support mostly rain at Cordova, with a
rain/snow mix more likely for Valdez and all snow at higher
elevations. As has been the case with other recent systems,
guidance has been trending westward with the spread of
precipitation into Anchorage and the Mat Valley. Therefore cannot
rule out another quick dusting of snow overnight for these areas. An
upper level trough will then lift through the region from the
southwest on Wednesday. This feature looks to be mostly dry for
now, thus limiting potential for any accumulating snow for most
areas. Still, there should be enough forcing to produce at least
a few snow flakes from Anchorage into the Mat-Su as the trough
moves through. Passage of the trough will then usher in a brief
dry period for most of Southcentral on Wednesday night before a
much more impressive system begins to approach from the southwest
on Thursday. This will bring another shot of mixed precipitation
to the Gulf coast and will also likely favor the western Susitna
Valley for several inches of snow. Meanwhile, the development of
cross barrier flow in the lower levels of the atmosphere looks
like it may keep the more downslope prone areas drier for a bit
longer at the onset of precipitation on Thursday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Another weak system moves into Southwest this evening, as a
reinforcing shortwave trough that will help spread some light
snowfall over the Bristol Bay and Lower Kuskokwim Valley areas
tonight. However, this system looks weak and quite moisture
starved, so snowfall amounts look to be around an inch or less.
Wednesday night into Thursday, a more substantial low and
associated warm front will bring in widespread rain and snow to
Bristol Bay before spreading northward Thursday night.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A warm front and associated weak surface low is currently
tracking inland this evening along the Bristol Bay coast,
producing some colder northerly flow and light showers over the
eastern Aleutians/Bering and AKPen tonight. A stronger warm front
and associated low will move into the Central and Eastern
Aleutians/Bering Sea Wednesday through Thursday, bringing gale
force winds and widespread rain as it tracks into Southwest
Alaska. A gale force occluded front will move into the western
Aleutians/Bering Wednesday night into Thursday, bringing another
round of precipitation and gusty winds to the area.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The long range forecast begins Friday with a broad upper level
trough situated over the Bering and a surface low over Bristol Bay
moving East/Northeast into Southwest Alaska. A large swath of
precipitation will spread across the southern half of the state
into Friday bringing warmer temperatures and snow to inland
locations with rain expected along the coast. For Southcentral,
southeast winds look to develop Friday leading to downsloping off
the Kenai and Chugach Mountains limiting the amount of
precipitation for typically downsloped areas. A shortwave ridge
builds in between systems over Southwest then Southcentral Friday
night into Saturday bringing a short break of decent weather. A
front, with parent low well west over NE Russia, will sweep
across the Bering and move into SW Alaska Saturday night before
elongating and falling apart as it goes over the Alaska Range.

Looking out into next week, long range models are showing less
uncertainty than the last couple days and are hinting at the
continuation of the upper level trough over the Bering through the
first half of the week, and hence a persistent unsettled pattern.
There are signs of a pattern change as we head into the latter
half of next week, with both the GFS and EC showing an upper level
ridge building in over the Bering/Western Alaska. This would lead
to the development of dry off shore flow over most of the state,
though depending on how far east the ridge axis sets up, could
bring down a good shot of cold air from the Arctic. Stay tuned for
details.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 170 173 174 178

&&

SYNOPSIS AND MODEL DISCUSSION...RMC
SOUTHCENTRAL ALASKA...CB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TP
LONG TERM...KVP