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Area Forecast Discussion (AFD)

Issued by Anchorage, AK (AFC)

                            
000
FXAK68 PAFC 130433 CCA
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
733 PM AKST Wed Dec 12 2018

.ANALYSIS AND UPPER LEVELS...

An elongated trough encompassing the Bering, mainland Alaska and
the Gulf is the main synoptic feature tonight. The trough is
associated with a vertically stacked low spinning near Norton 
Sound. Cold, arctic air is wrapping around this low and pushing
over the Bering and western mainland. Overtime, the trough has 
become more horizontal in nature with several short waves embedded
within the flow making it the "parent trough". A 180 kt jet 
stream is northwesterly oriented just south of the western 
Aleutians. Moving eastward, a meso low combined with a broad upper
level trough is producing snow across the Cook Inlet region and
MAT-SU valleys. A short wave is pushing over much of the Gulf. A 
vertically stacked low sits at the base of the shortwave in the 
southern Gulf which is easily detected in satellite imagery. Gale 
and storm force winds have been observed in the latest 
scatterometer pass near the center of the low. With the aid of
westerly flow around the parent trough, more cold air is being 
brought down over Southcentral.

&&

.MODEL DISCUSSION...
The primary models are in good agreement in the handling of the 
synoptic fields. Will be generally making adjustments to the wind 
fields using the NAMNEST. 
&&

.AVIATION...

PANC...Light snow will continue across the Airport complex through 
tonight with some clearing on Thursday. Expect periods of MVFR and
IFR conditions until then. Generally light winds. 

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2...Tonight 
through Friday Afternoon)...

Snowfall has diminished from Anchorage through the Matanuska and 
Susitna Valleys this afternoon, but will pick back up later this 
evening through Thursday morning. This is due to an upper level 
trough sliding north through the area tonight. Upper level 
vorticity will increase as this trough moves through, and a 
surface low will form over Prince William Sound that will keep 
wintery conditions around the area. POPs in these areas were 
increased higher than guidance due to the high confidence in 
snowfall occurring in these favorable conditions. Thursday 
afternoon should be a quiet break in precipitation before another 
system moves into the region from the south Thursday night and 
Friday. 

Cold air will continue to move into the area from the low in the 
northern Bering, with temperatures dropping to near normal for 
the first time in the last three weeks. For the Anchorage area, 
this is in the single digits to teens overnight, with high temps 
in the low to mid 20s. The cold temperature trend will continue on
Friday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3/Wednesday
night through Saturday)...
Cold air continues to be wrapped around the low hugging the
coastline and funneled into southwest AK. A change in the cloud 
cover forecast has precipitated a change in the temperature 
forecast for the next couple of days. Initially, it looked as 
though southwest AK would see onshore flow producing cloud cover. 
This is now not expected to be the case, as presented in the 
previous forecast. Went with continuity here as there isn't any 
newer data that would suggest otherwise. So, cold temps for 
southwest AK the next few days.

As the low pushes south past southwest AK Friday, northeasterly 
flow is then induced, which will pull in cold air from the Interior
of AK Friday into Saturday. The direction of this flow means very
cold air will make it all the way to the sea, with below zero 
overnight temperatures Friday into Saturday even along coastal 
areas. Wind chills will be something to keep in mind, but right 
now wind will not be strong enough to induce extremely cold wind 
chills. At this time wind chill advisories won't be needed.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through
3/Wednesday night through Saturday)...
Northerly flow continues to be the major weather feature for the 
Bering Sea. Therefore, not much has changed in the forecast. Just
some minor tweaks in wind speed, but nothing substantial. Since
this flow has persisted for a few days now, models have locked on
and are in good agreement. This means high confidence in the
forecast. Issues this pattern presents continue to be heavy snow 
showers making aviation forecasting difficult and freezing spray 
moving south in the Bering.

A couple of smaller interesting features are working their way
through the eastern Aleutians today. A compact polar low quickly
pushed southeast across the Aleutians this morning, bringing snow
and gusty winds with it. Models picked this up the past couple of
days, so was interesting to watch it pan out. There is currently 
a well defined wave working its way east over the eastern 
Aleutians, bringing snow with it. It will push east past the 
Aleutians tonight.

The weak low hugging the coast will keep working its way south, 
sustaining this pattern until Friday. Friday will see a shift in
the wind to a northeast direction, as the low currently working
south passes by and is absorbed into a much larger and deeper low
moving west to east on the south side of the Aleutian Islands. 
Even with this shift, the general northerly flow means not much 
change in the forecast.

&&

.LONG TERM FORECAST (Days 4 through 7)...

The forecast period commences with the longwave trough extending
from Southwest Alaska to the North Pacific and Gulf Of Alaska. On
the 16th of December at 12z the GFS at 500 mb, has a closed low 
churning over the Gulf Of Alaska and a second one south of Adak. 
Generally speaking, expect this pair of disturbances to push 
eastward. The primary sensible weather highlight for this forecast 
package will be another round of snow for Anchorage and Girdwood 
Sunday and early Monday. That said, not expecting much headway 
towards catching up to the normal climatological snowpack for the 
month of December. The ECMWF has come into line with the GFS and 
the GEFS ensemble mean early in the forecast period. However, the 
consensus begins to erode rapidly by 00z Monday at least south of 
the Aleutian Chain. By Tuesday, the global models ping into 
another round of cyclogenesis near Kamchatka which will bring a 
front into the Western Aleutians actually warming up places like 
Attu, Shemya and Amchitka.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales...119 120 130 132 139 150 155 165 172 174 176 177.
 Heavy Freezing Spray...140 160 181 185.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...RA/RMC
SOUTHCENTRAL ALASKA...AH/BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BB
LONG TERM...PJS