Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 161434
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
534 AM AKST Sat Feb 16 2019

.ANALYSIS AND UPPER LEVELS...

The radar at King Salmon has been active all night and the radar
at Kenai is just now starting to ping into a band of precipitation
just south of Homer as of 4:02 AM or 1302Z. One of the satellite
instruments has detected a deep fetch of moisture from Lake
Iliamna to Kodiak and the Barren Islands, then extending it
southwestward south of 50N. The longwave pattern features a deep
and broad trough over NE Russia, the NW Pacific and the Bering
with a high amplitude ridge over the Gulf of Alaska and into the
North Slope. There is a deep surface low over NE Russia with an
occluded front spanning across the northern Bering and reaching as
far as SW Alaska. There is a secondary surface trough extending
from the aforementioned low which almost resembles a secondary
push of cold air advection for the Bering. There is a wave along
the frontal boundary south the the Aleutians and another low
spinning up east of Hokkaido. The latest ASCAT/advanced
scatterometer pass has detected gale force winds over the Bering
in association with the low over NE Russia.

&&

.MODEL DISCUSSION...

The ASCAT pass from 00:21z was used to evaluate the initialization
process with the ECMWF and the GFS, and showed both to be a skosh
off with the low position near Kamchatka. The global models look
decent at the surface depicting the synoptic features through 48
hrs. However, at 500 mb there is a myriad of embedded vorticity
lobes that are slightly displaced amongst the NAM/GFS/ECMWF and
Canadian even early on. Beyond 48 hrs there are some timing
differences at the surface in the Eastern Bering. Additionally,
the models are out of sync with the new low east of Kamchatka
early Monday. The era of robust confidence that we have enjoyed
the past several weeks is waning.

&&

.AVIATION...
PANC...There is a frontal boundary moving up Cook Inlet. The cloud
shield keeps advancing towards PANC. The TAF package will commence
as VFR, but expect the ceilings to lower today. In addition to
reduced cigs, snow is anticipated today and that will impact the
visibility, potentially lowering the visibility to MVFR or even
IFR conditions. The latest data from AMSU/advanced microwave
sounding unit (which is a satellite instrument) has the plume of
moisture associated with this front greater than 200 percent of
normal. Very confident that precipitation move over the runways at
PANC by 20z. As the TAF period unfolds, the model soundings imply
that wind shear will become an issue after 6z.

&&
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...

The clear weather of the past few days is giving way to a complex
storm system that will send multiple fronts across the region
through Monday. The biggest forecast challenges are with whether
there will be significant downsloping in the lee of the Chugach
and Kenai Mountains, and precipitation type on Sunday and Monday
in the Cook Inlet region and north Gulf coast.

At this time it looks like there should be snow moving into the
area today with the heaviest sea level snow occurring in the
western Kenai Peninsula. The heaviest area of snow will be over
Cook Inlet, but areas from Sterling west will likely see 5 to 9
inches of snow from late this morning through the evening with
the heaviest amounts farther west near Kenai.

Snow and blowing snow are still expected in Portage Valley and
eastern Turnagain Arm as well as Turnagain Pass. There is a winter
weather advisory due to this.

Snow over Anchorage and the Matanuska Valley will be largely
dependent upon the downslope. While surface winds will be east to
southeast for much of this event, the upper levels from 850 mb on
up are all south-to-southwest with abundant moisture. This has the
ability to overwhelm the downslope and could bring more snowfall
than expected to Anchorage, but amounts in the forecast are the
most likely scenario at this point.

Perhaps the biggest forecast headache is actually with the winds
around Cook Inlet, including Anchorage for Sunday. Models and MOS
data all show southeasterly winds in Anchorage, which will bring
temperatures into the mid 30s or even upper 30s. The atmosphere
is still cold enough to support snow, and at this point if
southeasterly winds do occur, they will likely downslope out the
precipitation. However, there is an indication on the 06Z GFS that
the front moving up Cook Inlet Sunday will have a triple-point
low develop near the southern tip of the Kenai Peninsula (like
what happened about a week ago) which will keep winds in a
northerly direction in Cook Inlet. This would also keep
temperatures lower. All of this will need to be watched on the
next model run to see if adjustments are needed in the forecast.

The Susitna Valley is also in store for a prolonged snowfall
today through Monday.

&&

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

The frontal system currently stretching across and bringing snow
to the lower Kuskokwim Valley and eastern Greater Bristol Bay
area will continue east to stall and then shear apart over the
Alaska Range this morning. The next frontal system will lift north
across the Alaska Peninsula midday today and then move into the
Bristol Bay area this afternoon. Snow will continue to expand
north into the lower Kuskokwim Valley this evening and over the
Kuskokwim Delta tonight.

Along the western periphery of the front where temperatures are
colder, blowing snow across the Kuskokwim Delta (and possibly the
western capes) with both northeasterly winds preceding the
surface low and southwesterly winds behind it, is a potential
concern. Further to the east, warmer temperatures will make
higher snow fall amounts and visibility reductions due to higher
snow fall rates the bigger impact although winds will be strong
and gusty as well. Precipitation will taper off from west to east
Sunday as the low and frontal system lift clear of the area.

The next front, which is tracking in from the Bering, looks to be
a rather strong one and will reach Southwest Alaska Sunday night.
The parent low will make landfall across the Kuskokwim Delta on
Monday with this farther south track (as compared to previous
model runs) pushing the strongest of the west to southwesterly
winds behind the low and accompanying larger seas into the Bristol
Bay area Monday and Monday night.

&&

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

A complex frontal system is currently located south of the
eastern Aleutians and Alaska Peninsula with a secondary low center
further to the west, south of the central Aleutians. This multi-
center low pressure system will pivot today with the eastern low
tracking northeast across the western end of the Alaska Peninsula
this afternoon and then tracking onshore across Cape Newenham
tonight. The western low will continue to the east south of the
Alaska Peninsula.

A large, rapidly deepening North Pacific low will cross the
Aleutian chain near the 180 longitude line Sunday morning and
then continue east-north east across the Bering to reach the
Pribilof Islands Sunday night and the Kuskokwim Delta Coast
Monday. South of this low, a broad area of storm force winds will
track from west to east up the Aleutian chain Sunday through
Sunday night, with an area of strong Gales (possibly still
containing storm force gusts) pushing into the southeastern
Bering and Bristol Bay late Sunday night and Monday.

&&

.MARINE (Days 3 through 5/Monday through Wednesday)...

A relatively strong ~975mb low will be tracking west to east
across the south central Bering Sunday afternoon through Monday
morning bringing widespread gales to the Bering as well as a
swath of storm force winds tracking along the Aleutians. This low
will move onshore Monday afternoon and weaken as it moves further
inland Monday night with winds and seas diminishing in response
from west to east Monday night and Tuesday. Another low will
approach the central Aleutians from the south Tuesday, although
there remains uncertainty with the exact track and timing, well as
intensity. Gales and higher seas would likely occur on the east
side of the low where winds will be southerly, which would include
areas generally east of Dutch Harbor. Stronger high pressure
building in behind the low should force it out of the region
quickly though again the timing remains uncertain.

Marginal southwesterly gales and higher seas are possible over
the western Gulf near the Barren Islands Monday morning
associated with the same aforementioned low. Gales have the
potential to redevelop over the central Gulf Tuesday as the
pressure gradient tightens between a trough over the mainland and
ridge to the south. Model agreement begins to degrade on
Wednesday. One model solution gives widespread gales over the
western Gulf, but others keep them much further west over the
Bering. Confidence will remain low until the low`s track is better
agreed upon.

&&

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

Partly to mostly cloudy skies with light scattered showers are
likely for the Aleutians Monday as a broad low departs to the
north. Strong winds will develop on the south side of the low
over the southern Bering with cold air wrapping around from the
northwest. Winds will diminish late Monday night through Tuesday
morning as the low weakens over the mainland. A low and associated
warm front will approach the central Aleutians from the south on
Tuesday bringing moderate rain and increased winds to areas
generally east of Dutch Harbor, though confidence is low on its
exact track. A strong high building to the south will quickly
push the low out of the region through the afternoon and bring
fair skies for much of the central and eastern Aleutians Thursday
into Friday. Models are indicating a stronger low developing over
the western Bering Thursday with moderate precipitation possible
into Friday along the associated front.

A relatively active pattern appears likely for the mainland much
of next week. A warm front will lift northward Sunday night and
Monday. The associated low will move into western Alaska Monday
afternoon through Monday night and is expected to bring further
accumulating snow to that region. Another low will bring in strong
southwesterly flow to southern and western Alaska on Wednesday
and, as with Monday`s system, accumulating rain/snow appears
likely though amounts will depend on the exact track which is
poorly agreed upon at this time. High pressure will begin to build
in from the south on Thursday and looks to somewhat dry
conditions out by Friday, however another low in the Bering may
bring precipitation to the western mainland.

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 121 125 145 155 161.
MARINE...Gale Warning 119 125 130 131 139 170 171 178 185 412 413 414.
         Storm Warning 172 173 174 175 176 177.
         Heavy Freezing Spray Warning 140.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...PS
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JER
MARINE/LONG TERM...AP


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