Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 171405

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
605 AM AKDT Sat Mar 17 2018


Moisture moving northward in a relatively unstable atmosphere as
evidenced by the 12Z Bethel sounding is being lifted and
generating scattered showers, drizzle, and low ceilings across the
Kuskokwim Delta this morning. That same moisture is moving across
the rest of Southwest Alaska as well, but because the atmosphere
is significantly more stable through Bristol Bay, there is much
less in the way of shower activity. A dying upper level system
being sheared apart by encroaching upper level ridges to its east
and west is the catalyst for the clouds and showers. Over the
Bering, a 160 kt jet streak is moving over the western Aleutians
ahead of a strong upper level low along Kamchatka. To the east, a
persistent ridge remains in place across Southcentral, the Gulf
and western North America. The moisture moving into Southcentral
in the form of an overcast deck of upper level cloud cover seems
to have largely precluded the development of fog around Cook
Inlet. However, the Valdez and Cordova areas, being further east
and away from that cloud cover, are having fog and visibility
issues this morning.



The models are all once again in good agreement through day 2 as
regards the movement of the large synoptic features. Thus, once
again, the local scale differences between the models will be the
main forecast challenges. Among these are timing the next round of
precipitation moving into first Southwest Alaska, then
Southcentral Sunday and Sunday night. The foreign models indicate
some precipitation moving into areas from Anchorage north as early
as Sunday morning, while the American models hold off until Sunday
afternoon for the Susitna Valley, and Sunday night for much of the
rest of Southcentral. Another forecast challenge is precipitation
type for Sunday`s precipitation event. Even tiny differences
between the models are resulting in differences in placement of
the rain/snow line. The models have also largely been delaying the
end of the precipitation from previous runs, now suggesting that
precipitation from Anchorage eastward persists into Monday
morning. Because of all of these challenges, forecast confidence
is lower than it would otherwise be given otherwise good model



PANC...Widespread stratus with potential for it to lower to fog
will remain a threat through the early morning hours. Otherwise,
VFR conditions with light winds will persist for the rest of
Saturday and much of Saturday night. Low-confidence potential
exists for some snow shower activity to develop early Sunday
morning that may result in MVFR ceilings, though confidence is
higher for rain and/or snow showers Sunday afternoon.


With weak high pressure currently moving over the area, the
biggest forecast concern today will be for the development of
fog/stratus over Upper Cook Inlet and the North Gulf Coast this
morning. The most widespread fog/stratus should occur along the
North Gulf Coast where significant clearing of the upper level
cloudcover occurred overnight. For Cook Inlet, any fog that does
develop should be very spotty in coverage after cirrus clouds
moving over the area has prevented significant development from

The focus of the forecast then shifts to a weak front that will
begin to enter the Western Gulf late tonight. The largest impact
from this front will be for rain and snow that will spread into
Southcentral at some point on Sunday. The timing of the onset of
this precipitation is biggest forecast challenge in the short term
as there remains significant spread in the models between as fast
as mid morning (NAM and GEM) or slow as the early evening (GFS
and EC). The forecast continues to reflect the slower solutions
which have had much better run-to-run consistency. When the
precipitation does begin, it will likely begin as rain or very wet
snow for most areas outside of the Susitna Valley, before all
areas cool enough for just snow early Monday morning. This will
bring the possibility of accumulating snowfall into the Monday
morning commute, especially to the Matanuska Valley and along the
Glenn Highway corridor.



The latest water vapor image has mid-level moisture spanning from
the Alaska Interior across the Kuskokwim Delta and into Bristol
Bay. The surface low associated with this boundary will continue
to fill and lift northward. The models have showers tapering off
today. Upper level ridging briefly pushes in from the Gulf of
Alaska increasing the chance for the development of fog over
portions of Southwest. A mature low over the Bering Sea will
continue to close in on the region. The precipitation shield will
move into Hooper Bay and Kipnuk this evening pushing over
Dillingham and King Salmon overnight. The frontal boundary will
rapidly push inland and undergo decay.



There is a surface low near Kamchatka with an occluded front
bisecting the Bering and impacting the chain. This system will
continue to produce gales and enhanced seas. A secondary impulse
of energy will move about the base of the upper low bringing a
wave of colder and more convective precipitation to the Aleutians.
Late Sunday with a vortex of cold air over NE Russia and a dome of
high pressure south of the Chain, progressive westerly/zonal
winds will set up.


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

The mid/long term forecast begins Monday with an upper level
trough and associated surface low situated over the NW Bering
with a front extending out from the parent low and moving east
into Southcentral. Strong SW flow aloft will push moisture up the
Cook Inlet favoring the Susitna Valley and surrounding mountain
ranges with precipitation again. Warm air ahead of this storm
system will lead to a mix of precipitation types as it crosses the
southern mainland Monday, with snow briefly mixing in later in
the day as cold air advects in behind the departing front before
the precipitation shuts off.

In the wake of this system, a digging trough moving out of Asia
will lead to rapid building of a ridge over the Bering Sea Tuesday
through Wednesday. This will usher dry Arctic air from Siberia
southward across mainland Alaska and into the Gulf, leading to
below average temperatures and strong winds through the usual
terrain gaps along the Gulf coast and across the open waters of
the Gulf. Depending on the amount of cold air advection moving
into the interior, hurricane force gusts are not out of the
question through the typical wind funneled areas, including
Thompson Pass and Valdez, Tuesday through Thursday.  Models
diverge on whether this blocking ridge holds in place through the
end of next week or breaks down allowing the storm track to
meander back east into southcentral again. Regardless, all models
show ridging and drier conditions through at least Friday of next
week over the eastern two thirds of mainland Alaska


MARINE...Gale 174, 176, 178, 185, 411, 412.
         Heavy Freezing Spray 185.



LONG TERM...KVP is the U.S. government's official web portal to all federal, state and local government web resources and services.