Small Craft Advisory 299, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
522 PM AKDT Tue Apr 10 2018

.ANALYSIS AND UPPER LEVELS...
The gale force low that moved through the Gulf is now located just
to the south of Kodiak Island and is slowly beginning to fill.
This low and its associated front brought gale force winds to
much of the Gulf this morning, as well as widespread rainfall
totals surpassing 1 inch. The front from this low has stalled out
over the North Gulf Coast, bringing widespread rain, which has
been heaviest in the Portage Valley where over 2 inches of rain
has already been measured in the past 24 hours. Well east of the
weakening Gulf low, another developing low is present just south
of the Panhandle, which is expected to quickly develop to gale
force as it slings westward into the Central Gulf late tonight. To
the north, the persist ridge of high pressure over the interior
continues to protect most of interior Southcentral from the
widespread rainfall. This ridge will slide back southward into the
area later this week once low development in the Gulf begins to
end.

Over the Eastern Bering, the remnants of the front that moved
through the area yesterday has merged with the Gulf low and
created a deformation band stretching from the AKpen into the
Eastern Bering. This is generally bringing rain to much of that
area, though wet snow is mixing in to the west of Cold Bay where
cold air advection is also creating gusty northwesterly winds.
Just to the west over the Western-Central Bering the weather is
generally quieter but still gusty from a weakening gale force low
that is beginning to exit the Northern Bering. This is being
quickly replaced by yet another gale force low from the North
Pacific that will skirt the Western Aleutians late tonight.

&&

.MODEL DISCUSSION...
Model agreement has improved since last night with both the track
and intensity of the developing low currently near the Panhandle
that will track into the Gulf this evening. As a result forecast
confidence is beginning to improve for both the wind and precip
forecast from this system, especially along the Gulf Coast where a
majority of the impacts are expected. Afterwards good agreement
and forecast confidence continues into Thursday as the interior
ridge briefly moves back into the South Mainland, but this quickly
begins to break down by the end of the week. This breakdown
occurs as the models continue to struggle with how deep to dig (or
even in the existence of) a shortwave from an arctic trough that
will develop late this week into the upcoming weekend. In general
the models have trended slightly less aggressive with this trough,
so the forecast will continue to favor the EC and GEM which have
been the most consistent in the less aggressive solution.

&&

.AVIATION...
PANC...VFR conditions with light and variable winds will continue
into Wednesday morning. Beginning Wednesday morning there is a
chance that the incoming Gulf low will drop ceilings to around
5000 ft, however since this is a low probability for now on the
very edge of the low this layer was kept as a SCT deck for now.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Strong winds and heavy rain will taper off through the night over
the Western Gulf of Alaska and Kodiak Island as a fairly potent
surface low continues to track west then south of Kodiak Island,
before finally being ejected back east into the North Pacific.

For the rest of the forecast period, a broad upper level trough
over the Gulf and North Pacific will continue to dominate the
weather through the forecast period bringing widespread cloud
cover to most of the southern mainland and rain to the coastal
areas. An embedded shortwave trough will move north along the AK
Panhandle then north and west along the Northern Gulf Coast
tomorrow morning, quickly strengthening into a surface low. This
will bring another shot of gale force winds to the Northern Gulf
coast west towards Kodiak. Rain will again spread across most of
the coastal region tomorrow with heaviest amounts along the
Eastern Kenai Peninsula and Kodiak Island, with downsloping
keeping the Western Kenai, Anchorage, and Mat-Su Valleys dry and
warm. On Thursday, the flow over the Gulf becomes more zonal (East
to West) with overall lighter winds, though still onshore along
the coast leading to continued chances of rain through the end of
the week while more inland areas remain dry.

&&

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

Broad southeast flow aloft will continue to be the dominant
weather regime for the next few days as a series of shortwaves
moves across the Alaska Range and into Southwest. Downsloping and
a weak moisture stream will generally keep precipitation light
and sporadic, and will generally favor the Kuskokwim Delta and
Bristol Bay Coast. Some patchy fog will be possible tonight along
the Kuskokwim Delta as the pressure gradient continues to weaken.

&&

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

A deformation zone will remain in place over the Eastern
Aleutians, bringing continued rain (possibly mixed with periodic
wet snow. A low over the North Pacific will skirt the Western and
Central Aleutians, bringing a switch to easterly then northerly
low end gale force winds, but this low will quickly move out by
Wed afternoon with broad cyclonic flow then developing over the
Bering Sea through Thursday before a gale force low forms over the
Western Bering Sea Thursday night and Friday.

&&

.LONG TERM FORECAST (Days 3 through 7)...
After a brief break in the active weather for the Southern
Mainland as interior ridging moves back over the area on
Thursday, the focus in the long term forecast and questions
continue to surround and arctic trough centered over the Beaufort
Sea and how far south it will dig. Yesterday the GFS had been
very consistent in both the operational and ensemble mean in
digging a much deeper shortwave trough through the interior and
into Southcentral. Since then it has began to back off on this
much more aggressive solution, much more in line with the EC and
GEM which have been very consistent in keeping the bulk of the
energy and (more importantly) cold air well north of the area in
the Eastern Interior and the Yukon. As a result confidence is
beginning to improve that temperatures will stay generally near
normal with weak ridging in the area and little in the way of
significant precipitation chances for much of the South Mainland.

By late this weekend, model agreement is good that a broad trough
will setup over the Bering once again with ridging over the
mainland. There is reasonable agreement initially that this
ridging will keep much of the Mainland protected from activity
from the Bering through Monday, but possibility not much longer
into the week as the Bering trough continues to deepen. This will
be something that will need to continue to be monitored as the
models are typically too fast to break down Mainland riding, If
this trend continues active weather would begin to shift from the
Bering into the South Mainland for the remainder of next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales 119 120 131 150 172 351.

&&

SYNOPSIS AND MODEL DISCUSSION...DEK
SOUTHCENTRAL ALASKA...KVP
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA
LONG TERM...JR