Small Craft Advisory 96, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
339 AM AKST Tue Jan 30 2018

.ANALYSIS AND UPPER LEVELS...
Water vapor imagery in a loop tells the story quite well this
morning. Over the Bering Sea, a closed off high dominates the
column, while a closed low does the same over the eastern Gulf of
Alaska. Between the two controlling long-wave features, deep
northerly flow through the column, which is reinforcing the arctic
air mass already in place. No major disturbances are seen upstream
in the flow, the only active disturbance for our area is a
developing trough over the western Gulf as the longwave continues
to dig.

From a sensible weather perspective, clear, windy, and cold. The
Matanuska wind continues to blow from the northeast, keeping
temperatures elevated from the antecedent frigid state. Areas with
wind are hovering in the low 20s overnight, while protected areas,
or areas where the surface inversion is setting back up have
dipped back into the single digits, or negative. Winds in both the
Matanuska Valley and Valdez are have dropped a little from the
more extreme values of the last day or so, but continue none the
less.

&&

.MODEL DISCUSSION...
Same story with guidance. Good general synoptic agreement, but
some differences with a weak developing low in the Gulf.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist. The only
uncertainty lies in whether or not gustier northerly winds
continue for west Anchorage. The general trend should be downward,
however, the edge of the strongest wind from the north should be
hovering just offshore of Anchorage International.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
With low pressure to the south and high pressure continuing to
build in over the northern part of the state, outflow winds will
continue for the next couple of days. An upper level low will
move over the Gulf today from the northeast which will form a
surface low that will track toward the western Gulf tonight. This
setup will keep precipitation away from most of Southcentral
Alaska with the exception of Kodiak Island. As the low develops
in the Gulf it will bring moisture to the Island beginning tonight
and increasing Wednesday and Wednesday night. The precipitation
will begin as snow, but could mix with rain Wednesday afternoon
and evening with temperatures just above freezing.

&&

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

More of the same can be expected through Wednesday, before the
pattern starts to change some thereafter. The next few days will
feature a persistent offshore flow pattern as the region lies
between a strong mid-level area of high pressure over the northern
Bering, and a developing ares of low pressure aloft across the
Gulf. The Bering high will shift northward towards eastern Siberia
by Wednesday, as the strengthening Gulf low develops a surface
reflection.

This surface low will then retrograde while parallelling the
chain through Wednesday evening, before getting absorbed into a
strengthening Pacific low as it heads north towards the chain.
These systems will eventually bring a chance of precipitation to
the AKpen starting Wednesday, increasing each day through Friday.
Otherwise, look for temperatures to slowly moderate over the next
few days.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
(Today through Friday)

A repeat scenario is in store for the region for the next few
days, before things become slightly more interesting. As mentioned
above, the Bering high will continue to shift away from the
region, with a pair of lows making inroads towards the chain by
the end of the work week. What this means in the near terms is
mainly dry conditions will persist area-wide, aside from a few
areas of flurries or perhaps a snow shower for the eastern parts
of the chain. Winds will generally remain below gales.

Heading into Wednesday evening through Friday, an approaching pair
of low pressure systems and a warm front will gradually increase
precipitation potential initially for eastern areas of the
Aleutians and nearby waters, spreading westward with time. As this
occurs, winds will also be on the increase during this period
ahead of the incoming lows/warm front.

&&

.LONG TERM FORECAST (Days 3 through 7)...

Beginning the long term forecast period Wednesday night, there
will be a rather stout high latitude omega block over Eastern
Siberia with an elongated trough stretching from the Eastern
Aleutians to the Canadian Maritime. A very stout arctic surface
high will be in place over eastern Alaska, with a continuation of
the current dry, cold offshore flow pattern. Precipitation will be
relegated to Kodiak Island, but it is expected to be all snow, and
there may be some decent accumulations, especially along the
eastern part of the island. With time, the omega block will shift
east over northwest Alaska which will effectively elongate and
shear the upper trough over Alaska. By late Friday into Saturday,
a North Pacific warm front will lift north into the Gulf of
Alaska, bringing much heavier precipitation to Kodiak Island (snow
changing to rain). This is where uncertainty grows rather quickly
among the global models. The GFS has been consistently progging
the front to reach the North Gulf Coast by Saturday while the
ECMWF and Canadian GDPS favor a slower progression given the
strength of the arctic high. In fact, the ECMWF never progresses
the front farther north than Kodiak, keeping all of mainland
Alaska and the coast dry. For now, an approach favoring the ECMWF
and the ECMWF ensemble is preferred. However, it is not out of the
question that the GFS verifies with warming temperatures by early
next week...and increasing chances for precipitation (favoring the
coast). Stay tuned...

&&

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

&&

SYNOPSIS AND MODEL DISCUSSION...MTL
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...JA