Small Craft Advisory 851, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
456 PM AKST Tue Nov 7 2017

.ANALYSIS AND UPPER LEVELS...
The main features are a flattening ridge extending northeast
across Southcentral/eastern Mainland and a broad long wave trough
extending across much of the Bering Sea and Southwest Alaska.
Surface pressure gradients and supporting upper flow across
Southcentral weakened today and in response there was a
significant decrease in gap flows. Though, locally gusty winds
continued across the northern Gulf and eastern Kenai Peninsula
this afternoon. Otherwise, fair skies prevailed across
Southcentral.

A frontal boundary on the leading edge of the upper trough
over Southcentral brought rain or a mix to the Kuskokwim Delta.
Overall, the front is fairly weak and will likely continue to
weaken as it heads east. Prefrontal sounding over Southwest Alaska
showed a significant amount of dry air and will help in the demise
of the front. A series of weak troughs with associated showers
were evident across the Bering Sea.

&&

.MODEL DISCUSSION...
Overall, models are in good agreement through the much of the
short term. That said, the most notable of differences are the
handling of a low heading toward the Kuskokwim Delta Wednesday
evening. The 12z GFS and 18z Nam are carrying a more consolidated
low center rather than the open wave represented on the
ECMWF/Canadian models. We are leaning toward the GFS/NAM with
this feature with the main impact being the potential for gales
over Kuskokwim Bay.

&&

.AVIATION...
PANC...Generally VFR conditions and light winds will continue
through tonight. There is a slight chance of some patchy fog
developing during the morning hours Wednesday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A progressive pattern is on deck. The forecast period commences
with a ridge directly over southcentral this evening. With energy
moving onshore from the Bering, the ridge axis will quickly push
eastward overnight. Temperature inversions will set up tonight.
The Copper River Basin may experience fog development.  Some
portions of southcentral will be under clear skies and while a few
locations will see cirrus. Either way, expect radiational cooling
tonight and the temperatures will plummet after sunset. With the
remnants of the trough moving into the region Wednesday, the upper
level flow will become more westerly which tends to be a dry
pattern for Southcentral.

Elsewhere, cold dense air will pool up over southwest AK and then
get forced through the Kamishak gap resulting in gusty winds over
the Barren Islands Wednesday and Thursday. On Thursday, a new
front will approach southcentral with the GFS model bringing the
surface low over Cook Inlet and across the Kenai Peninsula. Bumped
up pops this forecast package. Sensible weather highlights include
gusty winds and another round of precipitation for Kodiak and
Kenai.

&&

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

A weakening front is currently crossing southwest Alaska, with
the latest analysis placing the front just east of a Bethel to
Port Heiden line. Regional radar loops and surface observations
show precipitation amounts have been near the 1/10 of an inch
range from Goodnews Bay north. Lighter amounts have been reported
further south due to drier air in the low levels. This has been
noticed in the observational data with higher cloud heights being
observed. This front will continue to trek east through the
night, spreading widespread light snowfall.

Our attention then turns to another fast-moving system that will
reach the western coast by Wednesday evening. A few changes were
noted in today's runs of the models. Virtually all of the short
range guidance has trended towards a more amplified trough aloft
crossing the Bering sea and southwest Alaska, with some
indications of a closed low developing south of Hooper Bay as it
moves ashore. The models also show a stronger frontal signature
with higher precipitation amounts along the front. This is a
consistent trend we've seen in the last few models runs as well.
As such, precipitation chances and amounts were bumped up in the
forecast, as were some of the wind speeds.

&&

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

A weakening front currently crossing eastern portions of the
Alaska Peninsula will move out of the region this evening, with
the next item of interest being a strengthening trough and
associated cold front for Wednesday into Thursday morning. As
mentioned above, we've seen an uptick in the strength of the
upper-level trough and it's associated surface cold front, with
some indications of a closed low developing along the trough axis
shortly before moving ashore across southwestern Alaska. With
increased confidence in this scenario developing, winds were
raised to gale force for marine zones 180 and 181, stretching
from the Kuskokwim Delta north towards Nunivak Island.

West of this region, gale force winds may need to be added to
later forecasts if this strengthening trend continues in later
numerical runs. Otherwise, look for widespread precipitation to
accompany this front as it crossing the Bering Sea and adjacent
portions of the Aleutian Chain.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The numerical models are in very good agreement for the long term
portion of the forecast, therefore the confidence today is higher
than normal. The basic pattern is anchored by a long wave ridge
that is negatively tilted from about 170 degrees west longitude
near the Aleutians to west of 180 in the northern Bering Sea. A
downstream long wave trough will reside over eastern Alaska into
the Yukon territory. Several short wave troughs top the ridge and
drop northwest to southeast across the state during the forecast
period. The strongest of these drops through on Sunday. The WPC
guidance uses a nearly equal blend of the GFS and ECMWF. The
forecast incorporates a considerable portion of the deterministic
solutions along with the ensemble means, especially in the early
periods.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...None.

&&

SYNOPSIS AND MODEL DISCUSSION...RMC
SOUTHCENTRAL ALASKA...PS/AD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...BL